metal melting furnace - 1991

5/11/2018 Metal Melting Furnace - 1991

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H O W T O B UILDA M ETAL M ELT ING

FURNACE

Pu b lis he d b y :T R END L IN E PUB LIS H ING COMPANY

( A D iv is io n o f T re n dlin e In te rn a tio n al P ty .lId .)G .P .O . B o x 1 8 89 , S y dn e y, 2 0 01 . A u str alia .

@Co p y rig h t 1 9 91 . T re n dlin e In te rn a tio n al P ty .lId .

R ep ro du ctio n p ro hib ite d b y a ny m e th od w ith ou t p rio r p erm is sio n In w ritin g fro m th e P ub lis he rs .

T h is m a n u a l is c o m p i le d f r o m a s p e c ia l p ro je c t h e a d e d b y J . T . A llo w yn a nd c o m m is s io n e d b y T re n d lin eIn te rn a t io n a l P ty . L td . f o r a d a p t in g t h e p a te n te d d e s ig n in to a c om p a c t lo w c o s t u n i t c a p a b le o f b e in g b u i l ta n d u s e d in th e h om e w o rk s ho p . T h e in fo rm a tio n c on ta in e d h e r e in is b e l ie ve d t o be a c c u ra te , b u t is s o ld a sin fo rm a t io n o n ly , a n d n o l ia b i l i t y is a c c ep te d b y e it h e r t h e P u b l is h e rs o r R e s e a r c h a n a ly s t s .

P U B L IS H E R 'S N O T EThis unit is protected by palent administered by our parent company, Trendline International Ply. Ltd. We have full publishing righls,and the purchase of this manual entities you to build one for your ow n use. However, if you intend manufacturing for resale, a smallroyally applies and you should write to Trendline International P ly . Ltd. and ask fo r a Royalty Agreement form to be posted to you.

W A R N I N GMe ta l m e ltin g is d a ng e ro u s. W e a r p ro te c tiv e c lo th in ga nd p ay p artic ula r a tte ntio n to a ll s afe ty p ro ce du re s.


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1.

INTRODUCTION.

The design of this furnace is relatively simple and straightforward, but never theless is highly efficient and capable of industrial quality output, and will giveyou manyyears of lasting plea lee and trouble free service.This is small enough for the homeworkshop, but big enough for small scalecOmffiercialproduction.It is capable of producing very high quality castings in alltypes of commonlyknowncasting metals, and will take up to a size AlOmeltingcrucible.(Melting pot). An AlOcrucible holds 2 litres, and is a big capacity forbackyard use.It is as big as can be safely handled by one manas far as aluminiumoutput is concerned. (It is advisable to have an assistant helping if you aremelting a .fu.Ll,charge of cast iron due to the extra weight, and the fearfulsearing heat of this white hot molten metal.)Wehave designed the furnace at this size because a lot of the castings that youmight want to do require this amount of molten metal to form one casting. Itemssuch as machinery casings,. mounting plates, and other specialised and customdesigned equipment such as lathes and milling machines which-yoll can buildyourself. Also, if you take up small scale production runs such as supplying carclubs with custom castings or reproductions of hard to get parts, you can pour allthe small separate castings in one melt, thereby saving considerably on time, fueland effort.This type of furnace is best described as a solid fuel, crucLb.l.eE\lrnace. Thismeans the fuel used is solid such as coke, coal or char. (Char is a type ofcompressed coke. It is often used instead of wood for stoves and heaters.) Theterm "Crucible Furnace" means the metal is melted in a crucible, ie. a meltingpot rather than melting in direct contact with the coke as is the case with themore popular cupola type furnace. The cupol~ type furnaces were more popular forcontinuous melting because the coke and metal is shovelled in on an alternatinglayer basis with the burning coke in direct contact with the metal. The moltenmetal flows to the base of the furnace where it is periodically tapped off. Whilethis type of furnace is better suited to continuous melting it does not producethe purity and quality of the standards which are possible in the cruciblefurnace. In the cupola, the molten metal tends to absorb impurities from theburning coke. An additional disadvantage is the difficulty in experimenting withchemical additives and steel alloys . the problembeing that the chemicals willeither change or vaporize with the direct flame contact, and steel will oxidizeand actually burn when exposed to the combination of .1ntense heat and the airblast. However, in the crucible furnace the molten metal is protected inside themelting pot, and in.simplified terms 'what you put in is what you get out'.It has been designed so that you can melt and cast all types of commonmetalsranging from the low melting point metals such as lead, zinc and aluminiumup tothe highest range which includes cast iron, stainless steel, carbon steel, mildsteel and cast iron / steel alloys where the pouring temperature reaches l550C.At this temperature the colour of the furnace interior, the melting pot and themolten metal has gone beyondbright red and light yellow to turn into a brilliantdazzling white.The construction of the furnace is relatively simple, and can easily be built bythe average homehandyman. It is basically a 60 litre (12 gallon) oil drumforming an outside casing which holds the furnace lining and internal components.The material used in the construction is high quality industrial refractorycement, knownin the trade as "Refractory Castable". This forms the main liningof the furnace, and incorporates the air blast grates, and an adjustablecentre post to support the melting pot. A sheet of 25mmceramic fibre is


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~d as additional insulation between the refractory cement and the drum wall.,~is particular type of refractory material is quite common in the furnaceindustry, and can be purchased from most furnace equipment suppliers. Therefractory cement is called "Duracast 1650", and the insulating fibre is called25mm Ceramic Fibre, LT900 grade. ( This ceramic fibre is the same grade as is usedin the portable stove ).If you have difficulty getting this material from your local suppliers, you canbuy it di rect fro m Field Furnace Refractories Pty. Ltd. CrescentSt. Granville, 2142 (Sydney). Ph. (02) 637-1001.They are in Goodlet &Smith brickworks in Crescent St. Granville.. They will supply you direct atdiscounted industry prices. Ask for Alan Jones. He will give you an up to dateprice, and will organise freight and delivery to anywhere in Australia.The prices as at the date of publishing this manual is $32-50 for a 25kg bag ofthe Duracast 1650. (You will need 5 bags ), and the ceramic fibre sheets measuring600mm x 900mm are $8-00 each. ( You will need 2 of these).T hi s p ar ti cu la r c om b. Ln atLen of materials has been chosen because it is easy touse, and will withstand around the clock working temperatures of up to 1650C.Although this cement is considerably more expensive than the lower temperaturegrades, it is essential that you use the Duracast 1650. This furnace reachestemperatures in excess of 1550C, and cement below the 1650C grade are likely tosoften, in some cases melt into a black treacle like substance which will ruinyour furnace and necessitate a partial rebuild. Note; It may be your intentiononly to melt low temperature metals such as aluminium. Even in circumstances suchas this it is still essential that you use the 1650C. The reason being that ittakes a while for the temperature in the melting pot to reach it's proper pouringtemperature. During this heat build up, the temperatues in the furnace,particularly around the centre post may be up around 1500-1550C. This isparticularly so when you are doing quite a lot of melts, and the furnacetemperature is well up, and a cold pot of a.Lumf.num is put in. The pot will becomparatively cool, yet when the air blast is turned on the centre post with itsretained heat content will be glowing bright red or even light yellow within a fewminutes. (Light yellow is around 1350C), and as the aluminium reaches its pouringtemperature of around 760C, the lower portion of the centre post will be upa ro un d 1 50 0 C.BASIC CONSTRUCTIONDETAILS .

This furnace is designed for easy low cost construction using readily availableparts. Apart from the refractory cement all other parts can be obtained for verylittle cost, and in some cases no cost at all.Basically the furnace is a 60 litre oil drum lined with refractory cement, andrunning on either charcoal, barbecue heat beads, coke, coal or any similiar typeof solid fuel. It is a forced draught unit, with the air being supplied by eitheran old vacuum cleaner in reverse cycle, ie. blowing air, or any other air blowingdevice. The lid mechanism is made using old water pipe or steel tube.

The trolley is optional. We have made up this trolley using old scrap angle iron,(see photo) with the wheels being salvaged from a garbage throw out, and the sandand tool trays made from the sides of an old washing machine. It could have beenall screwed, rivetted or bolted together, but we have welded it simply because itwas quick er a nd easi er.The trolley is useful for wheeling it in and out of the shed. However if you have


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3.a well ventilated weatherproof area to position and use the furnace as a permanentfixture, then the trolley is of no value. In fact, a permanent fixture is moredesireable because you can set up your sand area and molding trays for anefficient work output, and have all your refined ingots stacked and labelled forquick reference and selection.This furnace is designed for ease of construction. In accordance with our usualcriteria of design, scrap material is used extensive1y in our projects.In this particular project, the design is quite flexible. This means you can usewhatever scrap material will do the job in whatever layout you prefer to suit yourshed, and your style of work.However. there are certain essential requirements are far as design and .ateria1are concerned. ego The furnace interior design and specified materia1.Throughout this manual, alternatives in materials and design are given. Thisenables you to look around your shed and pick out a likely piece of scrap and useit. This is much more convenient than running around the shops chasing up newparts., and of course the big advantage is old scrap pieces cost you nothing.Where a specific design or procedure is required. then no options ora1ternative ideas are given. and in this case it is essentia1 thatour specifications are fo11owed.

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CONSTRUCTION PROCEDURE

The furnace consists of two main components, ie. the main furnace chamber and theair supply. The trolley and sand tray are both optional, and you can organizethese to suit yourself. (Suggestions are given later in this manual).

The Furnace Chamber.The furnace chamber is made using a 60 litre oil drum. (These drums are free frommost service stations and car repair places. They are stamped "Non Returnable" onthe bottom, and are usually thrown out). Before starting work on the drum, cleanit out thoroughly with hot soapy water, and allow it to stand with water in it fora few days. The reason being, the drums contain a variety of 1iquids,eg.automatictransmission fluid, degreaser, radiator rust inhibitor etc and in addition tothat, sometimes dirty petrol and other solvent waste is tipped into them. Emptydrums are dangerous. Always clean them thoroughly inside beore starting work onthem.The Procedure is as Follows;1. Mark a line around the drum, 75I,1I1IIown from the top. (The easiest way to dothis is to make a scriber. Drive 2 nails into a piece of wood, 75mm apart. Onenail is well out, while the second nail has only its sharp end protruding. Thelong nail rides around the top of the drum while the sharp end scribes a neat linearound the drum.2. Mark out a "V" shape as per the photos. The top of the "V" should be about100mm wide on the 75mm line, extending to a depth of about 125mm, measured fromthe 75mm line. The bottom of the "V" should be rounded to neatly cradle a I"water pipe or whatever size pipe you are using to extend the cast iron saucepanhandle.3. Cut around the scribed line, taking care to cut down around the "V" shape .

..This top section of the drum becomes the lid of the furnace. Put it aside for thetime being, and continue working on the drum attachments.

Making the Air Blast Inlet.The only specific requirements for the air blast inlet are;1. The bottom of the pipe should be 60mm above the floor of the drum. Thisenables a 60mm depth of cement on the furnace floor without interfering with theair flow.2. The inlet pipe should aim directly at the centre of the post. (This splits theair blast, and gives an even pressure in the expansion chamber under the grate).3. The inside diameter of the pipe should be not less than 25mm, and not morethan 35mm.We have used I" (25mm) water pipe offset to one side to give more space forstoring tools "and ingots on the trolley tray. An elbow is used to give the properentry into the drum. A piece of flat steel is welded to the elbow, then rivettedto the drum wall. Slots are cut in the small section protruding into the drum.This is to allow expansion of the metal without cracking the cement. Note; Apiece of PVC tubing is slipped over the end, and extends 90mm to the centre of thedrum. This is to form the inlet hole when the "cement is poured. (It burns awayleaving a neat inlet pipe in the cement wall. If the steel pipe extended throughit would cause cracking due to its faster expansion rate, and the possibility ofthe end drooping with the heat, and restricting the air blast.)


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5.Making The Ash Exit.The requirements for this are as follows ;1. The ash exit can be positioned' at any point around the floor, provided it isbetween the 75mm thick furnace wall and the 100mm thick centre post.2. A short piece of I" threaded water pipe is ideal, but a larger size is OK. Astandard galvanised cap screws on from under the drum to close it off airtight.Again, we welded a flat piece of steel to it, then rivetted the steel piece to thedrum. Note; We1ding is not necessary. As app1ies to the air in1et a1so, as ta nd a rd t h re ad e d c o .L La r -c a n be tightened on each side of the drtllll.a11. Ie, Thethreaded end of the pipe extends through the drum wa11, and a c o. l. La ri s t ig ht en edup on each side.3. A piece of PVC or cardboard tube 60mm long, with an outside diameter the sameas the inside of the ash exit pipe is positioned against the short protruding endwhen cementing. A short piece of old broomstick holds the tube in alignment.Note; The tube is not slipped over the pipe as is the case with the air inlet.The reason being when it burns away, a ledge is left which can jam or trap hardpieces of coke clinker. It is not a problem, just an annoying irritation whencleaning out the ash.

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The Inside Lining (Cement Formwork).This lining is made from any type of scrap to form a cylinder 225mm (9") indiameter extending from the floor of the drum to the cut off top. This leavesspace in the drum for a wall thickness of 75mm (3"). We used sides of old oildrums, and pop rivetted them together.T he r eq ui re me nt s a re ;1. The formwork has to be strong enough to take the cement being poured andr am me d d ow n.2. We rivetted the formwork together from the outside with small pop rivetts. Toremove the formwork after cementing, the rivetts were wiped off' with an anglegrinder from the inside, and the seam came open.3. Using scrap wood of any thickness over 8mm, cut out 5 or 6 circles of 225mmdiameter to brace the formwork from inside. (Drill a 25mm hole in the centre ofeach to facilitate easy positioning and removal with your fingers or a hook.4. Cut a hole near the bottom of the lining to allow the air inlet PVC to pokethrough.4. Cut out the "V" shaped opening taking care that it lines up with the "V" inthe drum, and the air inlet pipe.This is as far as you can go with the drum at this stage.


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Making The Lid.You will be opening and closing the lid quite a lot during the working hours ofthe furnace. It is therefore desireable, though not essential that opening andclosing the lid is an easy comfortable operation. It is for this reason that wehave designed the double hinge arrangement. We have it hinged on part of thetrolley frame. It is hinged for up and down opening plus sideways rotation. Thelid has to lift up to clear the "V" shape, then swing sideways so that the hotunderside of the lid is not radiating a fierce white hot heat up at you as you arereloading or refuelling the furnace.However, you are free to choose whatever hinge arrangement you have handy in yourscrap box. A single hinge arrangement works OK. ie, the lid lifts up and downbehind the furnace. The heat radiation will then be upwards at an angle, which isalright if you stand on the opposite side.Any type of hinge capable of handling the weight of the lid is OK so long as it ismade of metal, and a bit sloppy. (The heat can cause things to go slightly out ofalignment, therefore a tight precisely fitted hinge may not allow the furnace lidto seat properly under hot operating conditions).The Procedure to Follow is;1. Mark out a circle on the top of the drum of approx. 250-260mm diameter, thencut it out. This leaves a rim about 40-50mm wide around the top.2. Snip or cut this rim inwards to make little strips about 10-12mm wide. Bendthem downwards in an irregular manner, and give each a twist with a pair ofpliers. These twisted strips give a good strong key for the cement.3. Attach whatever hinge arrangement suits you best. We welded 2 pieces of flatsteel to a flat strip bent to follow the curve of the drum. This was bolted witht" bolts to another strip inside. Holes were drilled in the 2 welded pieces forthe upward hinge pin. The rotating part of the hinge was made by welding 2 piecesto the head of a 6" x 3/4" bolt which slides down into a short piece of 3/4" waterpipe welded to the angle iron upright.4. Attach a lifting socket to the lid. We used a short piece of old t" waterpipe. A piece of steel was welded to the pipe, then poked through a slot cut inthe shoulder of the rim, then pop rivetted.The rods running back are welded to the underside of the pipe, and welded to theinner flat piece where the hinge attachment is bolted.-Nofe; This method was used purely for convenience. Its purpose is to enable youto slide a rod into the pipe and lift the lid without getting too close to theheat.v, If you are not able to weld, then a piece of flat steel drilled to take a iorod could be ri vetted to each side of the lid., ie. one at the front. and one nearthe hinge. The lifting rod would then slide through both to give a strong liftingpoint without the need to use any strengthening struts, nor the need for anywe1ding.If you decide to use the pipe with welded struts, the welding should be under thepipe, and the struts no thicker than 6mm, and curved out wide so that they arealmost above the 75mm walls (Less heat expansion), and up towards the top of thelid, ie. covered by 25-30mm of c eme nt .Formwork for Cementing the Lid.The lid is the first part of the furnace to be cemented. It has to dry beforecementing the furnace walls. The reason being the lid has to sit upon the wetcement walls, (separated by thin plastic) to seat in, and form a perfect sealagainst the furnace walls.The Procedure for the Formwork is ; I1. Nip the point of the "V" in two or three \places, then bend it up so that thepipe handle of the cast iron saucepan will sit freely in the space when the lid isin position on the drum.2. Attach a strip of wire netting (we used the thin 25mm square type) to the bentup pieces of the "V" ,and wire it up at an angle to attach to either the liftingsocket : rods, or the twisted strips around the side. (The reason for the wire


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7.netting is to add reinforcement and impact strength to the "V" section of the lidin the event of it being accidently dropped down hard when shutting.3. Lay the lid upside down on a board. Cut out indentations for the liftingsockets, so that the lid lays f1a~ on the board. (Paint the board with any type ofpaint you have. This waterproofs the board and prevents moisture absorbtion fromt he c em en t, w hi ch w ea ke ns i t c on si de ra bl y. )4. Cut a 225mm circle out of a piece of board approx. 18mm thick. The purpose ofthis is for forming the heat sealing ledge on the underside of the lid. Cut out asection for the "V" shape, then paint all parts of the wood which will be inco ntact wi th the cemen t.5. With the lid sitting firmly on the board, place a 100mm diameter jam tin (orslightly smaller) in the centre. This becomes the exhaust hole. (We placed thetin open end down. When the cement was dry, we ran the angle grinder cutting discaround the tin slicing the closed end off). The tin sticks in the cement, so ifyou don't have an angle grinder, cut the bottom out with a tin opener after thecement dries. It does not matter if a jagged edge is left, because the heat will.quickly burn the tin out anyway.Mixing and Pouring the Ceaent.The procedure for the Duracast 1650 is exactly the same as that for ordinarybagged concrete.Pour the cement into a large tray, or non absobent flat surface, then add waterand thoroughly mix to the same consistency as a good well textured concrete mix.It is advisable to dry mix the contents of the bag before using it, becausesometi mes duri ng tra nsport the vib ration can separ ate out the chemic al aggre gatesin the bag.When mixed, the cement should be wet enough to "flow" with a bit of prodding andtamping.Thoroughly prod and tamp the cement in around all the reinforcing and metal tabsof th e l id, t hereby elim inatin g a ny possib le air bubble s.Shape up the "V" protrusion using a small hand trowell. Shape the pipe inlet witha piece of pipe the same diameter as the pot handle, taking care to incline it atthe same angle as it would sit in the furnace, ie. When the saucepan is latersitting flat on the centre post, the handle will be inclined upwards. (Remember,the lid is upside down when being cemented, so take care not to incline the handlethe wrong way.) Sit the painted 225mm cut out in position. It will sit flat on the cement filledlid, with the cemented "V" shape protruding. Fill with cement, prod tamp andsmooth it off.Leave it stand in a cool shady place for 2 or 3 hours, then cover with wet ragsand keep it damp for 24 hours. Remove the rags, and allow to dry for a day or so,preferably in the sun on the second day.You are now ready to begin cementing the main furnace walls.Cementing the Furnace Walls.The furnace walls are 75mm thick, made up of a layer of Ceramic Fibre 25mm thick,which will compress to about 18mm when the cement is rammed down. The ceramicfibre is an essential part of the furnace design. Without the fibre the heat losswould be too great, and it would be difficult if not impossible to reach the hightemperatures which this unit is capable of. The cement takes up the remainingwall thickness.The Procedure is ;1. The ceramic fibre is placed against the drum walls.. Cut it to fit beforemixing the cement. Cut out the air intake hole, and the "V" shape. Where thefibre jOins, cut it slightly oversize so that the fibre is "compressed together"where it butts up against itself.


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2. Sit the ceramic fibre in position, install the inner lining and position thePVC tubing for the air inlet. Give it a couple of taps with a hammer to locate itfirmly on the pipe stub, then poke a wooden rod to locate it firmly just in caseit slips off when tamping down the cement. Kote; To prevent the ceramic fibrefrom absorbing JIOisture from the c~t, either spray it with a fine water sprayfirst, or preferably slip in a strong plastic lining to separate the cera..ic f ibrefrom the cement. This incidently makes the cement filling and tamping mucheasier.3. Install the 225mm wooden circles. Space them out evenly using small blocks ofwood as spacers if they don't stay in position.4. Mix up a good sized batch of cement, mixing thoroughly by hand or in a cementmixer. Prod and tamp it down thoroughly as you shovel1it in thereby eliminatingair bubbles and bedding the cement in firmly. (Note; This cement "goes off"faster than ordinary concrete, so only mix enough to enable you to use it within20-30 minutes.)5. Slightly overfill the walls and form the "V" shape, again slightly oversize.(This allows for the squelching and positioning of the lid as it settles on thewet cement to mold its own shape for a perfect heat seal.)6. Lay a short length of pot handle pipe in position, and cover all the wetcement with a very thin plastic sheet. (Glad wrap or the very thin painters drops he et s a re i de al .)7.Now lift the lid into position and tap it with a rubber mallet or wriggle itfirmly pressing down hard so that the wet cement is forced into the exact surfaceshape of the lid.8. Wipe off any excess that is forced out, then cover the whole thing with wetrags, and leave stand in a cool shady place for 24 hours. Remove the wet rags,and allow to dryf()r 2 or 3 days.Kote; Any left mrer cement can be used for various accessories such as the" es ba us t r ed uc er s s ho wn l at er .- _ ~---Ceaenting the Base and Centre Post.After having removed the formwork from the furnace, you are now ready to cementthe base (which incorporates the air expansion chamber), and the centre post.You will require a few pieces of wire netting of about 25mm mesh size and a shortlength of 100mm PVC sewerage pipe. It is essential that plastic or PVC pipe isused instead of metal because when the PVC formwork burns away, it automaticallyleaves the correct clearance gap for the grates.Most harware and plumbers supplies will cut and sell the l O O m m PVC pipe in onem et re l en gt hs .The Procedure is ;1. Cut a 180mm length of the PVC pipe. Cut the pipe from top to bottom down oneside. Wedge the slot open about the width of a matchstick, then tape the join onthe inside with masking tape or any similiar strong tape. This allows anexpansion gap to take up any heat distortion as the PVC pipe heats up prior tob ur ni ng q ut .2. Cut 4 couple of pieces of wire mesh to cover the base of the drum, and extendabout 180mm up. (This helps strengthen the centre post against any possibility ofbreakage at the base due to rough handling. The base area is air cooled, so theheat will not be sufficient to melt the enclosed reinforcing at this point).3. Cut a 35mm length of 30mm diameter PVC tubing. Slit it down one side, wedgeand tape in a similiar manner to the l O O m m pipe. Now close off the end with tape,and poke a couple of small air release holes through the taped end.This short piece of tube is pushed down into the cemented centre post to form al oc at in g so ck et fo r re mo va bl e ce nt re po st ex te ns io ns a nd ot he r a cc es so ri es .4. You are now ready to mix and pour the cement


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10 .Cement Pouring Procedure ;1. Place in position the ash 'exit tube, locating it firmly in position with apiece of old broomstick.2. Pour about 2Smm of cement to cover the bottom of the drum.3. Position your wire mesh for reinforcing the centre post, and cover with another20mm of cement around the base.4. Position the 100mm pipe, ensuring that the wire mesh is centrally positioned,angled out at the base for maximum stabilizing strength.5. Using a small weeding shovel or trowell, put in a 75mm depth of cement, eachtime prodding and tamping it down with a rod or stick so that the cement settlesfi rm ly a ro un d t he re in fo rc in g.6. While tamping and prodding, slowly lift the pipe up. Continue this slow liftallowing the cement to bulge out to form a ledge about 6-8mm wider than the pipe.Stop lifting when the bottom of the pipe and the ledge so formed is 90-10Omm fromthe metal floor of the drum. (Estimated by your previous measurements and markingsfor the air inlet).7. Now fill the 100mm pipe up to a point where the distance from the top of thecement to the top of the furnace walls is 250mm.8. Push the short 30mm PVC in the end weighing it down with steel offcuts to stopit "floating" in the wet cement. The cement level will rise slightly, but this isOK. This distance of 250mm, or slightly less is to allow sufficient cle~rance foran A10 crucible with a lid, and allow a heat flow clearance space, andun re st ri ct ed ex ha us t e xi t.9. Tidy up the cement ledge, and scoop out a gutter about lOmm deep and using thisscooped out cement plus a little extra if necessary form up a ledge about 8-10mmwide against the furnace walls. The purpose of this ledge is to provide a supportfor the grate. The gutter so formed should be about 30mm wide and about 40-S0mmdeep. Bote. The ledges don't have to be perfectly formed because the cement grateswhen poured will follow the contours of any irregularities.The gutter serves the dual purpose of being an air expansion chamber, and an ashdisposal area.The air should flow without restriction into this gutter, then expands and thepressuraequalizes as the air is forced upwards through the grate slots to give anevenly distributed heat output around the centre post.10. Tidy up the entrance to both the air inlet, and the ash exit. ie. smooth thecement around the edges of these.11. Allow the cement to set for 2-3 hours, then cover with wet rags and follow theprevious drying procedure.Making the Grate.The grate is divided into 4 sections. The measurements given, and designspecifications are of absolute importance. The dimensions of the air blast holeshave been designed for the best possible performance, while at the same time beingvery easy to make. The size and number of slots has been carefully calculated toprovide AN EVEN OVERALL AIR PRESSURE AND BURNING PATTERN.Don't be tempted to make a steel grate on the assumption that it would provide abetter less restricted airflow through the bars. When the air is unrestricted overa wide area it follows the path of least 'resistance ie. it may all flow in anunrestricted manner up in one small section which is burning strongly on one sideof the post. This results in a drop in heat output and problems of uneven heatingand part "chilling" of the crucible. Also,perhaps. of more. significance is that asteel grate in this type of furnace cannot take the heat and will melt and burn inthe first full blast heat run. (We tested a steel grate, and after the furnace waswarmed up, the grate lasted 2 hours at full blast. The steel turned into a moltenmass in the ash pan and burned into a black porous coke like substance.)


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11.The Proc~dure to Follow is;1. Wrap a piece of thin plastic around the centre post extending from just belowthe ledge to about 60mm above the ledge. Do the same on the outer ledge. Thepurpose of the thin plastic is to allow the cement for the grates to follow theledge contours without sticking to them. (The grates are lifted out to enable theeasy cleaning o-f the ash pan. )2. Fill the air expansion gutter with sand until it is level with the ledges, thenwet the sand so that it settles into a smooth solid support for pouring the cementon. (The sand is later drained off to leave the clear gutter under the grates.)3. Divide the sand circle into four sections using pieces of wood 5-6mm t hic k, a ndwide enough 1; :0 seal it off. Use pieces long enough to grip for lifting outlater. Wrap a piece of plastic over the end and up each side so that they are easyto remove when the cement sets.4. In the middle of each section place another piece of wood about 5m m x 25mm withrounded corners. Use short pieces of wood because they will be hard to move whenthe cement dries and shrinks on them, and consquently it is easier to burn them outwhen drying out the cement. (Enough air will come up through the four dividers).5. Cut pieces of mesh to reinforce the four grate sections.6. Mix and pour the cement to a depth of about 40mm, ensuring that the fourdividers and the four lenthwise slot formers are in position. The reinforcing meshis placed just below midway, ie. it is covered by about 25mm of cement. (The airflow under the grates keeps them cool enough to stop the reinforcing from meltingi ns id e t he ce me nt ).7. Using some left over cement make up a "plug" for the saucepan handle hole. Todo this, lay thin plastic such as cling wrap in the hollow in the furnace wall,then wrap it over into a rough pipe shape with the plastic and shut the lid pushingdown hard. Any excess cement will squeeze out each end. Wipe this excess off. Itis useful to have a looped piece of wire in this "plug" to use as a lifting outhandle. The wire can be poked in after forming or laid in before forming it up.8. You have now completed the major part of the furnace construction. Make yourFurnace Accessories now so that they can be drying out while you are doing otherthings. ( Refer to .tIE "Furnace Accessories" section.)

THE AIR SUPPLY.There is almost unlimited scope for making up the air pump using a wide variety ofreject scrap items. You can even use your household vacuum cleaner without anymodifications at all providing you can blow air with it. (Most have a reverse cyclesocket which the vacuuming hose plugs into).We used the fan and fan housing off an old GEC upright vacuum cleaner driven by ahigh speed washing machine pump motor. (The original vacuum cleaner motor was burntout when the unit was salvaged from a rubbish dump).It is mounted on its side with a plastic 5 litre oil container for the motor cover,and an old Swimming pool pump hose enabling a flexible vibration absorbing hose tot he f ur na ce .The motor speed is controlled by an ordinary 50 0 Watt light dimmer. The air blastcan therefore be varied from the slightest whisper of air up to full blast.The Air Supply Requirements Are;1. The volume and pressure of air at full blast should be equal to a good qualityvacuum cleaner, (With a clean bag and filters blowing in reverse cycle).2. You need to be able to vary the air flow, either by varying the motor speed orby a baffle or release arrangement. (Refer details later).3. A flexible air line should attach to the furnace pipe to avoid transmittingvibration from the air blower.4. 200mm of metal pipe should extend from the furnace to handle any heatt ran sf er ri ng f ro m t he f ur na ce .


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12.

Various plower Options.There are quite a lot of blowers around that can be salvaged from wrecking yards orrubbish throw outs, which include;VACUUM CLEANERS: Old vacuum cleaners are ideal. If they have a reverse cycle socketit saves you having to modify it. (Most vacuum cleaners have this facility to blowair). If not, take it apart and reverse the ends so that the suction end becomesthe air intake, and instead of blowing the air into the dnst bag, it blows outthrough a pipe into the furnace. Be sure to clean all filters thoroughly for anunrestricted air flow. In some cases they can be thrown away because you are notinterested in collecting dust with a modified machine. (Refer to the "SpeedControl" section for details on controlling motor speed).CAR HEATER FANS. These generally provide a good air flow, and have a variablespeed set-up already built in. You only need to change the shroud and attach theair discharge hose. Usually it is quite straight forward. Use a few pieces ofscrap tin, pop rivetts and a bit of flexible putty or filler such as "No MoreGaps".OLD CLOTHES DRYERS. The blowers from electric clothes dryers provides a verygood air flow. Disconnect the heater element and use only the air blower.SPEED CONTROL.Most vacuum cleaners and small high speed motors such as in electric ~drills andhigh speed washing machine pump motors (Not the heavier drive motor) are the"uni versal brush type motor". ie. They are not induction motors. The speed ofthese universal brush type motors can be varied and set to run at any speed byusing a light dimmer as the speed control.However, the ordinary light dimmer is usually limited to 500 watts. If your motoris over 450 watts you will probably overload the dimmer and burn it out. You canbuy lamp dimmers up to 1300 watts and possibly higher from your local electricalwholesaler. Most wholesalers stock HPM products, or if not they can easily getthem. The HPM 5 amp, 1300VA Dimmer is a good all round unit. It is a bitexpensive when compared to the ordinary 500 watt unit, but it is solid and wellmade with cooling fins added. It is made to fit a standard mounting board and canbe connected up as an "in line controller". ie it can be mounted on a board withplug in sockets and a three pin plug. This means you can plug your ordinaryhousehold vacuum cleaner into it and vary the speed as required. Note; Thesedimmer units only work on the universal brush type electric motor with the standardon/off switch. If other electronic controls are incorporated in the appliance,check with the lIaIlufacturers first. These diDlDers will not work on the heavyinduction type motors.AIR BAFFLE OR RELEASE:You may have a constant source of air from an old blower powered by an inductionmotor where speed control is not practical, or an industrial air compressor wherethe air supply is fixed and constant. Depending on the blower type, the optionsare;1. If the motor does not rely on the incoming air flow to cool it, then fit anadjustable restrlctor to vary the incoming air, thereby controlling the outgoingair blast. Do not use this method if the motor is cooled by the inco.tng air flow.2. An air release set-up similiar to that used on the Blacksmiths Forge (ManualF-52) is probably the easiest, cheapest and most convenient way to control and varya constant air flow from a fixed speed fan, particularly if it relies on the airflow to cool the motor.Fit a short piece of I" water pipe to the outgoing air pipe. Screw a "T" piece toit so that the air flow can travel in an unrestricted manner through the straightpiece of the "T. Fit a gas tap or ordinary big bore water tap to this straightpiece. The right angled part of the "T" goes into the furnace.The principle here is air takes the path of least resistance which is straight outthe open air tap. Providing the bore of the open tap is not less than the pipe,then very little if any air at all goes into the furnace. When the tap is closed,


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14.the air blows into the furnace. The air flow into the furnace can be regulated bypartly opening. the tap which then allows some of the air to blowout which thenreduces the quantity and pressure blowing into the furnace. This method is cheapand convenient, and works well when your air supply is strong and constant.PRELIMINARY BURNOUT.The main furnace structure is complete and the air supply is in operation, so youare now ready for a furnace warm up and preliminary burnout.T he D ur ac as t 1650 refractory cement cures by a two part process. The cement setsby the normal setting and hardening, then the second stage is the ceramic bond orsintering process. ie. particles in the cement actually melt and bond together.The Procedure is;1. Lift out the wooden grate separators, and blow the sand out the ash exit. (Usethe furnace blower or compressed air.)2. Close off the ash exit, and connect up your air supply. Ensure that a strongair flow comes up through each of the four separator slots, then switch back to avery gentle air flow.3. Light a piece of screwed up newspaper and a few light sticks. The drying outprocess is very important. The heat 8USt be increased ~ry slaw1y, ie. at about40-50c pe r hour to avoid the retained .aisture in the ce.ent expanding as steaaand b10wing pieces of ceaent off. (Known as steaa spalling.)4. Keep the fire barely alight, ie. a few flickering flames licking around. Thenafter an hour start to increase it. The PVC around the centre post will start tosoften and buckle. Drop in a few more sticks and disconnect the air. (The firewill draw enough air from the short metal inlet to keep burning slowly) Then let itdie out, and leave it until next day. The reason being the PVC will buckle backfrom the centre post allowing trapped steam to escape. If the fire was allowed tocontinue, the heat against the exposed centre post where the moisture has beentrapped by the PVC would be too severe.5. The following day, light another fire with newspaper and sticks. Build it upto the previous days heat, then increase slowly. Let the air blow very gently, orcut it off completely to maintain what could best be described as a cool fire, iedrop in enough wood to absorb the heat then supply only just enough air to keep itgoing. After I i hours increase the air flow, and the smouldering charred wood willflare up. Give it a short burst of air, then cut it back to keep this higher heatat a reasonably constant level for another hour. Continue this heat at ep-upprocedure all day. Late in the afternoon you should have a normal hot free burningfire. ie, The fire should have only enough air to burn in the same manner as anormal open fire would burn.(Too much air will cause the heat to go too high.)Disconnect the air pipe, cover the exhaust hole with a brick allowing a small spacefor the smoke to escape and leave overnight.6. Next day clean out the ash. (Use a hooked rod to lift the grates out.) Theymay.be stuckvith the half burned PVC. down around the base of the post.If so ,vacuumand blow the ash out through the slots.7. Light another fire bringing it up to the previous days temperature, then addcharcoal, barbecue heat beads or Char. (Do not use coal or coke at this stage).Turn the air up to heat the fuel, then cut it back to a trickle and leave for anhour. Now add more fuel to bring it up level with the top of the post. Keep agentle air flow going for 2-3 hours. The heat will gradually increase until thecentre post is red hot. Maintain this heat, increasing it slightly over the nextcouple of hours. Shut the furnace off and leave it overnight.8. Next day clean out the ash gutter. (The grates should now lift out easily).Push out the charred wood remains of the middle slots.9. Block off the saucepan handle hole with the cement plug.10 . Clean out the charred rubbish in the locating hole in the centre post and fitthe temperature test crucible. (Refer Furnace Accessories). Put a piece ofcardboard between the crucible and the centre post. The cardboard burns awayleaving a thin film of carbon which prevents the crucible sticking to the centrepost.11. Heat the centre post and crucible to a bright red, and maintain this heat for


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15 .two hour~. Fill the. furnace with either coke or char ensuring that it is packed upas high as possible around the test crucible.12. Turn the air up to full blast, and put the 50mm exhaust reducer in position.Take the temperature up to a light yellow / white over a period of 2-3 hours.Switch off and poke the burnt fuel down and refill, again stacking it up higharound the crucible. Place a few pieces of cast iron, stainless steel, high carbonsteel ego pieces of an old file, and then add pieces of mild steel. eg pipe and baroffcuts, bolt heads etc. then add enough broken glass to form a layer 8-10mm thickwhen it melts. Turn the air up to full blast, again using the 50mm exhaustreducer. IMPORTANT NOTICEYOU ARE NOW:AP l ! l lDACl l IKG TIlE HIGHER METAL MEL'fiNGHEATS . - Refer tothe Safety Procedures WEAR GOGGLES AND PROTECTIVE CLOTHING.DO NOT UNDER ANY CIRCUMSTANCES ALLOW WATER OR ANY MOISTURE OF ANYTYPE NEAR THE FURNACE. THIS INCLUDES PERSPIRATION. WEAR A HEAD BANDIF PERSPIRATION IS LIKELY TO DRIP DOWN. A SIIall. droplet of .,isturefa11ing into IIDlten Jleta1 will cause an instantaneous explosion blowing superheated .,lten meta1 into your face. ("!'be water turns to steam instantly withsuch an explosive expansion that molten IIeta1 is blown out with great force.)

THE CERAMIC BOND SINTERING PROCEDURE.This second and final process of curing the Duracast 1650 starts at 1500C, andreaches its maximum strength at 1550C. This heat has to be maintained for onehour or longer. It takes about an hour for the heat to penetrate the furnacewalls. The easiest way to monitor the heat levels is by placing various metals inthe Duracast crucible.(Refer to "Furnace Accessories"). The heat has to penetratethe cement crucible to properly heat the metal. This will take quite some time,and as the furnace interior reaches a white heat, a darker colour range, ie.bright red and yellow will be seen around the crucible and its contents indicatingthat the heat has not fully penetrated the cement. NOTE: SWITCH THE AIR OFF,AND ALWAYS USE EXTREME CAUTION WHEN OPENING THE FURNACE, IE. WEARGOGGLES AND PROTECTIVE CLOTHING, AND DO NOT LET ANY MOISTURE FALL IN.Arrange the metal pieces so that you can identify which ones are melting. ego usea piece of angle iron offcut for the mild steel, an odd shaped piece of castiron sticking up etc. As they melt, they will disappear into a molten pool underthe molten glass. The glass will melt first, and form a thick sticky protectiveblanket over the molten metal thereby lessening the possible danger fromaccidental moisture drops. A secondary purpose is to assist in reducing theoxidising and burning of the steel.As the temperature rises and penetrates the cement crucible and its contents, theglass melts first followed by the cast iron at about 1250C. The stainless steelmelts at about 1400C, followed by the carbon steel at about 1460C, and the mildsteel will be last at about 1500C.When the furnace interior is one brilliant dazzling white mass without any darkershadows, the furnace has reached 1550C. At thfs point, all the contents of thecrucible would be a molten mass.SPECIAL NOTICE: DO NOT ATTEMPT TO REFUEL THE FURNACE AT THIS POINT.As explained previously, it is best to take the furnace and crucible to almostwhite heat, then poke down and refuel. The fresh fuel absorbs and shields the heatenabling you to position the metal pieces and glass fragments. The refuelling atthis point gives enough heat for long enough to bring about the sintering andceramic bond in the cement.When the mild steel melts, cut the air blast back enough to maintain the heatwithout burning out the fuel with an excessive air blast. Keep the blast goinguntil the fuel burns out.Allow to cool with the lid shut. When it has cooled down, lift out the crucibleand tap it face down on something solid. The solidified metal will probably fallout. If not, then break the crucible with a hammer to enable you to thoroughly


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16.check the metal to ensure that all pieces have melted. The top layer will consistof solidified' dross and slag mixed with blackened glass. Note; Do notexpect a perfect metal texture. This procedure is used ONLYTOMONITOREATPENETRATION.It is not a metal casting procedure. (Special addi ti YeS andtechniques are required for casting these high heat type metals. )FURNACEACCESSORIES.Apart from the obvious accessories such as a poker, lifting rod for the lid, hookfor lifting out the grates, and the saucepan handle hole plug explainedpreviously, you will need the following special accessories.1. THE EXHAUSTREDUCERS:The purpose of the exhaust reducers is to limit theheat loss through the exhaust. The 100mmexhaust is necessary when starting upthe fire, but when it is running at full blast with all the fuel hot and wellalight, the heat loss with the 100mmexhaust is so great that it is nearimpossible to reach steel melting temperatures. The exhaust reducers contain theheat while allowing a ful1 air blast into the fire. Note; depending on your airsupply, it may be necessary to experiment with a couple of different sizedreducers. The reason being an air compressor will supply constant pressure/volu.eregardless of fuel density and restriction fro. the reducer whereas a vacuumcleaner whilst capable of supplying volume cannot overcome pressure restrictionstherefore the size of the reducer mayneed to be smaller with a more forceful airblast~,such as that supplied by an air compressor.The two most useful sizes with a vacuumcleaner air blast is a 5 ( 1mm . and a25mmdiameter. The reducers are made from the Duracast 1650. They have a locatinglug 18mmdeep so that they sit in the exhaust hole. To make the formwork for thelugs cut a 90-95mmhole in a piece of 18mmboard, one for each reducer. Cut theend off a plastic container or plastic pot plant container to use as the outerformwork. Sit this centrally over the 95mmhole, and stand a rod of the chosendiameter upright in the centre. Cut small pieces of wire reinforcing, using acouple of loops to extend upwards for lifting hooks. then pour the cement., (Usesome type of burnable material for the 50mmand 25nun'centre formers, because thecement will shrink slightly and make them difficult to remove. Also remember topaint all absorbent material such as wood or cardboard which comes into contactwith the wet cement.)Whenthe cement is dry, sit the reducer in pOSition on the furnace during the warmup to dry out all moisture. (It maybe necessary to drill a i' hole through thecentre former if you are using solid wood. This enables the fire to burn throughmore quickly.) Thewoodenbase board will burn awayby itself. Do not attempt toprise it off, or you will crack and break the locating lip.THE TEMPERATUREEST CRUCIBLE: This test crucible is cast from the Duracast1650using any type of moldwhich-will give an outside diameter of 100-140mmand aheight of about 150mm.(Apiece of the 100mmPVCpipe is OK.)To makea locating lug for this crucible, dril1 a 25mmhole in a piece of wood20-25mmthick. Using a sharp knife or rat-tail file taper the edges out to justunder 30mm.Paint areas exposed to wet cement. Place the 150mmormer in positioncentred over the hole. (The hole forms the cement locating lug, the tapered edgesadd strenth and also assist in quick positioning.)Pour the cement, and push in a tapered vessel such as a small disposable plastictumbler. Push it down 50-60mm,adding weights if it tends to float. Tap theinside, or wriggle it to remove air bubbles which can be seen through the clearplastic. Note; Do not use glass tumblers or solid items because the tighteningeffect as the cement shrinks slightly during drying makes them impossible toremove.THE CENTREPOST EXTENSIONBLOCKS:The purpose of the centre post extension blocks are to enable you to use a varietyof different sizes in melting pots and cast iron saucepans with fixed extensionhandles. (The cast iron pots are for aluminium and other low melting pointmetals. High melting point metals such as cast iron and steel require specialcrucibles made from a clay graphite compound. They come in a range of standardshapes and sizes from as small as an egg cup up to a tonne or more.). This furnace


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17.all sLzed crucibles up to a maximum of size A IO . A very important additionalfeature is that the furnace is designed to take cast iron saucepans in the two mostpopular sizes that are normally stocked in army disposal and camping stores.The centre post extension blocks are made in varying sizes to suit the height ofcast iron saucepans, or to add height to the centre post to allow greater fuelreserves for running continuous melts without having to refuel so often. (This isparticularly useful if you are using charcoal as it burns away quicker than coke.)The lOOmm PVC pipe is used as the main former fol!'the. extension. The locating lugis made in the same way as the test crucible. The easiest way to measure the exactheight required is to cut a piece of wood and trim it to a height that allows thesaucepan to sit firmly and squarely upon it. Make the cement extension block tothis height taking care to allow for the locating lug sitting in the main post.THE SAUCEPAN HANDLE HOLE PLUG.As explained previously, it can be made using some of the left over mix. Simplylay a piece of thin plastic or glad wrap on the bottom, then half fill with cement.Lay a piece wire mesh along lengthways with a looped piece protruding for thelifting lug. Top off with cement, then lay another piece of plastic over the top,and shut the li4, ensuring that it closes down firmly.This will make a neat heatproof plug to close off the handle hole. This isnecessary for all uses of the furnace except when using the cast iron saucepan andother more specialised and advanced pieces of equipment which 'will be fullydetailed in another book due out soon.USING THE FURNACE.This furnace is easy to use, and has been designed with a heavy emphasis on safety.Metal melting is, and has always been dangerous. The major dangers are the moltenmetal itself, the gases given off from the melting process, and the fire dangerfrom heat or hot objects igniting nearby materials. It is essential that you payparticular attention to all safety procedures.******** SAFETY PROCEDURES *******# Always use the furnace in a well ventilated area.# Always ensure that the furBae.and. all hot o1J,3eets're well clear of inf1aJllllable_ter~.' Do not have' anyinf1.a1aable .aterial within 50 metres of the furnace.Sparks are often blown out the top particularly when the furnace is running atfull blast, and these can be carried quite some distance by a light breeze.# Use the furnace on a dry sand surface. If you happen to drop or spill theJmlten lletal, the sand will contain it. However if it falls on a surface otherthan dry sand it can splatter and erupt in small explosions blowing molten metalall over the place. (Even dry concrete can be dangerous. Hot metal, particularlycast iron will cause concrete to explode off on the surface blowing hot metal andconcrete surface fragJlents quite some distance. DRY SAND MUST SURROUNDYOUR WORKING AREA).# One of the major dangers in foundry work is the actual handling of the moltenmetal. Ina nor.al s.a11 crucible furnace, the crucible has to be grasped, thenlifted from the furnace using lifting tongs. It then has to be poured into aladle, then finally poured into a mold. We have designed the furnace to reducethis dangerous double handling of .,lten metal especially for the popular lowert~rature lletals such as zinc and alUlliniUll.The cast iron pot haa a fixed handle which is never removed. There is no chanceof the tongs slipping or the crucible being knocked O'ger during placellent of thetongs. lbe molten metal is poured direct frOll the saucepan thereby eliminatingdouble handling of the lletal. However although we have incorporated these safetyfeatures into our design, accidents can still happen and protective clothingshould be worn at all times.IYou should always wear a full face protective helmet with a wide hat so that inthe event of splashing or blowing high, they will not drop down your shirt collar.


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18.Weara leather or protective welders jacket and leather workgloves with your coatsleeves extending downover your gloves. Weara leather apron, leather leggingsandheavyworkboots.# .Never ever allow water, liquids or moisture of any type near the furnace. Weara head band if there is any possibility of perspiration drops falling into themetal. (As explained previously moisture of any type will instantly erupt in ablast of steamblowingmolten metal out with someconsiderable force.)IN GENERAL, BE CAUTIOUS AND USE COMMON SENSE.You will probably be apprehensive during your first pour, and to your amazementthe mol ten aluminium will pour in a docile deceptively harmless way. It willprobably be quiet, smooth and without hissing spitting or steaming. It willappear to be so quiet and harmless that you will be tempted to dispense with allsafety gear and procedures. DO NOT BECOME OVER-CONFIDENT. ALWAYSOBSERVE ALL SAFETY PRECAUTIONS. MOLTEN METAL CAN BEDECEPTIVE, AND IT IS DANGEROUS.THE CAST IRON SAUCEPANFor safety, convenience and low cost, the cast iron saucepan is the most suitablemelting pot. You will be able to melt and cast lead, zinc, die-cast alloys andaluminium. These metals are quite easy to cast, and scrap is cheap and plentiful.The cast iron saucepans can be purchased for a few dollars from most camping andarmy disposal stores. They have a standard t" threaded rod attaching a woodenhandle to the saucepan. (See photograph.)The saucepans are easi l y modi f ied for furnace use by unscrewing the handle, anddiscarding the wooden piece. Replace the threaded rod with the looped part at theend in a vertical position. (The high part of the loop helps support the pipehandle from inside). A metre long length of 3/4" water pipe is slipped over thisand welded to the saucepan. (Note: If you cannot weld, then cut a piece out of thepipe as per the diagram. Flatten the remaining piece, then bend it down almost atright angles to follow the angle of the saucepan wall. This piece is then drilledand bolted to the saucepan wall using t" b ol ts . A no th er t" hole is drilled throughthe pipe handle so that a bolt goes through the eye of the loop inside the handle.This method will safely and securely attach the long pipe handle to the saucepanw i th ou t w el d in g. )We have welded a t" water pipe spout into the saucepan, and a "retaining wall" tohold back the dross and slag when pouring. The pipe extends to within 5mm of thebottom of the saucepan. This enables clean metal to be poured from under thedross. The rubbish left remaining in the saucepan :i.S tipped out into the sandbefore the next melt.This particular modification is purely for convenience and ease of pouring. It isnot a necessary requirement. The saucepan with the extended handle can be used asis. When using the saucepan as is, skim the dross and slag off just beforepouring, then any small odd pieces can be held back wi th a small ladle allowingthe pure metal to be pour-ed cleanly, and without contaminat ion.FIRING UP FOR A MELTWith all safety procedures thoroughly understood and the furnace properlypositioned, ~lean out the ash pan, put the centre post extension block in positionand switch on the air to a gentle flow. Light it up, and stack with fuel up tothe top of the extension block. Note; You may need to use small wood scraps tolight the coke properly. Charcoal over burning wood gives a good start to coke andchar. Turn the air blast up to heat the fuel, then when it is well alight poke itdown and refuel. Put the saucepan in, and turn the air blast up with the 25mmexhaust reducer in position and the lid shut.When the saucepan shows a faint red, put in a few pieces of scrap aluminium chunksand run at about half blast. Check it after 2 or 3 minutes because in a hotfurnace the metal melts quickly, particularly if the quantity is small. Switchoff the air and either lift off the exhaust reducer and look into the pot, or open


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21/23

20.the lid. The metal often melts, but leaves a slag outline of its original shapecausing you'to think that it has not melted completely_ Use a dry steel poker tocheck the melt. When it melts, skim the dross off and pour it into either aproperly prepared mold, or hollows in dry sand to make ingots for later castings.Empty the remainder of the pot into dry sand, and the pot is now ready to ta~e thenext melt. The whole procedure is basically quite easy and straightforward.Note; When the furnace is hot, and the pot is hot aluminium will aeltwithin a few minutes. Keep an eye on it, and ease off the air if thepot becomes bright red. At full blast, this furnace has the power tomelt everything including the cast iron pot within a short time. DONOT LEAVE THE FURNACE UNATTENDED.MAKING THE MOLDS.The easiest and most foolproof way to make a mold is to use ordinary Plaster ofParis.In the accompanying photographs, the gear cog is suspended in a plastic foodcontainer. Plaster of Paris is mixed to a creamy pourable consistency, and pouredinto the container after spraying the cog with an oily fil~ eg.WD40 to prevent theplaster sticking to it. When it sets, the cog is lifted out and the plasticcontainer taken off, and the whole thing allowed to dry in the sun.Before using the mold, it should be dried for an hour at about 150C, ~nd then 2hrs at about 250C. This can be done in an ordinary kitchen oven, or in a tinover the furnace exhaust or even in the furnace after shutting it off for the day.If the Plaster of Paris is not dried to remove crystaline moisture just prior to amelt, the hot molten metal will cause steaming and bubb1ingln the mold .r.esu1tingin mold defects.As a general rule, the spots where air bubbles are trapped in the mold (ie.beforetapping and tamping the mixture to release them), are the exact places where steamand gas will be trapped causing defects. (Refer to the photographs.)Sand is the most common mold material. It is mixed with just enough clay. andwater to hold it together. The gas and steam escapes through the sand particles.This damp sand molding is referred to as "Green Sand Molding". It is fast, cheapand convenient for the experienced operators.It is however an art in itself and requires special procedures to prevent themolten metal washing the sand away, and various other problems which are easilyavoided with the proper knowledge, but can be dangerous and very disheartning tothe inexperienced beginner. (We expect to have a detailed manual on this shortly).Plaster of Paris is eaSily acquired from most hardware stores ~nd school projecthobby shops. It is however best to buy it in 20kg bags from plaster suppliers.It is called "Casting Plaster". Buy the fast setting gra:de. The mold is set solidwithin 20 minutes. It is sold by Gyprock and plaster board suppliers, and usedextensively for casting and creating ornamental features and special ceiling.cornices particularly in home restoration.REFINING SCRAP INTO INGOTS.As a general rule, it is best to refine dirty scrap pieces into clean ingots, .thenlater melt the ingots for the actual product casting.There is an enormous amount of scrap lying around, and whilst it may be dirty,greasy, oxidised or painted, it can all be thrown into the. pot. The rubbish willeither burn out, or come to the surface as dross which can be skimlned off anddiscarded. The molten metal can be poured into metal patty cake tins (paintedwith plaster of paris to stop thein sticking), or into small hollows in dry sand.The molten aluminium does not soak into sand, but tends to remain intact on thesurface in a solid shiny blob. Note; Do not overcook aluminium.Over heating and lengthy overcooking destroys the strength andtexture of aluminium. as soon as the alu.inium melts, ski. off thedross and pour it. In dim light the alu.inium, at the right pouringtemperature will be a dull red, and in bright daylight it will besilvery.


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21.Warning : Do not use magnesium alloys for melting. It can sometimes catchalight, and it is virtually impossible to put out quickly. The only thing to dois cover it with dry sand to slow the burn rate, and allow it to burn out.Magnesium alloys are found in aircraft salvage scrap and some lawnmower housings.It can sometimes be identified by sight as it tends to be greyer in colour, and isless dense than aluminium, but this is only a general rule of thumb and in notinfallible. If the metal is magnesium, a small drop of one per cent SilverNitrate solution will show as a black stain on a freshly filed surface, whereasthe aluminium and aluminium alloys remain unchanged. Silver Nitrate is availablefrom some Pharmacies, and from Laboratory suppliers.Good quality scrap is available from a wide variety of sources. Old car parts arean excellent source of scrap. Broken pistons, intake manifolds and heads on somecars, thermostat and water pump housings and so on. Often the aluminium is nearworthless for the non-ferrous metal merchants because steel is often attached toit, and the cost of separation in man hours exceeds the value of the scrap, so itis usually thrown in the rubbish bin. The quickest and easiest way to break upand separate tough scrap such as truck pistons is to heat it near red hot thengive it a few sharp smacks with a hammer and it will break up easily. If you sitbig pieces in the furnace to heat up, take care not to crunch sharp edges down onthe centre post, and watch the heat carefully. Aluminium does not give anywarning of the melting point. It will melt with a whoosh and immediately solidifywhen it hits the air grates. It can sometimes necessitate a partial rebuild oft he f ur na ce .The quickest and easiest and cheapest way to heat big pieces of scrap is to usethe Diese1 Air Torch (Manual DT54). It is like an oversized, supercharged blowlamp. It has a very powerful intense flame that can cover a wide area, and isable to heat big pieces very quickly. It is also used for heating weldingfabrications over a big area for uniform stress relieving of welds.Aluminium drink cans are a readily available source of scrap, and melt down intoshiny strong good quality aluminium. It takes about 50 cans to produce lkg ofr ef in ed m et al .When melting cans, it is essential that the molten metal be poured off quickly. Aseach load melts, pour it offregardless of how small the amount is otherwise itwill soon overcook and become nrittle and useless.Warning; Drink cans often contain .,isture. Do not drop thea into .,1ten .eta! inthe pot The safest way is to crunch and stack as many as possible into the saucepan, thenthe heat will dry them before they melt. When the cans melt, they leave theirshape as a flakey residue giving the impression that they have not, and will notmelt, whereas in actual fact the molten metal is hidden under powdery dross andash. A quick check with a poker will show whether the melt has begun. < If so,pour it off immediately and empty out the flakey remains. When the rubbish cools,retrieve all blobs of trapped metal, and throw it in a later melt for refining.Warning; Stay c1ear of ru.es, particu1ar1y wben. _1ting dirty scrap and cans. Thepaint and rubbish gives off unp1easant ru.es 1Ihich cou1d be dangerous.FURNACE FUEL.Coke and Char are about the best all round high heat fuel. Charcoal gives a goodheat, but burns out too quickly for high heat melts. It is good for aluminiumand of course it has the big advantage of being easily obtained and cheap if youmake it yourself. High grade charcoal such as that used last century is not aadeby burning wood. The wood is heated to a certain temperature which preserves theheat generating energy in the wood while driving off unwanted moisture and gases.(The gas given off actually feeds the fire heating the wood). Proper furnacecharcoal will give a solid ringing sound when tapped. Refer to Manua1-CC56 fo rfull detai1s.) Coke is the traditional furnace fuel. A good quality with asilvery tinge is available from Hills District Fue1 Supp1ies, 102 B1ood.1IDod.Rd.Arcadia.(Near Sydney),Ph.{02) 656-1429. They supply a good sized coke ideal for


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22.the furnace and forges. It is a bit smaller than golf ball size which is idealfor settling in the furnace. (They will arrange rail or road transport ifrequired.)"Char" is a smokeless fuel and is also the ideal size for the furnace. It gives ahigh heat, as goodor better than coke and has the added advantage of burning awayto a brown dust which is easy to clean out of the furnace. It is bagged anddistributed by Anthra Wholesalers, 45 PunchbowlRd, Belfield 2191. Ph. (02)642-2470 (Sydney). They will arrange road or rail transport to anywhere inAustralia.CONCLUSION.Metal melting is a fascinating and rewarding hobby. Youmay start out with theintention of only casting aluminium, but then the magic lure of other metals willdrawyouin.This book deals primarily with the furnace design and construction, and includesan introduction t:o casting aluminiumwhich enables you to put your furnace toimmediateuse.Themelting and casting of other metals is muchmore involved, and requires theuse of good quality crucibles, special procedures and additives for degassing andfluxing the metals. This will be covered in another book which is nowin theearly stages.However,if youalready have foundry knowledge, then the furnace is quite capableof producing very high quality castings of all commonlyused casting metals. Goodquality crucibles can be purchased from reputable suppliers such as HorganiteIndustrial Products, 65 BourkeRd, Alexandria HSW20l5,Ph. (02) ~69-5711.

* * * * * * * * * * * * * * * * ******PUBLISHERSNOTE:This ..anual is designed to convey practical usefulinforaation in asiaple straightforward .anner without the reader having to get boggeddown in fancy words and glossy pages.The names and addresses of suppliers listed in this aanual are forthe convenience of readerB only. We do not have any financialinterest in any of the Buppliers, nor do we have any coaaisBionarrangeaent with thea. They have been selected on the basis of goodreliable service, and reasonable prices at the tiae of researchingthis aanual. However if you are treated badly or the goods are notof a satisfactory standard, please let. us know and we will deletetheir naaes froa further issues. Likevise~if you have any coaplaintsabout our service, or this aanual please let us know. Our incoae isderi ved froa repeat business...... It is in our own interests thatyou are a sat~sfied custoaer.

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metal melting furnace - 1991

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