faceting machine project

7/28/2019 Faceting Machine Project

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"Tool Time" for Gemcutters!

Graphics Intensive. Sorry. It has to be. If you have a slow connection or server, this would be a good time to raid the refrigerator or

something.

The NEWEST Version, to be finished around May, 1999! Faster, cheaper, simpler!!
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A "Revision H" with digital facet head and digital tachometer.

Why build it when you can buy it?

Because.

1: There are many good, popular machines on the market. In order for the companies who make them to stay in business, and pay their employees, their G&A, and

their overhead, and maybe even make a profit, a good portion of the $2,000-$3,000 price simply cannot be shipped with the product.

2: If one builds it, one must face the Customer every day. Therefore, the motivation to do a good job is high.

3: The homebuilder chooses the components, and lives with them. Some cost-cutting accountant is not dictating the use of nylon bearings.


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Why Not.

1: You need a good metal lathe (Or a friendly machinist who REALLY likes jewelry.)

2: You must have some mechanical ability, but since you are already cutting stones, well..

3: Time and effort. It is much easier to write a check....if you will settle for someone else setting the rules in your shop.

Essentially a lapwheel with a dividing head which indexes the stone around a central axis to provide symmetrical placement of the facets. The head is adjustable for

angle as well, since different stones require various angles due to differences in refractive index. This machine is built on 1" thick 6061-T6 aluminum tooling plate,

The permanent magnet DC motor is reversible and the speed is controllable from 0-2,000 RPM with constant torque. The mast is 1" diameter type 316 Stainless

Steel, and employs a linear ball bushing , and all needle and ball-bearing construction to allow Zero Lash. Fine height adjustment is accomplished with a


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A bandsaw with an appropriate blade for 1" thick aluminum.

3: An operator for the above.

4: Rudimentary electrical skills.

5: Some money, checkbook, or other barterable medium of exchange.

6: About 40-100 hours.

Lessons learned from previous mistakes and disasters!

1: Use sealed (not sheilded) ball bearings. What happens to a precision bearing when rock dust and diamond powder gets into them has to be seen to be believed.

2: Laps must be at least statically balanced. No runout or "hop" can be tolerated, regardless of effort, because gemcutting is not an impact process.

3: Sleeve bearings/bushings of Nylon, porous bronze, etc. should be avoided. When abrasives get into them (not if), they make good laps, and proceed to ruin the

shaft. (The expensive part.) They are cheap, but otherwise, there is no excuse for using them.

4: Avoid V-Belt drives. There are hysteresis losses, and they are unbalanced. If spatial constra ints demand a belt drive and arbor, use timing belts. I use direct

drives because motor ball bearings are usually sealed Conrad Type R bearings, will support a thrust load, and can be quickly and cheaply replaced.

5: House Rule here :-) If it can be heard running ten feet away, fix it.

6: Be messy on the lap assembly if you must, as long as the spindle/arbor assembly runs perfectly true, and whose axis is per pendicular to the base to the limits of

your best efforts at measurement.. but on the faceting head, any runout or error will be transferred to every stone you ever cut. Zero runout or slop means "Zero",

or the resolution limit of the best dial indicator or other device you can buy, beg or borrow. (Includes laser interferometers. "Zero" means Zero. If you make theattempt, you will have a better machine than can be bought.)

7: Motors used are DC, either Permanent Magnet or wound field types, having a shaft diameter of at least 1/2" (12.7 mm), and using sealed ball bearings, rated at

least 1/5 HP (~150 Watts).


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Introduction:

This is not a Beginners' Project. By its very end use, the machine is intended to produce faceted gemstones of high precision, and it cannot produce something more

accurate than itself. Familiarity with mechanical principles is needed to complete this project. A proficiency with mechanical assembly practices and the use of

common shop tools is a must.


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For those unfamiliar with the operation of machine tools such as lathes and milling machines, it is best to get help from a machinist, toolmaker, or other skilled

craftsperson. Often, a local trade school can help greatly..sometimes they are grateful for a "special" challenging project. Of course, you will have to pay for

materials and supplies at the least. Another possibility is to enroll in a basic machine shop/ metalworking course. The skill will not be wasted (No skill ever is..), as

later on, you can make your own laps and other tooling, saving many times the cost of the course.

Design Principles: The baseplate can be aluminum tooling plate ("Jig Plate"), 5/8" thick, or Plexiglass, 3/4" thick. If the latter is chosen, it should be drilled andtapped so extruded aluminum channel can be bolted to the back surface as stiffeners. In either case, the materials must end up being better than 0.005" flat per foot.

Plexiglass is very easy to work with common woodworking tools, so many would prefer it. It is cast to sufficient flatness so it can be used. It does scratch easily, of

course. Both materials are illustrated later on.

A DC motor with sufficient torque is used so it does not slow down under load. A solid-state controller with feedback may be used for controlling the motor speed.

However, faceting does not put great demands on the motor, so a simpler speed control, such as a Variac, a rectifier, and a capacitor may be used with good results.

Very inexpensive controllers are available based on Triac or SCR switching, like a lamp dimmer. The output of these devices m ay be wired to the appropriate

transformer for the motor's voltage, rectified, and connected to the motor. For example, a 24 Volt permanent magnet motor can be run very well by buying a 24

volt transformer of sufficient current rating, connecting the output of a "dimmer" to the input, and connecting the output to a bridge rectifier and a

capacitor,(Rated 30-50 VDC, ~10,000 MFd.) through a reversing switch, to the motor.

Ball-bearings are used everywhere, of the sealed type. Should any diamond dust or grit get into them, they can be changed quickly and cheaply. The use of ballbearings eliminates most "slop", and the accuracy of the unit does not deteriorate over the years. Also, all the close tolerance work is done by the bearing

manufacturer...all for less than ten dollars.

The faceting head and quill assembly is all ball bearing. The bearings used in the trunnion assembly should last forever, as they are not exposed to spray. The gears

are off-the-shelf precision stainless steel, and are assembled to an accurately machined shaft adapter, so they can be changed in seconds . Because the gear is silver

brazed to the adapter, there will never be any indexing errors caused by slippage.

The mast assembly consists on a stainless steel centerless ground 1" rod, chilled into an aluminum baseplate. A Thompson or B arden ball bushing with Zero "slop"

is used on the head assembly for smooth and error-free adjustment and travel. The depth is set by a micrometer spindle which rests against a coarse adjustable

aluminum clamp, so a wide range of depths is possible whether cutting a table or girdle. The Base is straightforeward, and will be built first. If you decide to stop

when you are finished with the base, you can always build the faceting head and mast assembly later on if you want, and buy W ykoff's "Calibrated Jam Peg", and

with skill and practice, probably turn out better stones than I do, and still be far ahead in cost.

Materials Needed: 1 Sheet 3/4" plexiglass, or 1 sheet 5/8" aluminum Jig Plate 18" X 14", or whatever dimensions you might prefer. These are a judgement call by

the builder. Some might want to build it into and existing cabinet or countertop, others may wish to build a base for it so it will be "portable", and will sit on a

counter.


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Construction details

Revision " H" !...In Stainless Steel.

Motors used are DC, either Permanent Magnet or wound field types, having a shaft diameter of at least 1/2" (12.7 mm), and using sealed ball bearings, rated at

least 1/5 HP (~150 Watts).

Here is the c-face motor mounted to the underside of the acrylic base. Note the bolt pattern, and the machined splash and dust guard (Made from an old wire spool).

The face of the motor was machined dead perpendicular to the shaft, to remove errors caused by the manufacturer's paint. To do so, the motor shaft was mounted

to a live center on the lathe's tailstock, and the motor was chucked, after its wires were carefully taped to the body of the motor. This step may NOT be neccesary inmost cases. I just did not like the looks of the paint application, and I like to run the lathe, anyway. A "bump" of paint will affect the perpendicularity, just as a

shim would.


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Here is a simple fixture, accurately bored, which mounts on the spindle, and measures wobble relative to the top surface. Since the faceting mast assembly mounts

to this surface, and the motor is mounted to the opposite side, this is the datum plane from which measurements are derived, and the plane which is the angular

reference for every stone which will ever be cut. The parallelism specs for cast acrylic sheet are very tightly controlled.


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The faceting head assembly, plopped onto the base. It is secured with a single 3/8-24 bolt and clamping nut and washer, so it can slide in and out towards or away


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from the centerline of the spindle. You will find that if the perpendicularity is accurate (See section on the mast assembly below), it can also be swung through an

arc, allowing unlimited positions for operator comfort.

Mast assembly


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Faceting Head.

This is the challenging part. This section of the document will be growing over the months, as more sections appear in the Eclectic

Lapidary. Warning: Graphics intensive. If you have a slow server or connection, go get a cup of coffee or something.


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A Word About Gears.

This design uses gears with a Pitch Diameter of 2". You will notice in some of the preceeding photographs that I do not number the index gear teeth, except by a

binary pattern of dots. I think it's faster and more visual, but of course, you can number the teeth if you feel you need to. For translating cutting data for those who

need it, I use this

Worksheet and design form.

Variations between gears of different pitches needed to supply different numbers of teeth for a given design size are accomodated by the spring-loaded indexing

pawl. The "V" design of the indexing tooth accomodates different pitches (Tooth sizes). Here are some examples:

96 tooth, 48 pitch. PD 2.000, OD 2.041

64 tooth, 32 pitch, PD 2.000, OD 2.062

48 tooth, 24 pitch, PD 2.000, OD 2.083

32 tooth, 16 pitch, PD 2.000, OD 2.125

128 tooth, 64 Pitch, PD 2.000, OD 2.031
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This view is of the side usually away from the right -handed operator, showing how the dowel pin is installed. The pin is a rest upon which the angle stop rests. The

angle stop rotates with the trunnion shaft and locks to accept any angle setting.

That's it for the big pieces. Now all that are needed are two pieces of 1/2" shafting with a little milling and turning, a fairly simple angle stop clamp, a stolen

protractor, and a few little parts like the indexing pawl , a spring and stuff and then we can put it together and calibrate the protractor. When these parts are done,

you can start worrying about buying rough.


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Assembly and Calibration.

By now, all the critical major parts are done. The indexing pawl must be made, but this is straightforward. After assembly, with the index gear in place, clamp thepawl lever down so the gear is freewheeling. The a small hole (1/8") can be drilled. When freewheeling mode is needed, such as preforming the cone for a pavilion or

preforming the circle for a round brilliant, a small pin can be inserted in the hole, locking the pawl out of engagement.
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Coarse depth stop: This unit clamps onto the mast shaft. It does not mar the finish of the mast shaft, as a set screw would. The facet head/mast follower assembly

sits on this part, and the micrometer spindle tip rests on the top surface. This allows rapid height changes, from girdle cutting to table cutting, while the micrometer

head allows critical settings. NOTE: The 1" center bore must be perpendicular to the top and bottom surfaces.


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The angle stop clamp is a simple locking device which is installed onto the trunnion (angle) shaft.

When the thumbscrew is tightened, it then rests on the 1/4" dowel pin, acting as a "hard" stop for the angle. So first, the quill assembly with the mounted dopped

stone is allowed to hang down. The angle is set with the protractor with the screw slightly loosened. When the assembly is rotated to the correct angle when resting

on the dowel pin stop, the screw is tightened. Then the height is set coarsely with the above mast stop, and the head assembly slowly lowered with the micrometer

spindle until the stone contacts the lap. Now the starting height number from the micrometer stop may be noted, and cutting and polishing can begin.

CALIBRATION.

Below is a simple, but very accurate method of calibrating the machine's angle protractor. With a good flat lap, place two accurately surface ground gage blocks or

"V" blocks IDENTICAL IN HEIGHT on the lap as shown. Place in the dop holder a perfectly straight piece of centerless ground rod. With the gear locked in


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freewheeling mode, rotate the quill and observe if there is any runout. Using a combination of the height micrometer and the angle stop, adjust the faceting head's

angle and height the so that light is just visible between the rod and both gage blocks. This is a very accurate method, far exceeding the resolution of any protractor

or vernier. When uniform height is acheived on both blocks, lock the protractor in place reading exactly 90 degrees. Using a 1/16 tungsten carbide drill bit, pin the

protractor at this value with a 1/16 roll pin. This test may be repeated on the machine's baseplate. The reason for using the lap as a datum is simply because that is

where the stone will be cut. If you mounted the motor perfectly, the protractor will repeat the values at both surfaces, the lap, or the baseplate.

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