chapter 5 design for high speed automatic assembly and robot assembly 5/14/2015 dr. mohammad...

Chapter 5Design for High Speed Automatic Assembly and Robot Assembly

04/18/23

Dr. Mohammad Abuhaiba1

5.1 INTRODUCTIONFig. 5.1:The slightly asymmetrical threaded stud would not present significant problems in manual handling and insertionFor automatic handling an expensive vision system would be needed to recognize its orientation.If the part were made symmetrical, automatic handling would be simple.

04/18/23

Dr. Mohammad Abuhaiba

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5.1 INTRODUCTION if a part can be handled automatically,

then it can usually be assembled automatically.

When we consider design for automation, we will be paying close attention to the design of the parts for ease of automatic feeding and orienting.

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5.1 INTRODUCTION In automatic assembly:

Time taken to complete an assembly does not control the assembly cost.

It is the rate at which the assembly machine or system cycles.

If the total rate (cost per unit time) for the machine or system and all the operators is known, the assembly cost can be calculated.

We shall be mainly concerned with: Cost of all the equipment Number of operators and technicians Assembly rate at which the system is designed to operate

Apportion the cost of product assembly between the individual parts and, for each part; we shall need to know the cost of feeding and orienting and the cost of automatic insertion.

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Cost of feeding & orienting parts depends on:

1. Cost of the equipment required2. Time interval between delivery of successive

parts. Time between delivery of parts: reciprocal of

the delivery rate.1. It is equal to cycle time of machine or system.

Let Fr = required delivery or feed rate (parts/min), then the cost of feeding each part Cf is given by

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Rf = cost (cents/s) of using feeding equipment

CF = feeder cost ($) Eo = equipment factory overhead ratio Pb = payback period in months Sn = number of shifts worked per day The constant 5760 = number of available

seconds in one shift working for one month divided by 100 to convert dollars to cents.

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTINGExample: assume that a standard vibratory bowl feeder costs $5000, payback period is 30 months with 2 shifts working, factory equipment overheads are 100% (Eo = 2), we getRf = 5000 x 2/(5760 x 30 x 2) = 0.03 cent/sIt would cost 0.03 cents to use the equipment for 1 second.Taking this figure as the rate for a "standard" feeder and assigning a relative cost factor Cr to any feeder under consideration, then Eq. (5.1) becomes

Cf = 0.03(60/Fr)Cr (5.3)

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Feeding cost per part is inversely proportional to

required feed rate and proportional to feeder cost.

For otherwise identical conditions, it would cost twice as much to feed each part to a machine with a 6 s cycle compared with the cost for a machine with a 3 s cycle.

This illustrates why it is difficult to justify feeding equipment for assembly systems with long cycle times.

It would cost twice as much to feed a part using a feeder costing $10,000 compared with a feeder costing $5000.

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Fig. 5.2: The faster the parts are required, the

lower the feeding cost. This is true only as long as there is no

limit on the speed at which a feeder can operate.

There is an upper limit to the feed rate obtainable from a particular feeder.

Fm = maximum feed rate

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTINGExample: Let max feed rate from feeder = 10 parts/min.1.If parts are required at a rate of 5 parts/min, feeder can simply be operated more slowly involving an increased feeding cost.2.Suppose parts are required at a rate of 20 parts/min.

1. Two feeders could be used, each at a rate of 10 parts/min.2. Feeding cost per part using two feeders to give twice max feed

rate will be the same as one feeder delivering parts at its max feed rate.

3.If required feed rate is greater than max feed rate obtainable from one feeder, feeding cost becomes constant and equal to cost of feeding when the feeder is operating at its max rate. This is shown in Fig. 5.2 by the horizontal line.4.If multiple feeders are used for increased feed rates, then the line will be saw-toothed as shown.

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Eq. (5.3) holds true only when required feed rate

Fr<max feed rate Fm when this is not the case feeding cost is given by

Cf = 0.03(60/Fm)Cr (5.4)

Now max feed rate Fm is given by

Fm = 1500 E/l parts/min (5.5)

E = orienting efficiency for the part l (mm) = its overall dimension in the direction of

feeding and where it is assumed that the feed speed is 25 mm/s.

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The meaning of the orienting efficiency E:Consider feeding of dies (cubes with faces numbered 1 to 6).If no orientation is needed, the dies can be delivered at a rate of 1 per second from a vibratory bowl feeder.If only those dies with the 6 side uppermost were of interest,

a vision system could be employed to detect all other orientations and a solenoid operated pusher could be used to reject them.

In this case the delivery rate would fall to an average of 1 die every 6 seconds or a feed rate of 1/6 per second.

The factor 1/6 is defined as the orienting efficiency E and it can be seen that the max feed rate is proportional to the orienting efficiency (Eq. (5.5)).

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING If dies were doubled in size and that feed

speed on the feeder track were unaffected. It would then take twice as long to deliver each die.

Max feed rate is inversely proportional to length of part in the feeding direction [Eq. (5.5)].

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Eq. 5.4 shows that when Fr > Fm, the feeding

cost per part is inversely proportional to Fm. Under these circumstances, cost of feeding is

inversely proportional to orienting efficiency and proportional to length of part in the feeding direction.

Automatic feeding and orienting methods are only applicable to "small" parts.

Parts larger than about 8” in their major dimension cannot usually be fed economically.

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING When considering the design of a part and its

feeding cost, the designer will know:1. required feed rate Fr

2. dimensions of the part, l The remaining two parameters that affect

feeding cost, namely, the orienting efficiency E and the relative feeder cost Cr, will depend on:1. part symmetry2. types of features that define its orientation.

1. A portion of a classification system is presented in Figs. 5.3 to 5.5.

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5.2 DESIGN OF PARTS FOR HIGH-SPEED FEEDING AND ORIENTING Fig. 5.3: 1st digit of a 3-digit shape code. Fig. 5.4: 2nd and 3rd digits are determined

for a selection of rotational parts (1st digit 0, 1, or 2) and corresponding values of orienting efficiency E and relative feeder cost Cr.

Fig. 5.5: 2nd and 3rd digits are determined for a selection of non-rotational parts (1st digit 6, 7, or 8).

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FIG. 5.5: 2nd & 3rd digits of geometrical classification for some non-rotational parts

5.3 EXAMPLE

Part shown in Fig. 5.6 is to be delivered to an automatic assembly station working at a 5 s cycle.

Use classification system and database to determine feeding cost assuming that cost of delivering simple parts at 1 per second using standard feeder of 0.03 cents per part.

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5.3 EXAMPLE A = 30 mm, B = 20 mm, and C = 15 mm A/B = 1.5 and A/C = 2 Fig. 5.3: A/B < 3 and A/C < 4, part is

cubic non-rotational and is assigned a 1st digit of 8.

Fig. 5.5: part has no rotational symmetry about any of its axes.

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5.3 EXAMPLE Outline of part in X direction:

A step or projection in the basic rectangular shape This feature alone can always be used to determine part's

orientation. This means that if the outline in the X direction is oriented as

shown in Fig. 5.6, part can be in only one orientation Therefore, 2nd digit of the classification is 4.

However, either groove apparent in the view in Y direction and step seen in the view in Z direction could also be used to determine the part's orientation.

Select feature giving smallest 3rd classification digit; in this case it is the step seen in X direction.

Thus appropriate column number in Fig. 5.5 is 0. Three-digit code: 840

Orienting efficiency: E = 0.15 Relative feeder cost: Cr = 1

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5.3 EXAMPLE Longest part dimension l = 30 mm Orienting efficiency E = 0.15 Eq. (5.5) gives max feed rate obtainable from

one feeder Fm =1500 E/ l = 1500x0.15/30 = 7.5 parts/min

From the cycle time of 5 s the required feed rate Fr is 12 parts/min, which is higher than Fm.

Since Fr > Fm we use Eq. (5.4) Cr = 1 Feeding cost Cf = 0.03(60/Fm)Cr = 0.03(60/7.5)1 =

0.24 cents

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5.4 ADDITIONAL FEEDING DIFFICULTIES Fig. 5.7: If edges of

parts are thin, shingling or overlapping can occur during feeding, leading to problems with the use of orienting devices on feeder track

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5.4 ADDITIONAL FEEDING DIFFICULTIES Fig. 5.8: for each combination

of features, an approximate additional relative feeder cost is given that should be taken into account in estimating the cost of automatic feeding.

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5.5 HIGH-SPEED AUTOMATIC INSERTION If a part can be sorted from bulk and delivered to a

convenient location correctly oriented, a special-purpose mechanism or work head can usually be designed that will place it in the assembly.

Such work heads can generally be built to operate on a cycle as short as 1 second.

For assembly machines operating on cycles greater than 1 sec, automatic insertion cost Ci is given by

Fr = required assembly rate (or feed rate of parts) Ri = cost (cents/s) of using the automatic work head

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5.5 HIGH-SPEED AUTOMATIC INSERTION The equipment rate Ri is given by

Wc = work head cost ($) Eo = equipment factory overhead ratio Pb = payback period in months Sn = number of shifts worked per day

Assuming a standard work head costs $10,000, payback period is 30 months with two shifts working, and factory equipment overheads are 100% (Eo = 2), we get

Ri = 10,000 x 2/(5760 x 30 x 2) = 0.06 cents/s

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5.5 HIGH-SPEED AUTOMATIC INSERTION It would cost 0.06 cents to use the

equipment for 1 second. Taking this figure as the rate for a

"standard" work head and assign a relative cost factor Wr to any work head under consideration, then Eq. (5.6) becomes

Insertion cost is inversely proportional to required assembly rate and proportional to work head cost.

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5.5 HIGH-SPEED AUTOMATIC INSERTION

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5.6 EXAMPLE Fig. 5.6:

part is inserted horizontally into the assembly in the direction of arrow Y

It is not easy to align and position Not secured on insertion Appropriate classification is row 1, column 2 in Fig. 5.9 Automatic insertion code is thus 12, giving a relative

work head cost of 1.6. For a cycle time of 5 s, assembly rate Fr is

12parts/min and Eq. (5.8) gives an insertion cost of: Ci= 0.06(60/Fr) Wr = 0.06(60/12)1.6 = 0.48 cents

Total handling & insertion cost Ct = 0.24 + 0.48 = 0.72 cents

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5.7 ANALYSIS OF AN ASSEMBLY assembled at

a rate of 9.6 per minute

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5.7 ANALYSIS OF AN ASSEMBLY FIG. 5.11: Completed worksheets for high-speed automatic

assembly analysis of the assemblies in Fig. 5.10 - Original Design

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Part or sub or

opern No.

No. of

repeats

Handling

code

Orientation assem

bly

Relative

feeder

cost

Max feed rate

(parts/min)

Handling

difficulty

Handling cost (cent

s)

Insertion

code

Relative

work head cost

Insertion

difficulty

Insertion/

operation cost

(cents)

Total

cost (cents)

Figure for min part

s

High speed

automatic assemblyName of

assembly-value

Name of part, sub-assembly

or operation

IDRPHCOECRFMDHCFICWCDICICANM 11831000.2014.812.40.40001.06.30.380.691Housing21020000.40121.46.30.20021.50.560.560.631Plunger3100840.* ***.***.**.**Manual assembly required7.130Gasket4100800.*****.***.**.**Manual assembly required6.671cover52210000.901122.76.30.20391.811.30.681.440screw

5.7 ANALYSIS OF AN ASSEMBLY FIG. 5.11: Completed worksheets for high-speed automatic

assembly analysis of the assemblies in Fig. 5.10 – Re-Design

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Part or sub or

opern No.

No. of

repeats

Handling

code

Orientation Efficie

ncy

Relative

feeder

cost

Max feed rate

(parts/min)

Handling

difficulty

Handling cost (cent

s)

Insertion

code

Relative

work head cost

Insertion

difficulty

Insertion/

operation cost

(cents)

Total

cost (cents)

Figure for min part

s

High speed

automatic assemblyName of

assembly-value

Name of part, sub-assembly

or operation

IDRPHCOECRFMDHCFICWCDICICANM 11831000.2014.812.60.40001.06.30.290.691Housing21020000.40121.46.30.20021.59.40.430.631Plunger31000400.70326.318.80.61001.06.30.290.900Gasket41020000.40115.06.30.20380.85.00.230.431cover

5.8 GENERAL RULES FOR PRODUCT DESIGN FOR AUTOMATION The elimination of a part would

eliminate:1. a complete station on an assembly

machine-including the parts feeder2. special work head3. associated portion of the transfer device

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5.8 GENERAL RULES FOR PRODUCT DESIGN FOR AUTOMATION Automation can be facilitated by the

introduction of guides and chamfers. Figs. 5.12 and 5.13 Sharp corners are removed so that the

part can be guided into its correct position during assembly leading to:1. less control by the placement device or2. can even eliminate the need for a

placement device.

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5.8 GENERAL RULES FOR PRODUCT DESIGN FOR AUTOMATION

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Screws that tend to centralize themselves in the hole give the best results in automatic assembly:1. Rolled thread point: very poor

location; will not centralize without positive control on the outside diameter of the screws.

2. Header point: only slightly better than (1) if of correct shape.

3. Chamfer point: reasonable to locate.

4. Dog point: reasonable to locate5. Cone point: very good to locate.6. Oval point: very good to locate.

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5.8 GENERAL RULES FOR PRODUCT DESIGN FOR AUTOMATIONAssembly from above:Allow for assembly in sandwich or layer fashion, each part being placed on top of previous one.

Gravity can be used to assist in feeding and placing of parts.Work heads and feeding devices above the assembly station:

They will be accessible in event of a fault due to feeding of a defective part.

Assembly assist in the problem of keeping parts in their correct positions during the machine index period, when dynamic forces in the horizontal plane might tend to displace them.

With proper product design using self-locating parts, force due to gravity should be sufficient to hold the part until it is fastened or secured.

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Assembly from above is not possible:

divide assembly into subassemblies.

Fig. 5.15: Difficult to position and drive the

two cord grip screws from below. The two screws, cord grip, and

plug base could be treated as a subassembly dealt with prior to main machine assembly.

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Have a base part on which assembly can be built. Must have features to be suitable

for quick and accurate location on the work carrier.

Figure 5.16a: If a force were applied at A, part

would rotate unless adequate clamping was provided.

To ensure that a base part is stable, Arrange that its center of gravity be contained within flat horizontal surfaces.

Fig. 5.16b: A small ledge machined into part

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Fig. 5.17: Location of base part in the horizontal plane is often achieved by tapered dowel pins mounted in the work carrier to provide guidance

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5.9 DESIGN OF PARTS FOR FEEDING AND ORIENTING Most versatile parts feeder is the

vibratory bowl feeder. Three basic design principles:

1. Avoid designing parts that will tangle, nest, or shingle.

2. Make the parts symmetrical.3. If parts cannot be made symmetrical,

avoid slight asymmetry or asymmetry resulting from small or non-geometrical features.

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5.9 DESIGN OF PARTS FOR FEEDING AND ORIENTING

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deliberately add asymmetrical features for the purpose of orienting.

The features that require alignment are difficult to utilize in an orienting device, so corresponding external features are deliberately added.


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FIG 5.20a: a part that would be difficult to handle FIG 5.20b: redesigned part, which could be fed and

oriented in a vibratory bowl feeder at a high rate.

5.9 DESIGN OF PARTS FOR FEEDING AND ORIENTING Parts that are easy to handle

automatically will also be easy to handle manually.

Very small parts or complicated shapes formed from thin strips are difficult to handle in an automatic environment. Manufacture the parts on the assembly

machine or to separate them from the strip at the moment of assembly.

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5.10 SUMMARY OF DESIGN RULES FOR HIGH-SPEED AUTOMATIC ASSEMBLYRules for Product Design1.Minimize number of parts2.Ensure that product has a suitable base part on which to build the assembly3.Ensure that base part has features that enable it to be readily located in a stable position in the horizontal plane.4.Design product so that it can be built up in layers, each part being assembled from above and positively located so that there is no tendency for it to move under the action of horizontal forces during the machine index period.5.Provide chamfers or tapers that help to guide and position parts in the correct position.6.Avoid expensive and time-consuming fastening operations, such as screw fastening, soldering, and so on.

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5.10 SUMMARY OF DESIGN RULES FOR HIGH-SPEED AUTOMATIC ASSEMBLY

Rules for the Design of Parts1.Avoid projections, holes, or slots that cause tangling with identical parts when placed in bulk in the feeder.2.Attempt to make the parts symmetrical3.If symmetry cannot be achieved, exaggerate asymmetrical features to facilitate orienting or, alternatively, provide corresponding asymmetrical features that can be used to orient the parts.

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLY Three representative types of robot

assembly systems :1. Single-station with one robot arm2. Single-station with two robot arms3. Multi station with:

Robots special-purpose work heads manual assembly stations

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLY: Single-station system For reasons of safety it would usually be

necessary to transfer the assembly to a location or fixture outside the working environment of the robot. Robot place the assembly on a transfer

device that carries the assembly to the manual station.

After manual operation has been completed, assembly can be returned in a similar manner to within reach of the robot.

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLY: Single-station system

Two insertion situations:1. Insertion or placement of part by the

robot without it being secured immediately. followed by transfer of assembly to an external workstation to carry out the securing operation.

2. A special-purpose work head is engineered to interact directly at the robot work fixture.

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLY

To determine assembly costs, it is necessary to obtain estimates of:1.Total cost of all general-purpose equipment:

cost of robots Cost of transfer devices Cost of grippers.

Total cost of all special-purpose equipment and tooling: special-purpose work heads special fixtures special robot tools or grippers special-purpose feeders special magazines, pallets, or part trays

Average assembly cycle timeCost per assembly of manual labor involved in:

1. machine supervision2. loading feeders, magazines, pallets, or part trays3. performing any manual assembly tasks

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLY One classification & one data chart for

each of the three basic robot assembly systems. Insertion or other required operations are

classified according to difficulty For each classification, and depending on

difficulty of operation, relative cost &time factors are given that can be used to estimate equipment costs & assembly times

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLYFigure 5.21: a portion of classification system and database for a single-station one-arm robot assembly system.Part is being added to assembly, but is not being secured immediately.Selection of appropriate row depends on direction of insertion.Selection of appropriate column depends on:

whether part needs a special gripper, clamping temporarily after insertion

whether it tends to align itself during insertion

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Example: A part is to be inserted along a horizontal axis, does not require a special gripper, requires temporary clamping, and is easy to align.Code =12Relative robot cost AR =1.5If basic capital cost of an installed standard 4 DOF robot (capable of only vertical insertions) is $60,000, a cost of $90,000 is assumed.Relative additional gripper or tool cost =1.0Since the part needs temporary clamping, special tooling mounted on the work fixture would be required.Thus, if standard tooling or gripper cost $5000, additional tooling needed would represent a cost penalty of $5000 in the form of special purpose equipment.

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5.11 PRODUCT DESIGN FOR ROBOT ASSEMBLYExample: A part is to be inserted along a horizontal axis, does not require a special gripper, requires temporary clamping, and is easy to align.Relative basic operation time TP =1.0The basis for time estimates is the average time taken by robot to:

move approximately 0.5 m grasp the part Return insert the part when the motion is simple and no insertion problems exist

For a typical present-generation robot, this process might take 3 s.Since relative time penalty for gripper or tool change is zero, no additional time penalty is incurred and total operation time is 3 s.A further time penalty must be added when the part to be inserted is not completely oriented by the part presentation device.

The robot arm must perform final orientation with the aid of a simple vision system and an additional 2 to 3 s must be added to the operation time.

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In addition to cost of robot and special tools or grippers, costs of part presentation must be estimated.

In practice there are usually only two choices:1. The special-purpose feeder2. The manually loaded magazine, pallet, or part tray.

Costs associated with part presentation can be divided into: Labor costs:

material handling (loading parts feeders or magazines) system tending (freeing jams in feeders, handling parts trays, etc.) system changeover costs (changing of work fixture, feeders, and

magazines and robot reprogramming)1. Equipment costs:

feeder depreciation depreciation of special fixtures, special tooling, magazines, pallets, or

part trays.

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Bulk material handling costs are negligible compared with cost of manually loading individual parts one-by-one into magazines, pallets, or part trays.

3 significant factors needed to estimate cost of part presentation:1. Special-purpose feeders:

cost of a special-purpose feeder, fully tooled and operating on the robot system, is assumed to be a min of $5000.

Actual cost of a feeder, for a particular part, can be obtained from the data presented earlier in this chapter.

Manually loaded magazines:1. cost of one set of special magazines, pallets, or part trays for one part type is

assumed to be $1000.2. For large parts this figure may considerably underestimate the actual cost and

extra allowance should be made. Loading of magazines:

time needed to hand-load one part into a magazine can be estimated to be part-handling time, obtained from data in Chapter 3, plus 1 s.

a typical value of 4 s may be used.

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5.11.1 Summary of Design Rules for Robot Assembly

1. Reduce part count2. Include features such as leads, lips,

chamfers, etc., to make parts self-aligning in assembly.

3. Ensure that parts which are not secured immediately on insertion are self-locating in the assembly.

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5.11.1 Summary of Design Rules for Robot Assembly4. Design parts so that they can all be gripped and

inserted using the same robot gripper. Each change to a special gripper and then back to standard

gripper is approximately equal to two assembly operations.

5. Design products so that they can be assembled in layer fashion from directly above.

6. Avoid the need for reorienting the partial assembly or for manipulating previously assembled parts.4. These operations increase robot assembly cycle time without

adding value to assembly.5. If the partial assembly has to be turned to a different resting

aspect during assembly process, then this will usually result in increased work fixture cost and the need to use a more expensive 6 DOF robot arm.

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5.11.1 Summary of Design Rules for Robot Assembly7. Design parts that can be easily handled from bulk. Avoid

parts that Nest or tangle in bulk Are flexible Have thin or tapered edges that can overlap or

"shingle" as they move along a conveyor or feed track Are so delicate or fragile that recirculation in a feeder

would cause damage Are sticky or magnetic so that a force comparable to

the weight of the part is required for separation Are abrasive and will wear the surfaces of automatic

handling systems Are light so that air resistance will create conveying

problems

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5.11.1 Summary of Design Rules for Robot Assembly8. If parts are to be presented using

automatic feeders, then ensure that they can be oriented using simple tooling.

9. If parts are to be presented using automatic feeders, then ensure that they can be delivered in an orientation from which they can be gripped and inserted without any manipulation.

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5.11.1 Summary of Design Rules for Robot Assembly10. If parts are to be presented in magazines or

part trays, then ensure that they have a stable resting aspect from which they can be gripped and inserted without any manipulation by the robot.

If the production conditions are appropriate, the use of robots holds advantages over the use of special purpose work heads and some design rules can be relaxed.

For example, a robot can be programmed to acquire parts presented in an array—such as in a pallet or part tray which has been loaded manually, thus avoiding many of the problems arising with automatic feeding from bulk.

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chapter 5 design for high speed automatic assembly and robot assembly 5/14/2015 dr. mohammad...

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