Design for High Speed Automatic Assembly and Robot Assembly

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Chapter 5Design for HighSpeed AutomaticAssembly and RobotAssembly

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Dr. Mohammad Abuhaiba1


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5.1 INTRODUCTIONFig. 5.1: The slightly asymmetrical

threaded stud would notpresent significant problems inmanual handling and insertion

For automatic handling anexpensive vision system wouldbe needed to recognize itsorientation.

If the part were madesymmetrical, automatichandling would be simple.

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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 thedesign 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 theassembly 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 becalculated.

We shall be mainly concerned with:1. Cost of all the equipment2. Number of operators and technicians3. Assembly rate at which the system is designed to operate

Apportion the cost of product assembly between theindividual parts and, for each part; we shall need to knowthe cost of feeding and orienting and the cost ofautomatic 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 required

2. Time interval between delivery of successiveparts.

Time between delivery of parts: reciprocal ofthe delivery rate. It is equal to cycle time of machine or system.

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

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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 availableseconds in one shift working for one monthdivided by 100 to convert dollars to cents.

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Example: assume that a standard vibratory bowlfeeder costs $5000, payback period is 30 monthswith 2 shifts working, factory equipment overheads

are 100% (Eo = 2), we get Rf= 5000 x 2/(5760 x 30 x 2) = 0.03 cent/s It 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 Crto

any feeder under consideration, then Eq. (5.1)becomes

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

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Feeding cost per part is inversely proportional torequired feed rate and proportional to feedercost.

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

This illustrates why it is difficult to justify feedingequipment 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 feedercosting $5000.

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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 canoperate.

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

Fm = maximum feed rate

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Example: 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. Two feeders could be used, each at a rate of 10 parts/min. 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 feedrate.

If required feed rate is greater than max feed rate obtainablefrom one feeder, feeding cost becomes constant and equalto cost of feeding when the feeder is operating at its maxrate. This is shown in Fig. 5.2 by the horizontal line.

If multiple feeders are used for increased feed rates, then theline will be saw-toothed as shown.

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Eq. (5.3) holds true only when required feed rate Fr


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The meaning of the orienting efficiency E:

Consider feeding of dies (cubes with faces numbered 1 to6).

If no orientation is needed, the dies can be delivered at arate 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 toreject them.

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

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

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If dies were doubled in size and that feedspeed on the feeder track were unaffected.

It would then take twice as long to delivereach die.

Max feed rate is inversely proportional tolength of part in the feeding direction [Eq.

(5.5)].

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Eq. 5.4 shows that when Fr> Fm, the feeding costper part is inversely proportional to Fm.

Under these circumstances, cost of feeding isinversely proportional to orienting efficiency andproportional to length of part in the feedingdirection.

Automatic feeding and orienting methods areonly applicable to "small" parts.

Parts larger than about 8 in their majordimension cannot usually be fed economically.

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When considering the design of a part and itsfeeding cost, the designer will know:

1. required feed rate Fr2. dimensions of the part, l

The remaining two parameters that affectfeeding cost, namely, the orienting efficiency Eand the relative feeder cost Cr, will depend on:

1. part symmetry2. types of features that define its orientation.

A portion of a classification system is presentedin Figs. 5.3 to 5.5.

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Fig. 5.3: 1st digit of a 3-digit shape code.

Fig. 5.4: 2nd and 3rd digits are determined fora selection of rotational parts (1st digit 0, 1,or 2) and corresponding values of orientingefficiency 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 ofgeometrical classification for

some non-rotational parts


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5.3 EXAMPLE

Part shown in Fig. 5.6 is to be deliveredto an automatic assembly stationworking at a 5 s cycle.

Use classification system anddatabase to determine feeding costassuming that cost of delivering simpleparts at 1 per second using standardfeederof 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 andA/C < 4, part is cubicnon-rotational and is assigned a 1st digit of 8.

Fig. 5.5: part has no rotational symmetryabout 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 Ydirectionand step seen in the view in Z direction could also be used todetermine the part's orientation.

Select feature giving smallest 3rd classification digit; in this case

it is the step seen inX 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 fromone feeder Fm =1500 E/ l = 1500x0.15/30 = 7.5 parts/min

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

Since Fr> Fmwe 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 FEEDINGDIFFICULTIES

Fig. 5.7: If edges ofparts are thin, shingling

or overlapping canoccur during feeding,leading to problemswith the use of

orienting devices onfeeder track

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5.4 ADDITIONAL FEEDINGDIFFICULTIES

Fig. 5.8: for each combinationof features, an approximateadditional relative feeder cost

is given that should be takeninto account in estimating thecost of automatic feeding.

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5.5 HIGH-SPEED AUTOMATICINSERTION

If a part can be sorted from bulk and delivered to aconvenient 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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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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It would cost 0.06 cents to use theequipment for 1 second.

Taking this figure as the rate for a "standard"work head and assign a relative cost factorWrto any work head under consideration,then Eq. (5.6) becomes

Insertion cost is inversely proportional torequired assembly rate and proportional towork head cost.

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5.6 EXAMPLE Fig. 5.6:

part is inserted horizontally into the assembly in thedirection 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 Fris 12parts/minand 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 = 0.72cents

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5.7 ANALYSIS OF AN ASSEMBLY

assembled ata 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

repe

ats

Handli

ng

code

Orientat

ion

assembl

y

Relati

ve

feede

r cost

Max

feed

rate

(parts/

min)

Handli

ng

difficul

ty

Handli

ng

cost

(cents

)

Inserti

on

code

Relati

ve

work

head

cost

Inserti

on

difficul

ty

Insertio

n/

operati

on cost

(cents)

Total

cost

(cent

s)

Figur

e for

min

parts

High speed

automatic

assemblyName of

assembly-

valueName of

part, sub-

assembly or

operationID RP HC OE CR FM DH CF IC WC DI CI CA NM1 1 83100 0.20 1 4.8 12.4 0.40 00 1.0 6.3 0.38 0.69 1 Housing2 1 02000 0.40 1 21.4 6.3 0.20 02 1.5 0.56 0.56 0.63 1 Plunger3 1 00840 .* ***.* **.* *.** Manual assembly required 7.13 0 Gasket4 1 00800 .* * ***.* **.* *.** Manual assembly required 6.67 1 cover5 2 21000 0.90 1 122.7 6.3 0.20 39 1.8 11.3 0.68 1.44 0 screw


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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. 5.10Re-Design

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Part or sub

or opern

No.

No.

of

repe

ats

Handli

ng

code

Orientat

ion

Efficien

cy

Relati

ve

feede

r cost

Max

feed

rate

(parts/

min)

Handli

ng

difficul

ty

Handli

ng

cost

(cents

)

Inserti

on

code

Relati

ve

work

head

cost

Inserti

on

difficul

ty

Insertio

n/

operati

on cost

(cents)

Total

cost

(cent

s)

Figur

e for

min

parts

High speed

automaticassemblyName of

assembly-

valueName of

part, sub-

assembly or

operation

ID RP HC OE CR FM DH CF IC WC DI CI CA NM1 1 83100 0.20 1 4.8 12.6 0.40 00 1.0 6.3 0.29 0.69 1 Housing2 1 02000 0.40 1 21.4 6.3 0.20 02 1.5 9.4 0.43 0.63 1 Plunger3 1 00040 0.70 3 26.3 18.8 0.61 00 1.0 6.3 0.29 0.90 0 Gasket4 1 02000 0.40 1 15.0 6.3 0.20 38 0.8 5.0 0.23 0.43 1 cover


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5.8 GENERAL RULES FOR PRODUCTDESIGN FOR AUTOMATION

The elimination of a part would eliminate:

1. a complete station on an assembly

machine-including the parts feeder2. special work head

3. associated portion of the transfer device

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Automation can be facilitated by theintroduction of guides and chamfers.

Figs. 5.12 and 5.13 Sharp corners are removed so that the

part can be guided into its correctposition during assembly leading to:

1. less control by the placement device or

2. can even eliminate the need for aplacement device.

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Screws that tend to centralizethemselves in the hole give the bestresults in automatic assembly:1. Rolled thread point: very poor

location; will not centralize withoutpositive control on the outsidediameter of the screws.

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

3. Chamfer point: reasonable tolocate.

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

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Assembly 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 assemblystation: 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 theircorrect positions during the machine index period, whendynamic 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 fastenedor secured.

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Assembly from above is notpossible:

divide assembly intosubassemblies.

Fig. 5.15:

Difficult to position and drive thetwo cord grip screws from below.

The two screws, cord grip, and

plug base could be treated as asubassembly dealt with prior tomain machine assembly.

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

for quick and accurate locationon the work carrier.

Figure 5.16a: If a force were applied atA, part

would rotate unless adequateclamping was provided.

To ensure that a base part isstable, Arrange that its center ofgravity be contained within flathorizontal surfaces.

Fig. 5.16b: A small ledge machinedinto part

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Fig. 5.17: Location ofbase part in the

horizontal plane is oftenachieved by tapereddowel pins mounted inthe work carrier to

provide guidance

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5.9 DESIGN OF PARTS FOR FEEDINGAND ORIENTING

Most versatile parts feeder is the vibratorybowl 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 asymmetryresulting from small or non-geometricalfeatures.

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deliberately addasymmetrical features for

the purpose of orienting. The features that require

alignment are difficult toutilize in an orienting

device, so correspondingexternal features aredeliberately 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.

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Parts that are easy to handleautomatically will also be easy to handle

manually. Very small parts or complicated shapes

formed from thin strips are difficult tohandle in an automatic environment.

Manufacture the parts on the assemblymachine or to separate them from the stripat the moment of assembly.

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5.10 SUMMARY OF DESIGN RULES FOR

HIGH-SPEED AUTOMATIC ASSEMBLY

Rules 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 thatthere is no tendency for it to move under the action of horizontalforces during the machine index period.

5. Provide chamfers or tapers that help to guide and position partsin the correct position.

6. Avoid expensive and time-consuming fastening operations, suchas 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 symmetrical

3. If symmetry cannot be achieved, exaggerate asymmetricalfeatures to facilitate orienting or, alternatively, providecorresponding asymmetrical features that can be used toorient the parts.

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5.11 PRODUCT DESIGN FOR

ROBOT ASSEMBLY

Three representative types of robotassembly systems :

1. Single-station with one robot arm2. Single-station with two robot arms

3. 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 benecessary to transfer the assembly to a

location or fixture outside the workingenvironment of the robot.

Robot place the assembly on a transferdevice that carries the assembly to themanual station.

After manual operation has beencompleted, assembly can be returned in asimilar manner to within reach of the robot.

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5.11 PRODUCT DESIGN FOR ROBOTASSEMBLY: 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 anexternal workstation to carry out thesecuring operation.

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

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ROBOT ASSEMBLYTo 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.

2. 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

3. Average assembly cycle time

4. Cost per assembly of manual labor involved in: machine supervision loading feeders, magazines, pallets, or part trays performing any manual assembly tasks

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ROBOT ASSEMBLY

One classification & one data chart foreach 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 &timefactors are given that can be used toestimate equipment costs & assembly times

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ROBOT ASSEMBLYFigure 5.21: a portion of classification systemand database for a single-station one-armrobot assembly system.

Part is being added to assembly, but is notbeing secured immediately.

Selection of appropriate row depends ondirection of insertion.

Selection of appropriate column depends on:

whether part needs a special gripper, clampingtemporarily after insertion whether it tends to align itself during insertion

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ROBOT ASSEMBLY

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ROBOT ASSEMBLYExample: A part is to be inserted along a horizontal axis, does notrequire a special gripper, requires temporary clamping, and is easy toalign. Code =12

Relative robot cost AR =1.5 If 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 =1.0 Since 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 ofspecial purpose equipment.

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ROBOT ASSEMBLYExample: A part is to be inserted along a horizontal axis, does not require aspecial gripper, requires temporary clamping, and is easy to align. Relative basic operation time TP =1.0 The 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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ROBOT ASSEMBLY 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:1. 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)

2. Equipment costs: feeder depreciation depreciation of special fixtures, special tooling, magazines, pallets, or

part trays.


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ROBOT ASSEMBLY 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, isassumed 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.

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

assumed to be $1000. For large parts this figure may considerably underestimate the actual cost and

extra allowance should be made.

3. 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 count

2. 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 inthe assembly.


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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 layerfashion from directly above.

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

adding value to assembly.

If the partial assembly has to be turned to a different restingaspect during assembly process, then this will usually result inincreased work fixture cost and the need to use a moreexpensive 6 DOF robot arm.


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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 automatichandling systems Are light so that air resistance will create conveying

problems


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Rules for Robot Assembly

8. If parts are to be presented usingautomatic feeders, then ensure that they

can be oriented using simple tooling.9. If parts are to be presented using

automatic feeders, then ensure that theycan be delivered in an orientation from

which they can be gripped and insertedwithout any manipulation.Dr. Mohammad Abuhaiba

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Rules for Robot Assembly10. If parts are to be presented in magazines or part

trays, then ensure that they have a stable restingaspect from which they can be gripped and

inserted without any manipulation by the robot. If the production conditions are appropriate, theuse of robots holds advantages over the use ofspecial purpose work heads and some designrules can be relaxed.

For example, a robot can be programmed toacquire parts presented in an arraysuch as in a

pallet or part tray which has been loadedmanually, thus avoiding many of the problemsarising with automatic feeding from bulk.


Documents

Design for High Speed Automatic Assembly and Robot Assembly