Russell Davison

I originally presented this article, "A Software Aid to Product Design for Robot Assembly", as a guest speaker at an I.Mech.E. Congress on Automotive Technology ...

INTRODUCTION

Original work by industrial researchers into classifying and coding parts for automatic parts handling, more than 30 years ago, led to considerations of good design features for automatic handling. Further research work resulted in a classification and coding system for manual handling, manual insertion and automatic insertion. This work culminated in the production of assembly system designer guidelines and these were later converted to computer software package to help product designers in the Design for Assembly process. With the increasing interest in the use of industrial robots for assembly, an obvious extension to the work on product design was the development of appropriate classification and coding systems for assembly robots and the translation of this into a user friendly computer based system.

PRODUCT DESIGN FEATURES FOR VARIOUS FORMS OF ASSEMBLY

There are inherent design rules for all forms of assembly and these are independent of the assembly process being used. There are other rules which are process dependent. The more important considerations are:

Number of Parts:- For all forms of assembly, reducing the part count, through considering the potential redundancy of every part, leads to a reduction in assembly and component manufacturing costs.

Parts Handling:- A further example of a common design requirement is for parts handling where, although manual handling is completely different to automatic handling, both benefit from an increase in the symmetry of a part. Similarly, both methods cannot easily accommodate minor asymmetrical features, nesting and tangling parts, very small or large parts, etc. As a result of this commonality, features which allow a part to be automatically handled easily are invariably useful for manual assembly and, in general terms, unless there is a significant manufacturing cost penalty, parts can always be designed for automatic handling.

Parts Insertion:- The requirements for the various types of assembly vary significantly for some insertion operations. For manual handling, the emphasis is on access and sighting. For automatic assembly, the main features are alignment, ease of insertion and stability after insertion. An additional potential problem is the direction of insertion for robotic assembly. For fastening operations, regardless of assembly method, the most economic operations use integral fasteners and the most expensive require threaded fasteners.

Parts Gripping:- In manual and automatic assembly (ignoring the features already mentioned related to size), gripping doesn’t create technological or economic problems. In robotic assembly, however, gripping features can be very significant and parts should be designed so that the least number of different grippers are required. This reduces costs and often reduces non-productive assembly time.

Assembly sequence:- The optimum sequence of assembly is very much dependent upon the type of assembly. For single worker assembly, the sequence of assembly is not important and it’s often determined by operator preference. In manual line assembly, sequence is controlled by line balancing considerations. In automatic assembly, sequence is related to the basic logic of the equipment and is controlled by the quality and, under some circumstances, the cost of the parts to be assembled. The sequence of assembly is determined by gripper requirements in single station robotic assembly, where the emphasis is on reducing significant non-productive time, such as either gripper changing or turret indexing.

ASSEMBLY ALTERNATIVES

In manual assembly, the two categories are single worker and line, with many variations incorporating features of both methods. It is generally not too difficult to identify the most appropriate form of assembly.

For automatic assembly, the choice of equipment is limited and selection is based on the number of parts, cost and component quality. Again, it is not difficult to identify the most appropriate equipment.

In single-station robotic assembly however, the selection of the most appropriate equipment is more difficult. There are many assembly robot types with different characteristics. Additionally, there are many parts handling possibilities and various gripper options. Although basic design for robotic assembly is essentially independent of the particular assembly cell configuration, both the product designer and system designer need some help to evaluate the performance and economics of alternative systems. Software applications have been developed for robotic assembly to serve both these functions.

Firstly, the product design is analysed by investigating its operation sequence relationship, handling features, gripping features and its insertion features. The user is asked to configure a system by specifying the robot to be used. The cost and performance specification for three popular assembly robots is built into a robot data file and these can be increased by the user at any time. The software application determines the most appropriate assembly sequence, based on interdependencies and the type of robot to be used. It then evaluates various parts feeding options. These options are based on feeder characteristics built into the system and they can be increased to include new types of automatic feeders.

The software application offers re-design possibilities to reduce the handling cost, where only expensive feeding methods can be used or when only manual handling is possible. If the robot type is unsuitable, due to lack of capability, then this is reported and the application user can either modify the robot data file, enhancing the robot specification, or select another robot. If the number of grippers required is excessive then various re-designs for easier gripping are proposed. The application also takes into account existing equipment utilisation. This is important because greater utilisation reduces the assembly cost.

CONCLUSIONS

The product and system design software application is a useful tool for evaluating robotic assembly. It can be modified and extended to reflect advancements in robots, feeders and grippers. It gives a quick evaluation of the suitability of a product’s design and determines the effect of changing assembly system parameters. These tasks could be done manually, using data sheets, but it is time consuming because of the large number of permutations of the various equipment types.

I originally presented this article, "The Flexible Assembly of Automotive Components", as a guest speaker at an I.Mech.E. Congress on Automotive Technology ...

There is a requirement for a special kind of system to assemble products required in modest volumes with a degree of variety. A system which is as cost effective and efficient as hard automation, whilst providing the flexibility of manual assembly, is called a flexible assembly system. Within such a system, certain product parts may be required at a different rate to other parts. Some operations may require the flexibility and dexterity of a robot, or even manual labour. The resultant system would be a hybrid of many methods of assembly. This article recommends a technique to be used for the design of such a system, with the aid of a case study.

INTRODUCTION

The factory cost of a product is the addition of the manufacturing cost (e.g. casting, moulding, turning) and the assembly cost (e.g. manual, automatic, robotic). Industrial engineers continually seek new methods to reduce the factory cost of products. The current trend of exploiting cheap labour in developing nations, through “offshoring” creates a challenge for domestic manufacturers in the developed nations. Between 40 and 60 percent of the factory cost for many products is associated with the labour content. The majority of this cost is incurred during assembly. There are three reasons for this uneven split between labour costs in manufacturing and assembly.

(i) Manufacturing operations are usually done by, or with the aid of, a machine, i.e. turning, milling, drilling, etc. The manufacturing systems designer does not have the wide choice of the assembly systems designer because some degree of mechanisation must be used. It is then a logical extension to further automate the manufacturing process to reduce labour costs.

(ii) New processes have been developed which eliminate many manufacturing operations. Powder metallurgy is an example of such a process.

(iii) Most products are designed to be assembled manually. This often means that components are of such a design that they cannot be handled by automatic feeders. Additionally, many assembly insertion operations are too complex to be automated.

THE DESIGN OF FLEXIBLE ASSEMBLY SYSTEMS

The assembly process has two constituent parts and these are; the handling of components and the insertion of components. The design features of a part must be examined to decide if it can be automatically handled automatically or if it must be handled manually or placed in magazines. Similarly, the insertion process must be analysed to decide what type of workhead is required.

Various organisations have developed procedures that help the designer to estimate how easy it is to handle and orientate components by assigning a handling code to each part. The maximum feed rate and relative cost of the feeding method can then be estimated from this code. The parts which would require expensive automatic feeders or which could not be fed at the required feed rate can be identified. These parts must then be handled manually or in magazines/pallets. Additionally, certain parts cannot be handled automatically because they have other bad feeding qualities, e.g. they may be flexible or too light. The previously mentioned estimation systems also help the system designer to forecast the relative cost of the workhead required to insert a part into a part-built assembly. Those operations which require a complex path of insertion, or a large thrust, require more expensive workheads than for simpler operations. A list of parts (with their associated automated handling codes) and a list of operations (with their allocated automatic insertion codes) can be constructed from the preceding information.

If the product parts are listed in order of increasing handling difficulty levels then the most economical method of feeding a part to the workhead can be determined. Parts with low handling difficulty levels are fed by conventional vibratory feeders and, as the difficulty level increases, specially designed feeders/magazines/pallets/manual handling are used. The relationship between the handling difficulty level and the type of feeder to be used depends upon the required return on investment for the equipment.

The insertion operations can also be listed in order of insertion difficulty levels to determine the most economical method of insertion of a part into a part-built assembly. Greater difficulty levels can mean that the equipment is more expensive and, for assembly robots, more degrees of freedom are required for an insertion operation. If the difficulty level is too high then it’s necessary to employ manual workers for some operations.

When an assembly system is designed for a new product, the cost of parts handling and insertion can be reduced through re-design of the product. It’s usually not viable for an existing product to be re-designed, because of the tooling modification cost in the manufacture of the parts. Inevitably, therefore, the most economical method of assembly is limited to the existing product design, without design efficiency improvements.

The assembly handling and insertion codes determine which feeding method and insertion device are most appropriate for each part and operation. The part-built assembly has to be transported to each workstation between operations. This will either be synchronous or non-synchronous motion. Synchronous machines are generally less expensive than non-synchronous types, but they are limited by how many parts can be assembled on one machine. This is due to downtime and the space available.

It is desirable to construct a product from as many sub-assemblies as possible to achieve a high overall efficiency of the assembly system. These sub-assemblies should be common to all product styles, within the family of products. The variety can then be created in the final assembly of the product. If this approach is adopted then sub-assemblies will be required at a rate which is enough to justify the use of automatic indexing machines having dedicated workheads. The output from these machines can then be sent to the final assembly line via free transfer lines, to create a buffer stock of sub-assemblies. The buffer stock is necessary to minimise the effect of any indexing machine downtime.

This is a manufacturing study that I was asked to carry out for a Swedish world leading manufacturer of compressors, generators, construction and mining equipment, industrial tools and assembly systems. The company wanted to create a database of observed operation times for robot assembly tasks.

INTRODUCTION

The robot assembly of the pneumatic cylinder has been analyzed using the video taken recently. Activity times were related to the digital clock display on the video. The object of analyzing the assembly process was to create a data base. Information could be extracted from this database to evaluate the robot assembly of other products manufactured by the client.

PNEUMATIC CYLINDER ASSEMBLY

There are fifteen parts used in the assembly of the pneumatic cylinder and some of these are actually sub-assemblies. All the parts are presented to the robot on a pallet, with the exception of the screws. The cover screws and piston rod screws are automatically handled by vibratory linear feeders. The cycle time for the complete assembly is 166 seconds. This is substantially longer than predicted by academic estimation methods.

The speed of the robot is set at 60 percent of the maximum. An electric current in excess of that tolerated by the drive motor circuitry, at 100 percent, makes this action necessary. This high power consumption, at start-up, is caused by the mass of the turret being approximately five times that of a conventional gripper. The robot manufacturer is replacing the relevant circuitry to allow full speed of the robot. Additionally, they have modified the feedback circuit to compensate for the larger mass. The robot programmer estimates that an increase in speed, from 60 percent to 100 percent, would provide a reduction in the cycle time of no more than 20 percent. The activity times obtained from the current analysis are used for the purpose of design for robotic assembly, and the evaluation of the client’s other products.

SYNTHESIS OF ROBOTIC ASSEMBLY TIMES

The time taken to assemble a part by the robot has four periods :

1) The movement of the gripper from the previous assembly position to above the current part to be assembled.

2) The picking up of the part.

3) The movement of the part to above the place of insertion.

4) The insertion, and subsequent release, of the part.

The above time periods, when added together, make up the basic operation time for a single activity.

RESULTS OF THE STUDY

The total assembly time for the pneumatic cylinder is broken down into 54 steps to quantify the 4 constituent periods in each activity. The time study sheet is shown later in this article. From the time study sheet, a basic operation time of eight seconds is derived. It can be seen from the table that this basic operation time is equally divided between the four constituent periods. Notable deviations from the basic operation time are :

End-piece - The end-piece is used to form a sub-assembly with the half -piston. There is no insertion time for this part because it is integrated with the half-piston.

Cover Screw - There is a 50 percent increase in the basic operation time for this part. It is caused by the extra time involved with screw fastening and the transportation distance between the linear feeder and the work fixture.

Piston Rod - A significant increase in the basic operation time for this part is due to the additional operation of 'knocking down' the piston rod after insertion. This is necessary because of the technique used to lift this part from the pallet.

Piston Rod Screw - A 50 percent increase in the basic operation time is caused by the screw fastening operation and the transportation time from the linear feeder to the work fixture.

Completed Cylinder - The gripper is in an adverse position from the previous operation and this increases the operation time.

PREDICTION OF CYCLE TIME FOR THE ROBOT ASSEMBLY OF THE PNEUMATIC CYLINDER

The cycle time for the robot assembly of the pneumatic cylinder is predicted by using a basic operation time, multiplied by a factor.

assembly time = basic operation time * assembly process factor
where,
basic operation time = 8 seconds
assembly process factor = 1.0 for straightforward insertion
1.5 for screw fastening operations
1.5 for long parts requiring two insertions

Using the above approximations, a cycle time of 164 seconds is predicted. This is within one percent of the actual time of 166 seconds. It is not suggested that such a simple method could always achieve this accuracy. However, in the present case, the predictions for 9 out of 10 parts are within plus/minus 1 sec.

PART ACTUAL PREDICTED DEVIATION OUTSIDE
DESCRIPTION TIME TIME 1 SECOND
CYLINDER BARREL 7 8 NONE
END-PIECE 5 *4 * NONE
HALF-PISTON 7 8 NONE
COVER SCREW 48 48 NONE
PISTON ROD 13 12 NONE
HALF-PISTON 8 8 NONE
PIN ROD SCREW 12 12 NONE
END-PIECE 8 8 NONE
COVER SCREW 12 12 NONE
COMPLETED CYLINDER 10 8 2 SECONDS
166 164
**NOTE**

The predicted time of 4 seconds for the end-piece allows for the fact that it is not inserted into the part-built assembly. This part forms a sub-assembly with the half-piston.

There is a negligible amount of time lost due to gripper changing. The turret is indexed during movement from one operation to the next. A typical programming chart for a component is given at the end of this article and it shows that the basic assembly operation takes 8 program steps. Additional steps are required for screw fastening.

TIME STUDY FOR ROBOT ASSEMBLY OF PNEUMATIC CYLINDER

0:01 Gripper above cylinder barrel
0:02 Pick up cylinder barrel
0:05 Cylinder barrel above fixture
0:07 Release cylinder barrel
0:10 Gripper above end-piece
0:14 Gripper above half-piston
0:15 Insertion of end-pieces and half-piston completed
0:17 Half-piston and end-piece above barrel
0:19 Release half-piston
0:21 Arm 2 above cover screw
0:24 Pick up cover screw
0:26 Cover screw above barrel
0:34 Arm 2 above cover screw
0:36 Pick up cover screw
0:38 Cover screw above barrel
0:46 Arm 2 above cover screw
0:48 Pick up cover screw
0:50 Cover screw above barrel
0:58 Arm 2 above cover screw
1:00 Pick up cover screw
1:02 Cover screw above barrel
1:09 Gripper above piston rod
1:11 Pick up piston rod
1:14 Piston rod above fixture
1:20 Completion of piston rod assembly to cylinder
1:22 Gripper above half-piston
1:24 Pick up half-piston
1:26 Half-piston above fixture
1:28 Completion of half-piston assembly to barrel
1:30 Gripper above piston rod screw
1:32 Pick up piston rod screw
1:35 Piston rod screw above fixture
1:40 Completion of piston rod screw assembly to piston rod
1:42 Gripper above end-piece
1:44 Pick up end-piece
1:46 End-piece above fixture
1:48 Completion of end-piece assembly to barrel
1:50 Gripper above cover screw
1:52 Pick up cover screw
1:55 Cover screw above barrel
2:02 Gripper above cover screw
2:04 Pick up cover screw
2:07 Cover screw above barrel
2:15 Gripper above cover screw
2:17 Pick up cover screw
2:19 Cover screw above barrel
2:27 Gripper above cover screw
2:29 Pick up cover screw
2:32 Cover screw above barrel
2:36 All cover screws inserted
2:40 Gripper above barrel
2:42 Pick up completed cylinder
2:44 Completed cylinder above pallet
2:46 Completed pneumatic cylinder in pallet

BASIC OPERATION TIME OF THE ROBOT

FROM PREVIOUS PICK MOVE TO INSERTION OPERATION
OPERATION UP PLACE OF AND TIME
TO ABOVE PART INSERTION RELEASE (SECONDS)
PART
10) CYLINDER BARREL 1 1 3 2 007
09) END-PIECE 3 2 0 0 005
08) HALF-PISTON 2 1 2 2 007
07) COVER SCREW 3 2 2 5 012
07) COVER SCREW 3 2 2 5 012
07) COVER SCREW 3 2 2 5 012
07) COVER SCREW 3 2 2 5 012
06) PISTON ROD 2 2 3 6 013
05) HALF-PISTON 2 2 2 2 008
04) PISTON ROD SCR 2 2 3 5 012
03) END-PIECE 2 2 2 2 008
02) COVER SCREW 3 2 2 5 012
02) COVER SCREW 3 2 2 5 012
02) COVER SCREW 3 2 2 5 012
02) COVER SCREW 3 2 2 5 012
01) COMPLETED CYLINDER 4 2 2 2 010
TOTAL 166

This is a quick feasibility study that I was asked to carry out for a Swedish world leading manufacturer of compressors, generators, construction and mining equipment, industrial tools and assembly systems. They wanted a swift appraisal of the economics for the robot assembly of their pneumatic valves.

INTRODUCTION

The client company has committed to capital investment in a two-armed robot for the assembly of its range of pneumatic cylinders at one of its Swedish manufacturing plants. The robot will not be fully utilised and another product is required to economically justify the installation.

The manufacturing plant currently assembles two product families :

1) Pneumatic cylinders
2) Pneumatic valves

The feasibility of assembling a model of pneumatic valve is investigated for the client company.

VOLUME REQUIREMENTS OF THE SPECIFIC PNEUMATIC VALVE MODEL

The pneumatic valve annual production volume is 30 000 units. The valve is currently assembled manually and the client company assumes that demand for the valve will increase to 40 000 within 2 years. Three workers are currently required for product assembly. The product has a total of 63 separate parts, of which, 31 are unique parts.

MANUAL ASSEMBLY

The manual assembly of the valve has been studied to create individual times for the 97 operations. The manual assembly worksheet, shown below, gives the sequence of operations and their corresponding operation times. The worksheet shows that the cycle time for the complete assembly is 389 seconds. One worker can assemble 11 786 valves in one year, with single shift working at a labour efficiency of 70 percent :

225 working shifts per annum (single shift working)
= 5 940 000 working seconds per annum (440 minutes / shift)
= 4 158 000 working seconds per year at 70 percent labour efficiency
= 10 689 units assembled per annum. .

ROBOT ASSEMBLY

Certain assembly operations can be executed by the robot without a re-design of the pneumatic valve. The assembly sequence for the robot assembly is given at the end of this article and an estimate for the robot capital expenditure is :


(a) Cost of the robot and controller = 50 000 euros.

(b) Turret and eight grippers = 5 000 euros

(c) Fixture number 32 = 3 000 euros, fixture number 33 = 2 000 euros, fixture number 34 = 2 000 euros, fixture number 35 = 3000 euros, fixture number 36 = 2 000 euros, fixture number 37 = 500 euros, fixture number 38 = 500 euros, fixture number 39 = 2 000 euros

(d) Arm-2 0-ring tools (6 off), including tool holder = 1 800 euros. Arm-2 screwdriver bit and friction screwdriver bit = 200 euros.

(e) Greasing station = 2 000 euros

( f) Labelling station = 5 000 euros

(g) Cleaning station = 2 000 euros

(h) Eight vibratory linear feeders at 3 000 euros each = 24 000 euros

Total = 105 000 euros

CYCLE TIME

It is estimated that the cycle time for the robot and manual assembly of the valve would be 456 seconds. Using this estimate, one robot can assemble 5210 valves in one year, with single shift working at a robot efficiency of 80 percent :

225 working shifts per annum (single shift)
= 5 940 000 working seconds per annum (440 minutes/shift)
= 4 752 000 working seconds per year at 80% robot efficiency
= 10 421 units assembled per year (single shift)

The robot can assemble approximately the same number of products per year as one worker, considering single shift working. However, certain operations (using the existing product design) must be executed manually.

ANNUAL COST SAVINGS

The annual cost saving of using the robot is one worker per year. If the annual cost of an operator is 50 000 euros per year (including taxes, social charges, pension contributions, overhead contribution, etc.) then the cost saving would be approximately 50 000 euros per year.

PAYBACK PERIOD

The payback period for using the robot is 2 years, for the assembly of 10 421 units per year.

VALVE SPECIFICATIONS

If the valve is to be re-designed then it must have the following performance characteristics :

(a) It must achieve a flow rate of 2.2 litres per second for 10 000 000 cycles of operation, without leakage from port to port.

(b) The upper sealing gasket must not drop off when the body sub-assembly is transported between operations.

(c) The inner sleeve 0-rings must be stable during assembly of the inner sleeve sub-assembly to the valve body.

(d) The activation time of the unit must be better than 0.02 seconds.

(e) The operating air pressure for the double acting valve should be less than 1.2 kg/cm2 and less than 2.5 kg/cm2 for the spring return valve.

(f) The customer should have the option of achieving flow rates between 0 and 50 percent of the maximum and between 50 and 100 percent of the maximum, using a convenient design feature.

RECOMMENDATIONS FOR A RADICAL RE-DESIGN OF THE PNEUMATIC VALVE TO REDUCE THE NUMBER OF PARTS IN THE ASSEMBLY

The following design changes are recommended to reduce the cost of the assembly of the pneumatic valve :

(1) Eliminate the choke screw housing (6) by providing an internal thread in the valve body, where the choke screw housing sub-assembly is currently situated. This would involve the use of a choke screw and O-ring only, thus eliminating two parts. .

(2) Eliminate the gasket (2) and top cover (23) by moulding the airway into the integral body and top cover. This would eliminate two parts but would require the bodies to be stocked in two styles to accommodate single acting and double acting valves.

(3) Eliminate the piston sleeve (9) by reducing the bore of the body at this point so that the piston is guided by the body, instead of the sleeve.

(4) Eliminate the three sleeves (9), (12), (14) and integrally mould the three sleeves as one part.

(5) Eliminate the piston by integrally moulding it with the spool piece, for single acting valves.

(6) Eliminate the spool piece O-rings / sealing rings and locate them on the spool piece sleeves.

(7) Eliminate one end piece and integrate it with the valve body.

(8) Eliminate the indicator and integrate it with the spool piece.

(9) Eliminate the label and print it directly onto the valve.

(10) Eliminate the cover screws and incorporate (a) a bayonet fitting or, (b) a screw thread between the body and end cover.

(11) Eliminate the short spring by changing the following design features of the long springs :
(a) spring wire gauge
(b) number of turns per inch
(c) spring material
(d) external diameter of spring

CONSIDERATIONS FOR THE RE-DESIGN OF THE PNEUMATIC VALVE

There are many factors that must be considered when re-designing the pneumatic valve for assembly. The effect of changing one design feature of a part may have an effect on the design of the other parts. The performance of the valve can be reduced by adverse design changes, or there may be an increase in the manufacturing costs of the product parts, due to new tooling costs. A number of factors must be considered when the re-designed valve is being evaluated :

(1) A capital investment has been made by the client company in mould tooling for the valve body. If other part features are to be integrated within the body, or if the body is to be split into more than one component, then there will be an investment required for the new mould tooling.

(2) If the choke screw housing feature is to be integrated within the valve body then the body tooling modification cost, and the scrapped choke screw housing tooling cost, must be considered.

(3) The assembly of the O-ring seals to the spool piece presents the problem of expanding the O-rings over the part and then allowing them to contract into the o-ring groove. The task can be simplified by re-designing the joints between the spool piece sleeves. Unfortunately, the current design of joints has been carefully chosen to avoid the possibility of an O-ring passing over a joint between two sleeves. It would be very difficult to achieve this same performance, so that the valve would operate for more than 3 000 000 cycles without a loss in performance.

(4) The spool piece is surrounded by sleeves having a multitude of holes in them. It would be logical to eliminate the sleeves and to direct the flow of air from one port directly to another port. However, these sleeves are required to provide an even flow path around the spool piece to maintain the required flow rate. Larger ports could be moulded into the valve body, but this could cause the O-rings to be damaged as they passed over the ports.

(5) The piston is guided by the piston sleeve. This piston guide design feature could be integrated within the valve body. The inside diameter of the piston guide must be such that the air pressure required to operate the valve is no greater than that already required. Additionally, it must still be possible to insert internal parts to the valve body. If the piston sleeve is integrated into one half of the body only, for spring return valves, then all of the parts associated with the spool piece can be inserted from one end of the of the valve. This option would, of course, require there to be two valve bodies for the product range.

(6) The thrust from a new single spring must be such that it can overcome the action of the fluid pressure and the friction between the spool piece O-rings and the sleeves.

(7) The sleeves are moulded as separate components because, as an integral part, it would be difficult to get the correct distribution of plastic in the mould. If the sleeve must be split for this reason then it would be advantageous to situate spool piece O-ring seals between the sleeves.

(8) It must be impossible to inadvertently unscrew the choke screw out of the body. If the valve was to be re-designed so that the choke screw could be inserted into the body after assembly of the cover, or into a body with an integral top cover, difficulties may arise. If the choke screw can be inserted after the cover, or cover feature, then it could also be removed by screwing. The addition of a retaining part would be counter-productive and, therefore, a stamping operation would be more efficient. The tops of the choke screw holes would be deformed after insertion of the choke screw, thus retaining it.

(9) During manual assembly of the spool piece sub-assembly to the valve body, special tools are required to assist the operator. The outside diameter of the sealing ring is much larger than the inside diameter of the spool piece sleeve. A special tool is required to contract the rings before insertion into the valve body. The operation is so complex that it may not be efficient to carry it out by a robot, in its current state of design.

(10) The operation of inserting the spool piece sub-assembly into the valve body is so complex that it is not feasible for it to be done by the robot.

(11) The piston and lip seal can only be inserted into the sleeve in one direction. This is because the lip of the seal has a larger diameter than the inside of the sleeve. The piston could be inserted in both directions if the seal was an O-ring.

(12) If one of the end pieces were to be integrally moulded with the body then all parts could only be inserted into the body from one direction. The sealing of the indicator would create special problems. If the seal is inserted by the robot then it cannot be sufficiently located. Otherwise, the robot would not be able to assemble the seal. During movement of the indicator, the seal may be removed from its housing.

(13) Integration of the top cover would make it impossible to change the routing of the signal air because the gasket would no longer be present.

(14) The current pneumatic valve design currently has two springs to generate the required thrust.

(15) The pin is required for the stability of the long spring, during operation.

(16) For aesthetics, the top cover and end covers must be of aluminium, to give the impression of robustness to the product.

(17) The dimensions of the inlet / outlet ports must be kept the same for compatibility with complimentary and substituted products.

MANUAL ASSEMBLY WORKSHEET

1 = Part identification number
2 = Number of times that the operation is carried out consecutively
3 = Two digit manual handling code
4 = Manual handling time per part
5 = Two-digit manual insertion code
6 = Manual insertion time per part
7 = Operation time in seconds (2) x [(4)+(6)]
8 = Operation cost, centimes 0.4 x (7)
9 = Figures for the estimation of the theoretical minimum number of parts

_1 2 _3 ___4 _5 ___6 _____7 ___8 9
23 1 30 01.95 00 01.5 003.45 01.38 1 PICK UP TOP COVER
99 12.0 012.00 03.00 CLEAN THE TOP COVER
30 1 88 06.35 33 05.0 011.35 04.54 PEEL OFF LABEL
06 2 11 01.80 003.60 01.44 PICK CHOKE S. HOUSING
05 2 03 01.69 30 02.0 007.38 02.95 PICK UP O-RING
04 2 10 01.50 00 01.5 006.00 02.40 PICK UP CHOKE SCREW
AUTO. SCREW CHOKE
07 2 03 01.69 30 02.0 007.38 02.95 PICK UP O-RING
01 1 30 01.95 00 01.5 003.45 01.38 1 PICK UP VALVE BODY
29 2 33 02.51 40 04.5 014.02 05.60 PICK UP BLANKING PC.
01 1 98 09.00 009.00 03.60 TURN BODY UP/DOWN
2 30 01.95 31 05.0 013.90 05.56 PICK UP CH. SCREW S/A
02 1 38 03.34 43 07.5 010.84 04.34 INSERT GASKET
1 02 02.5 002.50 01.00 INSERT COVER TO BODY
19 1 10 01.50 001.50 00.60 PICK UP PISTON
18 1 10 01.50 30 02.0 003.50 01.40 INS. LIP SEAL TO PISTON
PICK UP O-RING TOOL
16 1 03 01.69 00 01.5 003.19 01.28 INSERT O-RING TO TOOL
17 1 00 01.13 30 02.0 003.13 01.25 INS. SPOOL TO O-RING
PICK UP MIDDLE O-RING
16 2 03 01.69 00 01.50 006.38 02.55 INS. O-RING TO TOOL
17 2 00 01.13 30 02.00 006.26 02.50 INS. O-RING TO SPOOL
15 3 03 01.69 44 08.50 030.57 12.23 SEAL RING TO SPOOL
24 2 10 01.50 003.00 01.20 PICK UP INDICATOR
20 2 03 01.69 30 02.00 007.38 02.95 INSERT O-RING TO IND.
22 2 30 01.95 30 02.00 007.90 03.16 INSERT ENDPIECE
09 2 10 01.50 003.00 01.20 PICK UP PISTON SLEEVE
08 4 03 01.69 30 02.00 014.76 05.90 INS. 0-RING TO SLEEVE
10 2 10 01.50 30 02.00 007.00 02.80 INS. SEAL TO SLEEVE
12 2 10 01.50 30 02.00 007.00 02.80 INS. SEAL TO PISTON
PICK UP O-RING TOOL
11 2 03 01.64 30 02.00 007.38 02.95 INS. O-RING TO SLEEVE
1 10 01.50 00 01.50 003.00 01.20 SLEEVES TO FIXTURE
14 1 00 01.13 001.13 00.45 PICK UP HALF SLEEVES
13 1 03 01.69 30 02.00 003.69 INS. 0-RING TO HALF-SLV
00 01.50 001.50 INSERT HALF PISTON
1 00 01.13 001.13 PICK UP VALVE BODY
INS. TOOL TO VLV. BODY
30 02.00 002.00 INSERT VALVE BODY
PICK UP O-RING TOOL
13 1 03 01.69 30 02.00 003.69 INS. O-RING TO TOOL
1 12 05.00 005.00 INS. O-RING TO VALVE
1 10 01.50 30 02.00 003.50 INS. SLEEVE TO BODY
1 10 01.50 30 02.00 003.50 INS. PISTON TO SLEEVE
2 00 01.13 002.26 PICK UP END PIECE
21 2 23 02.36 43 07.50 019.72 GASKET TO ENDPIECE
2 02 02.50 005.00 ENDPIECE TO BODY
25 8 11 01.80 00 01.50 026.40 INS. COVER SCREW
8 92 05.00 040.00 16.00 FASTEN COV. SCREWS
1 98 09.00 009.00 03.60 TURN VALVE BODY
1 99 12.00 012.00 LUBRICATE VLV. BODY
PICK UP SPOOL TOOL
1 00 01.13 31 05.00 006.13 PICK UP SPOOL PIECE
PUT TOOL DOWN
26 1 00 01.13 02 02.50 003.63 INS. SPRING TO SPOOL
27 1 00 01.13 02 02.50 003.63 INS. SPRING TO SPOOL
28 1 10 01.50 02 02.50 004.00 INS. PIN TO SPRING
1 00 01.13 00 01.50 002.63 INS. VALVE TO FIXTURE
02 1 23 02.36 43 07.50 009.86 INS. GASKET TO BODY
389.22

ASSEMBLY SEQUENCE FOR THE ROBOT ASSEMBLY OF THE PNEUMATIC VALVE, WITH MANUAL ASSISTANCE

Note - Operations marked with an asterisk (*) are carried out manually.

(1) Pick up the cover (23) from the pallet and insert into fixture (32).
(2) Automatically clean the cover.
(3) Automatically feed and insert the label (30) to top cover.
*(4) Pick up the choke screw housing (6).
*(5) Pick up the O-ring seal (5) and insert onto choke screw housing.
*(6) Pick up the choke screw (4) and insert into the choke screw housing.
*(7) Fasten choke screw in choke screw housing by friction screwdriver.
*(8) Pick up O-ring seal (7) and insert into choke screw housing.
*(9) Pick up valve body (1).
*(10) Pick up gasket and assemble to valve body.
(11) Pick up valve body and insert into fixture (33).
(12) Automatically feed and insert the blanking piece (29) into the valve body.
(13) Rotate the body in the fixture by 180 degrees.
(14) Pick up the choke screw sub-assembly and insert into the valve body.
(15) Pick up the top cover and insert into the valve body.
(16) Automatically feed the piston (19) and insert into fixture (34).
(17) Automatically feed the lip-seal and insert into the piston.
*(18) Pick up O-ring tool.
*(19) Pick up O-ring (16) and insert onto O-ring tool.
*(20) Pick up spool piece (17) and insert into tool.
*(21) Insert O-ring into spool piece.
*(22) Pick up O-ring tool.
*(23) Pick up O-ring and insert into O-ring tool.
*(24) Pick up spool piece and insert into tool.
*(25) Insert O-ring into spool piece.
*(26) Pick up O-ring tool.
*(27) Pick up O-ring and insert into tool.
*(28) Insert O-ring into spool piece.
*(29) Pick up sealing ring (15) and insert onto spool piece.
*(30) Pick up end piece.
*(31) Pick up end piece gasket (21) and insert into end piece.
*(32) Insert end piece sub-assembly into magazine.
(33) Pick up end piece and insert into fixture (35).
(34) Automatically feed and pick up end piece O-ring (20) and insert end piece.
(35) Automatically feed and pick up indicator (24) and insert into end piece.
(36) Automatically feed the piston sleeve (9) and insert into fixture (36).
(37) Automatically feed the O-ring (8) and insert into piston sleeve.
(38) Automatically feed the lip-seal (10) and insert into piston sleeve.
(39) Automatically feed the sleeve (12) and insert into piston sleeve.
(40) Automatically feed O-ring (11) and insert into sleeve.
(41) Pick up piston sleeve and sleeve and insert into fixture (37).
(42) Automatically feed middle sleeve O-ring (13).
(43) Automatically feed middle sleeve (12) and insert into O-ring.
(44) Insert middle sleeve and O-ring into sleeve.
(45) Pick up the body and insert onto sleeves.
*(46) Insert O-ring into valve body.
(47) Pick up sleeve and piston sleeve and insert into valve body.
(48) Pick up piston and insert into piston sleeve.
(49) Pick up end piece and insert into valve body.
(50) Automatically feed end piece screw and insert into valve body.
(51) Fasten end piece screw into valve body.
(52) Rotate the valve body by 180 degrees.
(53) Lubricate the valve.
(54) Insert spool piece tool into body.
(55) Insert spool piece into body.
(56) Remove spool piece tool.
(57) Automatically feed and insert long spring (26) into spool piece.
(58) Automatically feed and insert short spring (27) into spool piece.
(59) Automatically feed and insert pin (28) into spring.
(60) Repeat operations 49 to 51.
(61) Pick up the valve and place onto the test station.
(62) Pick up the gasket (3) and insert into the valve body.

I originally published this article under the title, “The Presentation of Parts for Robot Assembly” in the book “Advances in Manufacturing Technology”, Kogan Page, London, ISBN 1.85091.3951 ...

The presentation of parts for robot assembly involves the selection of the correct parts handling devices and it influences the robot degrees of freedom required. The design of appropriate feeders is discussed, with an emphasis on their flexibility. A classification system is described that allows parts to be categorised by their design features and physical properties. The performance of an automatic parts feeder is shown to depend upon the design of the part that is being handled. A selection procedure is described that enables the correct handling device and robot configuration to be chosen for a particular application. An expert system is shown to be the best method of acquiring design information about the handle-ability of a part. A software package that simplifies the selection of parts feeders and robot configurations is described. The importance of knowledge transfer between industrialists and researchers, in defining relevant handling devices, is discussed. The development of an enhanced CAD system is the subject of a further publication.

INTRODUCTION

The presentation of parts to a robot presents some of the most difficult problems in robot assembly. Single cell robot assembly systems may assemble a complete product consisting of several parts. These parts have to be presented to the robot at the correct rate and in a known orientation, or a limited number of known orientations. The rate of supply of parts to the robot cell is seldom a problem because cycle times are usually long. The orientation of the part, at the exit of the parts feeding device, is critical because this influences many other factors. The orientation of a particular design of part at the feeder exit can be predicted using knowledge of handling device design. Parts are classified according to size, geometry, etc. so that feeding device performance can be qualified. Using a standard parts coding system, feeder performance can be matched with that required for a particular design of part. The orientation of the part, at the exit of the automatic feeder, can be predicted and the need for extra robot degrees of freedom can be determined. The presentation of parts for robot assembly is a complex problem and it’s best carried out using a software application.

PARTS PRESENTATION TECHNIQUES

A multitude of automatic feeders are available to handle a wide variety of parts. However, only a small proportion of these automatic feeders are economically viable for robot assembly. For robot assembly, an automatic parts feeder must have a high general-purpose content and a low special-purpose content, so that the flexibility of the robot is not compromised by the inflexibility of its feeders. The vibratory linear feeder has a low cost special-purpose feed track that is mounted on a general-purpose drive unit and frame. The device is very flexible because changeover is effected by removing the current feed track and replacing it with a feed track for the next part. The vibratory bowl feeder consists of medium cost special-purpose tooling that is mounted around the periphery of a general-purpose bowl. The feeder is generally inflexible and the time associated with part changeover makes it unsuitable for many applications with small batch sizes. The horizontal pallet transfer system has low cost special-purpose pallets that move into, and out of, the work zone by a general-purpose transfer system. Flexibility is achieved by using different pallet configurations or by simply changing the pallet contents. The 'Hitachi' type feeder works on a similar principle to the vibratory bowl feeder, with the special-purpose tooling being replaced by a vision system. Within certain geometrical and size limitations, this device is highly flexible; using a vision system to identify part orientations. The programmable belt feeder uses special-purpose pushers and gates, activated by a vision system or sensors, mounted above a general-purpose belt. Product changeover is achieved by using a different vision system computer program or by replacing the pushers and gates.