eNotes: Design
   



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DI:14.2 DESIGN FOR ASSEMBLY (DFA)


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These techniques attempt to simplify products to ease the assembly process, without compromising functionality of the product.

First, consider the basic steps involved in assembly,

1a. parts are purchased, and put into inventory, or storage bins.
1b. parts are manufactured, and put into inventory, or storage bins.
2. batches of parts are often inspected for quality.
3. the batches are moved to the work station.
4. the partially completed assembly may be already at the work station, or the operator may accept it from another source (e.g., a belt on an assembly line)
5. the part base will be set in position.
6. The operator will pick a part from the parts bin.
7. the operators will (if not already) position the part correctly in their hand, and prepare to insert it into the work.
8. The operator will guide the part into the final position.
9. The operator will move the two parts so that they fit together
10. The operator will perform any fastening operations required.
11. Additional alignment or quality inspection steps may sometimes be included.

Each one of these steps has potential for problems, or improvement. For example, if one part can be modified to match another, we cut the need to perform steps 1 to 5 in half. For each part that can be eliminated we reduce steps 1-11.

One report of these techniques applied to circuit boards [Boothroyd and Knight, 1993] reports,

- manufacturing costs down almost 20-30%

- component costs down 10-20%

- component counts down almost 25-40%

- board densities down almost 5-20%

- problem parts down over 20-90%

- yield up over 30-50%

DI:14.2.1 Design rule summary

Part Design

1. Eliminate/minimize tangling between parts in feeders.
2. Use symmetry to reduce the orientation time during handling
3. If symmetry is not possible, use obvious features to speed orientation

Product Design

1. Reduce the number of parts when possible
2. Build the part in layers from the top on the bottom, using gravity to locate parts
3. Have the already assembled product stable on the work surface
4. Have the work lie in a horizontal plane
5. Use chamfers and fillets to ease mating of parts.
6. Use snap-fits, and other quick fasteners, avoid screws, glue, etc.

DI:14.2.2 Rules for Manual/Automatic Assembly

The basic strategies of DFA for automated assembly are,

1. Reduce the number of parts
2. Allow assembly from the top of a fixtured part
3. Develop symmetry for easy part orientation
4. Use guides to simplify part mating, such as chamfers
5. Aim for snap-fit connectors, avoid screws
6. Reduce handling problems

The basic rules of DFA for manual assembly are,

1. the number of parts should be reduced
2. parts should be standardized where possible
3. alignment operations should be reduced
4. locating and aligning features should be used
5. allow clear paths for parts being added to the assembly
6. add orientation features so that parts can only be assembled in the correct orientations
7. consider part feeding/picking from batches
8. introduce symmetries to reduce the need for reorientation
9. add orientation features to simplify orientation identification

DI:14.2.3 Reducing the Number of Parts

Designs often include more parts than are necessary

A set of questions must be satisfied for any two parts in an assembly to justify their being separated

1. Do the parts move relative to one another?
2. Must the parts be made of different materials?
3. Must the parts be separable for maintenance or manufacture?

Some simple ideas possible are,

1. Instead of attaching labels on plastic parts, add the lettering into the mold so that the letters are added at the time of molding. The completely eliminates a part, and the associated operations.



2. In sheet metal parts create features using sheet metal, instead of attaching them with other means. Some examples are,
- instead of adding hook to a sheet metal part, cut and bend hooks out of the sheet metal
- don't add screw standoffs to metal, but punch the metal to create a standoff, and tap the hole.
-
3. When possible use snap fits instead of screws. Most screwed connectors require 1 nut, 1 bolt, typically 2 or more washers, and possibly a lockwasher, as well as a great deal of time and dexterity to assembly. Snap fittings can be made very simple and fast. NOTE: press fits can also be considered for these operations, although their need for higher forces can be a negative.



4. If screws must be used try integrating washers with the screw heads, this will eliminate at least one part.




5. Replace separate springs with parts with thin sections that act as springs.



6. When screws are required (often for maintenance) try to reduce the number to a minimum.
7. Cables can be eliminated for a reduction in cost, and an increase in reliability, and access for maintenance. Card edge connectors, and PCBs will be slightly higher in material costs, but the boards are simply plugged together. If cables are strung between boards and other boards/components, they will require additional time for soldering, be the source of soldering quality problems, and make the boards tricky to orient, etc.

DI:14.2.4 Feeding and Orienting Parts

It must be considered that more complicated parts require greater handling time to properly orient them.

DI:14.2.4.1 - Part Tangling/Nesting

It should be considered that when small parts are shipped, they come in bulk lots. (large/more expensive parts are often shipped in pallets, or separately.

When the parts are stored together, they can sometimes tangle or nest.

Tangling - the parts get looped together, making them difficult to separate
Nesting - one part gets stuck inside another, much like styrofoam cups.

The obvious problem with this situation is that the parts often require additional costly human intervention to separate them, and this problem will greatly reduce the success of an automated parts feeder.

The problems of nesting and tangling can be significantly reduced through small design modification in many cases.

A few of the problems, and some possible solutions are shown,







DI:14.2.4.2 - Handling Parts

As parts vary greatly in size we must how it is to be manipulated. The basic categories are,



Other factors to consider when handling parts,

- are the parts sticky?

- are the parts fragile?

- are there any sharp edges?

- do the parts nest or tangle?

- are there any parts or tools that the operator must leave the work station to get?

DI:14.2.4.3 - Orienting Parts

When parts are to be fed, automatically and by human, the task is simplified if certain features are added.

Basically, symmetry is a major problem for automated feeders.

In general, a part is easier to orient if,

1. The orientation is based on internal features, and there are external features that can be used for reference.
2. Extra features are added to change the centre of mass, or create holding points for features.

Some examples of parts orientation are,









DI:14.2.4.4 - Locating and Aligning Parts

When we try to thread a needle, the thread is smaller than the hole in the needle, but this does not make threading the hole simple. In fact the process of threading the needle is simplified by the rounded opening on the needle. If the opening of the needle were square it would greatly complicate this problem.

Much like threading a needle, the problem of mating two parts can be simplified if the parts tend to align and locate themselves.

If we consider that for one part to mate with another, it must travel along an approach axis. In fact, when the parts are mated the parts will have common axis. We can add guides to the parts to align the axis to be parallel, and to locate the axis so that they are colinear. Hence, the terms aligning and locating.

Consider the cases given below, and the implications they have for alignment





Although screws are discouraged in DFA techniques, when they must be used, then we can add some features to help align and locate them.



DI:14.2.4.5 - Part Symmetry

Perfectly symmetrical parts need no rotation to orient them, completely asymmetrical parts require at most 360 rotation followed by a second 360 to put them in the same position every time.

There is also a recognition phase required by humans for every orientation. Therefor parts that are not symmetrical, but look as if they are upon quick inspection, will require additional inspection time.

consider the cases below,









Alpha symmetry is the largest angle a part would have to be turned about an axis perpendicular to the insertion axis.

Beta Symmetry is the largest angle the part would have to be rotated about the insertion axis for mating.

Alpha and Beta symmetry actually range from 0 to 360 (instead of the intuitive 0 to 180) because it is assumed that the worst case rotation is used.



DI:14.2.4.6 - Part Shape, Size and Thickness

We must consider the basic shapes of the parts being assembled. Two basic categories are prismatic and rotational.

Rotational parts tend to roll when placed on a surface, suggesting that they will need some sort of holding fixture. This also means that during assembly, they must be supported by hand if not in a stable position when working.

Prismatic parts tend to have at least one stable orientation that allows them to be rested on surfaces. Unlike rotational parts. If the prismatic parts are made to be stable when put in their final position, then they are much easier to fasten.

The size of an object is generally the size of its largest major dimension, and thickness is the smallest major diameter.

There are a number of criteria that can be used to determine how easily a part can be handled,

- a high size/thickness can be a measure of fragility
- large size values can indicate large weights
- small size values can indicate the need for special tools

DI:14.2.5 Mating Parts

There are a large number of methods for assembling parts. Generally a fastening operation is involved.

The best rule of thumb is that all assembly work is best done by setting down a large base, and slowing dropping more parts on top of the base. Each part should be fed by gravity, and the work base should not have to be moved to put the part on.

When mating two parts there are a number of possible combinations. The following table is an adaptation of Boothroyd [1979].



If a part must be supported or held down by hand while a fastening operation is done, this greatly complicates any operations. If this is the case, self securing parts should be used.

parts may also exert some sort of resistance to insertion. If this is the case, the force should be minimized

There are two type of obstructions that must be considered during assembly operations

- the operator has no clear view of the assembly site
- the assembly site is not in easy reach (i.e. the assembly axis is not clear)

A self nesting or self fixturing part is ideal. In effect the part will hold itself in location after it has been positioned.

DI:14.2.6 Adjustments

As an assembly is built, adjustments are commonly used to bring the shape back to proper specification. This can easily by the result of errors accumulated as parts are added in discrete steps.

This problem can best be avoided by,

- having parts positioned relative to one reference piece. For example pilots through layers of the work can be used for mounting parts.
- screw hole slots, instead of holes can allow play in position.
- loosen tolerances to the minimum levels
-

DI:14.2.7 Modular Assemblies

Designing in modules will allow reduction of the problems involved with any one assembly.

Each module should be functionally separate from the other modules

A module should have docking features to allow it to be connected to the main assembly

advantages,

- simplified assembly steps
- easier quality control
- simpler inventory
- easier to reconfigure a system
- suited to automatic assembly
- fewer adjustments are required on final parts
- simplified maintenance

DI:14.2.8 Standard Parts

There are a few distinct benefits to standards parts (as opposed to custom designed),

- lower development costs

- simple selection of vendors

- lower production costs (no special tooling required)

- quality levels are well established

- these parts are easy to approve for Acceptance Sampling programs

- automation tooling is available for many standard parts

DI:14.2.9 Part Fixtures and Jigs

Jigs and fixtures are often used when,

1. Doing manual assembly, with small or fragile parts
2. doing any form of robotic assembly (at present sensors are not yet available for reliable fixtureless work).
3. when designing self fixturing parts where the base part also acts as a form of fixture.

When parts are mounted on fixtures, we can pretend this is another assembly step, and apply all of the normal DFA rules.

The location of the part on the fixture is important for both alignment, and location in many cases, as the fixture has been set up as a reference.

For high accuracy in location, we are better to have (one, two, or three) point contact between the fixture and the part. For orientation, surface/surface contact (such as chamfered hole shaft pairs) will give better results.





DI:14.2.10 Bottom Up Layered Assemblies

The assembly should be done using a heavy and stable base piece at the bottom.

As the design continues we want to add new parts in layers.

The docking location for the new part should be a sort of mini fixture.

Pilots should be added to locate layered pieces.

This method also allows many parts to be put in place, and then a number of parts assembled in one assembly step.

DI:14.2.11 Examples

First, review the DFA handbook paying special attention to the work sheets and the tables.

Use the DFA handbook a) To do an analysis of the assembly below (assume dimensions with an overall length of about 8"). b) do a redesign of the assembly and reanalyze.



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