MFE O3 Design for Manufacture and Assembly

Answer key:

Section-I

1.Design for Manufacturing (DFM) is a methodology aimed at improving the manufacturability or producibility of a single product by reducing manufacturing costs

2. Functional dimension-is an important dimension that is involved with the correct working of a machine assembly.

Non Functional dimension is, relatively, an unimportant dimension in that it is not a critical dimension in the working of the machine.

3. This technique involves the selection of random observations with in the simulation model. It is constrained for application involving random numbers to solve deterministic and stochastic problems.

The underlying principle of this technique is

  • Replace the actual statistical universe by another universe described by some assumed probability distribution.
  • Sample from this theoretical population by means of random numbers.

4. Straightness, flatness, roundness, concentricity, squareness, parallelism, angularity, symmetry

5. 1.trial and error 2. Interchangeable assembly 3.selective assembly

6. False

7. The procedure to be followed when changing the datum face may be summarized thus:

  • Decide the required manufacturing datum face.
  • Decide the required manufacturing required manufacturing dimension
  • Determine the tolerance for each of the manufacturing dimensions
  • Set suitable limits for all but of required dimensions
  • Determine the limits for the final dimension.

8. True

9.

  • Uneconomical
  • Difficult to remove from the mould
  • Core removal difficulty

10. True

11. Machined holes

  • Round machined holes in a cast component may be produced by drilling or reaming or by boring.
  • Drill/boring is decided by the size of the hole
  • Machined holes, up to and including 30mm diameter to be drilled, holes over 30mm diameter to be bored.
  • Machined holes up to and including 30mm diameter to be cast solid. Machined holes over 30mm diameter to be cast/cored.

12.

  • Avoid placing welds in vulnerable cross-sections
  • Avoid laps, straps, and stiffening angles (with figures)

13. True

14. True

15. -Economic analysis-component design

-Ability to identify uneconomical design

-Ability to modify the design to eliminate the fault whilst retain the essential functional feature of the design

16.

  • unnecessary large machined areas
  • unnecessary amount of small tolerance machining
  • difficult or unduly lengthy machining features
  • slightly dissimilar components that could be standardized

17.

  • Six sigma is a business method for improving quality by removing defects and their causes in business process activities. it concentrates on those outputs which are important to customers.
  • This method uses various statistical tools to measure business process.
  • The main goal is continuous improvement
  • Six sigma is carried out as project.(DMAIC-Define,measure,analyze,improveand control)
  • Bottle-necks and problems identified.
  • Improvement program is defined and defects removed

18.A poka-yoke device is any mechanism that either prevents a mistake from being made or makes the mistake obvious at a glance: to control (for instance shuts down) or warn for abnormalities

No errors  no defects  Zero quality control (ZQC)

19.“A team based approach to identifying and eliminating waste (non-value-adding activities) through continuous improvement by flowing the product at the pull of the customer in pursuit of perfection”

A methodology to minimize waste at all levels through assessment of each activities of a company

20.

A Value Stream is the set of all actions (both value added and non value added) required bringing a specific product or service from raw material through to the customer.

Value Stream Mapping

•Follow a “product” or “service” from beginning to end, and draw a visual representation of every process in the material & information flow.

•Then, draw (using icons) a “future state” map of how value should flow.

•VSM serves as a -Communication Tool,Business planning ToolTool to manage any change process

Section-II

21.(i)

  • Tolerance must be placed on dimensions of a part to limit the permissible variations in size because it is impossible to repeatedly manufacture a part exactly to a given dimension.
  • A small tolerance results in greater ease of interchangeability of parts and less play or chance for vibration in moving parts.
  • Obtaining the best quality and performance from the design.
  • Minimizing manufacturing cost.
  • Manufacturing process is a combination of three elements man, materials and machine. If the process is under control,. All the assignable causes of variation are controlled or eliminated, the variations in sizes of similar components

will be within reasonable limits.

  • improves communication
  • provides better product design
  • increase production tolerance

Tolerances as Feature Attributes

  • Tolerance as an attribute

Size tolerance: attributes of dimension parameters

Form tolerance: attributes of edges or surfaces.

Position & orientation tolerance: attributes of feature relationships

21.(ii)Process capabilityanalysis:

  • Statistical techniques can be helpful through the product cycle
  • Development activities prior to manufacture, in quantifying process variability, in analyzing this variability relative to product requirements or specifications, and in assisting development and manufacturing in eliminating or greatly reducing this variability
  • Product capability refers to the uniformity of the process. Obviously, the variability in the process is a measure of the uniformity of output.
  • There are two ways to think of this of this variability: 1.the natural or inherent variability at a specified time; that is “instantaneous” variability.2. Thevariability over time.
  • It is customary to take the 6-sigma in the distribution of the product quality characteristic as a measure of process capability.
  • Process capability analysis as an engineering study to estimate process capability.
  • The estimate process capability may be in the form of a probability distribution having specified shape, center (mean), and spread (standard deviation).
  • A process capability study usually measure functional parameter on the product, not the process itself.
  • Process capability analysis is a vital part of an overall quality improvement program.
  • Prediction how well the process will hold the tolerances.
  • Assisting product developers/designers in selecting or modifying process.
  • Specifying performance requirements for few equipment.
  • Selecting between competing vendors.
  • Reducing the variability in a manufacturing process.

22. (i)

  • Interchangeability or random assembly is an assembly technique in which all the components assemble with any other mating components
  • Selective assembly if the tolerance limit of mating part is very, in the case of fully Interchangeability ,the demanded accuracy of assembly may not be obtained. thus we go for selective assembly technique, her all the parts are measured, graded and grouped according to size, and finally corresponding groups are assembled together.

Interchangeability / Selective assembly
Mass production system.
Specified tolerance limits.
Process cability is equal to or less than manufacturing tolerance allowed for that part.
Assembly time is reduced
Reduced production cost / Selective assembly where parts are manufactured to rather wider tolerances.
Only matched groups of mating parts are assembled

(ii)Laminated shim control of axial ply.

  • Larger component machining tolerance, when a critical assembly is requested, is the laminated shim.
  • This particular type of shim has the advantage of quickly and a accurately providing this assembly requirements No second stage machining is involved.
  • Only one shim is needed per unit assembly, thus providing an advantage over other shim system where a range of several shim thickness must be available.
  • Laminated shins – sheet of brass fail.
  • The choice of lamination thickness, either 0.051mm or 0.076 mm is usually determined by assembly tolerance involved, thus.
  • If the assembly tolerance is 0.051mm laminations, use 0.51mm lamination
  • If the assembly tolerance is 0.076 mm or more, use 0.076 mm lamination.
  • Min thickness can vary – depending upon the overall size and shape of the shim used.

Benefits of laminated shim.

  • Replace costly precision machining of making parts.
  • Provide flexibility for assembly line adjustments.
  • Make assembly line adjustments earlier and faster.
  • Smart shim with feel able precision of 0.001 mm is readily offered.

23. Grouped Datum plane, Spigot and hole assembly with grouped Datum plane, recess and pin (with figure)

  • Function:

The system controls the relative movements of two components in all 6 degrees of freedom, but rotation about 0Z less precisely than the others.

This system is used mainly as a datum group for positional features.

Geometric analysis:

The plane are principal datum’s with zero position tolerances.

The spigot and ranges are datum with zero position and square ness tolerances on MMc relative to the principal datum.

  • The hole and pin are datum having position and square ness tolerance MMc relative to the datum spigot and plane and the datum spigot and plane and the datum recess and plane.
  • Location accuracy:

Translational movement in the x,y and z direction is limited by the fit c between the spigot and recess and by the flatness of the planes. Take as before the Criterion.

C = C ± k

  • Rotation about ox and oy is limited by the flatness of the planes, about oz by the fit between the spigot and recess and the hole and pin.

The clearance between the latter including provision for position tolerance for the position tolerances for hole and pin.

  • Basic size:

Choose from data sheets as appropriates.

  • Fits, design sizes and tolerance. The fit between the spigot and recess should be chose n to suit the location accuracy required.
  • Hole basis is preferred.
  • Do not specify flatness tolerances for the planes unless unusually location is required.

24. (i) Comparison between geometric tolerancing and coordinate tolerancing

Drawing concept / Coordinate tolerancing / geometric tolerancing
Tolerance zone shape / Condition
Square or rectangular zones for hole location
result / Condition
Can use diameter symbols to allow round tolerance zones
result
less tolerance available for hole
higher mfg costs / 57% more tolerance for hole location
Lower mfg costs
Tolerance zone flexibility / Condition
Tolerance zone fixed in size
result / Condition
Use of modifier allows tolerance zone to increase under certain conditions
result
Functional parts scrapped
Higher operating costs / Functional parts used
Lower operating costs

24.(ii)Projected Tolerance Zone (with figure)

When dimensioning threaded holes (or press-fit holes), consideration must be given to the variation in perpendicularity of the axis of the hole relative to the mating face of the assembly.

The squareness error of the fastener (or press-fit pin) may result in an interference condition with the mating part.

Where design considerations require a closer control in the perpendicularity of a threaded hole than that allowed by the positional tolerance, a perpendicularity tolerance applied as a projected tolerance zone may be specified

Specifying a projected tolerance zone will ensure that fixed fasteners do not interfere with mating parts

Further enlargement of clearance holes to provide for an extreme variation in perpendicularity of the fastener is not necessary.

Zero true position tolerancing

  • Zero true position tolerancing is a technique adaptable to situation requiring functional interchangeability and maximum tolerance advantage in the feature size,form and position interrelationships.
  • Where mating parts and features are simply to mate up or “GO” and tangent contact of the mating features could occur, zero tolerancing is technically acceptable
  • Zero position tolerancing is not appropriate
  • True position tolerances are usually established on the basis of MMC size relationships of mating part features.
  • In zero true position tolerancing the same principles apply, except that the true position tolerancing stated is always a fixed “zero”, with all the tolerance placed on the same dimension.

25. (i) (i) Form design for welding rules: (with fogures)

  • Do not attempt to copy blindly from cast, riveted, and forged designs.
  • Provide for s straight line force pattern as far as possible
  • Avoid laps, straps, and stiffening angles
  • Use butt welds wherever possible
  • Limit the number of welds used
  • Make sure that ends to be welded together are of equal thickness
  • Avoid placing welds in vulnerable cross-sections.
  • Facilitate assembly by means of registers, shoulders,etc
  • Provide for easy ass to welds.
  • Allow for the effect of thermal stress
  • Distribute heavy loading over long welds in the longitudinal direction
  • Avoid subjecting welds to bending loads
  • be careful with the use of ribs
  • avoid large flat walls which tent to bulge and flex

25.(ii) cast holes-cored holes(with figures)

  • Holes of any kind in castings are produced either by casting or by coring.
  • Holes produced by the mould sand of the boxes are cast holes.
  • Holes produced by use of special sand core –cored holes

26(i). Identifying uneconomical design features

  • unnecessary large machined areas
  • unnecessary amount of small tolerance machining
  • difficult or unduly lengthy machining features
  • slightly dissimilar components that could be standardized
  • machined features restricted to one particular process
  • feasibility of assembly and dismantling

for casting, sand cores that may be eliminated

(ii) Modifying the desig

  • when redesign for manufacture the external features and dimensions of the unit should remain unaltered
  • Internal features may be adjusted in any way to meet manufacturing requirements, provided the functional features of the design are not impaired.
  • Provided the functional features of the design are not impaired
  • Whenever an assembly or dismantling difficulty is present in a proposed design it is essential that difficulty be removed, even expensive.

27. Gear box: Identifying uneconomical design features

  • unnecessary large machined areas
  • unnecessary amount of small tolerance machining
  • difficult or unduly lengthy machining features
  • slightly dissimilar components that could be standardized
  • machined features restricted to one particular process
  • feasibility of assembly and dismantling
  • For casting, sand cores that may be eliminated.

Base has an unnecessarily large machine area

With base as mould parting line, internal and external sand cores needed for the circular bosses.

Spot facing of holding –down bolt holes restricts machining to one processes

Joint face has unnescessarily large machined area

Different types of bearing bush

Different type of boss type

Assembly of layshaft is difficult

Acceptable modified design of the spur gear reduction unit where (with figure)

  • The machined base is reduced to small facings encompassing the holding –down bolt holes
  • The casting can be obtained without the need for sand cores
  • The holding –down bolt hole bosses can be machined by several different processes
  • One type of bearing bush used throughout
  • Splitting the box on the shafts axis permits easy/dismantling, and hence permits
  • Amalagamation of gears with shafts which eliminate an appreciable amount of small tolerance machimg-circlips and keys.

28. (i)

DFMA Applications

  • DFMA software provides a systematic procedure for analyzing proposed designs from the point of view of assembly and manufacture.
  • This procedure results in simpler and more reliable products that are less expensive to assemble and manufacture.
  • In addition, any reduction in the number of parts in an assembly produces a snowball effect on cost reduction, because of the drawings and specifications that are no longer needed, the vendors that are no longer required, and the inventory that is eliminated
  • DFMA tools encourage dialogue between designers and the manufacturing engineers and any other individuals who play a part in determining final product costs during the early stages of design
  • This means that team working is encouraged, and the benefits of simultaneous or concurrent engineering can be achieved.

Application of DFMA Software to the Design of a Motor-Drive Assembly

Application of DFMA at an Electronics Manufacturer

Design-for-Assembly Implementation at an Automotive Manufacturer

Manufacturability Benchmarking by an Automotive Manufacturer

(ii)Quality Function Deployment

 A Means of integrating the design process

–Voice of the Customer

–Priorities of Marketing

–Product design knowledge of the Engineer

–(Production Planning and Design)

House of Quality

  • Gather customer attributes
  • Group attributes logically
  • Assess relative importance of the attributes
  • Assess competitive performance on the attributes
  • Describe product in terms of engineering characteristics
  • Detail influence of engineering characteristics on customer attributes
  • Detail interaction between engineering characteristics

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