## Mechanical Design

Mechanical Design. Mechanical Design. Department of Engineering

ASSESSMENT BRIEF

Component Number: 1 of 2
Weighting: 50%
Learning Outcomes to be assessed:

Learning US – Underpinning Science and Mathematics
Outcome US3: Ability to apply and integrate knowledge and understanding of materials,
1 processes, inspection and design disciplines to define manufacturing specifications and
efficient and cost-effective manufacture.

Learning E – Engineering Analysis
Outcome E2: To be ability to identify, classify and describe the performance of manufacturing
2 design decisions through the use of systematic analysis, analysis and modelling
techniques
E3: To be competent in the use of commercial software packages to design, analyse
and model mechanical design systems.

Learning D – Design
Outcome D3: Identify and manage cost drivers that influence the choice of manufacturing
3 processes.
D6: Manage the design for manufacture process relevant to products and systems and
evaluate outcomes

Learning P – Engineering Practice
Outcome P6: Understanding of British and International standards appropriate to product design
4 and manufacture
P7: To be aware of the quality issues underpinning manufacturing design, eg
uncertainty, dimensional and geometric tolerances, surface finish.
P8: To be aware of how uncertainty applies to manufacturing processes and
measurement.

Marking Criteria:

See weightings of each requirement

The minimum pass mark for this element is D

You will pass the module if you achieve a minimum grade of D for this assignment and an average grade of D for the whole component.

1

Background

In order to set the scene for this assignment it may be useful to remind you of some definitions of the phrase ‘Design for Manufacture’:

‘Design for manufacture’ = how the design of a piece-part or product is influenced/affected/constrained/limited by the manufacturing and assembly production processes or the manufacturing system used. [Griffiths]

or:

‘Design for manufacture and assembly is attempting to foresee, at the product design stage, what the manufacturing problems will be and to quantify them so that they can be corrected on the drawing board, rather than later, during manufacture.’ [Boothroyd and Dewhurst]

The three-pin power Plug

Figure 1 below shows a standard UK three-pin power plug. Considering the function, what does matter is that the three pins must assemble easily into the base. Assume that the plastic base is injection moulded and the brass pins are machined. Note that both the pins and the base will have tolerances associated with them.

Figure 2 shows a schematic section of the plug base with a slot for one of the rectangular pins. The pin is inserted from above and protrudes below the bottom face ready for mating with the socket. The angular alignment of the pin is important to ensure easy and correct location into the socket. This angular alignment will be defined by a geometrical tolerance.

direction of pin insertion

top face

plastic plug base

bottom face

Figure 2

Figure 1

2

Assignment:
Part A: Write an essay that includes:
a) Explanation on DFM including: (30)

 A section explaining why at the design stage of a product, the cost expenditure may only be 20%, yet the committed cost could be as high as 80%. You should include a diagram to explain this. Relate this to the comment “throw it over the wall to be made”.

 A section explaining the general principles of DfM.

 A section explaining the importance of Design for Manufacture (DfM).

b) A review of a statistical method that helps in process control and capability. (10)

c)A discussion on ‘confidence’ in measurement. (10)

The essay must be at least 1500 words long and must include diagrams, graphs or tables. The diagrams etc can be in addition to the words.

Part B

Define three types of surface finish standard and explain where each might be used.

(15)

There is another page to this assignment

DIRECT REFERENCES MUST BE IN ITALICS AND BE LESS THAN 10% OF
THE DOCUMENT.

3

Part C

A pulley is designed to fit on a 90mm diameter shaft with a medium drive fit.

After a design review it is decided that a more positive drive transfer mechanism is required and a rectangular key is added.

This results in a fit between the hole in the pulley and the shaft being changed to a close running fit.

a) Explain the changes in the tolerance of both shaft and hole, and give an
indication of what this means to the manufacture of the parts. (15)

b) Redraw figure 2 sketch of the plug (using either a CAD package or by hand) and add to this drawing the following information using the relevant ISO symbols:

 A geometrical tolerance box showing that the centre of the slot must be contained within a 50um diameter cylindrical tolerance zone which is perpendicular to the bottom face. This is a perpendicularity geometrical

tolerance. (10)

 A dimension showing that the slot width is nominally 4mm with an h8 tolerance. This tolerance is to be shown not by the designation ‘h8’ but in

actual upper and lower tolerance values. (5)

 A surface finish tick mark showing that the root-mean squared roughness of the inside of the slot is to be a 5um as produced by the moulding process with

p(6)

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Mechanical Design

Mechanical Design

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