Manufacturing Technology (ME461) Lecture10

8/12/2019 Manufacturing Technology (ME461) Lecture10

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Manufacturing Technology

(ME461)

Instructor: Shantanu Bhattacharya


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Determining Machining Conditions and Manufacturing

times

Having specified the work-piece material, machine tool, and cutting tool, the question is what

can be controlled to reduce the cost and increase production rate.The controllable variables are cutting speed (v), feed (f), and depth of cut (d). Jointly, v,f, and d

are referred to as machining conditions. There are a no. of models specifying optimal machining

conditions out of which the two best ones are:

Co= cost rate including labor and

overhead cost rates ($/min.)

C1= tool cost per cutting edge, whichdepends on type of tool used

C= constant in tool life equation

V= cutting speed in meters/min.

f=Feed rate (mm/rev)

d=depth of cut(mm)

n= exponent in the tool life equat.t1=non productive time consisting of

loading and unloading the part (min.)

tc= machining time per piece

(min/piece)

T= tool life (min.)td= time to change a cutting edge (min.)

tac= actual cutting time per piece, which isapproximately equal to tc (min/piece)


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Determining

Machining Conditions

and Manufacturing

times


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Maximum Production Rate Model

Another criterion used to determine the optimal conditions is

maximum production rate which is inversely proportional tothe production time per piece, which is given by:


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Manufacturing Lead Time

Assuming that the lot size is Q units, then theaverage lead time to process these units will

be .

Lead time = major setup time + TuQ


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Numerical ProblemA lot of 500 units of steel rods 30cm long and 6 cm in diameter is turned on a numerically

controlled (NC) lathe at a feed rate of 0.2 mm per revolution and a depth of cut of 1mm.

The tool life is given by:

vT0.20

= 200The other data are:

Machine labor rate = $10/ hr.

Machine overhead rate = 50% of labor

Grinding labor rate = $10/hr

Grinding overhead rate = 50% of grinding labor

Work peice loading/ unloading time = 0.50 min/ piece

The data related to the tools are:

Brazed inserts

Original cost of the tool = $ 27.96

Grinding Time = 2min.

Tool changing time = 0.50 min. (The tool can be ground only five times before it is discarded.Determine the following:

(a)Optimum tool life and optimum cutting speed to minimize the cost per piece.

(b)Optimum tool life and optimum cutting speed to maximize the production rate.

(c)Minimum cost per component, time per component, and corresponding lead time.

(d) Maximum production rate, corresponding cost per component, and lead time.


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Solutions


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Solutions


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Solutions


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The principal process planning

approaches

The principal approaches to process planning are the manual experience

based method and the computer aided process planning method.

The manual experience-based planning method:

The manual experience based methods have the same steps for manually

generating a process plan as described earlier. However, it is a timeconsuming and inconsistent approach.

The feasibility of process planning is dependent on many upstream factors

such as design and the availability of machine tools.

Also, a process plan has a great influence on many downstream

manufacturing activities such as scheduling and machine tool allocation. Therefore, to develop a process plan, process planners must have sufficient

knowledge and experience. It may take a relatively longer time and is

usually expensive to develop to skill of planners.


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Computer aided process planning

The primary purpose of a process planning activity is to translate a design

of any product into manufacturing process details. This prima facie suggests a feed forward system in which the design

information is directly processed by a CAPP system to layout a

manufacturing plan. However, this does not rhyme very well with the

concurrent engineering philosophy where design is an integral part in all

steps of a manufacturing unit. So, somehow we have to integrate the CAPP system into the inter-

organizational flow.

1. For example if we change the design we should be able to fall back on a

CAPP system to generate cost estimates of these design changes.

2. Similarly, if there is a breakdown the CAPP should be able to suggest analternative plan so that the most economical situation can be adopted.

The two major methods that are used in computer aided process

planning: the variant CAPP and the generative CAPP method.


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Framework for a CAPP system


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The variant CAPP method

In the variant process planning approach, a process planfor a

new part is created by recalling, identifying, and retrievingan existing planfor a similar part making necessary

modifications for the new part.

Quite often, process plansare developed for parts

representing a family of parts. Such parts are called masterparts.

The similarities in design attributes and manufacturing

methods are exploited for the purpose of formation of part

families. A number of methods have been developed for part family

formation using coding and classification systems of group

technology (GT), similarity coefficient based algorithms, and

mathematical programming models.


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4-steps to variant process planning Define the coding scheme: Adopt existing coding or

classification schemes to label parts for the purpose of

classification.

Group the parts into part families: Group the parts into part

families using the coding scheme selected earlier based on

commonality of part features.

Develop a standard process plan: Develop a standard processplan for each part family based on the common features of

the part types. This process plan can be used for every part

type within the family with suitable modifications.

Retrieve and modify the standard plan: When a new partenters the system, it is assigned to a part family based on the

coding and classification scheme. Then the corresponding

standard process plan is retrieved and modified to

accommodate the unique features of the new part.

l d


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Some commonly used variant

approaches

One of the most widely used systems iscomputer aided process planning, developedby Mcdonnell-Douglass Automation company

under the direction of CAM-I (Computer AidedManufacturing International).

The other popular variant is the MIPLAN,developed by OIR (Organization for industrialresearch) and General Electric Company(Hurzeel, 1976).


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The generative CAPP method

In the generative approach, process plans are

generated by means of decision logic,formulas, technology algorithms, and

geometry based data to perform uniquely the

many processing decisions for converting apart from raw material to a finished state.

There are two major components of a

generative process planning system.1. A geometry based coding scheme.

2. Process knowledge in the form of decision

logic and data.


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Geometry based coding scheme

The objective of a geometry based coding scheme isto define all geometric features for all process

related surfaces together with feature dimensions,

locations, and tolerances and the surface finish

desired on the features. The level of detail is much greater in a generative

system than a variant system.

For example, such details as rough and finishedstates of the parts and process capability of machine

tools to transform these parts to the desired states

are provided.

P K l d i th f f


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Process Knowledge in the form of

decision logic and data

Documents

Manufacturing Technology (ME461) Lecture10