chapter eight tolerance charts.pptx

8/10/2019 Chapter Eight Tolerance Charts.pptx

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Chapter Eight

Tolerance Charts

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INTRODUCTION

A tolerance chart is a graphical method for presenting

the manufacturing dimensions of a workpiece or

assembly at all stages of its manufacturing dimensions

of a workpiece.

The chart provides an intermediate control system of

checks and balances to insure that processing

dimensions and tolerances will meet those specified on

part prints The tolerance charts are useful in sorting dimensional

problems on individual parts, and also useful in

processing assemblies2


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Purpose and Utilization

of Tolerance Charts

1. It permits the process engineer to determine in

advance of tooling whether or not the part can be

made to part print tolerances. A surprising number of

designs are actually dimensionally faulty and can notbe manufactured to print. If this condition can be

discovered in advance, costly scrap can be prevented.

2. It aids in developing proper manufacturing sequence.

3. It provides a means of establishing the proper working

tolerances for each operation in the sequence.

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4. It provides assurance that sufficient stock removal will

always be available for each operation in the sequence and

what that amount is. If sufficient stock is not availabletolerance charts will disclose the same.

5. When accuracy of the machine is known, the tolerance

chart will indicate whether or not it is capable of meeting

part print specifications.

6. It provides an intelligent instrument for negotiating with

product design when manufacturing specifications cannot

be met economically.7. It offers a convenient and useful check on alternate

methods of dimensioning the part for processing purpose.

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8.It aids the process engineer in determining whether the part

will arrive at its last operation with desired dimensions and

tolerances.

9. It helps to determine the practicability of combined tooling,

such as form tools-or combinations of working and

inspection gauging.

10. It provides a means of reducing dimensional errors whichare likely to occur if complex parts are processed without

the use of tolerance charts.

11. It aids in determining the proper raw material sizes and in

developing the necessary castings and forgings.12. Together with the process picture sheet, the tolerance

chart provides an invaluable aid in the development of

complete and accurate process routings.5


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Definitions and Symbols1. A working Dimensionis the distance between a

locating surface and the surface being processed.2. Stock removal is the difference between the

dimension that existed prior to machining andmachining dimension.

3. Resultant dimensionis the difference between twodimensions or a dimension and an intermediateresultant. It is sometimes called as a balancedimension. An intermediate resultant occurs whenadditional stock removed in later operation willaffect its size.

4. Total toleranceis the total variation from the basicstock removal dimension which can result from theoperation performed

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Assumptions

Rough forging dimension = 2 in

Nominal finish dimension = 1.920 in Difference = 0.080 in to be removed from two

surfaces B and G

Both B and G surfaces are to be machined,

hardened and ground, therefore let us assumethat 0.010 in is left for grinding for each surface.

Therefore stock to be removed from each surface= (0.080/2)-0.010= 0.030 in

No change in dimensions due to heat treatment processes will be assumed in this example.

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Operation 10

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Operation 10: calculations

Forging Dimension = 2.0000.010 in

Operation 10, W.D. = 1.9700.002 in

Stock removal = 0.0300.012 in

Min. of 0.018 in and max. of 0.042 in. can beremoved.

No final (resulting) dimension is resulted from

this operation.

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Operation 20

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Operation 20: Calculations

Three separate surfaces result from this operation. Location

takes place on surface G which was machined in operation10. Surface A is faced, the center hole is core drilled and

reamed, and the surface C is turned. Here, we concern with

surfaces B and C. Here, as in previous operation, a nominal

0.030 stock will be removed from A. The working toleranceis 0.002 as before. Therefore,

Operation 10, working dimension 1.9700.002


Stock removal 0.0300.004

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Operation 20 continued

Surface C must be produced as a continuation of

operation 20, using the same locating surface. Becausethe position of the surface C with respect to surface Gis affected by the amount which will later be removedfrom surface G by grinding, allowance must be made to

insure that the final relationship between these twosurfaces is maintained. Since 0.010 is allowed for stockremoval in grinding, the working dimension will be1.7800.002. Therefore,

Operation 20 (a), working dimension 1.9400.002 Operation 20(b), working dimension 1.7800.002


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20

Operation 30


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Operation 30: Calculation In the first step performed in operation 20,a nominal

working dimension of 1.940 was established betweensurfaces B and G. To determine the nominal workingdimensions for operation 30, two things must beconsidered.

1. The nominal dimension between surface G and surface F.2. The nominal stock removal from surface G by grinding inthe final operation. The nominal working dimension forthis operation is found by subtracting these twodimensions from the nominal working dimension from

operation 20.1.940-0.200-0.010=1.730 with working tolerance as

0.002.

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Because this operation is carried out with a form

tool, two surfaces are created simultaneously.This automatically fixes the relationship between

surfaces E and F. The form tool dimension which

relates the two surfaces can carry the full partprint tolerance. The resultant dimension between

surfaces F and G can now be calculated as:

Operation 20(a), working dimension 1.9400.002


Resultant (intermediate) 0.2100.004

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Operation 40

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Operation 40: calculations

Working dimension for this operation must take into

account the allowance for grinding after hardening.

Because the nominal depth of this milling cut is 0.250

and the nominal allowance for grinding is 0.010, the

working dimension fro this operation is will be

0.250+0.010=0.260 with working tolerance 0.002.

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Operation 50

The prints requires a finished hardness depth of0.0150.010. Actually, this depth can be

controlled within 0.005. Because 0.010 is to be

removed from each ground surface, the working

case depth is set as 0.0250.005. Since nothing

has been removed from the workpiece in

carburizing and hardening, no stock removal is

recorded. As indicated previously, there is nochange in workpiece dimension and hence no

change in the resultant occurs.

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Operation 60

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The working tolerance on grinding can beachieved as 0.0005. Because 0.010 has been

allowed for grinding, the working dimension that

must be held in this operation is 1.9300.0005.Thus stock removal


Operation 60, working dimension 1.9300.0005 Stock removal 0.0100.0025

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Operation 60 continued With the completion of operation 60, the dimensions from

surface G to , G to C, and F to E are now finalized. Since thedimension from F to E was established earlier by the form tooland is not affected by this operation, it can be recorded as aresultant as shown in figure. Operation 30 was the last to be

performed on surface F before operation 60. The resultantfrom G to F can now be recorded.



Resultant 0.2000.0025

The resultant dimension between surfaces C and G now

becomes final and can be checked. Its previous dimension wasattained in operation 2(b).

Operation 20(b), working dimension 1.7800.002

Operation 60, Stock removal 0.0100.0025


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The case hardened depth of G must be checked to

make certain the grinding operation did not cause

it to fall below the depth specified.

The case hardened depth is determined by the

difference between the initial case depth workingdimension and the stock removal in grinding


Operation 60, Stock removal 0.0100.0025 Resultant 0.0150.0075

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Operation 70

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Aside from balancing final tolerances, operation 70 completesthe machining operations on the workpiece, unless it is foundthat additional operations must be performed to correct

tolerance stacking. Stock removal may be calculated as Operation 60, working dimension 1.9300.0005



Other resulting dimension of B and D Operation 40, Working dimension 0.2600.002



Dimension of B (case hardened depth)

Operation 50, Working dimension 0.0250.005



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Balancing of Tolerance Chart

If the developed tolerance chart is accepted at thisstage, tight and costly processing increases cost of the

product. The first step in balancing the tolerance chart is to

examine the final resultant dimensions and comparethem with those specified the part print.

The resultant whose tolerance compare closest withthe part print tolerance will be the logical place tostart.

As can be seen in the developed chart, the resultantdimension between surfaces C and G is 1.7700.0045,

which compares with the print dimension of1.7700.005. This comparison is closer than the others,balancing will start from here.

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Balancing operation 60

Operation 60 can now be recalculated as: Operation 20(a), working dimension 1.9400.002



Operation 20(b), working dimension 1.7800.002

Operation 60, stock removal 0.0100.003

Resultant 1.7700.005 The preceding resultant is now identical with the

part print specifications.

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An intermediate resultant between surfaces F and Gwas established as a result of operation 30. The finalresultant was obtained in Operation 60 . Because the

relationship between surfaces E and F was determinedby form tool, an increase can be made in the workingtolerance of operation 30 without affecting othersurfaces. The working dimensions of operation 30 can

be changed to 1.7300.004. The correct resultantbetween surfaces F and G will now be:

Operation 60, working dimensions 1.9300.001

Operation 30, working dimensions 1.7300.004


The dimension 0.1000.002 remains unchangedbecause it was established from the form tool.

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Balancing Operation 70

Operation 70 now can be recalculated as follows:

Operation 60, Working dimension 1.9300.001 Operation 70, working dimension 1.9200.002



Operation 70, stock removal 0.0100.003


Operation 50, working dimension 0.0250.007 Operation 70, Stock removal 0.0100.003


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Completed Tolerance Chart

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