Tolerance Charts[1]

8/3/2019 Tolerance Charts[1]

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Tolerance Charts

Dr. Pulak M. Pandey

http://paniit.iitd.ac.in/~pmpandey


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Introduction

A tolerance chart is a graphical method for

presenting the manufacturing dimensions of aworkpiece or assembly at all stages of itsmanufacturing dimensions of a workpiece or

assembly at all stages of its manufacturing The chart provides an intermediate control

system of checks and balances to insure thatprocessing 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 assemblies


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Purpose and Utilization of ToleranceCharts

1. It permits the process engineer to determine in

advance of tooling whether or not the part canbe made to part print tolerances. A surprisingnumber of designs are actually dimensionallyfaulty and can not be 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 thesequence.


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4. It provides assurance that sufficient stockremoval will always be available for eachoperation in the sequence and what thatamount is. If sufficient stock is not availabletolerance charts will disclose the same.

5. When accuracy of the machine is known, thetolerance chart will indicate whether or not it iscapable of meeting part print specifications.

6. It provides an intelligent instrument for

negotiating with product design whenmanufacturing specifications cannot be meteconomically.

7. It offers a convenient and useful check onalternate methods of dimensioning the part forprocessing purpose.

8. It aids the process engineer in determining

whether the part will arrive at its last operationwith desired dimensions and tolerances.


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9. It helps to determine the practicability ofcombined tooling. Such as form tools-orcombinations of working and inspectiongauging.

10. It provides a means of reducing dimensionalerrors which are likely to occur if complex partsare processed without the use tolerance charts.

11. It aids in determining the proper raw materialsizes and in developing the necessary castingsand forgings.

12. Together with the process picture sheet, thetolerance chart provides an invaluable aid inthe development of complete and accurateprocess routings.


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Definitions and Symbols A working Dimensionis the distance between a

locating surface and the surface beingprocessed.

Stock removalis the difference between thedimension that existed prior to machining andmachining dimension.

Resultant dimensionis the difference betweentwo dimensions or a dimension and anintermediate resultant. It is sometimes called as abalance dimension. An intermediate resultant

occurs when additional stock removed in lateroperation will affect its size.

Total toleranceis the total variation from the

basic stock removal dimension which can resultfrom the operation performed.


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The circled dot denotes a locating

surface or centerline. The arrow

denotes the surface or centerline of the

surface to be machined.

Working dimension when tools are set

in a definite relation ship to oneanother. This occurs when combination

tools are used.

This figure represents a resultant

dimension between machined

surfaces, centerlines, or a centerline

and a machined surface.


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Rules for addingand subtracting

tolerances


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Example:

Cast steeldrive hub


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Conversion of asymmetrical tolerances into symmetrical tolerance


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Example of tolerance conversion

0055.00015.5

0055.0)0015.0000.5(000.5 007.0 004.0

=

+=+

Add L


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Developing Tolerance Chart


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Assumptions Rough forging dimension = 2 in

Nominal finish dimension = 1.920 in

Difference = 0.080 in to be removed from twosurfaces B and G

Both B and G surfaces are to be machined,hardened and ground, therefore let us assume

that 0.010 in is left for grinding for each surface.

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

No change in dimensions due to heat treatmentprocesses 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 be removed.

No final (resulting) dimension is resultedfrom this operation.


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Operation 20


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Operation 20: Calculations

Three separate surfaces result from this operation. Locationtakes place on surface G which was machined in operation 10.Surface A is faced, the center hole is core drilled and reamed,and the surface C is turned. Here, we concern with surfaces Band C. Here, as in previous operation, a nominal 0.030 stock willbe removed from A. The working tolerance is 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 ofoperation 20, using the same locating surface.Because the position of the surface C with respect to

surface G is affected by the amount which will laterbe removed from surface G by grinding, allowancemust be made to insure that the final relationshipbetween these two surfaces is maintained. Since

0.010 is allowed for stock removal in grinding, theworking dimension will be 1.7800.002. Therefore,

Operation 20 (a), working dimension 1,9400.002

Operation 20(b), working dimension 1.7800.002



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Operation 30


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In the first step performed in operation 20,a nominalworking dimension of 1.940 was established betweensurfaces B and G. To determine the nominal working

dimensions for operation 30, two things must beconsidered.

1. The nominal dimension between surface G andsurface F.

2. The nominal stock removal from surface G bygrinding in the final operation.

The nominal working dimension for this operation is foundby subtracting these two dimensions from the nominalworking dimension from operation 20.

1.940-0.200-0.010=1.730 with workingtolerance as 0.002.


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Because this operation is carried out with a

form tool, two surfaces are createdsimultaneously. This automatically fixes therelationship between surfaces E and F. Theform tool dimension which relates the twosurfaces can carry the full part print tolerance.The resultant dimension between surfaces Fand 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 operationmust take into account the allowance for

grinding after hardening. Because thenominal depth of this milling cut is 0.250and the nominal allowance for grindingis 0.010, the working dimension fro thisoperation 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 hardnessdepth of 0.0150.010. Actually, thisdepth can be controlled within 0.005.Because 0.010 is to be removed fromeach ground surface, the working casedepth is set as 0.0250.005. Since

nothing has been removed from theworkpiece in carburizing and hardening,no stock removal is recorded. As

indicated previously, there is no changein workpiece dimension and hence nochange in the resultant occurs.


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Operation 60


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The working tolerance on grinding can be

achieved as 0.0005. Because 0.010 hasbeen allowed for grinding, the workingdimension that must be held in this

operation is 1.9300.0005. Thus stockremoval


Operation 60, working dimension 1.9300.0005Stock removal 0.0100.0025


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With the completion of operation 60, the dimensions fromsurface G to , G to C, and F to E are now finalized. Sincethe dimension from F to E was established earlier by theform tool and is not affected by this operation, it can be

recorded as a resultant as shown in figure. Operation 30was the last to be performed on surface F beforeoperation 60. The resultant from G to F can now berecorded.



Resultant 0.2000.0025

The resultant dimension between surfaces C and G now

becomes final and can be checked. Its previousdimension was attained in operation 20(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 bechecked to make certain the grinding

operation did not cause it to fall below thedepth specified. The case hardeneddepth is determined by the difference

between the initial case depth workingdimension and the stock removal ingrinding





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Operation 70


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Aside from balancing final tolerances, operation 70completes the machining operations on the workpiece,unless it is found that additional operations must beperformed to correct tolerance stacking. Stock removal

may be calculated as Operation 60, working dimension 1.9300.0005 Operation 70, working dimension 1.9200.0005


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



C l t t l Ch t ( b l )


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Complete tolerance Chart (unbalance)


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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


The preceding resultant is now identical with the part printspecifications.


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An intermediate resultant between surfaces Fand G was established as a result ofoperation 30. The final resultant was obtainedin Operation 60. Because the relationshipbetween surfaces E and F was determined by

form tool, an increase can be made in theworking tolerance of operation 30 withoutaffecting other surfaces. The workingdimensions 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 1.7300.004Resultant 0.2000.005

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, stock removal 0.0100.003





Completed Tolerance Chart


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Completed Tolerance Chart

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Tolerance Charts[1]