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
Operation 20, working dimension 1.9400.002
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
Stock removal 0.1600.004
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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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Operation 30 continued
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
Operation 30, working dimension 1.7300.002
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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Operation 60: Calculations
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 20, working dimension 1.9400.002
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.
Operation 60, working dimension 1.9300.0005
Operation 30, working dimension 1.7300.002
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
Resultant 1.7700.0045
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Operation 60 continued
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 50, working dimension 0.0250.005
Operation 60, Stock removal 0.0100.0025 Resultant 0.0150.0075
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Operation 70
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Operation 70: Calculations
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
Operation 70, working dimension 1.9200.0005
Stock removal 0.0100.001
Other resulting dimension of B and D Operation 40, Working dimension 0.2600.002
Operation 70, working dimension 0.0100.001
Resultant 0.2500.003
Dimension of B (case hardened depth)
Operation 50, Working dimension 0.0250.005
Operation 70, working dimension 0.0100.001
Resultant 0.0150.006
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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 60, working dimension 1.9300.001
Stock removal 0.0100.003
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
Resultant 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, working dimension 1.9200.002
Stock removal 0.0100.003
Operation 40, working dimension 0.2600.002
Operation 70, stock removal 0.0100.003
Resultant 0.2500.005
Operation 50, working dimension 0.0250.007 Operation 70, Stock removal 0.0100.003
Resultant 0.0150.010
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Completed Tolerance Chart
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