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Machining Tolerances Calculator (limits and fits)

Limits and fits diagram for the machining tolerance calculator

Fig 1. Machining Tolerances

In order to ensure that any machined shaft and mating hole fit exactly as required, irrespective of where and how they were made, it is necessary to apply suitable tolerances to internationally recognised standards. The two best known standards for this process are ISO 286-1:2010 and ANSI B4.1 & 2

Some of the benefits of this standardisation are:

1) Optimum design life

2) Ease of assembly

3) Correct assembly condition

4) Correctly fitting replacement parts

5) Multiple suppliers

6) No more trial and error machining

Some standard fits are listed below:

ISO:

Fit Hole# Shaft# Description
Loose Running H11/c11 C11/h11 Wide commercial tolerances or allowances on external members
Free Running H9/d9 D9/h9 Not for use where accuracy is essential but good for large temperature variations, high running speeds or heavy journal pressures
Close Running H8/f7 F8/h7 Running on accurate machines and for accurate location at moderate running speeds or journal pressures
Sliding H7/g6 G7/h6 Not intended to run freely but to move and turn freely and locate accurately
Locational Clearance H7/h6 H7/h6 Snug fit for locating stationary parts but can be freely assembled and disassembled
Locational Transition H7/k6 K7/h6 Accurate location with a compromise between clearance and interference
Locational Transition (tight) H7/n6 N7/h6 More accurate location where greater interference is permissible
Locational Interference H7/p6 P7/h6 Parts requiring rigidity and alignment with prime accuracy of location but without special bore pressure requirements (can be separated)
Medium Drive H7s6 S7/h6 Ordinary steel parts or shrink fit on light sections. Tightest fit useable with cast iron
Force H7/u6 U7/h6 Suitable for parts that can be highly stressed or for shrink fits where heavy pressing forces are impractical

Hole#: hole governs fit/sizing
Shaft#: shaft governs fit/sizing

ANSI:

Fit Hole Description
RC1 H5/g4 Close sliding fit for accurate location and no noticable play
RC2 H6/g5 Parts made to fit, move and turn easily but not intended to run freely
RC3 H7/f6 Closest free-running fit (low-speed & ht pressures), but will seize under small temperature variations
RC4 H8/f7 Close running fit for accurate machinery under moderate speeds and bearing pressures (accurate location and minimum play)
RC5 H8/e7 Running fit for medium speeds and bearing pressures and exposure to small temperature variations
RC6 H9/e8 Free running fit for medium speeds and bearing pressures and exposure to moderate temperature variations
RC7 H9/d8 Free running fit for high speeds and bearing pressures where accuracy is not important and exposure to large temperature variations
RC8 H10/c9 Loose running for wide commercial tolerances
RC9 H11/c0 Loose running for wide commercial tolerances (maximum play)
LC1 H6/h5 Most accurate location where running and sliding is not required and ease of assembly is not important (least play)
LC2 H7/h6 Location clearance fit
LC3 H8/h7 Location clearance fit
LC4 H10/h9 Location clearance fit
LC5 H7/g6 Location clearance fit
LC6 H9/f8 Location clearance fit
LC7 H10/e9 Location clearance fit
LC8 H10/d9 Location clearance fit
LC9 H11/c10 Location clearance fit
LC10 H12/c0 Location clearance fit
LC11 H13/c0 Least accurate location where running and sliding is not required but easy assembly is essential (most play)
LT1 H7/js6 Location transition fit (lowest possible interference)
LT2 H8/js7 Location transition fit
LT3 H7/k6 Location transition fit
LT4 H8/k7 Location transition fit
LT5 H7/n6 Location transition fit
LT6 H7/n7 Location transition fit (highest possible interference)
LN1 H6/n5 Location interference fit (least interference)
LN2 H7/p6 Location interference fit
LN3 H7/r6 Location interference fit (most interference)
FN1 H6/c0 Light drive fit for low assemby pressures (thin sections or long fits)
FN2 H7/s6 Medium drive fit for ordinary steel parts or shrink fits on light sections (tightest fit for high-grade cast iron)
FN3 H7/t6 Heavy drive fit for heavy sections or shrink fit for medium sections
FN4 H7/u6 Force fit for parts that can be highly stressed or shrink fit where heavy pressing forces are impractical
FN5 H8/x7 Highest force fit for parts that can be highly stressed or shrink fit where heavy pressing forces are impractical

Holes govern sizing
RC = rolling clearance
LC = location clearance
LN = location interference
LT = location transition
FN = force interference

Machining Tolerances Calculator - Technical Help

You enter a basic diameter (Fig 1; Dia) and select your desired fit from the calQlation options and Limits and Fits will automatically calculate the machining tolerances (maximum and minimum diameters) for both the Hole and the Shaft.
When entering your diameter, you must ensure that you have selected the correct input units (millimetres or inches).
Click your mouse button on the blue text 'Diameter: mm' to change the units to Imperial ins
or
Click your mouse button on the blue text 'Diameter: ins' to change the units to metric mm

You can also create your own 'non-standard' fit by simply selecting the hole/shaft grades you require and the machining tolerances will be calculated for you.

Further Reading

You will find further reading on this subject in reference publications(2)

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