Monday, October 12, 2009

Standards of limits and fits for mating parts


METRIC STANDARDS FOR LIMITS & FITS



Definitions
1. Basic size is the size to which limits or deviations are assigned and is the same for both members of a fit. It is measured in millimeters.
2. Deviation is the algebraic difference between a size and the corresponding basic size.
3. Upper deviation is the algebraic difference between the maximum limit and the corresponding basic size.
4. Lower deviation is the algebraic difference between the minimum limit and the corresponding basic size.
5. Fundamental deviation is either the upper or the lower deviation, depending on which is closest to the basic size.
6. Tolerance is the difference between the maximum and minimum size limits of a part.
7. International tolerance grade (IT) is a group of tolerances which have the same relative level of accuracy but which vary depending on the basic size.
8. Hole basis represents a system of fits corresponding to a basic hole size.
9. Shaft basis represents a system of fits corresponding to a basic shaft size.

International Tolerance Grades
The variation in part size, also called the magnitude of the tolerance zone, is expressed in grade or IT numbers. Seven grade numbers are used for high-precision parts; these are

IT01, IT0, IT1, IT2, IT3, IT4, IT5

The most commonly used grade numbers are IT6 through IT16. For these, the basic equation is


where D is the geometric mean of the size range under consideration and is obtained from the formula

Basic size ranges (sizes are for over the lower limit and including the upper limits in millimeters.
0-3; for this range use Dmin = 1 mm
3-6
6-10
10-18
18-30
30-50
50-80
80-120
120-180
180-250
250-315
315-400
400-500
500-630
630-800
800-1000

Formulas for finding tolerance grades.
Grade - Formula
IT5 - 7i
IT6 - 10i
IT7 - 16i
IT8 - 25i
IT9 - 40i
IT10 - 64i
IT11 - 100i
IT12 - 160i
IT13 - 250i
IT14 - 400i
IT15 - 640i
IT16 - 1000i


Deviations
Fundamental deviations are expressed by tolerance position letters using capital letters for internal dimensions (holes) e.g. 20G7, 40F8, etc. and lowercase letters for external dimensions (shafts) e.g. 20h6, 16g7, etc.

The formula for the fundamental deviation for shafts is
Fundamental deviation = a + (bDg)/1000
where those 3 coefficients can be obtained from the separate table (not shown here).

Shaft Deviations.
For shafts designated a through h, the upper deviation is equal to the fundamental deviation. Subtract the IT grade from the fundamental deviation to get the lower deviation. Remember, the deviations are defined as algebraic, so be careful with signs.

Shafts designated j through zc have the lower deviation equal to the fundamental deviation. For these, the upper deviation is the sum of the IT grade and the fundamental deviation.

Hole Deviations.
Holes designated A through H have a lower deviation equal to the negative of the upper deviation for shafts. Holes designated as J through ZC have an upper deviation equal to the negative of the lower deviation for shafts.

An exception to the rule occurs for a hole designated as N having an IT grade from 9 to 16 inclusive and a size over 3 mm. For these, the fundamental deviation is zero.

A second exception occurs for holes J, K, M, and N up to grade IT8 inclusive and holes P through ZC up to grade 7 inclusive for sizes over 3 mm. For these, the upper deviation of the hole is equal to the negative of the lower deviation of the shaft plus the change in tolerance of that grade and the next finer grade.

source: google books

We will see more examples with excel file later in the next post [Standards of limits and fits for mating parts (Part 2)]

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