**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

**Basic size ranges**(sizes are for over the lower limit and including the upper limits in millimeters.

0-3; for this range use D

_{min}= 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 + (bD*

^{g})/1000where those 3 coefficients can be obtained from the separate table (not shown here).

**Shaft Deviations.**

For

*shafts designated*, 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.

**a**through**h***Shafts designated*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.

**j**through**zc****Hole Deviations.**

*Holes designated*have a lower deviation equal to the negative of the upper deviation for shafts.

**A**through**H***Holes designated as*have an upper deviation equal to the negative of the lower deviation for shafts.

**J**through**ZC**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*. 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.

**J**,**K**,**M**, and**N**up to grade**IT8**inclusive and holes**P**through**ZC**up to grade 7 inclusive for sizes over 3 mmsource: 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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