Tolerances and allowances

Tolerances and allowances

1. Tolerances and Allowances

2. Tolerance Dimensioning Tolerance is the total amount that a specific dimension is permitted to vary; It is the difference between the maximum and the minimum limits for the dimension. For Example a dimension given as 1.625 ± .002 means that the manufactured part may be 1.627 ” or 1.623 ” , or anywhere between these limit dimensions .

3. Tolerances The Tolerance is 0.001 ” for the Hole as well as for the Shaft

4. Examples Shaft tolerance = 1.248 - 1.247 =0.001 Hole tolerance= 1.251-1.250= 0.001 Allowance=1.250-1.248= 0.02 Max clearances=1.251-1.247= 0.04 handout 10a 4

5. Why Tolerances are specified? Variations in properties of the material being machined introduce errors. The production machines themselves may have some inherent inaccuracies. It is impossible for an operator to make perfect settings. While setting up the tools and workpiece on the machine, some errors are likely to creep in.

6. Types of Tolerances Unilateral Tolerance: When the two limit dimensions are only on one side of the nominal size, (either above or below) the tolerances are said to be unilateral. For unilateral tolerances, a case may occur when one of the limits coincide with the basic size. Bilateral Tolerance: When the two limit dimensions are above and below nominal size, (i.e. on either side of the nominal size) the tolerances are said to be bilateral.

7. Specifications of Tolerances Plus-or-minus Dimensioning Unilateral Tolerance Bilateral Tolerance

8. Allowances: Allowance is the difference between the dimensions of two mating part(hole and shaft) for any type of fit. It is intentional difference between the lower limit of the hole and higher limit of the shaft. It is the minimum clearance (positive allowance) or maximum interference (Negative allowance) between parts . Allowance = 1.250-1.248=0.002 Hole Limit=1.250 Shaft Limit=1.248 Allowance is associated with two dimensions of two parts that form a fit.

9. Interchangeable Fit

10. Difference b/w Tolerance and Allowance Tolerance Allowance It is the permissible variation in dimension of a part(hole or shaft). It is the prescribed difference b/w the dimensions of two mating parts(hole and shaft). It is the difference b/w lower and higher limits of a dimension of a part. It is intentional difference b/w the lower limit of hole and higher limit of shaft. Tolerance is to be provided on a dimension of part as it is not possible to make a part of exact specified dimension. Allowance is to be provided on the dimension of mating parts to obtain required type of fit. It has an absolute value without sign. Allowance may be positive(clearance) or negative(interference).