The table may also be used in reverse.

If you know what your given design

requirements are, look up these values in the table to find which part number you

should call out on the drawing.

(10) *Irregularly Shaped Curves*.

To dimension an irregularly shaped curve,

dimension the points that define the line. The more points you dimension, the more

accurate will be your definition. Figure 77 (on the previous page) illustrates a

dimensioned irregularly shaped curve.

b. * Tolerances*.

No dimension can be made perfectly.

Unless you are very lucky,

there will always be some variance. If, for example, you call for a dimension to

be made 5 inches long, you will not get exactly 5 inches on the finished part. It

may measure 5.0001 or 4.99999, etc., but it will not be exactly 5 inches.

It is not only impossible to manufacture perfect dimensions, it is also

unnecessary.

It is possible for a carpenter to build a house within the nearest

0.01 inches, but it isn't necessary for the structural soundness of the house.

Think of how much time such a constraint would add to the normal time required to

build a house, and then think of how this extra time would needlessly affect the

building cost of the house.

Because it is impossible to manufacture perfect dimensions, all dimensions must be

toleranced.

Each dimension must be considered separately in regard to how much

variance is acceptable to ensure a satisfactory finished product.

The final

judgment must be made by considering among other things manufacturing capabilities,

customer

requirements,

usage

requirements,

material

properties,

and

cost

constraints. It takes experience and practice to make such a judgment correctly.

Many companies have "standard" tolerances. That is, their shops will always work

to a given standard tolerance unless they are specifically told to do otherwise.

The standard tolerance is usually printed on the drawings as part of the company's

title block.

c. * Allowances*.

This is an intentional difference in the dimensions of mating

parts; i.e., the minimum clearance (positive allowance) or maximum