CNC Machining - Tolerances

How much does a Zero cost?

I have recently given several presentations on how a part is processed in a CNC shop. The talk discussed the decision making process for selecting the right machining process. It also discussed some rules of thumb and showed how to make a design more machinable.

The one issue I struggle with presenting is tolerance. Most engineers understand that a .001" (.025mm) tolerance is going to be more costly than a .01" (.25mm) tolerance. Sometimes,a small change in the tolerance can have a large affect on the part's cost. This is because of process tolerance; that is, each way of doing something will have an intrinsic tolerance.

For example, a total tolerance of .005" (.13mm) is easily obtained by drilling. However, once the tolerance decreases to .002" (.050mm) reaming is required. The time to produce that hole is effectively doubled. If you estimate a $1/minute machining cost this can add $.25 per hole.

It seems that there is no standard way that companies and engineers tolerance their drawings. This is inexcusable because ANSI B54.1 completely describes how this should be accomplished. There are plenty of resources on the Web that reference parts of ANSI B54.1, but for less than $100 you can purchase Machinery's Handbook. I purchased mine on CD-Rom and have the files on my computer, so that it's handy whenever I'm developing a new part.

Later I plan on trying to tie specific tolerance classes to manufacturing cost.