What's possible may not be reasonable

Sept. 2, 2004
Why have some engineers and designers forgotten about the impact dimensional tolerances have on the design process?

The oversimplified answer: computers. Computing technology has made zero defects a commonly accepted theory. The idea that machines can produce exact parts each time has caused the time-pressed engineering community to apply ±0.005-in. tolerances to every design dimension. The makers of machine tools and CAD software will rightly say this is no problem. A majority of the dimensions on my own drawings are ±0.005 in. But doing something just because you can isn't always the best approach.

For example, spec'ing a ±0.005-in. tolerance on the inside and outside of a machined box is possible. But how important is it that the outside be held to such a tight tolerance? If the end product is used in such a way that no other parts touch the outside during assembly or use, then it is not reasonable to hold these dimensions to a tight tolerance.

The concept of a "reasonable" check as a design tool should be part of every engineer's bag of tricks. It lets engineers balance design requirements with what is possible. And proper balance leads to simple yet elegant designs that minimize headaches for manufacturers.

In our machined-box example, tight tolerances on the outside are a waste because they play no useful role in the functioning of the part. And no matter how perfect the software and cutting-tool paths, statistically more parts with a tight tolerance will be rejected than those with a loose tolerance. Whether the machine work is done inhouse or outsourced, your company will ultimately pay the price in higher costs and delayed shipments.

To further complicate matters, let's paint our sample box with a coat of primer and a finish coat, but keep the tight tolerance spec on the outside surface. Good machinists &mdash seeing the fine-print note in the corner of the drawing that says dimensions apply after the finish coat &mdash consider paint build-up and the fact that it is not always predictable. So they factor a few test pieces into the quote. If this is a big run it will get lost in the "noise." But both time and cost of the test pieces become significant when only 20 pieces are needed in three weeks.

Then the quote will be high and the job will take too long to complete. This prompts purchasing to go with the lowest bidder, who probably ignored the note on the print. Of course the parts will be rejected, as time slips and costs mount. Either way, good machinist or sloppy, your result will be disappointing.

So what is reasonable for our painted box? Tell the maker of the box, in clear, unambiguous words that the dimensions apply after finish but over the average surface of the features. Then the extra paint that likes to build up along sharp edges doesn't put the entire part out of tolerance. Eliminate those features that make it harder to paint. Better still, do not paint parts that require tight tolerances.

The above example was for the design of machined parts, but the reasonable tool applies to other design challenges such as motor speed, pump design, pipe bending, and analysis. Make it a habit to measure your designs with the reasonable tool. When designing a new part, ask the person who has to make it if what you are proposing is producible. It is a truism that the people in the trenches know what works.

Modern design tools are more accurate than ever before, leading some to blindly follow the latest innovations. Remember, what is possible is always changing, but what is reasonable is always reasonable.

John Gibbons
Director of Engineering
JFM Technologies LLC
West Babylon, N.Y.

JFM Technologies LLC is a product design and development company (www.jfmtechnologies.com).

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