Critical design reviews are crucial. They help with dimensioning and tolerancing for parts and assemblies.
Years ago, I spoke with a machinist friend in his shop. We discussed our professions—mine in mechanical and aeronautical engineering. Soon, we talked about sensible design practices. He told a story about a mechanical design engineer. The engineer set a tolerance of plus or minus 0.0005 inches for a screw hole.
I pointed out that standard drill bits have a tolerance of plus or minus 0.003 inches. I explained the challenges of placing a feature on a part with such tight tolerances. Yes, you can do it, but you must have a good reason. You need special tools to create and locate the hole. This means using a reamer for precision and careful positioning on an end mill. We need to think about where to place the hole and how to tap the threads. You must do this using a precise reference point in both the X and Y planes.
We discussed the challenges of threading a hole. It’s tough to keep it accurate afterward. I suggested using dowel pins to align the two features. Then, use screw threads to hold them together. One dowel pin hole should be a press fit, while the other needs a tight clearance. Both may require reaming to different sizes or using tapered dowel pins.
Then I asked, “Why was this feature needed?” The answer made everything clear: “I wanted to see if they could do it,” said the design engineer when asked. I found myself shaking my head.
Why pursue such wasteful designs? The machinist might wonder why he or she needs these features. But the designer insists they are necessary. This leaves the machinist to carry out the task. The machinist may wonder why this feature is there. They might think about the design engineer’s reasoning.
It is one thing to require such features for prototype parts since someone will only build them a few times. But, if these specifications carry into manufacturing, the assembly costs rise beyond what is necessary. Specifying a tight tolerance in manufacturing is one challenge, but inspection becomes another. You may need special gauges. Standard equipment can’t reach this precision.
We must watch and control the temperatures during the feature’s creation and inspection. This is important due to tight tolerances. Additionally, you need a coordinate measuring tool for tolerance checks. Steel parts change size by 0.000006 inches for each inch of thickness. This happens for every degree Fahrenheit the temperature goes up or down. A 10-degree change in a feature 10 inches from a datum line may push it out of tolerance by 0.0001 inches. Can the measuring equipment even detect this small deviation? Inspectors must check the print. They might need to reject a part that is out of tolerance, even if it’s a tiny bit.
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org