ACME Screw Thread Inspection 1

[pdybeck]

I wanted to see if any one had any recommendations for inspecting ACME screw threads. My company has some parts with a 1"-5 ACME thread class 3G called out and we really don't have or know of a good way to inspect. I realize there are standards under ANSIB1.5 and others that define geometry of screw threads, but what are the common ways that these are inspected. My manager thought maybe we could measure the distance over wires. Possibly OK for the external thread, but what about internal threads. Any thoughts? Thanks in advance.

Pete

The pessimist says the glass is half empty. The optimist says the glass is half full. The engineer says the glass is twice as big as it needs to be.

 

[israelkk]

If this is an external thread then the best is to purchase ring (female) GO and NO-GO gages. If it is an internal thread then you need a GO and NO-GO male gages.

 

[earnest]

The 3G designation for both the screw and nut is v.important to the final function.

Aside from the usual thread variables: pitch diameter (which can be measured over pins) and thread pitch the major and minor diameters are also critical.

The reason is:
What are the radial loads on the screw?
If the design has no radial load, (like on a lathe lead screw)the root and crest clearances can be large. But if there is side load you must decide to take that load on the crest or root - if taken on the thread flank you get wedging - rapid wear and trouble.

This provision of a crest and root wear surface is one reason acme threads are chosen.

Of course, accurate go nogo gauges are nice but only justified for volume inspection. We have potted the screw/nut in hard epoxy then milled and polished a diametral section and scanned the image at 1200 dpi. Taking it into AutoCad allows good analysis.

Now, if anyone knows about lubricants for acme screws, I'm all ears...

 

[Spurs]

One question I would have is what frequency of inspection are you looking at.

If it is just one time for one piece, then you can use over pins or even optical methods. (For internals - use dental filling material (Reprocil) to produce a zero shrink cast reproduction of the internal thread as an external. Then measure the cast as a male thread, remembering that the tooth is really the space.

If you are looking at measuring on an ongoing basis, I would suggest you look into a gauge made by Johnson Gage to do this.

http://www.johnsongage.com