It's most likely that the tap and tap drill need to be slightly oversize to account for "springback" of the material. No need to change the callout, just have the inspector not accept parts that are made undersize - tell him that anything more than light fingertip pressure on the thread gage is rejectable.

Rejection of ill-formed threads _should_ cause your machinist to buy a brand new tap.

Suggest also that (s)he keep that tap separate from all the others, and never ever use it on metal, so that it will maintain the microscopic edge radius needed to properly cut plastic.

Mike Halloran
Pembroke Pines, FL, USA

As a designer your method for tightness of clearance between threaded parts is to change the class of the thread. An external thread will callout class 1A, 2A, or 3A. An internal thread will callout class 1B, 2B, or 3B. Class 1 is the loosest fit and class 3 is the tightest fit.

Achieving these classes in the finished part depends on using proper shop practices. Material is the biggest issue in tapping most materials. Softer material such as plastic and aluminum usually require greater hook angle in the cutting edge of the tap. Polished flute HSS fast spiral taps would be my recomendation. Now add reinforcement such as fiberglass into plastic or silicon carbide into aluminum to these soft materials the recommendations change dramatically. The tap changes to a carbide tap. Your 'standard tap' is probably a plug tap with a spiral point.

Read up on taps in the Machinery's Handbook and then talk with your tooling provider and get some expert advice on the preferred tap to use in your application. Availability may become an issue.

Bill

From my experience, the "spring back" of the material is the biggest issue, I believe.
Will the type of tap (material, finish and configuration) make a lot of different, I need to try it out.

There are standardized oversize taps made specifically for use in plastics. Check with your supplier.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

Yes, the type of tap (material, finish and configuration) will make a HUGE difference in tapping threads into a thermoplastic material. As ornerynorsk mentioned, there are taps specifically manufactured for thermoplastics (these materials generate long chips rather than the short chips from hard, reinforced plastics (glass-fiber reinforced, mineral filled, etc.). Emuge Rekord D-PVC/E is one example, but there are other tap manufacturers with similar products:

http://emuge.com/pdf/2007_catalog/IX-XXIV_Tap_Find...