SCREW PLUG GAUGES

[THREADBOY]

Need to find out what the actual purpose of a screw plug gauge is. Namely is it used purely to measure a drilled and tapped hole, is it purely for screw (lathe) cut threads, cold rolled etc. Any info is very much appreciated.

 

[yates]

Apart from the obvious application you mentioned (gaging tapped holes), screw plug gages are also used to check if ring thread gages are not too worn. They are also used to set to zero any adjustable external thread measurement device, be it a ring gage or a variable (teeth, rollers or segments) gage. They gage all threads irrespective of whether they were produced by rolling or cutting.

 

[Cockroach]

Gages are primarily used to measure the pitch diameter of the thread, in addition to other tooth geometries. What is of greater importance however, is the concept of interchangability between box (i.e. nut) and pin (i.e. screw) in a large manufactured run. Variablility of threading tolerances removes the necessity of matched box and pin sets when left ungaged.

Gages typically come in sets: Plug for box and ring for pin measurement. Typically a "GO" and "NOGO" plug/pin can provide the machinist with limits for acceptablility of threading. You need both to completely gage the thread.

Personally, I prefer the three wire measure, then use the pin to gage the box. This provides almost 83% accuracy and interchangability between box and pin threads in a manufactured at a fraction of the cost.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada