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What is the 6 in a #6-40 tap?

Question asked by Steve Krause on Jan 16, 2018
Latest reply on Jan 17, 2018 by Christian Chu

I'm designing an aluminum tube with  internal threading for a delrin retaining ring.  The retaining ring has outside threads that are spec'd as SM2 (2.035"-40).  So in the tube part I am drawing a 2.035 circle and extrude cutting to make the ID.  Then I go to the Hole Wizard and select threads and select an Inch Tap. From there I assume I want to select a 40 threads per inch size.  From a long list I see that there are some #4-40, #5-40, #6-40.   All three seem to draw the same threads within the hole.


With a screw the 4-40 screw is a smaller diameter and tighter pitch than a 6-32.  The 4 is the diameter and the 40 is the threads per inch.

1) With a tapped hole what does the 4, 5 or 6 choice do?  I can select any of them and get the same 40 threads per inch tap on the inside of the hole.


2) If the retaining ring part calls out a 2.035"-40 thread on the outside, should I be designing some larger size hole before I select the 40 threads per inch tap?