TAP OD.

Just bought one of these from the Exhibition.

Outside diameter is 0.637" !

Is this normal?

Please specify manufacturer/ supplier if you have one at 0.626" +-0.001"

Hi there,

The explanation for what you have found depends a bit upon which thread form is involved.

If, say, your tap is to cut metric thread form, the male screw OD is the nominal OD but the female thread (i.e. the nut) has clearance to ensure it clears the crests of the male screw. In order to cut this clearance, the tap diameter has to exceed, by a small amount (* see below) the nominal OD of the thread.

Before posting this, I consulted Tubal Cain's 'Model Engineers Handbook', pages 57 & 58. He shows the profiles for both nut and bolt for several thread forms. His diagrams show the matter more clearly for metric threads than for, say, Whitworth form but there has to be crest clearance built in to the standard somewhere.

What you have found often goes un-noticed because of the difficulty of measuring the OD of a three-flute tap whereas your tap is four-flute where the OD is more easily measured.

I had the same experience when I made a drilling/tapping jig to put the 1/4" BSF screw-holes in the side of my ML7 bed to accept the fixing screws for the quick-change gear-box. I carefully drilled the holes in the tapping jig 1/4" only to find I had to open them up to let the tap enter.

I recommend Tubal Cain's book but I'm sure there are lots of charts of screw-thread profiles on the web.

I hope this helps.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 24/05/2016 21:16:54

This is true for most taps. If I'm tapping say M6 in a 20mm thick plate I always drill 6mm from the back so I'm not tapping a deep hole unncessarily. Then put the tap in from the back so the 'clearance' hole aligns it and you see a scratch where the tips of the tap have cut the 6mm diameter. If you suspect the thread form try micrometer over wires in the vee of the threads. The principle is shown at **LINK**

There is another reason for unusual size taps, Helicoil ones. Been there, done that! :>(

Then there is the oversize taps for items to be plated or galvanised.

Steve

Thanks for your replies.

The male thread form is on a dead size , bearing fit, shaft.

The nut willl ' hold on' a propeller which is trying to get off the shaft ,if you see what I mean.

The missing 0.012" is not really desirable in this case.

The thing is yout make thread will have rounded cress so there will be no difference in the "grip" of the thread if the nut have pointed vallys as it is the flanks that do the work.

If you want it any other way then screwcut it.

Look at  the lower sketch, the nut is the top form with pointed vallys, the bolt the lower form with rounded crests as explained above.

Edited By JasonB on 25/05/2016 10:24:14