Tapping question ??...

One thing that I do not see in this discourse is the desirability to BORE the hole to the size required and not rely on drills which are likely to result in a non concentric part.
 
The MK2 Atlas shown above actually has MT1 taper in the tailstock. Same as the 618.
MS
 
True, if you want 100% thread contact. It might be hard to thread the two together, though. 60 to 75% is norm, depending on material and usage.
Not really, this generally yields a 75% thread, also the question is about taps where the size is fixed by the tool, the Pitch Diameter of taps are not easily changed.
 
Last edited:
My Starrett card does say 5/16 is the tap drill for 3/8 16. Hmm.
I have no idea why I remember this, in order to determine the Major Diameter of a number sized screw thread multiply the number by .013 then add .060, whoever thought this was a good idea in 1870 something deserves to be forgotten.

You will also notice that with a metric drill/tap chart the tap drill size is the Major Diameter minus the lead of the thread, for example an M5 .08 thread tap would like a 4.2 MM hole and so on. The metric screw thread standards and Unified Screw Thread standards are both 60 Degrees included angle, they differ in diameter and lead which cannot be mixed.
 
Last edited:
Thanks for all the help & suggestions...I got it threaded and it actually works !! Took a bit of head scratching though, I ordered a cheap set of HSS metric bits and drilled the hole to the needed 13mm then had to figure out how to lock things down for the LH thread. Wound up taking a small block of wood and 1/4in. drive ratchet extension and placing the extension in the 4-jaw chuck key hole and block of wood on lathe bed to protect it. Placed the 14x1.0mm LH tap in the drill chuck on the tailstock and got it to start a few threads before the drill chuck tried to unscrew itself again. At this point I started looking locally for a tap handle that would fit and came across one at Harbor Freight that fit the tap. It's a ratcheting type and seems fairly heavy duty (despite HF bad rep.). Started the tap back into the workpiece and using the extension & wood block to lock the 4-jaw to keep it from loosening (which it initially tried to do), threading it was straightforward from there. Other than backing up the tap to break the chips and making sure the handles didn't contact the lathe bed while turning the tap handle it threaded great. Here's a pic of the muzzle nut on a cut barrel piece that has the male threads I keep on hand to check threads...

 
Back
Top