Cut a small LH internal thread

[Pete.]

I was part-way through making some Delrin cheek plates for my new bench grinder wheel (the stock pressed ones were impossible to adjust for true running) when I had a good clean-up in my workshop. Somehow I lost the LH thread nut in the process so needed to make another one, but the problem with losing the nut was I couldn't make a tool bit for cutting a new thread as the wheel was off the grinder. On the other side I have a trued-up diamond wheel I didn't want to disturb and isn't any use for grinding HSS.

I do have a bigger grinder with a large friable wheel on it which was no use to ut a cutting edge on HSS but good enough to grind with, so I used that to grind down a M12 x 1.75 tap. I ground off three sides and ground back the underside to leave just a little bit on there as the side rake of the teeth would be the wrong way. I stuck it in my toolpost and used it to cut a LH thread by running the lathe backward and cutting on the reverse side. Made a nice thread in some brass hex.

[DavidA]

Well done Pete.

I've use the ground-up tap technique when wanting to cut fine threads in small bores; 40 TPI etc.  Works well.

Dave.

[Pete.]

Cheers David. I thought I had attached a pic of the finished nut, but I guess I missed that.

I had to make a few spring passes, did you find that when you used the technique?

[Jonny]

Just check your taps first before hacking away.
One piccied is neutral and surprised it worked without feeding in tap one pitch per rev.  Most have the ground thread at same angle as the pitch in which case after one rev it will pull itself in.

[DavidA]

Yes,  spring was an issue,  but not really a problem'

I used this on a largish bore hole so there was no need for me to grind the non cutting teeth away;  I could still use the tap as normal afterwards.
There was a reason why I didn't just use a single point tool as normal,  but I can't recall it at the moment.
I think the hard part was lining the tap up properly right at the beginning.

Dave.