Engineerists – Can I shorten a hardened steel tap?

I’ve needed to re-tap a thread on a machine I have to change the pitch of the thread (M16 left hand) which is a one time thing so I didn’t want to spend £50 on a tap.
Bought one from China for £10 shipped but it’s a taper tap rather than a bottom tap (I can only get to one side of the thread) so it’s not quite reaching the bottom of the thread.
Is it possible for me to cut/grind this end of the tap down to remove the taper section effectively making it a bottom tap?

An angle grinder will go through it, then dress the end on a grinding wheel.

Don’t see why not -it’ll be bloody hard though, so grinding will be the best way. Two initial thoughts:

– If you cut it off, there won’t be any run-in on the tap thread, so be careful it goes in square/may be hard to start it

– You will lose a lot of the thread on the tap that would be otherwise engaged in the hole as you cut the thread. If you need to put in a lot of torque to make the new thread you could pull/’drill’ the thread out of the hole (badly phrased, I know…)

^^what they said basically –
You can cut it with a grinder but it won’t be pretty – however if you’ve already ‘started’ the hole using the tap at its full length, the ‘cut down’ tap should pick up on the threads OK.

Yup, definitely try doing it in stages, don’t just cut a big lump off. Normally you use about 3 taps to get a hole threaded to the bottom.

I like many people have shortened many taps in my time. Just get a bir rowdy with the tap wrench – that should do the trick. :(

Cheers guys. I have already tapped the hole and it’s just the last 5-10mm I need do so the tapered section shouldn’t really be needed now. I’ll take a small bit off first and leave a bit of taper to see how that goes.

Edit: Just checked again and I think the tap has gone far enough so no need for cutting :)