Thread: Using an internal tap with a lathe

The few times I use an internal tap to thread work in the chuck, I generally mount the tap in the tailstock and let the tap pull itself (and the loose tailstock) into the work.

It works OK for me, and I'd never considered any other way.

Then recently I saw a video where a guy had the tap mounted in the toolpost and appeared to be driving it into the work - ie. set the change gears for the thread pitch to match the tap entry speed.

Anyone else do this?

Rob

No, but I've done the reverse, using a jaw from an old threading machine head as a chaser to cut a 1 5/16 x10 TPI Whitworth form thread to make a dummy spindle nose for the old Brackenbury and Austin lathe. The lathe gearing was set to cut 10 TPI. The photo shows the partly cut thread. The finished thread is a beautiful fully formed thread with the correctly shaped Whitworth rounded crests and valleys, just like the original spindle nose thread.

Frank.

No but I'm intrigued by the idea. Can you link the video Rob?
What would the advantage be over letting the tap feed itself? More accurate pitch? (Prolly not on my lathe.)
Besides that, when you backed out, would backlash in the gear train/leadscrew bite you?
Seems like the tap would no longer be in sync with the thread.

Originally Posted by Bryan

Besides that, when you backed out, would backlash in the gear train/leadscrew bite you?
Seems like the tap would no longer be in sync with the thread.

I might have to give that a try with a tap. On my AL960b that is the only practical way to do metric threads (that I know of) as it has an imperial leadscrew and thread dial. Power it in half nuts engaged, feed out and power reverse that puppy with half nuts still engaged. Rinse and repeat.. I have not had any issue with backlash messing up the threads. I have cut threads with no discernible play in them. I think the leadscrew backlash issue seems to be overstated. I was initially concerned until I cut some threads this way, and they turned out great. That said my machine is new.. An old worn out lathe and it is probably an entirely different story.

Backlash could be a big problem with a tap. Single point threading is a little different because the tool is not touching the work whilst you are reversing. With the tap though the spindle may rotate somewhat before the leadscrew does, with all the clearances in the gears. If the half nuts are a good fit the tap will not be able to push the saddle back and something will have to give

Power it in maybe, though i don't see why you really need to or what advantage you get over using the T/S like i imagine most of us do. But i would disengage the half nuts and let the tap push the saddle back.

Ew

I just had a thought.....

He wasn't using it like an internal thread chaser? Cutting say a 20x2mm thread with an M16 tap?

Ew

The only other reason I can think of for doing this is if the finished product had a very short thread in a blind hole, so you dont want to risk a little "drilling" with a bottom tap before it started. Though I think if I was that worried and I had more than a couple to make, I'd make a tap holder connected to the tailstock with a bolt of the thread the same pitch as I was cutting.

Stuart

Originally Posted by Bryan

No but I'm intrigued by the idea. Can you link the video Rob?
What would the advantage be over letting the tap feed itself? More accurate pitch? (Prolly not on my lathe.)
Besides that, when you backed out, would backlash in the gear train/leadscrew bite you?
Seems like the tap would no longer be in sync with the thread.

Easy fixed Bryan,
just release the tap from the tool post and wind it out with a tap wrench like normal hand tapping.
The tap has already cut to full depth so another cut wont be required anyway.

Phil

I actually contated this guy and asked him how he was doing it. His English is a bit poor and it appears he sometimes drives the carriage, and sometimes lets it just pull itself in.

It got me thinking about using the change gears, and also even about feeding into the job (and out) by hand with the carriage.

I am going to make up an adapter for this and give it a go.

The site is Romanian (I think) and he's an interesting rough and ready sort of guy.

http://www.youtube.com/watch?v=jdqMR...1nJBgHdxPdvkjQ

Rob

G'Day all, I vaguely recall seeing an article about this technique, and if I were to guess, it would have been a tip sent out from Village Press, home to "Machinists Workshop" and "The Home Shop Machinist". It was some time ago, and I thought that I had bought it to the attention of this forum in a response to a question someone had about threading using a lathe. I have not been able to find the thread here, and neither have I found the original article, but from memory the author claimed that it improved thread quality in difficult materials. He was using a tap of the size he wanted the thread, not doing internal thread chasing as Uee was wondering. When I have the time I will check the magazines from V.P. to see if there was an article in one of them on this subject.
Rob.

I have only just looked at the video. An M45 tap may be a bit of a different story to what most of us use and certainly what i was thinking of, plus he is using either an intermediate or plug tap.

Interesting job, all the drill rig rods i have seen have a tapered thread for fast fitting and removal.

Ew

Originally Posted by nearnexus

The few times I use an internal tap to thread work in the chuck, I generally mount the tap in the tailstock and let the tap pull itself (and the loose tailstock) into the work.

It works OK for me, and I'd never considered any other way.

Then recently I saw a video where a guy had the tap mounted in the toolpost and appeared to be driving it into the work - ie. set the change gears for the thread pitch to match the tap entry speed.

Anyone else do this?

Rob

Is this a similar idea to Harold Halls tapping jig where he uses the appropriately threaded Bolt to drive the tap into the workpiece ?

See attached.

Bill

I had a look on Google and can't find any pictures or links to a tool post tap holder, such as we are discussing.

Interesting concept.

I can see it could be a big help starting threads using the carriage.

The tap mount would either have to be self centering, or very accurate to prevent tap beakage from mislignment.

In the video it appears to be fairly lightly made in comparison to the tap diameter.

Rob

It would require a decent amount of power to do that ? My little Sheraton would have a hernia if I tried that Mike