View Full Version : Power tapping on a lathe?

06-15-2009, 11:30 AM
I have a bunch of 3/8" holes to drill and tap in the center of 2" square blocks of 6061, 1" thick. I don't have rigid tapping capabilities on my mill.

Is it possible (feasible, stupid, crazy et al) to put a tap in my boring bar holder and set the power feed to the tpi and power it in? I was thinking of using spiral flute taps and wd-40. I have tap adapters for my mag drill that will keep the tap from spinning.

Any thoughts?


06-15-2009, 11:48 AM
It's more commonly done by putting the tap in a chuck in the tailstock. One hand is on the Fwd/Rev switch and the other slides the tailstock up to the work. Don't lock the tailstock to the bed, it has to be able to slide freely. As soon as the tap starts to bite it will pull the tailstock along as it taps.

Be prepared to switch to reverse instantly. A blind hole can be a hair raiser :eek: but if you mark the tap or put a piece of tape around it for depth you can be fine power tapping on the lathe.

For higher production work, and for small taps (or dies) there are some other refinements. The simplest is to have a sliding mount for the tap or die that can rotate freely, that is, the chuck or die holder are not locked to the tailstock quill. You can use your hand to stop rotation of the tap as it's slid up to the hole with the spindle rotating. So long as you hold onto the chuck it will continue to tap forward, but you can let go at any time and let the tap spin freely when you're reached the depth you want. Withdrawing the tap is the reverse. I've also seen a friction clutch setup with a handle which is easier on the palms than holding the chuck.

And at the end of that continuum you have devices like the self-opening die heads found on screw machines and fortunate HSM shops.

06-15-2009, 11:56 AM
The holes are through-holes so bottoming is not a problem. I'm just concerned my worn out tailstock chuck will not hold the taps.


06-15-2009, 11:59 AM
Hmm. Sounds like a built-in but not-necessarily-adjustable torque limiter to me.:D

06-15-2009, 12:25 PM
I have tapped holes many times using the tap in the lathe tailstock chuck. It works very well so long as your lathe has a nice low speed. Mine is 70 rpm.

Another issue is the percent of thread that you are seeking. If the tap is very tight in the hole, obviously, the chances of breakage increase. If you are like most machinists, you would be using a two or three flute tap. I've totally abandined the typical four flute taps that are available at the local Hardware store since they are prone to break. But I assume that you already know that. I order mine from places like McMaster-Carr and MSC, etc.

Again, as mentioned above, a loose chuck could be an advantage since it would provide some give in the event that the tap binds.

06-15-2009, 04:09 PM
Another way to do it is to put the tap in a straight 90 degree tap handle, then put a block of wood on your carriage. Set the tap in the hole firmly with the handle resting on the wood. Turn the lathe on SLOW and when your tap is through the hole, hold the handle and reverse the direction.

06-15-2009, 05:10 PM
Another reason to put a VFD on the lathe.

I didn't realize it when I did the conversion, but tapping is now:

1) Set RPM at zero.

2) Press tailstock (with tap) against workpiece.

3) Regulate RPM in a cosy, overseeable way.

4) Slow down to zero when ready.

5) Switch to reverse and let the tap out.

Same applies to milling if the spindle can behave like a drill press.

Stainless or whatever, it works like a breeze.

06-15-2009, 05:11 PM
The holes are through-holes so bottoming is not a problem. I'm just concerned my worn out tailstock chuck will not hold the taps.


Worn out chucks are the best for "power tapping". They act as a nice torque limiting device and if you screw it up even worse, who cares, now you have an excuse to buy another and keep the trashed one for tapping, because odds are that its going to spin.

06-15-2009, 06:00 PM
Use a gun tap and drag the tailstock.

06-15-2009, 06:56 PM
This is assuming he's got a small enough tailstock to pull with a 3/8 tap. I think a sharp 3/8 tap has enough bite to pull my Pacemakers' tailstocks, but I know 1/4" cannot - even a good OSG tap wouldn't do it. It just "strips" the hole. 'Course the tailstock is 200 lbs... One of the items on my long to-do list is to make a tapping head for the tailstock for tiny taps.

What do you mean you don't have rigid tapping capabilities in your mill? I put an old drill chuck in the mill and use that to drive the tap. A 5/8" import ball bearing chuck had enough grab to power a 1/2" 13 through a piece of 1/2" 304 SS with ~75% engagement, so it should be able to grip a 3/8 tap for aluminum :D Just use some lube to keep it from getting sticky.

06-15-2009, 06:59 PM
I hope you don't have a threaded spindle nose.

06-15-2009, 07:48 PM
If I recall correctly, J Tiers showed a very neat fixture that he made for his tail stock that should work for tapping in the lathe.

It had a setting that limited how far the tool etc. could travel before it disengaged and spun freely until the lathe was reversed to back the tap/tool out of the hole in the job in the head-stock chuck.

I can't recall/find JT's post thread but it was some time ago.

There were several other similar devices on the same thread.

Perhaps, if JT - or someone else with the link/s - can re-post the tool and/or the thread.

06-15-2009, 08:08 PM
Should work just fine in the lathe, bottom speed, loose tailstock, safety glasses and try Relton A9 for lube. Peter

06-15-2009, 08:38 PM
Everyday job for me:
B/P Mill: Use a gun tap with about a 100 rpm speed.
Use lots of WD-40. Drill chuck to hold tap
Power down watch carefull and reverse out.
(tap will pull the quill down on a B/P, but hand power it as well)

Colchester Lathe: Again gun tap and about 100 rpm
Lots of WD-40 Tap in drill chuck in tail stock
Lots of oil on lathe bed.
Power in and quick reverse out.

Note: I am assuming that your machines can go in reverse quickly.


06-15-2009, 09:10 PM
Everyday job for me:

Note: I am assuming that your machines can go in reverse quickly.


If they can't, the Bridgeys are still pretty easy to use. For most jobs, the momentum of the spindle/pulleys/etc is enough to cut the threads. Flip off the motor and then keep one hand on the brake as you feed into a hole. That's how I do blind holes and small taps.

06-16-2009, 12:32 AM
A releasing tap holder with axial float.
Use the carriage with threading lever engaged or tailstock.

Releasing holders with float are very common in the screw machine industry. So they can be had dirt cheap.

06-16-2009, 09:20 AM
I've done the wrong job, with the wrong tool, the wrong way countless times. I was so happy to turn this around, if only once, that I made a movie of it.

3/8" tap, 1/2" 6061 aluminum, Lagun mill, tapping head.



Took over 40 years from the first time I saw that done to being able to do it. Worth the wait.

06-16-2009, 07:53 PM
I made a simple tap and die tool for use in my lathe, mill, and drill press. It's driven by a dog clutch arrangement and allows easy power in to a point (perhaps defined by a quill stop), reverse the motor, pick up the dog, and power out.

06-23-2009, 11:30 AM
...My mill has been stripped of the quill handle and I only have power feeds via Mach3. I guess I could write a g-code to make it work. 3/8-16 NC would mean .0625" of z movement per revolution.

At 75 rpm would be .0625 * 75 = G01 Z-1.00F4.6875

But I can't reverse the spindle so I'm screwed.

Perhaps a tapping head is in my future.

Or, if I put it in the lowest speed and release the quill stop, it would feed itself; getting it to push the quill back up may be a problem, but I could try it.

My lathe is a Logun (Wards) with step pulleys so quick reverse is not an option. However, with the short depth of the tap, I would have plenty of time to stop the chuck.

Thanks for all the suggestions.