Power for Turning 3/4-6 Acme Tap?

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SteveHGraham
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Power for Turning 3/4-6 Acme Tap?

Postby SteveHGraham » Sun Jul 16, 2017 8:51 pm

Today I finished tapping a cast iron wheel with a 3/4-6 Acme tap. I don't have a tap wrench big enough for the tap, so I held the wheel in the chuck with soft jaws (Harold), put a live center in the tailstock, inserted the center in the hole in the end of the tap, and turned the tap with a wrench.

It worked, but it took forever.

My question: should I have tried to use the motor? I could have chucked the tap by the round part in a 3/4" ball bearing chuck and put the lathe on 20 RPM, but it took so much torque to turn it, I figured it would definitely spin in the chuck. As it was, I twisted the wheel out of the soft jaws twice. I didn't want to overtighten the chuck, because I think that's abuse.
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John Evans
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Re: Power for Turning 3/4-6 Acme Tap?

Postby John Evans » Sun Jul 16, 2017 9:23 pm

You need 3 hands ,one to push the TS ,one to hold a die stock or wrench on the tap square and one to operate the jog button. Normally no lube in CI but I think with the Acme some Moly Dee or lard would help. I have a special chuck that holds taps by the square end ,Morse taper to fit TS on the other end.

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SteveHGraham
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Re: Power for Turning 3/4-6 Acme Tap?

Postby SteveHGraham » Sun Jul 16, 2017 9:26 pm

I was driving the tailstock ram, turning the tap with a wrench, and standing on the lathe brake. Very busy.
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Harold_V
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Re: Power for Turning 3/4-6 Acme Tap?

Postby Harold_V » Mon Jul 17, 2017 1:58 am

Power tapping with large taps can get you in trouble if something binds, and it often does. Unless you have a means of allowing the tap to free-wheel, something is going to get broken. Said another way, don't try power tapping by holding the tap with a tap wrench, or any other type wrench, unless it can turn freely when the tap hangs up.

One more thing to consider. If the tap is not ground for power tapping, unless you tap a thin section, pretty good chance you'll load the flutes to the point where the tap seizes. All hell breaks loose when that happens.

Don't worry about tightening the chuck enough to hold the wheel. Unless you have a lousy setup, you can get down on the chuck with both hands and not do damage. Don't use a cheater---that obviously will overload the scroll.

H
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liveaboard
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Re: Power for Turning 3/4-6 Acme Tap?

Postby liveaboard » Mon Jul 17, 2017 2:53 am

I've power tapped with a drill press up to 16mm, and with a handheld power drill for smaller work. In steel plate with a through hole.
This has been mostly successful, although I did break a tap or two...
But a 3/4 acme thread?
Yikes!

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SteveHGraham
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Re: Power for Turning 3/4-6 Acme Tap?

Postby SteveHGraham » Mon Jul 17, 2017 8:39 am

If power is out, I suppose I need a bigger tap wrench. The biggest one I have takes a 1/2" square shank.
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tornitore45
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Re: Power for Turning 3/4-6 Acme Tap?

Postby tornitore45 » Mon Jul 17, 2017 9:01 am

Why not single point thread some of the metal away. Start with a 60 degree, then go with a squarish growing. Do your geometry to make sure you are still "metal safe". Finish with the tap. It may take longer but eliminates the brute force factor. Brute force has its place but if things go wrong they go really wrong.
Mauro Gaetano
in Austin TX

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SteveHGraham
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Re: Power for Turning 3/4-6 Acme Tap?

Postby SteveHGraham » Mon Jul 17, 2017 9:06 am

Isn't that what the tandem tap is for?

I have a threading tool I made for internal Acme threads, but I bought the tap so I could quit using it.
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earlgo
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Re: Power for Turning 3/4-6 Acme Tap?

Postby earlgo » Mon Jul 17, 2017 9:17 am

What tornitore45 said. When I did the back plate for the ATLAS chuck, the threads were single pointed until almost there and then the tap was run through. That is a 1 1/2-8 tap and there is no way I could have turned it in a drilled hole.
Back plate tapping.JPG
The 4 jaw is held in a bench vise.
--earlgo

tornitore45
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Re: Power for Turning 3/4-6 Acme Tap?

Postby tornitore45 » Mon Jul 17, 2017 10:19 am

Isn't that what the tandem tap is for?


Sure but you have options. Sometimes you have no option like when the part cannot be mounted on the lathe.

Consider a manually tapped hole may be off axis.
Mauro Gaetano

in Austin TX

Harold_V
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Re: Power for Turning 3/4-6 Acme Tap?

Postby Harold_V » Mon Jul 17, 2017 3:18 pm

SteveHGraham wrote:If power is out, I suppose I need a bigger tap wrench. The biggest one I have takes a 1/2" square shank.

Assuming one can mount the object to be tapped in a lathe, a large adjustable (Crescent) wrench will serve nicely. The tap is followed with the tailstock, using a live center. While the spindle isn't powered, it should be in the lowest possible gear, to prevent rotation. If that fails, one can use the chuck wrench to aid in holding the chuck stationary.

I heartily agree with tornitore45. Removing the bulk of the metal is always a good idea, as it relates closely to the concept of roughing. Finish cuts are light, often yielding a better finish, with the added benefit of yielding a closer fit, to say nothing of the roughing establishing the often required concentricity.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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SteveHGraham
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Re: Power for Turning 3/4-6 Acme Tap?

Postby SteveHGraham » Mon Jul 17, 2017 4:33 pm

The reason I would like to get a tap wrench is that you can grab both handles, so you're not pushing the tap off center as badly, and it's easier to apply force.
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