Tips for hand-tapping aluminum tube?

I need to hand tap some 3/4-14 NPT threads into a piece of 6061 aluminum tubing which is 1.125" OD and 0.995" ID (this ID is larger than the normal ID for tapping 3/4-14, this is fine for my purpose). I've tapped smaller stuff before - up to 5/16" - in aluminum and steel without problem, but this job has some issues:

First, how to grip the tube? The tap is extremely difficult to turn, and a strap clamp proved useless. I can't get a good enough grip with the pipe wrapped in a protective sleeve of leather, with vise grips or channel locks. The only tool I have at my disposal that grips tightly enough is a pipe wrench, but the teeth on this cut deeply into the soft aluminum. I have a 4-jaw chuck, but I don't think I can't get a tight enough grip without distorting the tube.

Second, how to align the tap? I don't have a T-handle tap wrench big enough to hold this tap (the base is maybe 11/16" across the flats). My trial run doing this involved using a pipe wrench for the tube and a vise or vise-grips for the tap. Used plenty of cutting fluid, and a new tap which feels pretty sharp. Backed the tap out a half turn occasionally. The result was deep marring on the tube from the pipe wrench, some distortion/crushing of the end of the tube from the amount of torque I was using, and threads that are noticeably off from true alignment with the axis of the tube.

Any tips on tools or jigs/fixtures or technique for doing this job, without spending a lot of $$? I came into this thinking that tapping threads (even 3/4" threads) in an oversize aluminum hole should be a cinch, so I'm wondering if I'm doing something wrong.

Do you have a lathe?? How long is the tube?

I have an old lathe, but not interested in lathe-cutting the threads. The tube is maybe 8" long. Will need to thread both ends.
I thought of gripping in the 4-jaw on the lathe, but I don't think I have anything grip the tap with. Wait - I could used the dead center inserted into the hole in the end of the tap for alignment, and turn the tap with a wrench?

HandyNotHandsome said:

I have an old lathe, but not interested in lathe-cutting the threads. The tube is maybe 8" long. Will need to thread both ends.
I thought of gripping in the 4-jaw on the lathe, but I don't think I have anything grip the tap with. Wait - I could used the dead center inserted into the hole in the end of the tap for alignment, and turn the tap with a wrench?

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Now your getting somewhere

Two words . . . .
WD40

any hand tapping in the bigger sizes i use an impact wrench helps a lot with holding it and keeps it straighter. actually I use a small impact down to .25. thats what 8 point sockets are for, square headed stuff. I tried tapping an 1.25 pipe thread in the lathe broke the wrench and threw it past my head in 2 pieces I can rough it in with a single point then true it up with the tap way faster than fighting that crap

Try using a steel mandrel inside the tube at the four jaw location to prevent the aluminum tube from distorting. You may need to thread the end of the mandrel to withdraw it from the tube using a nut.

Another tip, is to use a tapered pipe thread reamer to prepare the tube for the tap. This greatly reduces the torque required at the tap. As Robert suggested, this should be done in a lathe using the tailstock with a dead center. Ideally a 6 jaw chuck with mandrel is the correct setup.

Find a tapered pipe reamer and a larger tap handle on ebay. It will make it easier. I use Tap Magic aluminum but I don't know what's best.

rickseeman said:

...I use Tap Magic aluminum but I don't know what's best.

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Kerosene works good. I'd bore some aluminum soft jaws and chuck the tube, then use the tailstock and a wrench as previously mentioned.

Might get away with wrapping the tube in a time card and using hard jaws.

Absent a tapered reamer, it's not a big deal to set the compound and bore that taper.

Probably your tap is shot. A brand new hardware store tap is already shot (maybe shit). A ground thread tap is so much sharper. The price might be objectionable.

You could try improving the grip of a 4 jaw with a wrap of coarse emery cloth or maybe that mesh type grit screen abrasive...not sure what its called, but comes in rolls of various widths. It may just shred, though.

Make a tap holder for the toolpost. A lot easier than holding a tap and wrench against a center. Even better: use the lead screw to advance the tap.

make a plug for the end of pipe held in the chuck so you can grip it tight without crushing the tube, and use a tapered reamer. use the center in the lathe to align and if you can't get a proper sized tap wrench use a adjustable wrench not vise grips. Also an interrupted tooth tap might help.

Illinoyance said:

Make a tap holder for the toolpost. A lot easier than holding a tap and wrench against a center. Even better: use the lead screw to advance the tap.

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Have you done with with success? Rigid tapping from the toolpost has always made me nervous.

Also they make pipe taps with skipped teeth,cuts down required effort to turn the tap way down. + 0ne on using a reamer also.

Just buy a good quality manufactured suppressor.

you're going to have real problems mounting it straight.

HuFlungDung said:

Probably your tap is shot. A brand new hardware store tap is already shot (maybe shit). A ground thread tap is so much sharper. The price might be objectionable.

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Hardware store tap is what I was thinking, and yeah they suck.. However, this past week
I NEEDED a 1" pipe tap, and they had one down at the hardware store.. Hanson brand, which
as we all know means *Garbage*..

Anyways, cut off wheel on a dremel to actually sharpen the teeth, and it did what I
needed it to do.. Once sharpened, it should on aluminum no problem.. Out of
the package, complete crap.. Though not as bad as some other hardware store taps
I've seen, at least a pipe tap is large enough to hand sharpen easily..

If it wasn't a Saturday morning, I would have bought a real one.

Cole2534 said:

Have you done with with success? Rigid tapping from the toolpost has always made me nervous.

Click to expand...

Have not tried it yet. So far my power tappng was done with the tap in a chuck in the tailstock, letting the tailstock float.

Thanks for the tips. I'll try it on the lathe, with a mandrel and the tap on the dead center.
The tube ID is already oversize for 3/4-14 NPT, so I don't think there's much to ream.
Not making a suppressor, if it makes any difference.
If this doesn't work, maybe I'll look into grinding some teeth down to make a skip tooth?

How many of these do you have to thread? If a bunch I would buy a skip tooth tap. If you are only doing a couple get a spring loaded center so you can back up often with the tap you have. You will use it in the future so the cost will be over a spread over a life time. The skip tooth taps I have would not be easily duplicated with any kind of hand grinding. I would not go there.