How to tap 1 1/4" - 7 threads in a lathe (with a tap)?

[dsmdude]

I have a 12" x 36" mannual lathe with a 1 1/2" spindle bore and a mt3 taper on the tail stock. I'm trying to tap 1 1/2" diameter DOM tubing with a 3/16" wall thickness. The tap is a 1 1/4"-7 held in a collis tap holder which sits in the mt3 taper. We atempted to tap the first piece of tubing today with out much luck. Either the tap would turn the tap holder in the mt3 taper or the piece would turn in the lathe. I have the lathe running about 82 rpm and simply pushed the tail stock with tap in to the end of the tube (which has an id just slightly larger than the recomended hole size for the tap) The tap is a normal hand tap. Should I be looking at a different type of tap or is my whole operation screwed up? One company suggested we power feed the carraige with a special clutch style tap holder. It just seems like there are some crazy forces required to turn my current tap. We tried doing it by hand and couldn't muster the strength to turn the tap in with a rather large wrench. Should I be using a tapered tap? Do I have to have a special power tap for this operation or would my hand tap be sufficent? I belive the tap I have now is 3-5 threads starting. Thanks in advance for all advice and help reguarding this problem!

[DSL PWR]

First off what grade of DOM is it?... Regardless of the grade, DOM is Drawn Over Mandrel, which work hardens the steel, making it darn hard to push a tap through. Why don't you cut an internal thread with your lathe?

[yoopertool]

if you can figure out how to keep the part from spinning in the chuck, you could use the tail-stock with a center to hold the tap centered between the tail-stock an the piece. Then just get the biggest wrench you can find to turn it. I have had to do the same thing a few times, and while I longed for a better/easier approach, sometimes brute strength and a bit of sweat gets it done faster. Good Luck!

[Kiwi]

Yes, you would be better off with a tapered tap.
Suggest you part cut the thread in the lathe and finish off with your tap.

[Geof]

You have chosen a bit of a difficult one for a small lathe. Power tapping on a lathe that small with a tap that large is approaching impossible; I wouldn't even try. But I would hand tap in the lathe.

Gripping the piece is the first challenge and the best way to do this is in a sleeve that gives contact all round the periphery of the DOM. The sleeve will allow you to really reef down on the chuck key without distorting the DOM; the sleeves walls should be about 1/4". Possibly a setscrew will be needed in the sleeve that will come up against a jaw to stop it turning.

The tap I would drive by hand with the end supported in a center in the tailstock. If the tap does not have a center hole you need to make a little cap to go over the end with a center hole.

The chuck is blocked from rotating with a piece of wood under one jaw down to the bed.

As the tap is turned in using a big adjustable wrench on the square end just crank the tailstock in to keep the center in the center hole so the tap goes straight.

[DareBee]

Another word of caution.
Your lathe will likely stall first but you could potentially break the clamping plate off of your tailstock and rip it off the lathe.

My method would be like Geof's turn about 80% deep by eye and then finish tap.
This is also my prefeered method for running a threading die on an external thread.

My favourite way by far is to punch 1.25 UNC into the lathe control and push the green button.

[Geof]

I did not mention in my other post but I think you might be astounded at how much torque you have to use to drive that size tap into steel. This is why I aid use and adjustable wrench...it will have to be a 12 or 16" wrench. I have tapped 1" - 8 in brass and leaded steel and for that you need to have eaten your wheaties.

[dsmdude]

Thanks fo the info! We need to tap about 100-200 of these tubes per month. Would internally threading the DOM without the tap be an efficent way of doing this? How many passes would be required to cut the threads? Is there another machine or process I should be looking at? Would doing the threads with a fine pitch make anything easier?

[HuFlungDung]

That is quite a bit of work to do that many parts regularly, on a manual lathe.

Finer pitch (UNF) would be somewhat easier, except you most likely would have to bore the tube to obtain the proper starting diameter. That will take some time. You could probably rough in the UNC thread in the same amount of time, and then finish by hand. The roughed in thread will make it possible to start the tap without a guide, and you can probably begin with a 'plug' tap, instead of the 'taper' tap.

I would arrange a secure workbench, and a pipe vise to hold the roughed in threaded tube. Then, devise a very long handled wrench, that two guys can walk around and finish the thread.

Don't sweat it, use as much leverage as is necessary to make it easy to turn.

Is farming the work out to a cnc job shop not a possibility?

[Geof]

For that many I second Hu's farm it out suggestion.

Threading that size on a small lathe in DOM, whether or not you finish by tapping, will be a pain. You will probably encounter a lot of chatter.

There is one alternative you may look out. See if you can find a gap toothed tap. I don't know the proper name but these are taps with alternate teeth missing so they do not take such a heavy cut.

[DareBee]

You have already RFQ'd for the mating part, you might as well RFQ for these as well.

[RICHARD ZASTROW]

dsmdude, Often taps that size come in 3 tap sets. Each tap removes less material than 1 would, requiring less force each. Usually results in a good final thread.

Personally, I'd farm it to a CNC lathe.