Cutting with a large tap

I have aquired a 40mm*1.5mm tap for forming the threads on accessories for my large bore Myford.Can I bore to size and tap without screwcutting or is that going to be to difficult.I have never used a tap this size before so I am seeking advice.

Frank

I would screwcut first just under the required depth and follow up with the tap. Having said that I have screwcut 1.5 in 8 tpi female to fit the spindle nose of the Atlas with no problems and no need for a tap.

Russell.

you will need a decent sized tap wrench..

There are a few sites that quote tapping torques and it looks as if the requirement will be something in the range 50-100lbft in old money. That's a hefty torque, so although it may be possible to do it (by hand in a large vise with a very sturdy tap wrench), it's not going to be easy. Just holding it on axis while you heave it will be challenging. Certainly won't be able to do it in your Myford.

The approved method might be to screwcut in the lathe close to finished dimensions and then run the tap through as a sort of chaser to finish off.

you will need a decent sized tap wrench..

About 6 feet long, a 3/4 inch thick bar, one man on each end and the job will be a doddle

Definitely screwcut on the lathe first, accuracy is essential, then use the tap as a chaser

Thanks for your replies,I will screwcut and use it as a chaser as sugested.

Frank

I used to watch heating engineers, assembling school heating systems, putting threads on 1.5 & 2 inch steel pipe & they did not seem to have any trouble. The more senior in years among us may well remember the heating pipes running round changing rooms & classrooms with reducers at the old CI radiators. Not allowed now for fear of scalding kids

So how come a tap is harder to use than a die?

Better to bore, thread and then tidy with the tap. More likely to be all square and concentric!

I saw them at work too, up to 4 inches from memory, It is probable, the first cut was with the die open, seem to remember that they used two part "square" dies that slide up and down in the die stock.

Frank

How long was the 'die stock' arm?

Tony

That sized tap will be hard work in anything other than free machining. As been suggested, screw cut to size, but with a full form threading insert makes internal thread cutting that size quite a simple operation. It will take quite a bit to even run the tap through to clean up the thread to size. Buying a 1.5mm internal threading insert and holder will be worth while if you are making quite a few accessories. The advantage of screwcutting is that the thread will then be true to the faced reference and location diameter.

Neil

Not as long as one might suspect. If you google " rigid pipe threaders" you will see that the head is not particularly big & handle is about 20 inches long.not very long when one has 20 threads a day to cut even in modern times. I have to admit when i last watched a chap he had a sort of electric drill with die at right angles to cl of normal drill drive

I get this out when the hit & miss engine models need a large pipe thread.

Holding the part is usually more of a problem as our plumbing friends don't think twice about using a pipe vice with its heavily grooved jaws which would chew up our sort of work.

Edited By JasonB on 04/02/2016 19:02:12

Taper pipe taps and dies are a lot easier to drive compared to a regular tap. Also most pipe is free cutting mild steel.

Neil

And having screwed 2" pipe by hand as an apprentice ISTR it can be darn hard work. Was the cause of much mirth among the very large middle-aged tradesmen as my 75kg frame struggled to get the darn dies started and then keep going.

Thank goodness for Rigid pipe threading machines, like a mobile lathe that grips the pipe and has the dies mounted on a toolpost sort of arrangement and you just press the button and go. Had to wait a few years before I got to use one of them though.

For your puposes, +1 on screwcut in the lathe and then use your tap for a very final clean up, just to remove tool marks etc. Be sure to bore your hole a little oversize before starting to screwcut, otherwise you make a lot of work for yourself getting that final fit. Most taps will not be made to give 100 per cent thread engagement. More like 90 per cent or less.

Last time I had to cut a 4" pipe thread I had the boy at one end holding the pipe-vice down while dripping oil on the thread. Me with the technical stuff and a 4 foot pipe handle extension. Not very difficult except for being in a hole.

Don't forget, the bore size can be a little larger than theoretical as you probably only need 70% or so engagement on the thread - Others will correct me on the percentage engagement required.
BobH

Iron water pipe is by no means free cutting, it's quite ductile, and tears quite easily, leaving a ragged threaI,v never measured my die stocks, but they would have about 2ft arms, one takes 2 piece dies up to 1", the other takes pipe dies to 1 1/2" (I think), If you want a bit less swinging of arms, use a ratchet type.

I bored and threaded a back plate for my lathe with no thought of using a tap, it had to be square to the face, and for a 2 1/4" x 8TPI thread there was no other way.

Ian S C

Edited By Ian S C on 05/02/2016 10:04:49