Cutting large threads with a die
I have a 7/8 UNF die coming because my lathe cannot cut the thread (My change gears are in 10’s and not 5’s” and was wondering what would be the best setup for cutting the thread.
This thread is quite coarse and deep so was wondering what would be the best way to cut it as square and true as I can.
I was going to make up a piece of bar that was internally threaded for m18x1.5, thread the valve into that, then clamp that in the lathe chuck jaws. Then using the tailstock face, push against the back of the die and using a spanner on one of the chuck jaws and holding the die handle, cut the threads this way. It might not be the best but It worked for the m18x1.5
Dave
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I’m in the UK you see 🙁 the die didn’t come yesterday so it better come today. I’ve got 2 other valve bodies if I fuck this one up.
Dave
If it doesn’t work send it over. Be happy to thread it single point and leave it light so you can just chase the die over it to complete to finish depth.
aloha,
walt
Thanks for the info guys, The Die arrived today but im not at home to try it yet.
I will update with how it went. UK time
Dave
Voltar pretty much summed it up I have done this in the same matter I really don’t like to use dies other than to clean up threads but I to have had to do it this way, also you can’t beat cutting threads on a lathe as you probably well know 😉
I have threaded things using the same method as you have described Dave, using the tailstock spindle as a “square” face to keep the die tru-ish.
Normally with a taper lead on the workpiece, but the tip about have a “start” machined to the thread depth so that the die sits on it is a handy tip voltar, I shall try that one out.
My lathe’s headstock is easy to lock just by engaging the back gear, and I sometimes have a piece of plate or flat bar in between the diestock and the tailstock spindle if the surface between the die and tailstock spindle is mismatched, this has to be removed as the die progresses to its width. This is much easier done with a mate pushing the tailstock, while you attend to the threading ( my lathe is quite big, and very old, and my ML7 hardly ever gets used)
If you are going to internally thread a piece of bar M18 x 1.5 to hold the workpiece, don’t remove it from the lathe to attach the workpiece or you will lose the datum. Keeping it in place will help the 7/8″ thread more likely to be cut along the same rotational axis.
Sorry if I am stating the obvious, but it doesn’t hurt.
The die is brand new and cost me £29 = $44
The piloting idea seems good and that is what I have done, the un,threaded part at the front of the valve was turned to the minor diameter of the thread. Atleast I got something right 😀
Tanks for all the help, I will be sure to update on how it goes, die should get here on monday
Dave
Brass…… well I think you would do that dry and only with a very sharp and new die. A trick I do to start dies straight is to turn a section the minor diameter of the die and that will act as a pilot to help keep it square. Then use your drill chuck with the jaws retracted to follow it on to the work.
I do not use power for dies as by the time it bites you can’t get stopped in time to back up.
Walter….
Yeah exactly what I was thinking, The material is brass and should chip away.
Its gunna be a bitch but should be able to do it, I will only start it in the lathe chuck and after that, stick it in the vice to get some real torque on it.
Thanks for the advice
Dave
Dave, that will take a pile of torque and you will be doing real good if you can hold the workpiece from turning.
Go slow and back up every 1/8th of a turn to keep the chips small you might be okay.
Good luck there hope it works for you.
Oh yes if you are threading steel see if you can get some of the old RapidTap cutting fluid amazing stuff that the greenies pretty much made unobtanium.
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Hey Dave,
Keeping a die that big flat against the work with the tailstock alone may be tough. A die head is the best way. My first die head was a piece of round stock with a hole bored in it to accept the die. It had two set screws to hold the die from rotating. I then put a dowel pin in the back of it and stuck that in the drill chuck of my tailstock. Flats were put on the die head and held with a large open-end wrench. Worked until I got a proper set up.
If the die is adjustable, open it up and cut it in several passes.
Hope that helps.
Jim