Need Help Threading

[stuby]

Hello, I have a customer looking for some threads on the O.D. of a 99.9% Tungsten bar. Im using a Leadwell LTC-15 CNC lathe, with a Fanuc-OT controller. I am cutting a 1.25x18 thread. I have a KD100 Diamond threading insert. I did a test thread on some hardened 4140 and the thread was sweet, right to size. I put the piece of tungsten in and take my cut. The Root of the thread is sweet, the mating piece feels like it wants to start; BUT the peaks of the thread through out the whole length of the thread just broke right off; and quite frankly I don't feel like I can sell it. Does anybody have any input at all? Ive asked all local machinists and even asked the old guys and they all say you can't machine tungsten at all. Any help would be appreciated. Thanks a lot.

[Mcgyver]

edm?

[RoboElvis]

Are you using a full thread form insert? Sounds like the crest is tearing. Also, are you turning down the major diameter? We've had this problem before if you leave too much material around the major.

[stuby]

You are correct in saying that the crest is breaking off; however as I said before the root of the thread is awesome. As far as turning down the major diameter, the stock we are provided with is already the corect size. It is a 1.25x18 thread. And the stock is 1.2499. What do you mean by Thread Forming insert? These are external threads I need to cut. Are you thinking maybe a roll tap ( Die ) ? When we got this piece of tungsten somebody had cut a thread into the I.D. and it looked sweet; we haven't been able to figure it out yet though. I am using a grade KD100 Kennametal Diamond threading insert. Thanks for the input thus far, hopefully somebody will shed some real light on the situation. Thanks again.

[g-codeguy]

I am going to assume that you are making a 1.250x18-2A thread. Major diameter tolerance is 1.2485/1.2398. If making the more uncommon 3A thread, the tolerance is 1.2500/1.2485.

Thread forming inserts machine the top of the thread. Leave the diameter at current stock size, and the threading insert will take .002/.003 off that diameter (if a 2A topping insert) leaving a nice rounded crest. Also creates more tool pressure.

I've never worked with Tungsten so I can't help you with any suggestions. Sorry.

[Mcgyver]

you didn't like the the edm idea?

[stuby]

McGyver, I don't have an EDM machine, however even if I did how would that work? Doesnt EDM cut just on the Z axis?

G-Codeguy, If Im not mistaken, there are only roll form taps, correct? Ive never seen an "insert" if you will that will roll an O.D. thread? Maybe Ive just never seen what you are talking about, but Im not quite following. Thanks again.

[Mcgyver]

Originally Posted by stuby McGyver, I don't have an EDM machine, however even if I did how would that work? Doesnt EDM cut just on the Z axis? .

i don't have one either so i guess that knocks me out the expert catagory, but you can certainly thread with an edm, although it may have be the right edm; ie sophisticated enough. I just mentioned it as it is the only technique I can recall hearing about that will put threads in tungsten - if it is the only way you may have to outsource that operation

[RoboElvis]

It's isn't rolling the thread. Full thread forming inserts are also referred to as topping.

With a 60 degree insert, you're only cutting the minor and pitch diameters. The crest of the threads don't have anything cutting them. The result is an oversharp thread and, depending on the material you cut, fine burrs are lifted. Hence the "fuzz". A full form insert will also cut the crest to the correct height. But you have to order them specific to the tolerance you're cutting. i.e. 1.25x18-2A

You can try to fix this by turning the major. But you risk pushing material back into the threads by doing this.