What's wrong with my threading technique?

[J Tiers]

'K, we were at cross purposes.... no issue, talking "past each other".

[Richard P Wilson]

Okay, so I went back and started over with standard threading technique and have been able to make several threads that accept commercial nuts with a very good fit. The only issue now is terrible surface finish, especially in steels. These were done with carbide inserts, and I'd read that HSS gives a better finish than carbide at low speeds, and that it's tougher, as in it doesn't chip as bad (which I haven't had problems with). So tonight, I tried a HSS threading insert from AR Warner, and it left terrible surface finish too, and the tip broke on a pass with 0.005" in feed on the compound (set to 29 degrees). It broke mid pass on a 1/2-28 thread in what I think is 4140. I know 4140 isn't the easiest steel to machine, but it's a fairly common steel for rifle barrels, which is my ultimate goal. In fact, this was a junk rifle barrel I was practicing on. I guess I'll stick to the carbide inserts, but I sure wish I could get a better finish.

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I'm surprised you managed to chip an HSS tool, they are pretty resistant to that sort of thing. 0.005" isn't an excessive cut in the early stages of a 1/2" x28 thread, though you'd want to be down to 1 or 2 thou and spring cuts towards the end.Try grinding a single point tool from a decent quality HSS blank. It isn't difficult to grind a 60 degree point using a thread gauge, and you are going to have to learn how to grind your own tools sooner or later anyway. Flood with decent cutting oil, feed in on an angle if you must, personally for a thread that small, I'd just plunge cut it. Check the basics - is the work well supported, is the tool set on centre height? What speed are you using?

[txfireguy2003]

I'm surprised you managed to chip an HSS tool, they are pretty resistant to that sort of thing. 0.005" isn't an excessive cut in the early stages of a 1/2" x28 thread, though you'd want to be down to 1 or 2 thou and spring cuts towards the end.Try grinding a single point tool from a decent quality HSS blank. It isn't difficult to grind a 60 degree point using a thread gauge, and you are going to have to learn how to grind your own tools sooner or later anyway. Flood with decent cutting oil, feed in on an angle if you must, personally for a thread that small, I'd just plunge cut it. Check the basics - is the work well supported, is the tool set on centre height? What speed are you using?

Work was within an inch of the chuck and held with the chuck and an outboard spider, so I think well supported. The took was centered just prior to starting the cut, I thought dead on center, but maybe a hair high? I don't think so, but who knows at this point. I was turning 60RPM and using a good dose of high sulfur oil. The "chips" that came off on the first two passes were not even really chips, more like metal dust suspended in the cutting oil. On the third pass, I was getting the same thing for about half the pass, then it snapped off.

Two possible issues, but I dont really think so: this insert and holder came with the lathe, I bought it used from the original owner's family, he died 13 years ago and it still had the rifle barrel he was working on chucked in it, so everything than came with it is at least that old.... but I'm not seeing how the insert could go bad, it seemed sharp. The only other thing I can come up with is geometry as the holder holds the tool in a negative position, but it seems like that would make the tool less likely to snap, but maybe I'm wrong there.

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[lakeside53]

How are you determining "center height"? If you don't have a preset tool to do the easy way is just lightly face a piece of work with the insert - you see where it is at the center point.

[RichR]

... The "chips" that came off on the first two passes were not even really chips, more like metal dust suspended in the cutting oil. On the third pass, I was getting the same thing for about half the pass, then it snapped off. ...

That sounds like you were not cutting but possibly just rubbing.

[J Tiers]

If you are too high, the tool "rubs" and does not cut well. But with a negative rake, that may not be so true... w negative rake, when high, will act much more like a neutral rake, which it actually will BE at a particular height.

I have no experience with the Warner HSS. No idea if it has a sharp tip, or a flatted tip for the stand up (the lay down seems to have a radius). I have knocked the tips of plenty of HSS tools until I decided to put a flat on them. A sharp end on the tool will be easy to break, and also heats up fast, potentially getting dull and then breaking off due to the forces of trying to push a dull tool through tough material.

Also, 4140 and HSS is not a great combo, although in my experience it takes a while to wear down the HSS to where it does not do a good job. Especially if you are not getting blue or brown chips.

A negative rake... that might have an influence, since it effectively makes the cutter act duller. Are you sure that is the correct holder for the threading insert? Never mind what the previous owner did..... Check and make sure.

The cutter might have to be ground specially for a negative rake, because the angle changes the cut angle so that the actual cut is not the same as the angle of the ground tip.

I looked in the catalog (last one 2014) at http://www.arwarnerco.com/catalog%202014.pdf and they have both stand-up and lay-down insert holders. Could not make sense of the codes, even looking at the catalog explanation, at least not as regards threading and the angle the insert is held at.

So, while the report you give of negative rake seems unlikely, and the geometry of the inserts seems wrong for that, I can't prove it.

[MattiJ]

If it is a mystery insert on mystery insert holder it is also possible that the helix angle is not correct or it's even wrong-handed.
Quite difficult to eyeball, difference is pretty small.

[Richard P Wilson]

If its just coming off as dust, theres something wrong with the tool, its either blunt, too high or the helix angle is wrong, and its rubbing, not cutting. Grind an HSS blank, put some top rake on it, a little helix angle and try again. For small threads like yours, I use a 1/4" dia HSS toolbit clamped in a 3/4" square MS holder, so twisting round to get the helix angle is easy.

[lakeside53]

A picture is worth 188 posts! Send picture.

[Yondering]

I was turning 60RPM

IMO this is part of your problem with surface finish. You don't have to thread at full recommended carbide surface speeds, but you should see a big improvement stepping up to 200-300 rpm or more. It's really not that difficult to stop at a shoulder at those speeds; some guys talk about it being scary but it's really not with even a little bit of practice, especially on a 28 tpi thread.

Cutting your relief groove at the shoulder before threading helps, then you can be mentally prepared to drop the half nut lever as soon as the threading tip enters the relief groove. There's nothing wrong with making the groove a little wider if you want some extra margin for error.

[txfireguy2003]

A picture is worth 188 posts! Send picture.

I'll work on that when I get home from work this evening. I've got 2 more edges on this insert I can try anyway.

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[txfireguy2003]

Okay, here are some pics from this afternoon: This is the tool I was trying to use. Here's a barrel stub cut down to just shy of .500" (.490 by my micrometer, not sure what's up with my DRO, was shooting for .495"). A scratch pass with 28tpi thread gauge showing correct machine settings. Ready to go with oil on it.

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[txfireguy2003]

1/2 way through first cutting pass, with .005" in feed on the compound, cross slide set back to zero, the to breaks off the insert.
A pic of the broken tip.
So I swapped back to the carbide, picked up the original thread path, and finished up. It's a little loser than I'd like, but it locks up tight to the shoulder and turns true. I'm thinking if I had hit the .495 major diameter from the start, it would have been a better fit.

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[danlb]

Well, there's your problem. That is the wrong holder for that insert. The top surface of the insert is sloping down at what I would guess is 30 degrees. It should be horizontal. The edge that touches first is the cutting edge and it MUST be right at the centerline to get the best results.

When you have that much rake you are not cutting as much as you are plowing your way into the metal. I suspect that the carbide insert is working because it's in a different holder.

I'm often amazed by how much "machining" you can do with the machine set up all wrong.

Dan

[MattiJ]

Well, there's your problem. That is the wrong holder for that insert. The top surface of the insert is sloping down at what I would guess is 30 degrees. It should be horizontal. The edge that touches first is the cutting edge and it MUST be right at the centerline to get the best results.

When you have that much rake you are not cutting as much as you are plowing your way into the metal. I suspect that the carbide insert is working because it's in a different holder.

I'm often amazed by how much "machining" you can do with the machine set up all wrong.

Dan

Yup, and the extreme relief angle shown is equally bad