Hard-to-machine thread

[tex]

Need help with a funny-looking thread.

Does anybody have any idea how to machine a thread with different inner and outer radii, like the one on the attached screenshot? It is supposed to be machined with a plain CNC lathe (like HAAS SL-30) and with a plain insert since there are no dedicated inserts with adequate radii.

Looks impossible to me...however, I've decided to ask for help anyway.

[Bear]

Hey TEX
We use TOPPING inserts for such thds.
check with MSC or with REX or your tooling sales rep.
Bear

[tex]

Originally Posted by Bear Hey TEX We use TOPPING inserts for such thds. check with MSC or with REX or your tooling sales rep. Bear

Thnx....unfortunately, the guys who are about to try to do this kind of thread are not able to get their hands on any kind of special inserts...what they want is to machine the thread with a standard triangular insert...

However, I would really appreciate if you could share a picture of a "topping insert" here, I've never had a chance to see one.

[duluthboat]

If this can be done with a standard triangle insert I would like to know how. I can conceive using multiple passes to get the form but you would still have scallops. I don’t know everything but I think a form tool is required. If the tool isn’t available then grind one.

[WayneHill]

A special ground insert is the right tool for the job.

[Bent]

Hello

There is a standard insert that will make round threads. whith a little grinding maybe you could get the radii you want. It can be found at www.coromant.sandvik.com go in under products - turning - thread profiles it´s called round

Regards Bent

[tex]

Thank you all who made suggestions about the round inserts...I am aware that there are round ones, but they are, unfortunately, not an option...

I would like to know what TOPPING inserts are, though...

[Bear]

Tex
what Bent said
Bear

[ghyman]

I've always liked the old-school top-lock (kennametal / sandvik) threading tools, simply because it is the same holder for the grooving inserts.
For this, I would use a full-radius grooving insert (size depends on what the internal radius on the part is), then write a threading routine with multiple X- and Z- starting points...

X.375 Z-.475
M98 P1234
X.372 Z-.455
M98 P1234
X.360 Z-.435
M98 P1234
X.33 Z-.415
M98 P1234
X.249 Z-.395
M98 P1234
X.223 Z-.375
M98 P1234
X.207 Z-.355
M98 P1234
X.197 Z-.335
M98 P1234
X.190 Z-.315
M98 P1234
X.188 Z-.295
M98 P1234

And then a sub that looks like this
O1234
G32 Z1.5 F.375
G0 U.2
Z-.5

Of course, you'll have to trig out the X/Z starting points, and you will have to see what kind of stepover you'll need to get the best-looking profile. And, it may even be necessary to do a regular threading cycle with the groover first to rough out the center of the thread.
This can make for a LOT of code, but with pretty good results.

I've donew something similar before... a customer wanted square threads; I used a regular grooving insert, but added some 'custom' passes to put a .02" radius on the corners. It was a lot cheaper than making custom tools, and the cycle time wasn't that much longer.

But the customer goes "ooh-aah", and that makes everything good!

[tex]

Originally Posted by Bear Tex what Bent said Bear

This looks like a standard threading insert to me...meaning that the inner and outer radii would be exactly the same (or I can't see well enough).

[tex]

Originally Posted by ghyman <snip> For this, I would use a full-radius grooving insert (size depends on what the internal radius on the part is), then write a threading routine with multiple X- and Z- starting points... <snip>

This looks way out of my league...congratulations, man! I suppose that this routine is going to make a threading tool (in case a smaller radii insert is used) "jump" along the X axis as it moves along Z with appropriate feedrate that corresponds to thread pitch.

I will give this code a big thought...maybe I'll even figure it out somehow...
Anyway, I have to ask one (probably stupid) question: what does the "U" command stand for...?...incremental X-axis motion?

[ghyman]

Yes, "U" is an incremental "X" move... Of dourse, you could use an X value that would be clear of the OD.

As far as the start point migration...

check the attachments:
1.jpg is a drawing of the thread profile. (Or at least *some* thread profile!)

I started it with the right-most point at .100" from zero, just to give myself time for the tool to get to threading speed before engaging the part.

2.jpg is the same thing, but with some equally-spaced lines thrown in.
3.jpg shows the dimension in X and Z to one intersection point.
(Obviously, you would have to do this with your actual dimensions...)

That intersection point would be the X/Z starting point for one pass. The threading pass that would be cut, starting from that point, would cut that part of the thread (2/3 of the way down the left wall) all the way across.

Starting at one extreme, and working across the profile one pass at a time, will let you make a nifty-looking "lighbulb" thread.

As duluthboat pointed out, there will be a small amount of "scallop" between passes, so it becomes a trade-off between profile and machine memory!

Actually, if the tool radius is big enough, then the stepover (distance from one start point to the next) in the center of the thread can be pretty large; the stepover at the top of the thread would have to be closer together.

And of course, never underestimate the value of a softwheel to beautify things!

I hope this makes sense... this is something I did on the fly once; I've never had to sort of spell it out for anyone... definitely a different animal!