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Cutting long threads and taper on Haas Lathe

turbo23

Plastic
Joined
Mar 20, 2008
Location
Lancaster, PA
Im currently at work trying to cut a 3/8-16 thread 1.250 long in 304 SS. Problem is I cannot use a center for this application, or have the threads rolled. So far all I have been able to come up with is running a die over the threads to remove the taper. Or going in and changing the depth amounts to have less tool pressure. Help?
 
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Im currently at work trying to cut a 3/8-16 thread 1.250 long in 304 SS. Problem is I cannot use a center for this application, or have the threads rolled. So far all I have been able to come up with is running a die over the threads to remove the taper. Or going in and changing the depth amounts to have less tool pressure. Help?



Either add the taper value, as viewed from the last thread, to your threading cycle line (I0.00?) or program the thread with G92.

Good luck!
 
Because it is 304SS, it gets pretty springy and will not straighten up easily if you try to cut it straight with light cuts.

Take your best guess and program a tapered thread with aggressive cutting parameters. Leave about .004" to finish and no spring passes. Instead, shift your Z start point .002 left or right, and program a single cutting pass on each flank of the thread. Even these passes might have to be cut on a taper.

It might take a few experiments to get it down pat. Cut aggressively to help keep the tool under the chip.
 
No-matter how you go about it - IF you can make this thread to print - you will constantly be chasing offsets and R values to keep it in tol.

Had a very similar part a number of yrs ago in 410. Not allowed to roll them there either. I tried everything. Even single pointing "roughing" it - and then running a round adjustable die over it for clean-up, but nodda.

I had one flavour of round adjustable die (Poland) that did a swell job. (I had tried many diff flavours.) Untill the supply ran dry. Next batch that came in would not produce possative results.

Tried a die-head as well.

My thread was only 5/16, and I don't recall the length. It may have been a little longer/D than yours. ??? If it would have been a fine thread - life would likely have been much better. ???

I ended up giving up. That is the one - or possibly 1 of 2 parts (not sure what the other one would be???) that I could not produce. :o (Even tho I did ship quite a few early on, but then couldn't replicate in a timely fashion.

Only thing that I don't think that I tried was altering the material via HT. I don't remember if that was allowed or not. ???

Now if the whole part is only 3/8" round then a no-brainer would be to stuff it in a Swiss, but if the body of the part is bigger'n 3/8 - then that trick won't help either...

I never understood the reasoning for the no-roll rule. Can some-one shine some light there?



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Think Snow Eh!
Ox
 
Not being a CNC guy, this is a problem that seems common but isn't addressed well on anything but a Swiss-turn machine. I've had to do some small and very long threads, far worse than what you're describing. When I do them manually, I use a follower rest. Both the follower and the cutting tool can be independently adjusted (custom made setup). I don't know of any good way to do this on a CNC- it needs another axis that isn't there. For production quantities the part went overseas where it's probably made on a tool grinder like a Rollomatic or such. No one here had the equipment to do it well enough, much less at a reasonable price. Your part sounds like maybe it could be jobbed out to someone with a Star or similar machine.

CH
 
Actually - 3/8 - 16 just MAY be OK!

[think think think]

You CAN use a center in it!

IMO your L/D ratio would easilly allow you to add some length to the end of the part to use your center, and then cut it away when done. (Possibly face part plung cut with endmill?)

It would take an awfully small D nose on the center to still clear the threading tool, but ....


----------------------

Think Snow Eh!
Ox
 
Usually I can program in a taper to compensate for the parts spring. I'll add the taper to both the threading tool and the turning tool. Tweaking till I get it right.

Now for real long parts where the diameter is a good deal larger to start with...I make in two steps. First I'll turn the first half or so of the thread length with a 45deg shoulder then thread that portion...then finish cutting the diameter and thread that part...if I can a light finish pass on the entire length to make a perfect blend...but 304...maybe not. The larger stock adds the needed support...kind of a cheap Swiss. It does take a bit longer...but I have had success where the parts can run consistent and unattended so I do not mind the extra time...just need good parts.

I've also put a Geometric die head in the turret, situated it in such a way the turret rotating against the Collet Chuck closed the head for threading, so it runs automatically. Very quick to run...good results...but the setup can be tricky.
 
Use G92 instead of G76. Use about 4 spark out passes at minor diameter at the end of the cycle. You shouldn't have any taper. If there is some, program some counter taper into the G92 cycle. For example:

G00 X.5 Z.1
G92 X.375 Z-1.25 U-.001 F.0625
X.365
X.355
X.345
X.335
X.325
X.315
X.305
X.300
X.297
X.297
X.297
X.297

Insert any D.O.C. you choose...just toss in some spark out passes at the end.
Just curious...what spindle speed are you currently using?
 
Just looked at the post again...your thread is not all that long... tweaking the program with a taper should work fine. John Mulherns Post looks pretty good...I'd consider more taper with a more aggressive cut an lose the spring passes...304 don't like being tickled...outside gets hardened and the tool wears...creates a few good parts then it's downhill.

I also find myself using a higher shear inserts such as the Carmex tools over the molded chipbreaker style tools...less push.
 
If you have a few to make why not just set up or make a steady
This will eliminate your taper problem

Cheers
 








 
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