Help with thread finish on lathe, nothing I have ever seen before

[qualitymachinetools]

Hey Guys, I have someone I am trying to help with the thread finish on his machine. Please see the attached photo.

Has anyone ever seen anything like that before? Material is 4140 and you can see the cutter being used. We have covered everything I can possibly think of with the lathe. He says that a year ago on this machine, this was not happening, but now it is. Finish when using a turning tool is fine, this is only when threading. Which is so strange to me, because turning and threading is the same thing, other than the shape and using the lead screw instead of the feed rod.

I have tried to duplicate this thread, and the only way I could do it on my machine was to just dig way too deep during passes. Not the same finish, but bad.


This is a list of what we have covered, so many hours over the last few weeks:
1. Spindle bearings
2. Its a 3 phase motor, was on a phase converter, now on a VFD. Seems to have improved some with the VFD, but still not good.
3. Belts
4. Tool post and compound, cross slide, all tight, he can not feel any vibration if he puts his hand on when cutting, but the finish is there
5. No strange noises coming from the machine, all shafts and bearings feel smooth
6. Chuck jaws not loose, also tried another chuck, same thing


Any ideas at all? Has anyone ever seen anything like this?

Thank you in advance!

 

[Rooster]

The tool holder looks to be sticking out quite away's ?.

 

[mksj]

Looks like an issue of rigidity of the cutter. Try a pass with the carriage/compound locked, recheck the height of the cutter to the work piece center. I would check the gibs on the back/front of the saddle, I had mine loosen after a period of time and noticed some increased chatter. The 4140 can be stingy and is sensitive to speed and feed, too slow and lack of sufficient SFM/feed can cause poor finish. Might try a different insert, I recommend the Carmex (Iscar) available through Zoro.

 

[JimDawson]

I have also had some problems threading 4xxx material. I would experiment with some different tool shapes. Hand grind a HSS tool bit. Looks like the material is tearing rather than cutting. Tool height might be a factor. Also surface speed might be a bit low. And last but not least, a heavy sulphur cutting oil might be helpful.

I don't think it's the machine.

 

[markba633csi]

I don't think it's the machine either Matt. It's tool or material or both, I've seen regular 1018 steel tear like that. The tool may not have the proper profile also
Mark

 

[stioc]

Does this happen with a softer more machinable material too?
Does this happen with a completely different TPI setting?

 

[T Bredehoft]

I'd suggest backing the tool up to the shoulder of the tool holder, just to eliminate that as a source of movement.

 

[4ssss]

Looks like an issue of rigidity of the cutter. Try a pass with the carriage/compound locked, recheck the height of the cutter to the work piece center. I would check the gibs on the back/front of the saddle, I had mine loosen after a period of time and noticed some increased chatter. The 4140 can be stingy and is sensitive to speed and feed, too slow and lack of sufficient SFM/feed can cause poor finish. Might try a different insert, I recommend the Carmex (Iscar) available through Zoro.

I second that.

 

[benmychree]

Personally, I never use carbide insert tooling for threading, it is not necessary for manual machines where there is not the ability (of the operator)to react fast enough to take advantage of carbide's ability to finish at high cutting speeds; at lower speeds it can have the tendency to tear and leave poor finishes. Insert threading tooling seems to be popular with hobbyists that do not want to learn how to grind HSS tools. There are alternatives, such as the Aloris threading tools that are sharpend ONLY on top and come in several sizes for V threading and acme threads. For a cutting fluid, I use MagicTap

 

[Bamban]

I saw similar thread looking like the photo after I tore the apron to replace the star knob with a lever on the SBL13.

Part of my checklist after the teardown was to go through the basic operations to make sure all is well. Turning, facing were all good. Threading the conventional way; catch the thread dial, engage half nut, disengage after pass, crank carriage back to starting point, repeat steps. The threads were just awful, looks like the previous pass was almost erased by the next.

I tried threading SS 416, and Aluminum, the same results. I chucked a fresh material and used a extra fine point sharpie just to see whether the passes as marked by the sharpie superimposed - they did not. I even tried catching the same number on the thread dial, the same results. Sometimes the mark superimpose, most of the times, they did not. Again, turning, and facing were non issue, so narrowing it down to the threading operation.

Next step, with the sharpie still in the tool holder, I made passes with the half nuts engaged the whole time (cumbersome without VFD) -- the marks superimposed.

So, what was causing the horrible threads?

It turned out when I bolted the thread dial after tear down, the gear was not fully engaged to the leadscrew, it slips around so when catching even a particular number on the thread dial, the actual gear engagement does not repeat. I loosened the thread dial and reset gear to full engagement... problem solved.

May not be the same issue experienced by the said user, but worth looking into.