[rhinsen]

This is about an LB-12. I was at a friends shop today and watching one of the guys threading a big stud, about a 2 1/2- 4 thread with about 8" of thread lenght. The customer was having trouble with these breaking off so they were trying some really mean alloy. Well I'm looking at the machine as its cutting this thread and right in the cut the machine seems to just have a brain fade. I expect a big bang, as stuff would normally hit the fan, but it just continues on like normal. It finishes the cut, goes back home and feeds back in and makes the next pass. Really freaked me out. The part didn't spin in the chuck. the piece was in chuck as far back as it would go, so the tool didn't move it. The tool was tight, it didn't break it. Is it possible that the toothed belt jumped a cog and just continued on????? The speeds were about 100 R.P.M. with about .002 cut per side. This happened when the thread was 85% done with no digging in or scrapping the part.

[SQT18MS]

If the thoothet belt jumped, the encoder for the z-axis should have shut the machine down right away. And the load are lower when the thread are so far done (85&#37 that the cutting forces shouldent either be a factor here.

Sure the cutting cycle didnt just fool youre eyes?

[rhinsen]

I saw it and the guy running the machine saw it. That's why I'm here asking this question of how this happened without wrecking the machine or as you've stated not stopping. I do disagree on the threading, the machine was working real hard and the thread was about .200 deep with more to come off the side of the angle of the thread.

[neilw20]

Did it go into current limit and the spindle slow a bit?
The command for the Z feed comes from the shaft encoder and it sounds like it worked exactly as it should, even though it slowed a bit.

Are you starting at a slightly different Z position each pass so that the tool is cutting on the lead side? This will reduce power requirement.
There is some g code to control the approach angle, but I can't remember it. See the programming manual.

Possibly the brushes on the tacho generator on end of spindle motor need servicing. Tacho is fragile, so only service it if you know what you are doing.

Use of a recommended coolant/cutting compound suited to the material and the tool can reduce power requirement.

As for spinning in the chuck, which I have had with tough materials here's my fix:

Remove the jaws and get a piece of carbide a car battery and clip leads.
Connect one side of battery to the jaw, and the other side to the piece of carbide. Drag carbide back and forwards across the jaw surface making lots of sparks. Every spark deposits a nice tiny bit of carbide in/on the surface, and now the jaws will grip better. Use a few feet of wire, not too heavy in section to limit the current to a practical level, or but a car headlamp in series.

[rhinsen]

Thanks Neil,
It's possible it did the current limit thing ( It left no message or alarm ) The spindle did slow down and that was very, very noticeable. It was almost a stop for just a second then continue on type of thing. No loss of where it was or anything like that. Just that moment of "Brain Fade". It didn't spin in the chuck, but that is a good tip on laying on a bit of carbide.

[neilw20]

If it only did it once, it may have been a brown out.
The electronics is very good in such low supply voltage conditions. Good design.
You might never see it again !