Interrupted Cut On The Back Gear

[Tersti]

Interrupted cut on the back gear, should it be done?

I've been using an ML7 on the back gear to do facing off operations and will certainly be using it for other interrupted cuts in future. The problem is that when I'm performing any kind of interrupted cut on the back gear there's a troubling CLACK from the back gears as they re-engage when the workpiece engages with the tool. Is that something I should be worried about? Should I be avoiding interrupted cuts on the back gear?

The bearings which I'm still in the process of getting right could be part of the problem. I've found the previous owner of the machine lost/discarded some of the shims in the plain bearings and the amount that I tighten down the tops of the bearings can tighten the spindle so much that it can't be turned by hand, so I've made some home-made shims which prevent that effect and make sure the spindle runs very freely in the bearings with little play (under .001"). Could the looseness be a factor in allowing the gears to become disengaged in the first place? There's a lot of momentum even in just the small chuck so it seems reasonable that the spindle, chuck, and workpiece could spin round freely and pull away from the teeth on the back gear. I also find that a bit of resistance in the system, a hand on the smooth surface of the chuck, means the gears never disengage and the clack goes away, but I don't know if there's still enough impact to damage or wear the teeth on the back gear prematurely.

Maybe the white metal bearings should be tight enough to introduce that resistance themselves? Maybe the resistance should or could come from somewhere else?

Any experience or ideas appreciated. Thanks.

 

[Mark_f]

It is my experience that you do get that "clack" on an interrupted cut in back gear. Do you have to use back gear for some reason? My preference is to try not to use back gear in an instance like that. It may not hurt anything but the noise bothers me. Unless it is a large diameter you should not need to go that slow. If you do, try taking light cuts. A heavy cut may "hammer " the gear teeth too hard.

The bearings being under .001" sounds ok to me. I set about .0005" when pushing the spindle with a short bar. This is all just my opinion. I'm sure there are other opinions that are just as good and valid.

 

[Paul in OKC]

I also prefer light cuts at a higher speed than in back gear. The clacking is relatively normal, in any machine when taking interrupted cuts. Little more rpm and light cut keeps the tool pressure down, lessening effect.

 

[markba633csi]

I'm sure you will get many opinions on this, so here is mine: It IS hard on the backgears, no doubt about that. You can trust your "machinist's sense"- In fact, if you were to observe the gears with a high speed camera you would find that each clack is actually several events happening in rapid succession. Your hand on the chuck is acting as a damper or shock absorber. If I was doing this I would put some very heavy grease on the gears.
I agree with the other posters; better to go faster and lighter cuts. Back gears are best for heavy non-interrupted cuts like thread cutting.
Mark S.

 

[T Bredehoft]

Your hand on the chuck is acting as a damper or shock absorber

That was going to be my suggestion, hand friction on the chuck to hold it back, so it doesn't bounce ahead when the tool leaves the metal.

Be cautious, turning things can be nasty.

 

[markba633csi]

I didn't want to recommend it, cuz it puts one's fingers in danger.
MS

 

[Tersti]

Yeah, I try not to normally mix hands with moving parts at all but the difference it made was huge. Thanks for the opinions, I'll try to avoid it like everyone agrees but if I need to do it again to face off something large I'll probably go with my wrapping a rag around a stick idea to brake the chuck with as-well as some lithium grease(?) on the back gears.

 

[epanzella]

The lathe I have now doesn't have a back gear but on my previous lathe for any interrupted cut in back gear I would lay one end of a 1x2 on the bed under the spindle and lift the other end up so the middle of the piece rubbed against the chuck. The drag took the play out of the gears and quieted it right down..