Overhang on back side of chuck

[higgite]

I've heard the rule of thumb to not have more than 3 diameters of the work piece protruding from the chuck without using a center or a rest. How about on the back side of the chuck? Is there a rule of thumb to not exceed a certain length without a spider or some way to limit any "whipping" action of the part of a long work piece that protrudes back into or through the spindle? Or do you just play it by ear?

Tom

 

[Tony Wells]

I've seen a 5/8" bar dig a groove in the concrete at about 3500 RPM because it was long enough to reach the floor. The operator steadied it with his hand and was satisfied that it was straight enough to run balanced......WRONG. It bent as soon as he started his cut.

I've also seen a rotor from a 2 7/8" mud motor disappear from view about the time the operator was bringing the tool up to touch off for facing. These rotors are about 9 feet long. Bent it up pretty good and beat the control cabinet for the M-5 which was positioned just to the left of operators position. I would blame the lack of support in the spindle and/or low chuck pressure on it.

In both cases, a headstock spider was not in use. Personally, I don't think it would have made any difference. I have run small diameter stock, which of course normally likes high RPM, sticking out of the back of the chuck but not the spindle, fast enough to flex over and stay in contact with the spindle bore. Vibration was noticeable, but the lathe heavy enough to handle it. I didn't like it though, so stuffed some shop towels around it just enough to keep it roughly centered.

The 3:1 "rule" is more for control of pushoff and chatter, IMO. If you are just center drilling, you can get away with much more if you are careful. Just no high speeds. Turning or boring, etc, the less the better is all I'll say. As far as the back of the chuck, you have more leeway, and the concern there is likely more about control of concentricity along the axis of the part/material. Mostly a judgement call, other than case similar to what I mentioned above.

 

[cathead]

I think it's pretty much common sense. Back when I was first interested in lathes,
I was at an old machinist's shop and asked what the hole in the wall on the other
side of the room was for. It was winter time and there was a rag stuffed in the hole.
He replied: It gets used when I have to turn a real long shaft and he was NOT kidding.



Just find a way to keep it supported safely. An unsupported shaft at high
speed could be lethal!

I've heard the rule of thumb to not have more than 3 diameters of the work piece protruding from the chuck without using a center or a rest. How about on the back side of the chuck? Is there a rule of thumb to not exceed a certain length without a spider or some way to limit any "whipping" action of the part of a long work piece that protrudes back into or through the spindle? Or do you just play it by ear?

Tom

 

[Bill Gruby]

Always support work when it hangs out the back of the spindle. On the Automatics (Bar machines) we used "Flopper Stoppers" to control bar whip. Even if you can prove the bar is straight it can go out of round while it spins. Picture a bar 3 inches in diameter hanging 2 or 3 feet out the back and spinning. If you don't think it will bend, you are absolutely wrong. Also the overhang will grab anything that comes close, even you.

My rule --- If I can see it, it is toooooo much.

"Billy G"

 

[Holescreek]

I saw the aftermath of an 8' long 1" diameter bar that bent where it was hanging out of the rear of the headstock on a CNC Okuma lathe because the operator didn't want to set up the bar feeder for a "quick job". It lifted the lathe off the floor, luckily the operator had walked away after hitting the cycle start button. All kinds of bad things happen when operators get distracted or in a hurry.

 

[mattthemuppet2]

A while back I read about a machinist at a company who was killed by an unsupported bar whipping round. I think it was in the US somewhere. Even one of those paint roller support stands with a C-clamp around the bar (to make a "captive space" for want of a better phrase) would be better than nothing

 

[Bill C.]

I've heard the rule of thumb to not have more than 3 diameters of the work piece protruding from the chuck without using a center or a rest. How about on the back side of the chuck? Is there a rule of thumb to not exceed a certain length without a spider or some way to limit any "whipping" action of the part of a long work piece that protrudes back into or through the spindle? Or do you just play it by ear?

Tom

I had a bar of aluminum sticking out the back end about three feet. That stock started whipping before the spindle got up to speed. By the time it stop spinning I had a "L" shaped piece. You can add a tube stand behind the spindle to support the bar. One shop I worked had a turret lathe. They build a gravity fed tube feeder to support the stock.

When in doubt think safety first,

 

[george wilson]

A near accidental death was happily avoided in a silversmith production shop when a long piece of pewter was left sticking out of the rear of a lathe. Of course,it immediately bent and beat up everything in reach when the lathe was started. They did not have,or KNOW that a "deadman" was needed to handle long rods.

 

[higgite]

Thanks for all of the responses. Consensus seems to be pretty much what I thought, use some common sense, but I thought it was worth asking if there is a rule of thumb I missed along the way. I'm generally pretty good for common sense, just a little short on machining experience.

I do like Billy G's rule... "If I can see it, it is toooooo much."

Tom

 

[w9jbc]

we had a job running on an old model 60 new Britain yrs ago that ran 13.5 ft bars in a 12 ft stock reel. we left the feed tubes open for the first dozen or so parts when we loaded that. just provided more support for that 3/4" bars until we cut a few pcs off then closed the tubes up. they used the longer bars because there was less wasted bar for the part.