Hardened shaft slips in lathe chuck, help?

[rsr911]

I know the movie just can't place the name. Anyway a little OT about instincts and getting back on the horse so to speak:

About 8 years ago when I was still drag racing the owner of our local track took a liking to me and my car, having climbed from street trophy straight into Super Pro. Anyway one weekend I launched the car (stick shift) at about 6000rpms against the stutter box (starting line rev limiter) and I had lined up a little out of the groove. Upon launching the car went wheels up like always but pulled hard towards the christmas tree and guardrail around it. Instinct kicked in and I pushed in the clutch, letting go of the gas would have slammed the car back to the ground, and turned away from the tree. I grabbed second gear any drove down the track to the first turn off and up onto my trailer. A few minutes later I'm still in the car with my heart beating out of my chest when the owner comes over to ask if I was OK. I told him I was fine but done for the day and he told me to get my butt back in line before I let what happened scare me too much. He later told me he'd seen very few drivers avoid a wreck in that situation and that I had the instincts and reaction time to be a great driver someday. Well a newborn changed all that as it does for many racers but this year I'll be taking the P-car to a roadcourse to get my racing license. The point I guess is it's OK to lose, just don't lose the lesson.

Wondering about whirly jigs....

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-Christian D. Sokolowski

[Peter S]

I don't think instincts have got anything to do with it - normally something like that will hit you before you have even begun to suspect there is something wrong....

In this case there was a little bit of warning, but don't count on it!

I have seen an entire compound slide/toolpost assembly shear its bolts and rocket up and past an operators ear - he didn't get time to blink. Instead of taking off his head it took out a hunk of concrete when it hit the floor. (Lathe was running in reverse, tool dug in).

[rsr911]

New steel 6.25" Bison came in today as well as the 12" D1-8 backing plate off ebay. I've got to make an adapter to fit the small chuck to the plate. Looks like it's going to be a fun project. I machined a recess in the backing plate as well as a short taper in the hole, the chuck adapter is going to have a #5MT taper machined in so I can use the backing plate as a face place to turn between centers. This may or may not work but it's only a little extra work, I'll post pics of the progress.

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-Christian D. Sokolowski

[spope14]

Comments, and a solution. Immediately, the solution. Instead of using sandpaper or notebook paper, bore out a 2 inch aluminum split bushing and hold direct in the larger chuck. The aluminum is somewhat stickier than the high carbon polished shaft, thus creates better "grip" than hard steel on hard steel. been there, done this many times. Your large chuck holds only to 1 1/4", 2 inch would work well, or even 1.5 diameter.

Problem analysis:

Maybe I am a bit off here, but the idea of chucking a smaller chuck in a larger chuck seems scary enough to start with, especially when using a larger chuck to chuck a smaller chuck. 1800 RPM's? First off, I would be leary of thise speeds in chuck work on chucks over 6 inch to begin with. Your larger chuck had to be larger than that. What was the RPM rating of that large chuck? My 8 inch hold at 24oo RPM's or less. what was the rating of the 4 jaw chuck? probably less than 1800 as 4 jaws are speed rated less than three jaws from my experience. Just FYI, I held onto the "power ratings" of my 8 inch compared to RPM's, and they start to decrease at 1200 RPMs (Buck).

Second, the larger chuck - were the jaws starting to extend outside of the body? This immediately starts to lose chuck holding power. At 1800 RPM's, this loss of power increases substantially. Thus a 10 to 12 inch chuck with a rating of probably less than 1800 or right there - the jaws extending out really cuts this out - INERTIA.
Third. I would question the practice of double chucking. I have done it, but it is a very last choice, and probably counted on one hand in my career. I have a 3 inch chuck that I have chucked in my 8 inch four jaw, and a couple of times in the three jaw, but the reasons to do this are few. My thought process looking at your mangled four jaw are this did chucking in the OD of said four jaw interior chuck create some stresses in your chuck that were released when the chuck might have sprung the stresses when the shaft slipped, heat occured throughout the set-up. Or just plain mechanical shifting or inertia, or one jaw being closer to an outside clamping jaw - and a more soild part of the body being further away....yah de yah...... Maybe the chuck started to crack while gorilla torqued? Maybe......Chucks just are not made to be "double chucked" all said and done from my experience. Maybe the chuck went just enough "off kilter" in the main chuck when some binding occured to lose the total grip length on one jaw.

Maybe I am wrong....to note again, but It would seem to me to find a better method than double chuck set-ups. Too many scary issues to deal with. I look forward to responses on this thought, for if I am off base, I would love to hear ideas (not flaming). This is why I am here, to share and hear experience.

Just some thoughts. Glad you survived. I actually talked this over with a guy I work with, we thought a bit of this out. Just our discussion. Once again, glad you lived to post.

My post is not to flame, but to share my thoughts.


Use a split bushing next time.

[Derek13]

Whirly Jig, Spin Jig, Spin Fixture etc... used for grinding round workpieces on a standard surface grinder... i have seen them with small 3 jaw chucks or with collets... i'll try to find a picture if i can..

[rsr911]

Derek, I'd love a pic or link if you have it.

spope,

My large 4 jaw is an 18", the lathe came with it and a 16" 3 jaw. The little chuck fits just fine with no jaw overhang. Lathe speed range is 18-1800 rpm with a 20HP motor.

Split collar was mentioned earlier in the thread and it's one of those "why didn't I think of that?" moments.

The clamping force of the large chuck is most likely what caused this to happen. I've chucked up a short piece of 4" hot roll without using the tail stock and taken amazing deep cuts with high feed rates without any slippage in the jaws and little if any chatter even 8-10 from the chuck. From my experience with this lathe it's easier to rotate the QC tool post under a heavy cut than slip in the jaws, which is why I think I'll use the split collar idea next time. The jaw screws at roughly 1.25" should grip like nobody's business in that setup.

I purchased a new 12" backing plate from ebay and a new steel 6" Bison 4-jaw. I'm making an adapter to adapt the small chuck to the back plate. I'm also going to turn down the OD of the backing plate to 10".

I turned a taper on the ID of the backing plate for the adapter to fit into. The adapter will also have a #5MT taper cut into it so I can remove the small chuck and turn between centers if need be. Slots will be milled in the steel adapter for a lathe dog. This will be a quick accurate way to clean up existing parts that have good centers already. Unless this lathe as a very shallow taper in the headstock I'd say it's a smooth bore just under 3" ID.

I used Machinist Toolbox to calculate speed and feed, I found a needed to go a few hundred RPM higher with a small DOC (0.010") to cut the shaft. I was using carbide insert cutting tools.

As for the chuck's ratings I'm reasonable certain they are rated high enough for the lathe's top speed, they were included in the original sale and from comparing them to manufacturer's specs they both seem to be rated over 2000 rpm.

I appreciate everyone's feedback on this and plan to post pics of the backing plate, adapter and small 4-jaw sometime this weekend once I've finished machining everything.

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-Christian D. Sokolowski

[spope14]

Chucks are often bought from suppliers to the machine companies, and do have "holding drop off points" lower than the speed ratings. Something I learned while buying machines for many a shop. It is really interesting to look at these charts when available.

One additional idea. My reservation to "Chuck on Chuck" is the lack of solid clamping. Chucks have cavities and such. Can you make a very solid adaptor for the lathe chuck to clamp on - such a 6 to 8 inch solid postpost, then mount the chuck on this like a backing plate, but for being about 4 inch length? This would give that positive clamping force much like your clamping on a 4 inch piece of solid. I am actually going to try this idea out on another machine in my shop.

[winchman]

That's some crash, and I'm glad you're OK.

In the photo of the broken chuck, there are two short pieces of round steel(?) on the floor just behind the jaw that's on it's side. Both of the pieces have what looks like half a hole on the end, and it looks like they may have been one piece.

Are they part of the broken chuck? If so, where did they fit, and what did they do? My four-jaw doesn't have anything like that in it.

What brand of chuck was it?

Thanks,
Roger

[rsr911]

Winchman, your chuck probably has similar pieces. Those two pieces hold the screws laterally. This was a cast iron Buck Chuck.

I need to make a correction to an earlier post. Today I had the chuck off and measured the spindle, it has a #7MT.

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-Christian D. Sokolowski

[winchman]

Got it now. Thanks.

Roger