square stock in a 3 jaw

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Lew Hartswick
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square stock in a 3 jaw

Post by Lew Hartswick » Sun Mar 16, 2008 12:47 pm

I had a bunch of residue square (more or less) aluminium from some
salvage place and wanted to turn it to as large a round as possible.
the 4 jaw weighs a "ton" and I'm a 130 lb weakling so I gave it some
thought and came up with the following: :-)


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Some more stuff on P B if you want to look
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Hope this worked.
...lew...

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MikeC
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Post by MikeC » Sun Mar 16, 2008 1:10 pm

Cute trick, Lew.
18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

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JimGlass
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Post by JimGlass » Sun Mar 16, 2008 1:11 pm

Lew;

Nice concept and safe as well.

Seeing your post made me think of an accident I had in my shop a couple of years ago.

I had chucked up a square piece of steel in the 3-Jaw chuck and converted it to a piece of round stock. Worked fairly well. Another day I chucked up a piece that was not square but more rectangle. Guess what happened? It flew out of the chuck without warning and struck me square in the upper lip. First thing I did is look in the mirror on my truck for a damage report.

Injury turned out to be very minor. A little more RPM or a little heavier piece and I might not be here to make this reply.

If you chuck a square piece in a 3-jaw chuck follow Lew's example and not mine.

Never hurts to pass on a safey tip.
Jim
Tool & Die Maker/Electrician, Retired 2007

So much to learn and so little time.

www.outbackmachineshop.com

Jose Rivera
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Not the best thing

Post by Jose Rivera » Sun Mar 16, 2008 2:24 pm

Chucking non cylindrical shapes with a self centering 3 jaw chuck can damage the chuck and from there on will never run true.

The scroll moves the three jaws at a circumference always.

Just because you "think" you're grabbing all three faces, you're not.
Most likely that you're holding tight only two. Specially with just a band-saw cut finish.

One of the first lessons I learned while serving my apprenticeship was to never use a cheater bar on a 3 jaw chuck or clamp stock that is not round.

As you clamp on two jaws only, the pressure (the more the worst) will damage the scroll in you chuck.

I have to admit that your procedure is very smart.

You're much better of turning between centers.

Also while turning square stock is best on the first cut to go deeper that the length of each corner and into the flat by some .015".
Use a fast spindle speed (as fast as the material/type of cutter will allow) and a fine feed.

Trying to round square or hex by just taking light cuts will beat up the lathe big time. Carbide will most likely chip the cutting edge off.

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Lew Hartswick
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Re: Not the best thing

Post by Lew Hartswick » Sun Mar 16, 2008 6:51 pm

Jose Rivera wrote:Chucking non cylindrical shapes with a self centering 3 jaw chuck can damage the chuck and from there on will never run true.

The scroll moves the three jaws at a circumference always..
I guess, Jose then you would never use a 3 jaw on a hex peice of material.
:-)
...lew...

Jose Rivera
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3 jaw chuck

Post by Jose Rivera » Sun Mar 16, 2008 7:10 pm

Hex stock is normally extruded from a precision die, plus yeah, it may not apply to a hexagon, good point.

You comment is valid and in theory my posting is correct according with machining practices I learned in more than one place where the same warning was given to me.

A piece of cold or hot rolled steel have out of roundnesses that can damage three jaw chucks I has told.

Maybe my elders where wrong! :D

Harold_V
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Re: 3 jaw chuck

Post by Harold_V » Mon Mar 17, 2008 3:51 am

Maybe my elders where wrong! :D
I'd suggest they were, at least in this fine example. The method proposed by Lew should offer excellent chucking capacity with little or no side thrust on the jaws. The forces would be extremely light in view of the fact that aluminum is being machined. What little there may be would be insignificant considering the gripping surfaces are spaced uniformly @ 120°.

My opinion would be that as long as the three jaws share the forces equally, which they would in this example, it should work flawlessly.

(Although I'd have left a few of them square. You never know what your needs may be.)

I think it's a damned good tip---one I'd never tried.

Thanks, Lew.

Harold

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Bill Shields
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Burst Bubble

Post by Bill Shields » Mon Mar 17, 2008 6:02 am

I hate to burst your bubble guys, but with a 3 jaw chuck, it doesn't matter what the shape of the stock is, all 3 jaws will grip unless there is something wedged BETWEEN the jaws that keeps them from closing in on their radial path toward the theoritical center.

Basic geometry 101...3 points makes a circle...and only 1 circle..

It doesn't matter if the stock is round, square, hex, dodechahedron or pentagon, a physical 'center' of the 3 point gripping circle will always exist, and the jaws will grab....unless the condition described above exists....and it isn't going to damage your chuck 1 iota.

Now if you are foolish enough to spind something out of balance with its center of rotation at high speeds...well we can look you up on www.darwinawards.com

Harold_V
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Re: Burst Bubble

Post by Harold_V » Mon Mar 17, 2008 3:02 pm

Bill Shields wrote:It doesn't matter if the stock is round, square, hex, dodechahedron or pentagon, a physical 'center' of the 3 point gripping circle will always exist, and the jaws will grab....unless the condition described above exists....and it isn't going to damage your chuck 1 iota.

I'm afraid I have to agree.

The only place I've noticed that gripping is critical is in machining soft jaws. I use a spider, which is narrow as it relates to the area of the jaws. If it is not gripped perpendicular, and on center, the outcome of the jaws isn't consistent. I attribute that to the spider, not being on center, shifts the jaws in the slides a miniscule amount. Once so machined, they do not repeat as well as if they were supported on dead center. The difference manifests itself in concentricity, and equates to less than a thou. No big deal under many circumstances, but can spell the difference on close tolerance work.

Harold

Jose Rivera
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3 jaws

Post by Jose Rivera » Mon Mar 17, 2008 3:16 pm

I would have to heat this one! :oops:

This was said to me working at a place where 98% of the work was forgings.

In this case one would be clamping with two jaws more likely. The stock is not round at all but with many flat marks where the press or hammer had beat on it.

I still feel that the elders where right in this case.

Harold_V
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Re: 3 jaws

Post by Harold_V » Mon Mar 17, 2008 11:17 pm

Jose Rivera wrote:This was said to me working at a place where 98% of the work was forgings.

In this case one would be clamping with two jaws more likely. The stock is not round at all but with many flat marks where the press or hammer had beat on it.
Two jaws, unless they are gripping by the sides of the jaws, can't grip anything. There must be an opposing force in order for the chuck to grip, and that is created by the third jaw. Until all three make contact, the part continues to move towards average center of the chuck.

Even a forging, assuming a circle was intended, will have three points that are common to a circle. The reality is as long as the center of the jaw is in contact with the item being chucked, it should find average center of the part and load the jaws quite uniformly. Bill's comment of three points making a circle are on target in that regard. Were it not, three jaw chucks would be relatively useless. Even cold rolled material isn't perfectly round, to say nothing of hot rolled material.

A chuck with tight slides would likely not notice minor difference, nor would it be effected if they existed, assuming there's been no crash. Once a chuck has been through a crash, all bets are off, as they might be with the bearings in the headstock. All depends on the nature of the crash, and the damage done to the chuck jaw slides.

Damage to three jaw chucks, or even four jaw chucks, is more likely to occur by chucking items only by the tips of the jaws, where mechanical advantage tends to deform the slides. Rarely does one find an older chuck that has seen extended service that has not been so sprung.
still feel that the elders where right in this case.
Only in the case of a forging that was badly out of round, so much so that only two jaws could grip, because the sides of the jaws were making contact instead of the gripping surface. I wouldn't want to stand by the machine when it started in that scenario. Two jaws, unless diametrically opposed, will have no holding power-----and even then parts are subject to side movement. It takes support in no less than three places in more than 180° before a part is secure.

Harold

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