Post By David J.
4 Jaw Chuck Runout
I am a lathe newbie - I purchased a 9" South Bend lathe with a 48" bed this summer at an auction. Recently I have been experimenting with it and practicing with my 4 jaw chuck. I have a 10" length of precision ground round stock that I can chuck with about .0005" runout near the jaws but when I indicate near the end of the rod, I get .012" wobble. I have tried this a few times and consistently get .012" wobble at the end. I believe the rod is true as I indicated it on a flat precision ground surface.
I also indicated the horizontal spindle and vertical shoulder to be near zero. Does anyone have any suggestions as to why I am getting this wobble?
because the jaws and other parts of the chuck are worn.
man, im having a hard time with decimal places.
+1. My kind of answer. Concise, brief, exact.
Originally Posted by Mulva
This can be alleviated (or diminished) by chucking on a very short length of the diameter. Then bumping the work around until it runs true.
You will not have the rigidity you would with chucking the entire length of the jaw. It will make life more difficult to fight the wear in the chuck. It may be more difficult to scrape up the Krugerrand necessary to buy a new one.
Most likely the chuck is at fault. Clamp a long bar, 1 to 2' lightly in the chuck, grip the end and flex. See if you find where there is motion. Most likely the jaws in the chuck body or the inside ends of the chuck jaws on the rod.
My first thought would be the chuck mounting arrangement. Check for burrs, crap or dings where the chuck mounts to the spindle. My D1-6 setup looked great but produced run-out. After closely checking I found a little ding that prevented the taper from seating true. A little work with a stone and I was back in business.
It could be the chuck but it could also be the mounting.
You have to know what to expect from a chuck. You are probably "swallowing" the work - seating it all the way in to the body. Not a good thing to do. All 4 jaw chucks have to be treated as though their jaws were slightly sprung. Very few have the gripping surfaces precision ground in place. Therefore there is no guarantee a 4 jaw independent chuck will grip straight. Neither do three jaw universal chucks once they've been in a few small wrecks but that's another subject.
Swalllowing the work is poor practice bexasure you can't bumber the uverhang into concentricity. Even if you do the slight wirks in the grip while the cuts are in progress causes the grip to drift back to the original orientation.
The solution to dialing in finished work in a 4 jaw chuck is to grip only a little of it or grip it in a short ring of copper. Make sure the ring of copper is in the plane of rotation. If it is not the gripping force will make it impossible to dil in the wobbling far end.
I wrote a couple of tutorial articles for the 3 jaw and 4 jaw chuck. Send me your email and I'll send the .doc files to you.
I've dialed in long high-precision work in a lathe to 0.0003" in a worn out 4 jaw chuck in a 36" lathe using the copper ring trick (Is the spindle sleeve for a 340T G&L horizontal boring mill precise enough for you?) Padding chuck jaws to protect the work and to aid dialing in the far end is technique old as the metalworking lathe.
OK, once you have gripped the precision work in a copper ring and you have ensured the ring lies in a radial plane, dial the work in close the the jaws. Then move out a ways and bump the far end of the work into concentricity being careful not to jar the indicator or its delicate guts. Use a lead or copper hammer or maybe wood or hard plastic. Soft face rubber or plastic hammers are too soft. They lack the essential shock needed to nudge the work in tiny increments.
Adding: My version of a "copper ring" is an incomplete circle of annealed #6 bare copper wire snipped from a longer piece I keep handy by the lathe. So long as you get copper under all four of the jaws all is well. The jaws of my chuck are serrated. #6 copper wire happens to fit the serrations. YMMV
You guys with smooth chuck jaws may benefit from a piece of copper pipe or a 3/8" wide strip rolled to approximate diameter. Lead is too soft, Plastic will crush or simply squirt out, aluminum is a little slippery. Soft iron works OK for hard work but will indent unheat treated steel and most all non ferrous metals. Paper is too thin, cereal box cardboard works OK but you might need a couple of layers. Gasket material is hard to beat for some applications but it has to be 1/16" or the jaw serrations will punch right through it.
Go back and forth dialing in close to the chuck and tapping in the far end. You will find that a little practice leads you to dialing in precision work to 0.001" TIR or better if you wish. When I was an apprentice we had to go against the clock. If we could dial in a hunk of 4" stock bother ends within two minutes we got that box checked off. Most of us got it with time to spare.
The next thing to address is security of the grip. One might think gripping work in a narrow band of coppr is precarious or even dangerous. There's truth in that so you have to proced with due caution. That's why you use a steady rest or a center once the work is dialed in.
This is part of lathe work. You cannot always depend on a sure immovable grip. The pressure of the jaws may force thin-walled work out of round. You may have to face off the end of a long part that cannot be supported and center drill it. You may have to face work parallel and there is no way to access the back side for dialing in except between the jaws. There's dozens of seemingly impossible-to-solve lathe problems. There are solutions to all of them. Time and experience will bring the answers to you. Many you will solve for yourself. The others us decrepit old timers will help you. Have fun with both alternatives.
Last edited by Forrest Addy; 12-19-2011 at 10:26 PM.
Get a hammer and whack it strait, Whats the problem?
Shoot you dont have a problem other than being a noob and there is nothing wrong with your chuck, its all operator error.
Make sure the numbers on the jaws match the numbers on the chuck body slots.
Then indicate the face and outside body of the chuck. If the chuck body wobbles, then you will find your bar wobble follows from that cause. Slot wear may add to the problem.
If the body runs true and the jaws are in the correct slots, then the jaws may be ground to correct wobble. A surface grinder can do the job, but you have to make careful measurements and careful setups of the jaws in the grinder.That method is still more certain than trying to use a toolpost grinder.
The other advice is good; I have just added more detail.
Originally Posted by L Vanice
On a 4 jaw that stuff dont mean a thing, not anything at all.
Everyone read what Forrest posted because he is dead right...
I have a high quality Pratt Bunerd and a high quality Bison...
Neither will hold work straight with the spindle axis when you swallow the work...
Of course once you bump, tap or beat the outboard end into alignment
you'll still need either a steady or a center to keep it there.