A Quick and Dirty Way...

[carrdo]

Hi All,

Ok, I shouldn't say this, but a quick and dirty way to show how good your 3 or 4 jaw chuck jaws are. It is also a very good test to perform if you are buying any used lathe chuck.

Chuck a free machining piece of 1" diameter steel, aluminum or anything else in the chuck which is stuck out about 3/4"-1'' from the face of the chuck jaws. Face the end of the bar and at the same time take a light skim cut along the length of the bar. Then cut the piece off, reverse it in the chuck setting the piece back about 3/4 of its length into the chuck and face the remaining end. Remove the test piece from the chuck and with a micrometer mike the length of the bar every 90 degrees at its outer diameter.

Are all the readings the same to within 0.001" or better? I will bet that 95% of all test pieces will mike not the same by 0.002".

 

[mmcmdl]

I will bet that 95% of all test pieces will mike not the same by 0.002".

How much ? If the jaws are sprung in any way , you'll get this with only 3/4 inch stick out . You'll also get it with a 3 jaw . If the part was thru the entire length of the jaws , bet 100% of the time it would be within .002 . Jus sayin . It's why they make collets .

 

[Dabbler]

Cool test, but doesn't give you a reliable metric.

You can, however turn a piece any size that has full engagement of the jaws, - you only need about 1/2 inch of turning, and clean it up. Mark the piece on jaw 1, then loosen and rotate 180 degrees. Retighten to the same torque.

Now you have something you can reliably measure. Get your indicator, and measure the new runout of the turned section. Divide by 2. That is how far out your 3 jaw chuck is.

AFAIK this is the industry standard ay to 'prove' a chuck. when you buy a 10K$ ultraprecision 3 jaw chuck this is the first thing you do with it after adjusting it. This can also be used to adjust a 'set-tru' chuck for a particular diameter.

 

[Mcgyver]

alignment of jaws under load to the lathe's axis is why a four jaw needs to be your most accurate chuck. The easy test is chuck something known to be round. Get it to a tenth and chuck then indicate it a few inches out from the chuck - this is indicating it in two planes.

I've done a few projects recently where it was critical to get it aligned properly. While the experienced man may only take a minute to get with in a tenth in one plane, it can take hours (talk about tedious) to do so in two. I groan in the face of such work, so tedious, but sometimes it is necessary.

 

[Susquatch]

While the experienced man may only take a minute to get with in a tenth in one plane, it can take hours (talk about tedious) to do so in two. I groan in the face of such work, so tedious, but sometimes it is necessary.

Yes, sometimes it is necessary. This is why I have several purpose made chucks in my arsenal. They facilitate axial alignment in two planes, and one of mine will do it in 3 if you can accept distorting the part to achieve it. I demoed them at our Toronto Meetup. Too bad you couldn't come......