What is acceptable chuck run out?

Having purchased two 100mm 3 jaw chucks with MT 2 spigot mounting plate for use with my rotary table both examples had to be returned. The first needed two hands to turn the chuck key and the second had 0.4mm axial run out on the chuck. The back plate mounting was fine. The suppliers were requested to check the third one before dispatch and were happy to do this but would not give any figure when checking the replacement, simply stating that it was OK.

The supplier was a very well known seller of all types of machine tools and if often referred to on this forum. So what is acceptable run out?

if you don't have certs for all the items what do you think is acceptable?

What run out are you measuring?

Surely, what is important is what run out, radially and axially does the chuck impose on work held in it.

Having the chuck body moving axially is of lesser importance, unless used as a datum.

Don't expect even a new 3 jaw chuck to hold work concentric to better than 0.75 - 0.125 mm.

If you think about the clearances,;scroll to body, scroll to each jaw, each jaw to it's slot, (in both planes ) that is pretty good.

Could it be that the chuck is not mounted properly on the 2Mt arbor, and that is what is causing the axial run out?

If the chuck axis is at an angle to that of the 2MT arbor there will be axial movement and eccentricity of both chuck body and work held in it.

Some more measurements may help find where the eroor lies.

It could just be indifferent, or incomplete fitting, or burrs.

Howard

Measured with the 15mm diameter length of Silver Steel I use as a test bar, the chuck supplied with my Chinese lathe has run-out averaging 0.04mm (worst 0.05) . I measured the run-out 50mm away from the chuck jaws.

Not sure why the run-out varies, could be a mix of the scroll not tightening evenly, faulty micrometer technique, and perhaps the Silver-Steel rod is imperfect; it's diameter is on the small size and it might be slightly bent. Nonetheless the values are within expectations - average run-out 0.04mm is somewhere between 1 and 2 thou.

My example being ten times better than the 0.4mm measured by Bryan suggests something is wrong. Perhaps the chuck is a lemon. However, as always, check everything. The chuck may be last straw rather than guilty party; possibly it's weight causes something underneath to move. Does the backplate measure OK if the table is pressed down hard by hand?

Dave