This is a 4in chuck that I hadn't used for precision work until now because the 3 jaw that came with the minilathe was so good! On that basis I wasn't expecting problems. Come the day I needed an accurate set up there was a wailing and a gnashing of teeth…..Having not ever experienced problems with a 4 jaw chuck before, until someone pointed out it might be an issue with the chuck I had assumed it was some fundamental operator error!

Eventually checking the jaw mating surfaces alignment with the chuck body when tightened on a short round of silver steel, using a square off the back of the chuck shows the following.

• short jaw surface looks reasonably normal and parallel to the square – caveat as it is only a short length and quite heavily serrated. – so no problem with one side of jaws
• Long jaw surface – two are quite severely and obviously not parallel – around 20 thou out! the other 2 are also not parallel but to a lesser extent. In addition, by varying amounts, the serrated teeth do not form an even surface when viewed against a straight edge.
• the two worst jaws are roughly parallel to each other but not to the chuck body
• the other two effectively create a bell mouth.

'Go figure' as they say….

It would appear that the long surface of the jaws were not ground originally parallel and there has been some bodged attempt to correct the problem at the factory (possibly explaining why the clamping surfaces are much wider than expected).

The result is it is almost impossible to get an acceptable axial setting for longer work using the jaws with the long edge holding the work.

In other respects, the chuck is not to bad, useable with the jaws reversed, the axial surface of the jaws is true and the jaw fit in the slots good. with smooth movement.

I will attempt a regrind, but first I will need to make some sort of height adjustable fixture to hold a rotary tool in the toolpost – no slotted table on the CML otherwise I could use a vertical slide.

The approach I would use based on a posting above is to lightly clamp up a washer in the gap between the rearmost two teeth. with the chuck stationary and spindle locked, grind all the forward teeth of the jaw to something resembling an even height and hopefully parallel to the lathe axis, repeat for other jaws.

Then go back in with the grinder to grind down the tooth that was behind the washer to below the level of the newly ground jaw surface so it will not impact clamping geometry. This may mean that the minimum stock size that is chuckable increases from 6mm to about 8mm, but I don't think that is going to be a particular issue.

I would like to think the vendor might offer to replace it, but it has been over 18 months since purchased – moral here is if you get some new stuff give it a good teardown and test the day you get it!