To grind or not to grind...

I've got a nice ML7 that I'm getting used to and usually use a 4 jaw chuck as I can set it up fairly quickly and get good accuracy, but I also have a Burnerd 3 jaw chuck that I'd like to use just for the convenience when I don't need accurate repeatability. However, I've measured the runout on a test bar 50mm from the jaws and it's 0.1mm. There's virtually no runout at all on the spindle nose, the backplate shows about 0.02mm runout on the perimeter, and the lathe body shows about the same, so I've assumed the major cause of the runout is inaccurate centering of the jaws. They are all in the correct slots and swapping them around simply makes the runout worse.

Bearing in the mind the total amount of runout I've got is it worth grinding the jaws or would most folks just live with 0.1mm? I'm tempted to try grinding as it gives me an excuse to convert an old router to a tool post grinder but pre-loading the jaws looks like it's going to take a bit of ingenuity!

I wouldn't grind the jaws, a 3 jaw chuck is not a precision work holding device. Of course some who think they know better will disagree.

Tony

Unless the jaws are worn so they do not grip along their full length I would leave as is, 0.1 is not too unreasonable for a three jaw chuck and even when ground they will probably not hold that accuracy through the full range of the chuck. If it is required that a job needs both ends machining and a high degree of concentricity then turn between centres or clock it up in a four jaw.

Mike

Thing is see that the three jaw is entirely dependant on the accuracy of the internal scroll that screws the jaws in and out. You might put a 60mm or a 40mm bar in and have the runout in an entirely different place. I was taught that so as not to damage the scroll we should be moderate with the force that we use to tighten the 3 jaw up in case we damaged the scroll , so no hide mallets or lengths of pipe on the chuck key then...... It might be worth removing the jaws ( which are normally numbered) and cleaning the scroll in case there are any chips or bits of swarf caught in it. Then lubricate before refitting the jaws. I use ACL, which is fluid grade of grease.

Edited By stevetee on 22/01/2016 19:59:47

Thanks all for the advice, think I'll leave it alone - but I'm still going to make that toolpost grinder! Just finished my second lathe project (first one was a spindle handle), a ball turning tool knockup up fropm scrap steel and aluminium, weel pleased with it. First job was to turn a ball for the handle, very, very satisfying. I actually used the 3 jaw chuck to hold the handle with the blank mounted on the end and it worked just fine.

I think you would find that if you put a number of different diameter bars in the 3 jaw, you will get a different run out for each diameter, some may be as near to another not to matter, and some way out.

Ian S C

You may also find that tightening the jaws using different keyholes will give different runouts,

Roy.

If you had soft jaws for your 3 jaw and machined them to size you can still get run out away from the chuck if you are not careful when setting the part in the chuck. The parallel grip of opposing jaws in a 4 jaw chuck reduce setting errors like this. If you machine a part in a 3 jaw chuck and mark its position to replace it back in the same place at some future time you will more than likely find it will not be running true when reinserted in the chuck.

Martin