Mounting a new chuck

That sounds like a reasonable explanation Mick. As a novice I can even hope to answer that but it sounds logical enough. Still, as others have said, there are options open to me.

Option 1 is to see if this backplate and new chuck run true. That solves the hole discussion except for the mounting options (back to front or front to back).

Option 2 is to buy a new backplate and machine it on my machine as a snug fit. Drill and tap holes in backplate. Good concentricity but no way to compensate for chuck run out.

Option 3 is to use the old backplate sans spigot and make the chuck “moveable” to a degree to enable me to compensate for the chuck run out.

Runout on a three jaw scroll chuck is a fact of life, it’s nice to have a small runout but if possible a job should be planned so the runout doesn’t matter, if concentricity of a second operation is important then it should be set up in a 4 jaw independent. I have no idea what the runout on my three jaw is because it doesn’t matter. I think the register for the chuck to the backplate is in part to stop the chuck moving when the bolts are torqued to a normal setting and provides a convenient repeatable position when the chuck is dismantled for maintenance. The griptru chuck is adjustable because it is impossible to be true at all diameters. You could just grind the jaws if you could maintain the same tolerance across the working range of the chuck. I think a griptru is useful for a second operation on multiple identical parts that are too large for collets and the tolerance can be met by the chuck.

Mike

Mark you should mount your chuck to the backplate so that there is no runout on the body of the chuck, be that by turning a register on the backplate or by nipping up the bolts and bumping it into place using a dial gauge to check then tightening it up once you're satisfied. The chuck body has considerable mass and any runout will cause an imbalance when run at speed, and the more the imbalace the greater chance it will show up in the finish.

As Mike Poole rightly says, runout is a fact of life for a 3-jaw - the better and newer the chuck the less you'll have but they all wear with use.