Fitting a chuck to a backplate

I've been ferreting about this website, reading various articles that I think might help me in the future, and I came across "The editor's workshop part 1 - Fitting a chuck to a backplate". Everything seems to make sense apart from the following statement....

"With a three jaw chuck, the outside register of the chuck should fit against the turned step and the centre part of the chuck should be clear of the backplate.
On a four jaw chuck, the reverse is true."

I can imagine why this is true for the 3 jaw chuck but never having used a chuck with a backplate, I can't picture why the 4 jaw chuck should be treated differently. Can anyone enlighten me?

Bodger Brian

Could this be because of where the retaining screw holes are. The threaded holes could be either on the rim or the spigot area of the chuck and the mating faces where the bolts are should be a perfect flat mating fit.

I'm searching my memory here, the last time I did this was in 1963, but perhaps it's a start.

Ian

BB

The key to this is simply the position of the holding bolts on the backplate. On a 3-jaw they are on a circle near the perimeter of the chuck. On a 4-jaw their circle is near the centre.

The point is that the backplate should touch the chuck where the holding bolts will be applying their pressure.

I've seen an independent 4 jaw that can be mounted the same way as a 3 jaw usually is so stating must on a 4 jaw really depends on if the fixing holes are within or outside of the register on the back of the chuck.

I think lathes co uk add a further point - engaging a centre with the face plate when it's being turned. This does makes some sense especially if the centre is truly accurately aligned. I suspect the idea is to make sure that the spindle is running true, or more true when there is a bit of headstock bearing wear. It should also help push the spindle back fully into the bearings. Both aspects should improve the results,

John

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Edited By John W1 on 04/09/2015 18:18:49

Thanks guys, makes sense now. I hadn't considered that the mounting holes might be inside the chuck register.

Having said that, I've no idea what this means....