With a little clearance in the register fit,you can gently tap the chuck body into line while the bolts are loosened slightly. It is a hit and miss method, and needs to be done after the workpiece is clamped tightly in the jaws. The runout should be rechecked after tightening the bolts. A different diameter of workpiece may run differently, depending on how the scroll is worn and wear generally. At the museum, we have a Smart & Brown model A which has the same size spindle fitting as your machine, and over the years I have ammassed about 6 three jaw scrolls and 2 four jaw independents. Only the four jaws and one of the three jaws have a tight register, the others can be adjusted if necessary. The ones that can be adjusted are all very good and have only been adjusted once or twice, their runout at different diameters is well within my idea of a good chuck.

Watch the video before doing anything to the chuck.

I have watched that video some time ago, and it gives a lot of very valuble tips on how tighten up work properly.

Edited By old mart on 20/03/2021 19:11:26

Edited By old mart on 20/03/2021 19:15:46

Another possibility is the jaws are worn. New jaws, or try regrinding them by clamping the jaws against an outer ring and truing up the ends. I expect someone else will be able to explain better! If the chuck body is worn or damaged, replace the chuck

Before doing anything too wild though, strip it down and for a good clean. Might be swarf in the works, especially at the register surfaces

Dave

First, do your jaws have the same number stamped on them? Next, does it make a difference if you change the jaw positions by one slot each way (not all chucks have the slots numbered).

Then try the ideas above. There are very few self centring chucks where the workpiece can be removed and remounted without losing concentricity. Collets go a long way towards, but turning between centres is the time-proven method for items which need dismounting and remounting.

i too have a smart and brown model m mk 1 i get similar numbers to you when that is not good enough i use the 4 jaw

You want to check a lot of different sizes. The scroll on some early lathes is 8tpi so every 1/4 inch size will likely be the same runout. So 1/4 bar is often the same runout as 1/2 inch. But 5/16 and 3/8 are on a different part of the scroll. Is it the same when held by the full jaw length and at 1/2 the jaw length. Older chucks can have damage to the chuck body by gripping too tight on short pieces, and racks the jaws track in the chuck body.

Another option is to loosen the holding screws, and the spigot, then put Belleville washers under those screws or nuts. It will allow you to make small adjustments to the part being held , a bit like a grip true does, but without the adjuster cams.

Neil

Mike Poole is absolutely spot on. I struggle to believe that so much has been written here about backplates, grinding jaws etc on a three jaw chuck.

When yo turn something in your three jaw chuck does it produce correct size concentricity, straightness etc? If it does then you are doing fine.

Having part turned your piece, turning it in a three jaw chuck is a totally pointless exercise. You have no chance of the second end running true to match the first turned end. So either use a collet on both ends, mount the piece between centres, turn the piece complete and merely part it off leaving a simple facing operation to finish, or if real accuracy is not required bore out a set of soft jaws to accept the first turned end. Yes you could turn the part and mount it in a four jaw chuck but the methods I have describes are more consistent. HTH. Happy turning.

I bought a 4 jaw self centering chuck with jaws that were mixed up and didnt match the body but we're numbered 1 to 4 with diferent set numbers. It was cheap at the Doncaster show. I mounted it up on a new back plate and with a test piece there was about 0.65 mm run out and needed grinding. I made a set of holders for the jaws, a four hole cloverleaf plate for the inside grinding and a set of rings for the outside faces. After grinding the run out was less than 0.02 mm. You have to load the jaws in the direction of his they are used and direction of clamped.

I borrowed a toolpost grinder from a friend and ground both inside and outside faces on the jaws.

David

Edited By David George 1 on 21/03/2021 08:05:10

Never used a 3 jaw chuck that is spot on accurate. To ensure concentricity I always use over size bar and turn down work to diameter if in a 3 jaw. Anything else is first clocked in a 4 jaw. Others may disagree but I reckon it's a waste of time fiddling about trying to improve the run out of a 3 jaw chuck apart from occasional grinding of the jaws when worn, if you have the kit.

Have only ever come across one three jaw chuck with Zero eccentricity. The worst, very well used, and worn. gave 0.036"

Depending on how the scroll is worn, the eccentricity may very well vary according to which part of it is being used to clamp the material.

Wear between the jaws and the chuck body could also affect eccentricity.

Even grinding the jaws will not guarantee that eccentricity will be absent at all other diameters.

As S O D says the turned diameter will be concentric to the axis of the Mandrel (Assumes that the bearing are in good condition and properly adjusted ) but not necessarily to the OD of the raw material.

If you are going to part it off, will it matter?

If you want two or more diameters to be concentric, turn them all without disturbing the work in the chuck.

Otherwise, mount in a 4 jaw and clock to obtain the level of concentricity that you consider to be acceptable.

The eccentricity is then your decision.

Howard

I'm late to the party… my suggestion was going to be a good clean anyway!

Neil