Re-chucking bowls problem

[Steve Blackdog]

Ok, chaps what am I doing wrong?

I mount a blank (say 7" dry oak) on screw in SuperNova 2 and turn back of bowl. No problem so far. Last thing I do is cut a dovetailed recess in the bottom of the bowl to fit the 2" jaws.

When I re-chuck the work piece by expanding the jaws into the dovetail recess, I find it impossible to get the bowl in balance. It will be out on a 7" x 3" bowl by about 5mm at the front edge. When the work piece is in rotation, you can see the jaws as well as the work piece visibly being out of round. I can deal with it on a fairly chunky piece, as I can turn the inside of the bowl fine, but the axis/centre is not the same as the outside. If I were to try to turn a piece with thin walls, I would have an unacceptable variation of thickness in the walls.

I have tried everything I can think of. This is really annoying me and, as you know, annoyance and craftsmanship do not mix! #-o

All help very gratefully received.

Cheers

Steve

 

[jumps]

the most obvious candidate is a bent screw, followed by a poor location of the screw in the chuck

first check points for me would be to mount the screw in the chuck as normal, then bring up the centre and rotate by hand to see if there is any issue.

next is that if using a recess the upper faces of the jaws will be creating the allignment - check them for absolute flatness with a straight edge.

 

[Mark Hancock]

Steve

Simply get the bowl mounted on the chuck then finalise the outer form so it's running true then hollow.

 

[alexf]

I would try drilling a 2" hole in the blank insread of a hole for the screw chuck. This would allow you to use your chuck without the screw.

 

[L2wis]

I have this problem when using a faceplate or glue chuck, always struggle to get the bowl true once turned round for hollowing.

Hope you find an answer as hopefully the problem will be the same for me! I've always wondered if it is a problem with the shaft of my lathe? Or Chuck manufacturing torrance?

always the rechucking

 

[Paul Hannaby]

Hi Steve,
Are you sanding the bowl while on the screw chuck? If so, it is likely that you will take more wood away from the side grain sections in comparison to the end grain sections so the bowl will become slightly oval. How much will depend how much wood you take away with sanding.

When you form the dovetail recess, make the size so it will fit the jaws when they are open far enough so there is roughly a 6mm gap between the jaws. this should make the jaws form the optimum circle when mating with the recess and that should improve the centring.

The other possible cause of misalignment might be the blank moving between you turning the outside and turning the dovetail. Make sure it is tight against the screwchuck face or chuck so it doesn't move during turning, which might affect concentricity of the recess. Even if the screw was bent or off centre it wouldn't matter as long as the blank didn't move.

 

[Steve Blackdog]

I would try drilling a 2" hole in the blank insread of a hole for the screw chuck. This would allow you to use your chuck without the screw.

I'll give that a try and see if it makes a difference.

I've just had a thought :idea: I tend to cut my wood with a chainsaw so the top and bottom of the blank are rarely flat, I wonder if the action of tightening the blank onto the bowl pulls the screw out of alignment. However, I'm not sure it can be anything to do with the screw - as by the time I have finished the outside of the bowl it is completely and perfectly round, so the dovetail recess should be dead centre, despite whatever is going on with the screw.

I think I'll have a go with a spigot instead of a recess and see if that makes a difference. :? :?

Steve

 

[Steve Blackdog]

Hi Steve,
Are you sanding the bowl while on the screw chuck? If so, it is likely that you will take more wood away from the side grain sections in comparison to the end grain sections so the bowl will become slightly oval. How much will depend how much wood you take away with sanding.

When you form the dovetail recess, make the size so it will fit the jaws when they are open far enough so there is roughly a 6mm gap between the jaws. this should make the jaws form the optimum circle when mating with the recess and that should improve the centring.

The other possible cause of misalignment might be the blank moving between you turning the outside and turning the dovetail. Make sure it is tight against the screwchuck face or chuck so it doesn't move during turning, which might affect concentricity of the recess. Even if the screw was bent or off centre it wouldn't matter as long as the blank didn't move.

Paul

I bet you've hit the nail on the head. I have been sanding this oak quite a lot. I had some standing up end grain which I sanded out, so I suspect I sanded about 2-3 mm off, which, if it had taken more off the straight grain than the end grain, might indeed have made the outside oval.

I'll try it on an unsanded bowl and see if that makes a difference.

Thanks for that one.

Steve

 

[CHJ]

Are your chuck jaws mounted in the correct order? 5mm out sounds like it might be a jaw out of sequence.

I use a 8mm screw chuck for initial shaping of bowl blanks on a regular basis and as long as the dovetail recess is deep enough to allow maximum support of the blank when reversed see no significant run out.

 

[Steve Blackdog]

Are your chuck jaws mounted in the correct order? 5mm out sounds like it might be a jaw out of sequence.

I use a 8mm screw chuck for initial shaping of bowl blanks on a regular basis and as long as the dovetail recess is deep enough to allow maximum support of the blank when reversed see no significant run out.

Thanks, Chas

I will have a look at the jaws sequence.

You know a bad workman always blames his tools, so it would be fitting if I could blame the chuck!

Steve

 

[Deejay]

Afternoon Steve

Another possibility is that the jaws themselves are not exactly the same size.

Close them until they touch and check to see if there are any 'steps' between them.

This was the case with my Patriot chuck. Sorby explained that all the (identical) jaws were made to tolerances and my set was made up of three at the 'small' end with one at the 'large' end.

They replaced them FOC.

I'm not familiar with the Supernova chuck, but might this happen if the jaws are not correctly fitted?

Cheers

Dave

 

[dickm]

When the work piece is in rotation, you can see the jaws as well as the work piece visibly being out of round.

Sounds like there is definitely something wrong with the jaws - either tolerance errors, or order of assembly as suggested by others.
The effect of sanding would surely only make the bland elliptical, not off centre?