Off-Centre Turning

[manodav]

:(
I photographed the attached off-centre turning which I thought was very attractive and challenging.

I have my home-made off-centre jigs, for standard off-centre turning, which should also suffice for the attached design. But, I tried it, and came unstuck - I wasn't able to get an even finish at the bottom of the bowl - I turned six off-centre circles around the circumference to a common depth, and then tried to turn out the bottom on regular centre turning, but it came out uneven and untidy.

Can anyone give me some tips/guidance on how to do this off-centre turning to reach the result shown on the attached pics - particularly, the order of the turning, and how to get an even surface across the bottom inside the bowl

267989267988

[hughie]

Hmm not really my forte, but I would probably look to your jigs to start with. The jigs need to able to repeat settings accurately, not only in moving the turning circles around but to grab it at the same height as well.

The best thing would be to add a few pics of your jigs/lathes/set up etc, so those do this often will have the full story and can offer up some detailed advice.

Have a look at these links

How to Turn an Arc Bowl. Mounting Off Centre - Paul Hannaby (http://www.hannaby.com/mounting-off-centre.html)
Vicmarc - Eccentric Chuck, Off Centre and Off Axis Wood Turning (http://www.vicmarc.com/default.asp?contentID=699)
http://www.sawg.org.nz/wordpress/wp-content/uploads/2011/09/eccentric-bowl.pdf

[wheelinround]

Hmm not really my forte, but I would probably look to your jigs to start with. The jigs need to able to repeat settings accurately, not only in moving the turning circles around but to grab it at the same height as well.

The best thing would be to add a few pics of your jigs/lathes/set up etc, so those do this often will have the full story and can offer up some detailed advice.

Have a look at these links

How to Turn an Arc Bowl. Mounting Off Centre - Paul Hannaby (http://www.hannaby.com/mounting-off-centre.html)
Vicmarc - Eccentric Chuck, Off Centre and Off Axis Wood Turning (http://www.vicmarc.com/default.asp?contentID=699)
http://www.sawg.org.nz/wordpress/wp-content/uploads/2011/09/eccentric-bowl.pdf

Some good links there Hughie :2tsup:

Here's the OTGA's jig (http://www.otga.org.au/Gallery_files/8th%20June%202010%20Update%20The%20OTGA%20Off%20Centre%20Bowl%20Chuck%20FULL.pdf) a PDF plan.

[wheelinround]

:(
I photographed the attached off-centre turning which I thought was very attractive and challenging.

I have my home-made off-centre jigs, for standard off-centre turning, which should also suffice for the attached design. But, I tried it, and came unstuck - I wasn't able to get an even finish at the bottom of the bowl - I turned six off-centre circles around the circumference to a common depth, and then tried to turn out the bottom on regular centre turning, but it came out uneven and untidy.

Can anyone give me some tips/guidance on how to do this off-centre turning to reach the result shown on the attached pics - particularly, the order of the turning, and how to get an even surface across the bottom inside the bowl

267989267988


Turning the centre out would be no different to turning a normal base for a bowl and flattening once the individual bowls have been turned away as this is done in normal mounting mode at centre.
A scraper would be ideal and maybe a little more thickness to the base internal and turn off the excess from the outer base after the inside is finished.

[Skew ChiDAMN!!]

As Hughie pointed out, accuracy in remounting is extremely important in cases such as this. Especially considering the way that all errors in the radial direction are effectively doubled. (That's the nature of round, spinning things. :rolleyes: )


You haven't said how you turned the outside in an offset manner.

That would help, in that it's at this stage you need to scribe mark to help with remounting the piece accurately for hollowing.

At the least I'd want one mark in the overall blank centre and a set of marks clearly delineating the centres of each 'cup,' preferably a scribed circle including 'em all in one hit. ('Cept the very centre mark, of course!) Ideally, these marks would be on top and bottom of the blank, but just on the bottom would do in a pinch.


I can think of three different methods for mounting to hollow this dish...

1. Mount each 'dish' centre in the middle of the faceplate for hollowing. This requires at least 6 dis- & re-mounts on the lathe, each with a chance of inaccuracy creeping in.


2. Mount a sacrificial ply disk to the faceplate. Transfer the diameter of circle containing the 'cup' centres onto this faceplate disk and carefully drill a hole somewhere along the circumference. Using this offset hole, place a worm screw (or ordinary wood screw) to mount the centre of the blank onto, so it's all mounted offset on the faceplate.

2b Before tightening the screw all the way, bring up the tailstock and carefully rotate the blank until the centre mark for one of the dishes aligns as accurately as possible with the tailspur centre. Tighten everything down & hollow. I'd also screw a number of scrap, sacrificial blocks around the outside of the blank to prevent any catches from 'twisting' it off-centre. (Not necessary for the method to work, but I consider it insurance.)

2c The next hole is just a matter of removing and chocks, slightly loosening the worm screw and rotating the blank around so the next dish falls in position as per part 2b above.

This method has the advantage of giving you a constant wall thickness (assuming that you mounted the centre accurately) for all cups... and as the piece is being rotated tangentially - not radially - then any positioning errors are only as bad as they actually are, not doubled as they would be in the first method.

(All of which probably doesn't mean anything to most people without further explanation but I've already typed way more than I intended... :doh:)


3. Involves a simple jig, but I'm keeping this method to myself until I get time to play in the shed & actually test it before I start blabbing... I think it should work and would be as accurate as method 2, albeit a step more complex... but I'm thinking it'd also be more versatile when it comes to other shapes.

:think: