Sorby Patriot chuck - a problem ?

[heronviewer]

I've been using a Sorby Patriot chuck with the pin jaws fitted (RSJP) to turm some small laburnum bowls. Small meaning about 3" outside diameter (hopefully) when it is finished.
I initially mounted the piece of wood by drilling a 1" hole in it and then turned the outside concentric, flattened the base and then made a 1" dovetail so that I could refit it on to the same jaws.
When I turned it round to clean out the inside, it wasn't concentric any more. I tried moving it into different positions but couldn't get it concentric.
So I have had to re-turn the outside and in the process making the small bowl even smaller.
Is there any reason why the concentricity is not maintained ?

Robert.

 

[CHJ]

Yes, the problem is not unusual, in fact I would say it is the norm with most woods.

The socket or spigot you have formed is in uncompressed wood and in most instances will have both end and cross grain components, or at the very least differing growth ring compression characteristics.

When you engage the jaws the wood will compress at differing proportions dependant upon the grain orientation and the partially turned piece will move off centre by this proportion.

When turning pieces that require strict conformity of inner and outer surfaces in the majority of woods it is necessary to leave the final outer shell finishing until re-mounted for the inner hollowing.

 

[heronviewer]

Thank you ! Sounds the correct reason. I'll just have to remember that and not try too hard before reversing in future. If I take enough off so there is no vibration, that should be the best way to do it and I will not loose too much of the final diameter.

Robert.

 

[CHJ]

One other factor that helps as experience grows is to re-clamp with just sufficient pressure to hold the piece just securely enough for the subsequent turning, many folks apply more pressure on the clamping jaws than is necessary which compounds eccentric alignment.

If really soft wood is encountered try soaking recess or spigot with thin CA glue to strengthen and stiffen the wood against compression.

 

[heronviewer]

Thanks. I think the laburnum I'm turning is fairly hard - it is certainly very well seasoned and the turnings are crisp and dry. I don't think I'm over tightening but the glue tip is a good one !

Robert.