Reverse chucking more accurately.

[BearTricks]

I've been having trouble reverse chucking my bowls to clean the foot up. Essentially I'm using a faceplate or screw to turn the outside, then when I put the foot in the chuck I usually have to do some kind of work on the outside to true the bowl up again.

Once I've turned the inside and make some kind of jam or friction chuck in order to clean up the base, I usually have to true up the outside yet again, which can cause some problems as the bowl isn't as secure as it was on the faceplate or the wood screw and it means that I end up with a smaller bowl than I wanted.

Am I doing something wrong? I feel as if it should be more straightforward considering I'm using a few hundred pounds worth of machinery. Instead the process becomes a bit more rough and ready the closer I get to the end. Does anyone have any tips, or do I need to invest in some kind of doughnut chuck?

 

[Dalboy]

When you have the bowl on the screw chuck and before you turn to do the inside make a small dimple in the centre of the chucking point this will aid in taking the foot off later. Also make sure the chucking point is the correct size and fit to the chuck jaws as this will make sure it runs true when turning the inside. Once the inside is complete and you are ready to remove the foot whether you use a jam chuck or make a doughnut chuck bring the tail stock up and locate the revolving centre into the dimple you made at the start of the turning this will help to centralise the bowl.
Also try and turn the bowl in one session as leaving it overnight the wood can move and distort it

 

[CHJ]

As Dereck says, try and complete the item body outside and inside in one session.

If using a screw chuck or faceplate just ROUGH turn the outer and form the holding spigot or socket.

Then either:-

• Mount it on the holding spigot or socket and true the outer then move into the inner, but LEAVE the OUTER ALONE as soon as you start on the inner as it will start moving out of round as you go.

Or

• 
  Alternately work down the outer and inner in stages towards the base but DO NOT GO BACK TO THE TOP EDGES with the gouges, the wood will have moved.

The odd millimetre difference in wall thickness is irrelevant on most items, If it's critical the wood choice and working methods need optimising.

When you reverse, as Dereck says, align the chucking spigot with the tailstock centre and ignore the odd few millimetres of run out of the body or outer chucking area, just clean up the chucking area, if it should be a couple of millimetre out of true it will not be apparent in the finished article.

The piece will generally move about several millimetres in concentricity when finished anyway as the humidity changes and wood stresses sort themselves out.

 

[duncanh]

When you reverse turn all you need to work on is the foot. It doesn't matter if the outside/inside isn't running true as long as the foot is more or less square with the headstock (and Dalboy's suggestions of putting a dimple in the bottom should help with that). Even if the base isn't exactly running true it isn't critical as once the item is placed on a flat surface it's unlikely to be noticed.

 

[KimG]

If the bowl is not a natural edged one (they are tricky) then what I do is use a large flat disc of ply with a larger diameter than the bowl (I made one earlier!) mounted in the chuck, the rim of the bowl is placed against it (with a cloth or tissue backing to protect it) and the tailstock located in the centre mark of the foot, sufficient pressure is applied from the tail stock to keep it in place (roughly centre is fine a little wobble won't show) the turn the foot down and sand it, but pretty slowly, 500 - 600 rpm, this helps to keep it all stable and controllable.

 

[Brianp]

I also do what KimG explains. I've a bit of grip-pad on my disc, but I dont think it makes a huge difference what you use to protect it.

Mine is mdf though, and it's liable to warp I would say. I should get around to making one from ply.

Given that faceplates are (relatively) inexpensive, its a good idea to make one up.