It’s often necessary to reverse a bowl to run true in the lathe to work on its foot, whether to reshape the foot, remove chuck marks, or remove it altogether. Reversing bowls on a vacuum chuck can be troublesome, because they don’t automatically centre the bowl.
If you know you will be reversing the bowl, and plan ahead, you can make a small centre divot with a skew chisel while first shaping the foot. This happens anyway if you use the tail stock. Then, when the time comes to reverse the bowl, the tail centre in the divot will line it up ready for the finish turning. This method works for friction drives as well as vacuum chucks.
You can incorporate the divot in a decorative feature, for example rings cut with the skew point. But if you don’t want a divot, a pencil dot can be made, which can be aligned to the tailstock without making contact.
Without a divot or pencil mark to help, the usual alignment method is trial and error. On a good day, that works well. On other days, not so much. Adjust the position of the bowl until it looks right, then check that it is running true. Adjust some more, then check again. Repeat and repeat until it’s right.
Aligning the bowl
To make reversing bowls on a vacuum chuck easier, I made some MDF discs in various diameters. They are primarily for positioning the work rather than protecting it. I pinned scraps of MDF against the spindle nose with the tail centre. Then turned discs to the correct fitting diameter of the scroll chucks used for gripping the foot earlier.
I can then bring up the tail stock, with the appropriate disc between it and the foot. The tail centre locates the disc, while I adjust the bowl until the edge of the foot lines up with the disc. If the foot is a little bigger than the disc, acceptable accuracy can quickly be achieved by eye. Even easier is to temporarily stick the disc to the foot with Blutack or similar. Then the lining up can be done off the lathe.
When the job is lined up, I turn on the vacuum and remove the tailstock and the disc. I then have full access to the surface.
This is a really practical fix. I’ve mostly relied on trial and error with a vacuum chuck and it can eat up a surprising amount of time getting the bowl running true. Do you find the MDF disc method still holds up on bowls where the foot isn’t perfectly round (e.g. natural edge pieces), or do you end up custom-shaping the disc to match in those cases?
Hello Aneeq. I haven’t used this method on green turned pieces, because the bowl will likely be too warped for the vacuum to hold. If I did, I would not shape the disc – that would take longer than the trial and error method. A better alternative would be to centre the disc on the foot while off the lathe. Find the best position for the disc by eye, then pencil a circle round the edge of the disc. In the lathe, centre the disc with the tailstock and align the circle to the disc.