Thursday, January 24, 2008

Remounting dried bowl blanks

Six months ago you started off with a green log and roughed out a bowl blank and left it to dry. It is now a cupped oval shape, and you need some method to secure it on the lathe for final turning. Any spigots or recesses you left are now oval too. If you used a coring system, there are probably no spigots or recesses anyway:-

There are many options available, and will depend to some extent on what equipment you have available, the size of the bowl, and of course your personal preference. Today I'm going to show you the approach I took recently while preparing 100+ bowl blanks for final turning.

Ultimately I want to be able to mount the bowl on a chuck, gripping inside the rim, with no tailstock support. This way I can turn the outside of the bowl without having to make awkward cuts to work around the tailstock. So what I want is a trued-up recess just inside the rim that I can grip with cole jaws. Here is a picture showing how I cut this recess with a square scraper. I also make a slight dovetail cut with a spindle gouge.

I have the bowl blank sandwiched between the chuck and tailstock. I am using just two large Nova Power Grip jaws to drive the bowl, but they aren't really holding the bowl, just supporting it and driving it. The corners of each jaw press into the outside surface of the bowl blank. These four points provide a very stable platform for the blank. I use just two jaws so that the oval blank sits nicely between them. I align the bowl so that the jaws are 'gripping' across the short axis of the oval. This way there is no way the bowl can turn and come loose.

This larger bowl needed a block of wood (a reject candle holder; I new I would find a use for it one day) between the bowl and the tailstock.

In a later post I will show how I turn the outside of the bowl by mounting them on my cole jaws fitted with customised wooden jaws.


Philip said...

Thanks for another great idea and set of illustrations. Look forward to the next set.

Derek Andrews said...

Thanks for the feedback. It's always encouraging to be reminded that someone is reading this and finds the content useful.

Len said...

Hi Derek,
Thanks for all the web work you do for wood turners. I would like to know if you have re-mounting ideas for some one like me who as a stricktly outboard lathe with no tail stock. Len

Derek Andrews said...

Len, that is a bit trickier and less forgiving. The only time this is a problem for me is when I turn bowls too large to swing over the bed, which I don't do too often.

These big bowls will invariably be the outside one from the coring operation and will have a hefty foot and screw holes from previously being mounted on a faceplate. So it is not too difficult to remount it reasonably accurately on a faceplate again. I then turn it totally in this orientation, using a pull cut from the center to do the outside. This I find to be rather inefficient and slow, and I can't achieve as good a tool finish as I can with a normal cut. (Other than the lack of really big cole jaw extensions, there is no reason I couldn't mount these big bowls on the faceplate, turn a recess inside the rim, and mount on the cole jaws to turn the outside.)

When it comes to the internal cores, these will not have a flat on the bottom for a faceplate, so you really have a problem. Vacuum chucking might work, but I have no experience with this. Cole jaws with extensions (I'm working on a post about those soon) might be sufficient to grip the outside of the rim well enough to turn a tenon or recess for a chuck. Another option to consider would be to saw and sand a flat on the bottom and glue on a flat scrap of wood that was previously shaped to fit a chuck

It all gets rather complicated, and depends on which jaw sets you have, the size of the bowls and how many you have to make.

Anonymous said...

I use a modification of method three, leaving an attached post (mandrel) in the center. Since the hole shrinks, I drop the proper size bit back through to enlarge and mount on the same device to turn the rear of the piece. It's easy to guide the bit because the long grain dimension remains unchanged, and acts as a guide.

If you had a pin set for left hand operation you could use it outboard as well. Had a bi-directional model for my old lathe, and it worked, but required extra care, because bi-directional isn't as positive a hold as single.