Jam Chuck.. safe?

[Brian McDermaid]

I've got a nice little Tasmanian Myrtle Burl bowl (6" x 4") i made a few weeks ago. After having some time to look at it, I've decided i want to put it back on the lathe and re-do the bottom.

At first i thought I'd make a jam chuck and just shove the bowl on and start turning... but now I'm having 2nd thoughts about safety.

Whats the proper way to re-work a bowl? Its the end of the month and my hobby cash is in the negative column so i cant buy any fancy faceplate or cool toy.

Is a jam chuck safe? Am i just being a chicken?

~Brian

PS. I should point out the bowl has quite a few natural cracks that make it a bit fragile.

[Reed Gray]

Brian,
There are all sorts of jam chucks. One is a ring cut into a flat board. Another would be a rounded form where the bowl is pressed up to the form, and the tailstock is up against the bottom of the bowl. You can use a piece of plywood, draw circles on it and center the bowl on it. Secure with tape, rubber buttons that are screwed into the plywood, tape, or even hot melt glue which can be difficult to remove. Properly made jam chucks are safe, and in all cases, always have the tailstock up against the bottom of the bowl until all that is left to do is remove that last little nub. For the bowl you have, I would want one that grips on the rim, where you are pressing it into a recess, rather than fitting it over a tenon, if that makes any sence. If you press onto a tenon, it will put pressure outwards (expansion) on the rim where it is more likely to crack that it would be if it was in compression. I prefer a softer wood like pine when possible, because it will compress a bit and give a better grip. Do turn at slower speeds, and take small light cuts.
robo hippy

[curtis rosche]

one way to do a jam chuck for weak bowls. take a fale plate put plywood on it, and then mount 1/2 a ball to it, foam or otherwise, that will basically fit the inside shape of the bowl. then cover it with 2 sided tape, stick the bowl on and put the tail stock up and go.

im in the proccess of making a vacuum chuck cause i dont like the idea of a jam chuck very much either, to make bowls have come off when there was more than just friction holding them on.

[Jeff Nicol]

Jam chucks are nothing to be afraid of they work wonderfully! They are easy to make and can be made from just about any wood. Like Reed says take your time and turn slow and you should have no trouble

Good luck,

Jeff

[Wally Dickerman]

Long before 4-jaw chucks, vacuum chucks, etc. existed, jam chucks were a standard way of reverse chucking a bowl. Attach a round piece of MDF, plywood or a flat board to a faceplate. Using a parting tool, cut a groove to as near the size of the dia of the bowl rim as you can. Pop the bowl into the groove and you have a jam chuck. If the groove is a sloppy fit, use some kleenex or TP to pad it. Bring up the tailstock. If the fit is a good one you won't need the tailstock. If you don't want the tailstock point to mar the bottom, put a tiny piece of wood between the tailstock point and the bowl bottom. After the turning is done, except for the small area under the wood piece, remove the tailstock and holding your hand against the tuning bowl carefully complete the bottom.

It's a good idea to bore a small hole in the flat board that's doing the holding. If your groove created a tight fit with the bowl, you'll experience a suction that makes the bowl difficult to remove. The hole eliminates that.

The flat board can be used over and over, If the bowl is a different size, cut a new groove.

If your bowl has a rim, such as a flared one, that makes it difficult to use this type of jam chuck there are other ways. You'll need to find the exact center of the foot. to center the tailstock. You can use your chuck if you have one or use a rounded block of wood attched to a faceplate or screw chuck. Apply some foam padding such as a scrap of carpet pad or a mouse pad. Place the bowl over that, bring the tailstock up and center it. Use very light cuts with a small bowl gouge, a spindle gouge, or even a scraper with a light touch.

I've used these methods and more many times. Carefully done with a light touch with the tools, they work.

Wally

[charlie knighton]

PS. I should point out the bowl has quite a few natural cracks that make it a bit fragile.


Reed and Wally have given great advise, for safety sake you may want to wrap the bowl with siren wrap or duct tape to keep it together. sometimes it is possibe to use a paper towel between the wrap or tape and the bowl depending on the shape. i would second the use of the tail stock.

there is a woodchucker mandrel that craft supply sells that instead of just 2 points of contact with a jam chuck, the woodchucker mandrel has 3 points of contact counting the tail stock. being you have natural cracks i would still wrap or tape the outside of the bowl

[Allen Neighbors]

This is a good, safe, way to re-do the bottoms of bowls.
This is my rendition of one that Herm DeVries built. It works.

[Art Kelly]

I jam-chuck bowls and vases when they are nice and round and balanced (i.e., no punky spots, no big knots) and worked the foot until there was a 1/4" stub remaining between the tailstock and the work, then finished the stub by hand (Dremel, sandpaper).

When more comfort is called for (I'm old, you know), I use this compression chuck.

In this case, the mouth of the bowl is so irregular it is held completely away from the base (in this case a homemade Cole-type chuck, but a piece of plywood screwed to a faceplate would be OK). There's a 2" dowel-post going to the center of the bottom of the vase.

I've posted elsewhere about NOT using MDF for the chuck plates. I had a lapse of judgement and one detonated; it was not pleasant.

If the threaded rod sticks far enough behind the baseplate to be a hazard to derfingerpoking, put some 4" pieces of blue masking tape on them and let them flap around. They make a harmless but frightening noise if you poke your finger in that direction, and they make a very nice breeze right where you need it.

My $0.02.

Art

[Dick Sowa]

Plenty of good advice so far. I use jamb chucks quite often. If I am at all concerned about the tightness of the fit, or even if I am not and just want to be careful, I will wrap duct tape on the assembly to hold it together.