Expansion vs. contraction mode? (Nova Jaws)

[Fred Oliver]

Which is better... expanding chuck jaws into a dovetailed recess, or contracting around a spigot?

The SuperNova2 instructions seem to indicate the expansion into a recess is better - at least the listed maximum size of the bowl is larger for expansion mode than for contraction mode.

That seemed counter-intuitive to me.

Now that I've done some bowls both ways, with 50mm and 100mm jaws, it seems to me that contraction mode gives a stronger grip. At least I feel like I've had more bowls come out of the jaws in the expansion mode.

Forming a cleanly cut recess seems a bit more difficult than forming a spigot (tenon) - unless there's a trick or technique I'm missing out on.

Plus it's harder to accurately judge the bottom thickness with a recess. (DAMHIKT.)

OTOH, my wife (and head-critic-in-charge) really prefers the looks/lines of the recessed tenon/foot.

What's the consensus on this forum regarding the advantages/disadvantages of these two fixin' methods?

[Jim Becker]

Expansion is only as good as the amount of wood you have available to support the tension you are putting on the recess. That's why this method is best suited to platters, rather than deep bowls and hollowforms. Personally, I prefer a tenon outside of platter work.

[Bernie Weishapl]

I use a expansion on platters and tenon on bowls.

[Kevin McPeek]

^^ What they said.
For me it's quicker and easier to make a tenon and they are for more robust in bowl work. I think it easier to see the form too, if you turn away the recess to finish the bottom it could change the form. But maybe thats just me.

[Richard Madison]

Always a tenon except for flat, platter-like piece. Is possible to split the workpiece expanding into the recess if same is not well supported by surrounding wood. Always mark #1 jaw position on the tenon (which will be removed later), in case of need to remove from chuck and then rechuck.

[Jason Clark2]

Ditto all of the above, tenons on everything except for platters. It's almost impossible (almost) to crush a tenon, but it's surprisingly easy to split a bowl completely in half with a little too much pressure on a recess.

Jason

[robert hainstock]

I have used both methods with equal success. When I expand, I use a two inch forstner hole Bob

[Reed Gray]

I use a recess exclusively for bowls and platters. A recess can be incorporated into the design and look good, especially for utility pieces. This is more difficult to do with a tenon. For hollow forms, I use a tenon, because the end grain is more likely to split when expanding, and part it off after completion. The main thing to remember when using either method is that the foot of the bowl is usually 1/3 to 1/2 the diameter of the bowl, so a 12 inch bowl should have a foot of 4 to 6 inches. Your tenon should be in this size range, and for a recess, the foot is the same size, with the recess inside it. If you use a 2 inch tenon on a 12 inch bowl, you may get away with it, but it is a leverage thing, and even the slightest catch will tear it off. For either method, the closer the fit of your jaws is to the tenon or recess, the better your grip is. This includes matching the profile of your jaws if they are straight, or dove tailed. If you have big jaws, say 4 inches, then you want a 4 1/4 inch tenon. If you have 2 5/8 inch jaws for a recess (my big Vicmarc), then you want a 2 3/4 inch max recess. I can turn and core bowls up to 18 inch diameter with this method without needing the support of the tailstock. It is important to have enough wood around the recess to support the pressure from expansion. For a 18 inch bowl, I will have an 1 1/2 inch or more (making the foot about 6 inches). I believe that a dove tail holds better than straight jaws. If your jaws are straight, with a small dove tail on the end (which is how the SN jaws are, I think) you only want that much depth. You do not want the tenon or recess to bottom out on the face of the chuck. For tightening, you don't need to tighten it as much as you possibly can. Over tightening can crush fibers, and blow out a recess. If you use a tenon, especially if you are using smaller jaws on a larger tenon, orient the jaws so they are 45 degrees to the grain orientation. Try to avoid having 2 jaws on the end grain, and 2 on the side grain. The end grain won't compress nearly as much as the side grain, and the hold will good on 2 jaws, but not on the others. Do use both of the keyways on your chuck, and snug it up a couple of times in each key hole. This is an old machinest's trick. If you get it as tight as possible, if you have a catch, the bowl will fail at the point of the most stress, which is often the attachment point. I have never split a bowl in half using the expansion method. I have split out a few shoulders from over tightening, and from there not being enougn wood to support the pressure, or not enough depth on the recess (on bigger bowls, 3/16 or so, on others 1/8 or so, 1/16 is too shallow). I have broken a couple of pinion gears on my chucks, and one of the main gears, but that was on one chuck, and I never did figure out why. I will admit to being one who believed that 'when all else fails it is nice to be able to rely on brute strength', but I have found that it isn't necessary here. For cutting a recess, I use a dove tailed scraper that is available in the catelogues, or you can take a small square scraper and grind it to fit. The angle on my skew chisel was too blunt to match the angle properly. I can usually get a good enough of a cut that I can sand it smooth with 220 on an angle drill. You can also get a small spindle gouge to go down the side and across the bottom to get it a little cleaner.
robo hippy

[ROY DICK]

I agree with what reed said.

[Fred Oliver]

Great info, Reed. Hadn't thought about tightening both keyholes, although I do that on the drill press.

Also hadn't thought about grain orientation in the jaws, but that makes sense, too.

I''m sure I've been overtightening the jaws, which has contributed to the indentations around the tenon or recess. Hopefully as I get better with the gouges, i.e. fewer catches, lighter cuts, I'll need less pressure and can use smaller/shallower recesses and tenons. I'm seeing that already after just 50-60 bowls. (Got my DVR mid-February.)

Sometimes I even remember to measure the recess depth before chucking and subtract that amount when approaching the inside bowl bottom.

I've been starting the tenons and recesses with a diamond parting tool, then cleaning up with a 3/16" shallow gouge. Also using the 3/16 gouge to clean up the indentations and black marks from the chuck jaws - mounted in the Cole jaws - although that's problematic with green bowls, as by that time they're eccentric.

Reed, what do you mean by this: "You do not want the tenon or recess to bottom out on the face of the chuck." I thought the faces of the jaws were supposed to rest against the shoulder of the tenon or bottom of the recess for additional support.

Good idea, Richard, about marking the jaw location in case there's need to re-chuck.

Does everyone agree with the Teknatool literature, which seems to indicate the expansion mode is a stronger grip?

Thanks, again, to all for your helpful responses.

Fred

[Reed Gray]

Fred,
By saying that you don't want the bowl to bottom out on the face of the chuck, I mean that you don't want the recess or tenon to be so deep that the recess or tenon rests on the face, or flat part of your chuck. The jaws do the holding, and if part of the bowl is resting on the chuck, the fit is never exact, and there will always be a little bit of play/slop in the fit. It is also a waste of wood, as you don't need more than 1/4 inch depth of a tenon or recess for a good grip.

As far as expansion or a recess getting a better grip, if the fit is good, I think they have the same holding power. I do feel that dove tail jaws hold better than straight jaws.

robo hippy

[Richard Madison]

Have not read the Tech. literature, and perhaps they have done some credible testing, but must disagree that expansion hold is stronger. With expansion you get four line contacts at the centers of the jaws, with little distortion of the wood, so the hold relies almost entirely upon friction. With contraction onto a tenon you get eight lines of contact at the corners of the jaws, and usually some indentation of the wood. So you have twice as many lines of contact plus eight points where the jaws are mechanically interlocked with the wood. I have no scientific test data to support this opinion, but it makes sense to me.

Edit: Agree with Reed re. dovetail jaws, as they tend to increase the contact pressure between the work and ends of the jaws.

[Skip Spaulding]

Got to go with Reed, I have a SN2 and had some problems until I took some advice from this site and ground a dovetail chisel to match the angle of my chuck jaws. My wife hardly ever hollers down the stairs " ARE YOU STILL ALIVE ?!", any more.

[Ralph Lindberg]

As usuall Robo (Reed) gives such GOOD advice