Expansion chucking vs. Compression chucking

[Rick Smith]

I've only done "a tenon into my chuck" compression-type mounting of my fun (how can anyone call this "work"?). In searching the threads, I don't see a "comprehensive" conversation on this. Can someone give me an idea as to when I would use a recess and expand the jaws into it, rather than the tenon method? And how deep a recess? Are the smooth jaws on the Super Nova right, or do I buy the ones with the ridges on the outside? I usually turn a tenon that is right at or just slightly less than the depth of my jaws (so the shoulder isn't quite touching).
Thanks in advance!

[Bernie Weishapl]

Rick I use the expansion of my apprentice chuck which has ridgids on the outside of the jaws when doing platters and I want a concave bottom with no foot or tenon when done. I cut the recess about 1/8" or so. I use the tailstock till I get most of the wood removed then move it to finish with light cuts. Haven't lost one yet.

[Jim Becker]

A recess should ideally only be used when there is a lot of material to support the pressure caused by the jaws...platters are good examples as they offer a lot of material surrounding the recess. Most bowls are not good candidates as they generally offer minimal material to support the jaws. A tenon with the required shoulder is usually best for bowls and vessels when it comes to strength. Sometimes you need to make an exception on the latter for artistic reasons or because of limited material...but in the latter case you always have the option of a glue block to extend your blank, provide a tenon and still get maximum usage of your more expensive turning stock.

[Steve Schlumpf]

Rick, I would add one thing to what Jim has stated and that is to cut the tenon so that it is less than the height of your jaws. You want the jaws to seat on the shoulder where the tenon meets the bowl and not bottom out in the chuck.

[George Tokarev]

Disagree with the conventional wisdom. The recess offers greater depth potential, since we normally cut with the heart up. Where we would not have a shoulder and a properly sized (oversized) tenon, we could already have a good dovetailed recess, which does not have to be turned away later.

When the piece has dried, a tenon will end up smaller in diameter to get back to round, a recess will still be nearly the same size. Makes the width of the support moot, and still preserves depth in the piece because of the normal curvature.

What you have to put aside is the idea of "grip," replacing it with the idea of "snug" for support. If things loosen up, and it rarely happens when you snug, shake and snug, don't reef on the key, loosen, reset on the flat interior of the mortise (shoulder if using a dovetail tenon) and resnug. Since you have a smaller force per unit of area, you do not need to distort the wood or exceed its surface strength and dig in.

Couple of things which are different with a mortise. First, you can't just start on a spur center and spindle turn a tenon on the other end. Of course, I think a spur center is the wrong choice anyway. You can start on a faceplate, pin chuck or pin jaws, making sure you're snug after rounding to support the piece as you make the recess. My preference for a pin chuck is based on ease of preparation, which requires just a 1" hole, safety, because the blank will never dismount, and expense, because you have a $70 device which you never have to change jaws on. Better than another chuck body, though one chuck body should have pin jaws permanently installed.

Last, if you like a bit of jazzing, you never have to reverse chuck after hollowing to trim up a mortise, you just do it in advance.

[Reed Gray]

I use a mortice or recess exclusively on my production bowls. I prefer dovetailed jaws as I think they hold better. On the top of the bowl, I drill a recess the proper diameter for my jaws to expand into. I no longer use the faceplates. On the bottom I use a dovetailed scraper to get the proper profile. With minimal sanding it makes a simple but not ornamental finished bottom.

When using the recess, there are a few things to keep in mind;

One is the size of the recess. You want the recess to match the size of your jaws as closly as possible. The more metal there is on the wood, the better your hold is. I have dedicated calipers that are glued in place to measure the recess size. You don't want a 3 inch wide recess for 2 1/2 inch wide jaws.

You also need ample 'meat' or shoulder around the outside of the recess. For bigger bowls, say 14 to 18 inches, I will have up to 2 inches around the recess. You can use less for asthetic purposes, but if you are using a 1/4 inch shoulder on a 6 inch bowl, you can't hog off material.

Depth wise, on bigger bowls, I will go in 3/16 or so. I don't measure, I just do it by eye, and have done it for so long that I just know when it is right. You can use 1/8 inch for smaller things, but I wouldn't even consider less than that. You don't want to bottom out and have the face of the chuck on the wood. The jaws should bottom out in the recess.

Tightening: you can over tighten. I did have a problem with this for a while. Being on the big and strong side, I was way overtightening. You can blow out the sides of the recess by doing this, just as you can crush the sides of a tenon if you tighten too much. If you have a keyed chuck, use both the key holes to tighten. Tighten until it is snug, then go to the next hole and tighten some more. On bigger bowls, I will do this a couple of times in each hole, especially if the wood is wet.

Does it hold as well as a tenon? I think so, and will even say it works better. I can core bowls up to 18 inch diameter, without using the tailstock, with no problems. My thinking on this matter is that I have more wood to hold with. There is more wood on the outside of my 2 1/2 inch recess than there is on the inside of a 4 inch tenon.

A tenon has to be removed, a recess can be worked into the design.

Experiment and see what works for you.

robo hippy

[Jim Becker]

George, starting between centers means you can manipulate the material to find the optimum axis orientation before you commit to a mounting method. Using a faceplate or screw chuck requires you to commit to a top and bottom before you know what's in the wood. But that's off-topic for this particular thread... ;)

[Paul Engle]

I agree with George and Robo Hippy... i use recess 99.9% of the time, never lost a bowl , and loose minimal wood/space to tennon. I like the look of the piece seemingly to float on the surface vs a tennon to hold it up. I use a screw chuck / ts. rough it out, remove the ts cut the recess and flip it , re rough and spiner up to 1600 - 2000 or more to final shape the outside, then start the inside at 1200 and up to 1600 to 2000 ( for final cut) as it smooths out to rough the inside ( up to 10 in bowl) 12 in only run about 1600 to finish cut . even on big green birch bowls ( using the 100 mm jaws) no problems and do not use a tennon , if making a foot I make the shallow foot (1/8 in or less) while still on the screw and clean it up after fliping to the recess , other wise I slighlty concave the area around the recess and make it the " foot ".

[Neal Addy]

What Bernie said.

I prefer a compression grip when possible but expansion is necessary in some situations (like platters).

The depth of the recess is important. You don't want it too deep. About 1/8" to 3/16" is plenty. The key is to make sure the edges of the jaws 'seat' against the bottom of the bowl (while gripping the recess). This lends support to the jaw's grip.

[Kaptan J.W. Meek]

I use the screw chuck to turn the outside of the bowl.. and a small or shallow recess to mount into the chuck. when finished hollowing, I flip it around again, and remove the recess, by turning a slightly concave bottom..
I think it was Rafkin, who said, "The finest peices, show no evidence of how they were ever mounted on the lathe".. I liked that, it just made sense to me..

[George Tokarev]

George, starting between centers means you can manipulate the material to find the optimum axis orientation before you commit to a mounting method. Using a faceplate or screw chuck requires you to commit to a top and bottom before you know what's in the wood. But that's off-topic for this particular thread... ;)

Y'know I hear this a lot. Never had much cause to worry about it, since I evaluate the blank and select my preferred conformation in advance. Those few times, and it's less than two hands, where I called a bark pocket too shallow or such are easily plugged with a 1" dowel and re-bored. Certainly nothing that will ever outweigh the ease and safety advantages, not to mention the easy re-truing after drying.

[Mark Pruitt]

Think about it like this. Is it easier to break a piece of wood apart by applying outward tension (which includes pulling across the grain!), or to crush a piece of wood by applying compression to a tenon? To do the latter, you would have to have either sapwood or punky material in your tenon, both of which can be easily avoided. To do the former, all you have to do is tweak that chuck key "one more little time"....CRACK!

[George Tokarev]

I find it as easy to split off a tenon along an annual ring as anything else. Of course, that's with a catch or high pressure cutting, not holding, which never wants any more pressure than required to hold the piece firm to the nose of the chuck. Since the chuck exerts pressure in four directions equally, there is not a weak spot when snugging which isn't balanced by a pair of strong ones, whether holding inside or out.

If you're cutting ring-porous woods you might consider reinforcing the annual ring structure in the hold area with some water-thin CA either inside the mortise or outside the tenon. Soak, true, and hold.

[Mark Pruitt]

Since the chuck exerts pressure in four directions equally, there is not a weak spot when snugging which isn't balanced by a pair of strong ones, whether holding inside or out.
That's not how it works IME. It's not about direction of tension being balanced but rather about the tension seeking a point of least resistance. What I was saying earlier is that that point is present in every piece of wood and it is more easily exploited by pulling apart than by pressing together.

[Reed Gray]

I don't see how pulling or pushing apart the wood fibers makes any difference, six of one, half dozen of the other. As near as I can tell, it depends on how much wood (size of the tenon, or size of the shoulder of the recess) you are pulling or pushing, not the direction the forces are being applied from.

robo hippy

[George Tokarev]

It does make a difference in long cantilever pieces, because you can put a lot of pressure on the hold by cutting at the far end. There you prefer the external hold for the greater resistance to lateral movement provided by a broad shoulder at right angles to the axis of rotation. I like the wide faces on my 75mm NOVA jaws, or if the piece is large enough to use them, the Power Grip types for this kind of work. For bowls, makes little difference.