Forming the recess for the chuck to hold

Yes I am a newfoundlander and we have some very excellent woodturners here in this province.

You're thinking in the wrong direction. You make the recess so that the chuck jaws wedge themselves against the bottom of the recess, not the sides. Woodworking 090 tells you don't try to put a round peg/nail/chuck in a round hole in wood, it'll split.

You want the dovetail in the wood to have at least as acute an angle as the dovetail on the chuck - but since it is unlikely you'll match the angle perfectly, make the dovetail in the wood more acute than that of the chuck. If the point of the chuck's dovetail contacts the wood and the rest of the metal dovetail isn't in firm contact with the wood the wood piece will wobble and come loose. Better to have the point of the metal dovetail not in contact and the metal side and bottom contact the wood further back. That way you have two firm points of contact all the way around and the more you tighten the chuck the more these points of contact come to bear. This gives you a firm grip on the wood.

In the ascii scetch below, imagine the inner part (the chuck) in actual contact with the bottom and you get the idea.

/ / / / / / /________

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Keith

I have found that I get better results when I use a tennon instead of a recess - the chuck can get a bettr bit in the compression mode versus the expansion mode - you may want to try using a tennon and then jam chucking the bowl to finish the bottom

Ray

Thanks George. Wrong direction heh.

Thanks again.

Keith, some more information might be helpful, like the type of wood, its green/dry status, bowl size, and the diameter and depth of the recess. Does the bowl fall off of its own accord, or does it take a catch:)

In my experience the diameter of the recess is more important than the angle of the dovetail. I say that because I do a lot of first fixings into a shallow hole drilled with a forstner bit, so I have no dovetail at all. Remember that the jaws have only one true diameter, that is when they form a true circle. When expamded beyond that diameter they grip only at the center of their arc, and at smaller diameters they grip at the ends of their arc.

One other tip is to make sure the chuck is clean of dust and operates smoothly and easily. If it gets gummed up, a lot of the power you apply to the key is expended overcoming friction, and it is difficult to get any feel for how tight the jaws are. You really shouldn't have to crank too hard on the key to hold a bowl. I periodically blow all the sanding dust out of the chuck and give the threads a little graphite lubricant. The jaws should then operate with just the lightest touch.

For final fixing I make the dovetail with a small detail gouge so the edges are nice and clean. The bottom of the recess is finished with a flat scraper, the finished shape of the recess erring towards concave.

[Posted and mailed]

To make the recess as secure as possible, cut it only just large enough to accept the closed jaws. Under cut the recess to form a wedge shape so the jaws do not slip out. mount on the jaws and open them to tighten against the recess wall. this way you will have almost continuous contact with the wood giving a tighter hold with less stress on any point of the recess, with no jaw marks inside the recess. If you use a larger recess there will be limited contact between the jaws and the recess walls with the risk of the wood flying off - CATCH!!!

Doesn't seem to make a difference if you bottom it. Or at least it doesn't at my house.

Only if you don't bottom it. I assume you are doing so (as I do also) with your first fixing.

The jaws don't need to "grip" anything unless you're using them in a non-dovetail recess. Check the pin jaw setup at the pages I referenced. Even there, the difference is in the bottoming.

SNIP

You really shouldn't have to crank

Follow-up. The pin jaws on the NOVA are circular at 1.125", the maximum depth of dovetail is 3/16. Quick math, verified after getting back from the ambulance garage shows a maximum jaw edge only contact area of .663 sq inches. Jaw faces are slightly greater than .25 broad, offering a contact area of 2.76 sq inches. In reality, reduce each by about 15% for gaps.

More to the point, the mechanical advantage of the .5625 arm resists movement much better than the .1875 at nearly right angles to it, especially when the force of cutting is nearly perpendicular to it when running from bottom to rim or rim to bottom. I would never recommend an edge presented perpendicular to the axis of rotation.

Bottom those jaws, that's where the money is.

With my Nova chuck (although the original author has a compact version), I get MUCH better grip in expansion vs. compression. Don't get me wrong, the compression is great, but in expansion, I get more solid holding with less visible marring of the wood.

I rarely, if ever, make the recess (or a tenon, for that matter) the full depth of the jaw's dovetail. I just make sure that the angle of the recess/tenon is such that the farthest point of the dovetail is what makes contact first, and then things go great. Good enough, in fact, that a year or so ago when I first tried purpleheart and made a rather stupid mistake, I broke the tenon off of the piece before the grip of the chuck gave up.

steve

Without having seen each other's turning methods, it's hard to say. : )

I should say that the difference I get isn't so much because the chuck itself grips better in either mode, but because when I turn in expansion, I usually have better geometry on the wood. Like you, I've had the tenon break before the chuck gave out - but the way I use it in expansion, there's typically more wood behind the grip, so to speak, and I've never had a break that way.

steve

As easy as 1,2,3.

1) The way to low stress turning is to cut the wood as it wishes to be cut. When you do that, the chips fall where they are released from the piece, they do not fly. If not so, make it so.

2) Energy does equal Mass times velocity squared, so turn at slow speeds, making less energy available to stress the recess in the event of a catch. You need only the energy required to sever the fibers, and have instant feedback from the shavings, which should present a clean edge as they are released - long side smooth.

3) Remember that the recess is just a means for wedging the face of the jaws snugly into the bottom of the piece. You need snug, not tight. Tight is the same as forcing a round peg into a round hole - it splits wood. That's why nails are clipped, pegs are rough whittled, or square.

Of course, I do know that long side smooth is for interior, short side smooth for exterior. I just didn't think to say it.

To turn endgrain N.I.Pine, I use a faceplate, waste some wood and complete the piece without reversing. It's academic, but If I did use a scroll chuck on softer timber would it not be better to expand the jaws toward the stronger periphery instead of compressing toward the log's weaker soft center or mushy pith? I'd think that compressing a tenon or expanding into a dovetail would largely depend upon whether the center or the periphery is strongest.

For first holding on/in a dovetail which doesn't allow the blank to reach the chuck body or base of the jaws, what determines the blank's axis; the axis of the dovetail walls or the face that the jaw's forward rims register against? OK to sigh, but don't groan. :)

Turn to Safety, Arch

Fortiter,

Hi there Arch

Arch ,my opinion, if everything's equal, the larger surface has the most strength, that's the outside. The other thing is, give me the oneway chuck jaws, they hold better hands down.

Have fun and take care Leo Van Der Loo

Arch wrote: