wrote in message
oups.com...


I can feel a storm brewing G and no I won't duck, people use the
other chucks all the time and there are times and places where a smooth
jaw chuck use is preferably, as are other kinds of chucking and holding
devises, a perfect for all and every use holding devise we do not have,
and who wants to admit that they should have thought more about what
they where going to buy, or that spending a little more for a better
thought out and flexible use product would have been the wiser choice.

What Leo has difficulty comprehending is that the jaws he describes damage
the wood, precisely because the contact area is _less_ than the smooth
jaws.

Strength of the wood is measured as a resistance to a force per unit of
area. Total Force = Force/unit * number of units. Stands to reason that
if you have knife ridges as your primary contact points, there is a
tremendous amount of force on a small area, which is why they penetrate and
deform the fibers. The smooth jaws provide a greater contact area, depress
the resiliant surface rather than sever the fiber, and since the total force
is a multiple, they require less crank on your tightening device. All you
need is snug, not grip.

If you can live with mangled wood fine and dandy. Crank down and chew
things up. But when you look at what you've done afterward, notice that as
the fibers are pushed away by the ridges, they also push you out of register
with the face of the jaws, denying you the resistance to deformation
perpendicular to the axis of rotation which is what prevents a catch from
dismounting the piece. The dovetail draws the wood in toward itself as it
is expanded or contracted, making that contact firm and sure.

People who grab things with serrated jaws are the ones who complain about
having to re-true the outside after reversing, because they have difficulty
getting a full seating. They are the ones who tell you you have to "settle"
for a certain degree of error. People who use smooth dovetails can mount as
true as the shoulder they have made for an outside grip, or the interior
shoulder they have made for an inside grip. They can take the piece off,
and if dissatisfied, re-mount and trim where the design does not please,
because their hold is _still_ circular and undamaged, available for use.
They don't have to try and make the serrations fit into old grooves, or,
after realizing that's not possible, try to get a whole fresh area to
crunch.

Additional advantage to inside dovetails, if you're trying for depth on a
bowl made with the outside of a log as the bottom, is that you need only cut
away enough to allow entry for your expansion jaws. Gives you as much extra
depth as the length of the tenon you would have made, only to turn away
later. With an external dovetail you don't have to trim away ugly
compressed and chewed wood if you can make the groove part of your design.

Take a careful look at
http://s35.photobucket.com/albums/d1...t=bb09ce26.jpg

Look at 5:00 o'clock and you will see the artifact from the interior hold,
as yet unsanded. The artifact of the exterior hold, because this piece was
done as a demo, are around 2:00 o'clock. No vacuum chucks, no cole jaws,
just a quick sandpaper cleanup and then a finish.