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Grain orientation for expansion chucking

Jamie Straw

Jamie Straw

Joined
May 28, 2015
Messages
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Location
Bainbridge Island, WA
Well, hello everyone! I've missed the forum, just really getting back into the swing of things after my first 2 months on BOD, yikes. Am making a platter today, the first one unsupervised after the Tom Wirsing workshop last fall. For compression chucking, I know how to orient the grain of the wood to get the most even compression. For this expansion compression, though, it seems I should try to reduce the chance of blow-out in a 3/16" deep recession. Could use some guidance -- piece is maple, not soft, has a bit of figure, but some not-so-close grain also. Yes, I will use the tailstock as much as possible, but still....

While you're at it, a primer on making a tool for the dovetail profile inside the hole for the chuck? I took an old, cheap skew and fooled with the edge to get something to do the job today, but it's not ideal.

Thanks!!
 
A skew on its side does a nice job of making a doevetail recess.

I rarely use a recess. When I do, I line up the jaws at 45 degrees to the grain but I don't think grain orientation of the chuck jaws matters much on a recess. You basically have a circle inside a circle.
 
I have used a diamond point tool and a skew on its side works about the same.
If you work the skew from the center towards the outer diameter of the recess you
can get a nice clean dovetail. I like using a recess mounting when turning bowls this
saves some anxiety later on when trying to finish the bottom of the bowl. I have always
had good luck with maple wood for bowls and platters when using a recessed mounting.
 
Jamie, I cover that here:

https://www.youtube.com/results?search_query=mounting+things+on+the+lathe+robo+hippy

I have seen the dovetail tool that Mike shows up above, but prefer the one I use in the video so the angle is perfect. Just couldn't see the other one being as accurate, but maybe with use, it would be. I do prefer chuck jaws to be oriented at 45 degrees to the grain rather than 2 at the end grain, and 2 at side grain. The wider the chuck jaws are closed the better. If you are going over 12 to 14-inch diameter, then 4 inches or so are better.

Hey, coming down to the Oregon Woodturning Symposium???

robo hippy
 
When you all are referring to the jaws being oriented at a particular position, do you mean the space between the jaws or the center of the jaw?

When we grip a tenon with jaws in compression, the pressure on the tenon is greatest at the ends of the individual jaws, or the "space"
When we expand the jaws into a recess, the pressure on the sides is greatest at the center of the individual jaws.

Can someone clarify?
 
If the tenon and/or recess, match almost exactly the size of the chuck jaws, that difference is minimal to non existent. If you have a 3 inch recess cut, and 2 inch diameter jaws, when expanded into the recess only the center of the jaw hits the wood due to the different size circles. Same difference if you have a 3 inch tenon and 2 inch jaws, only the corners of the jaws contact. Hope that makes sense.

robo hippy
 
Dean, your description sounds clear to me. But maybe people sometimes forget to consider where the pressure is being applied. How much difference it makes is hard to say. The only times that I've had a tenon break was due to unrelated causes where a different grip wouldn't have helped.

The ideal situation for either a tenon or mortise would be at the perfect circle diameter because there is uniform contact all the way around.If I have to use a larger diameter tenon then I prefer using Oneway's profiled jaws.
 
robo hippy said:
Hey, coming down to the Oregon Woodturning Symposium???
Click to expand...
Spent my symposium money to visit sis and bro in California -- health problems abound with them, needed to go down and be supportive. Will save up for 2018 though, the big one.

robo hippy said:
I do prefer chuck jaws to be oriented at 45 degrees to the grain rather than 2 at end grain, and 2 at side grain.
Click to expand...
That's what I remember for compression chucking. I'll take a picture of what caused me concern when I was chucking the other night -- grain in relationship to the jaws. Project still in chuck -- got distracted chainsawing some big maple that came my way, that's all I've done for the last 2 days.

robo hippy said:
The wider the chuck jaws are closed the better.
Click to expand...
Does that description match the following for expansion chucking? I've been cutting the recess to fit the jaws when they are almost closed. If, for design appearance, it later needs to be bigger, that's done on final bottom turning.
robo hippy said:
If you have a 3 inch recess cut, and 2 inch diameter jaws, when expanded into the recess only the center of the jaw hits the wood due to the different size circles.
Click to expand...
 
In expansion mode, if the recess is very much larger than the jaws, you have only four points of contact (in the middle of the jaws), and they rely only on friction for traction, i.e. small round against large round internally. Not a good situation.

Contraction mode is more forgiving. If the tenon is "too large," you still have eight points of contact (at the corners of the jaws), and substantial traction. This applies even if the tenon is very large, because the larger radius provides a longer lever arm against torque. That's the only reason I can think of for having the large adjustment range of scroll chucks.

FWIW, I'm a structural engineer.
 
Jamie, see you in Portland then... Maybe some time if you are passing through you can stop on by. I am building house and shop, should be done next year about this time... I will have more room then.

robo hippy
 
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