Plank camber
O.K. you guys are heading out to scream around up north, and I'm just getting around to gluing up my new plank.
I'm following the procedure written by Paul Goodwin on the DN BBS, I have the ash and clear pine slats cut planed, joined and glued to form the skins and core, and I have a good staight I-beam to clamp them to, but I don't feel I have a good handle on the amount of camber, expected spring back, and amount (if any) gull wing.
Anyone got 2 cents?
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Final results
Thank you for your input!
I ended up with 2 1/8" camber (slightly less than target)
So that was 5/8" springback
After clean up and routing the edges I have 1 3/4" deflection with my weight (right on target).
My alignment triangle is at a friends to be welded,. Throw on some finish, mount the chocks, shim the runners...maybe with luck I'll be ready by the weekend.
My guess..
I made an Excel spreadsheet to calculate plank deflections a few years ago. Paul Goodwin posted it somewhere but I don't know where anymore.
Playing around with it, assuming your plank is 7.5" wide, it looks like you should see a spring rate of 110 lbf/in so you would get 1.65" deflection with your 180 lbs . Spring back out of the jig would be about 11%. All this assumes the plank is rectangular. Any rounding of the laminations edges will soften it.
John Bushey
John Bushey
Plank spreadhseet
The plank spreadsheet is posted along with an article Paul wrote on the IDNIYRA.org.
There's also a good discussion in the old DN bulletin board archives.
There is also a spring-back formula in the WEST-System "Shop Tips" section.
Cheers,
Geoff S.
Particulars
5/16" ash skins
1/2" clear pine core
180 lbs skipper
camber
I've got this drawn up (on another computer). Will send Monday.
numbers
I have a pdf that shows where to set the blocks, how far to spring it, etc., etc., but I'm too lame to post images.
I can forward if I had your email. Have confidence that the numbers come from someone else -- a certain gold fleet member who has, let's just say, won his share of just about everything.
If you don't want to post your email, then here it is in narrative:
Screw two blocks 60" apart on a table that's about 8.5 feet long. The blocks should be 8" long, 1" wide and 2" tall. These are the two points that you'll bend the pieces over to make the crown.
Cut two more blocks of similar height and 8" long and place on the table where the two ends of the plank will touch the table when pushed down. These are put in place so you have room to get your clamps under the plank. Glue it up using best procedures for clamping, gluing, etc, but don't tighten everything too tight . .. yet.
Now, clamp both ends down to the table and don't forget the spacers.
Next, using even pressure, pull the center of the plank down toward the table. The center should be 2 3/4" off the table PLUS the height of the spacer blocks you put on each end. (BASED ON 185# skipper) Use a level to be sure you're level front-to-back. Now tighten all the clamps and then check for level one more time. Be sure to check the level at the ends as well as the center.
Every piece of wood is different so the spring rate will be different each time. You can tune the plank to the bend you like with a plane and sander.
I'd like to see that pdf.
Would you mind emailing it to me at jabushey (at) mac.com?
Thanks
John Bushey
John Bushey
Ditto for me:
Ditto for me: us5156@geoff-s.net
Plank Spring Back
Most of the 3 layered planks I've built with 2 1/2" camber seem to spring back about 1/2-5/8". The less camber you start with the less spring back you'll get. The last plank I built I was looking for a fairly stiff plank with about 1 5/8" of final camber and it only sprung back about 3/8''. No two pieces of wood are the same so don't expect a finite result.
BOB