take the warp out of plywood - Woodworking Talk - Woodworkers Forum
 
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post #1 of 16 Old 09-26-2009, 10:06 PM Thread Starter
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take the warp out of plywood

Can I take the warp out of a piece of plywood that is 18 X 56. I was thinking of cutting a kirf the width and inserting pieces of wood slightly bigger than the kirf cut spaced about 3 inches apart. The plywood is bowed about 1/4 " over the 56 inches. I was planning on cutting the kirf all the wat through to the top ply aprox 1/8".
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post #2 of 16 Old 09-26-2009, 10:51 PM
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Exactly what are you making, how thick is the plywood and how hard is it to press flat?

The reason I ask is that it may not be too much of a problem to 'pull' flat against a frame such as a table apron.

Anyway, you might try wetting down the concave side with hot water, a LOT of hot water, then flip the board over and add weights to the bulge of the convex side. This air movement will help shrink the upper convex side and swell the bottom concave side to flatten out the board. It should work in theory but it never worked for me. If it is an important piece of a project, I would unquestionably use a different piece and use this piece latter on down the line when you need small pieces. I learned a long time ago that trying to use something 'inferior' to save money will cost you a lot more in the long run.

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post #3 of 16 Old 09-27-2009, 12:08 AM Thread Starter
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They will be use for the front doors of a cabinet which are 3/4" thick.

Last edited by Bob Willing; 09-27-2009 at 12:10 AM.
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post #4 of 16 Old 09-27-2009, 05:25 AM
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If the cabinets were for my shop, I would give it the hot water treatment first and see what happens. If the Cabinets were for the inside of my home, I would use a different board and not take the chance.

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post #5 of 16 Old 09-27-2009, 06:22 AM
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What I'd like to know is

What kind of plywood? Doug Fir, interior/exterior, Birch or oak hardwood, from Home Depot, Lowes, Lumber yard, plywood distributer, etc. Some of the "cheap" stuff is warped and that's just the way it is. If it's not a critical piece of cabinetry, I like your idea of saw kerfs, but I'd make 3 parallel saw kerfs, just with an 1/8" of the good side, 4 1/2" apart down the length to relieve the stresses, then insert thin strips that fit in the kerfs easily without the glue, but snugly with the glue and then weight the whole piece down and see what happens. It's a simple fix if it works. Of course on the backside of the piece, and use a good glue or epoxy, let them be proud just a tad and plane and sand down flush. They will probably never show running with the grain as suggested, if done with a similiar wood. bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #6 of 16 Old 09-27-2009, 06:37 AM
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Some plywoods may have been fabricated that induces a deformity. Others may have been stored in a fashion that creates a bend. But, a 1/4" for that length isn't all that much.

You could also try just "overbending" it the other way, but do it dry. You would support the ends up and apply weights in the middle, and just try it over a period of time.

Wetting the plywood to bend may create more problems, as the depth of penetration may not activate a "stress relief". Some conditions are more involved than just expansion and contraction of concave/convex sides.






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post #7 of 16 Old 09-27-2009, 07:03 AM
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+1 for TonyB and Cabinetman.

G
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post #8 of 16 Old 09-27-2009, 09:05 AM Thread Starter
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At the moment I am trying cabinetmans suggestion I'll see what happens. Another thought is how to prevent this in the future. When I purchased the plywood (3 sheets of birch) they all appeared true and straight it wasn't until I had cut all of the pieces that I noticed the bowing. This was over the time period of about three days. I stored all of them up right and I did not lay one on top of the other. I purchased them from Menards and not HD because I compared the quality beforehand. I also looked at the same grade at one of the local yards and it compared to Menards. In fact the veneer was thinner and the price was 20% more. I bought the grade that had the thickest veneer and best core.

What should I look for if I purchase another sheet to do it all over again? I was thinking about using an aspen glued up pannels. I am making a cabinet for a friend who is going to just paint the cabinet when they use it rather than stain.

Don't forget I live in the UP of Michigan and the big boxes are no closer than 38 miles away for HD and 78 miles for Menards and Lowes.
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post #9 of 16 Old 09-27-2009, 11:58 AM
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I'm in agreement with C-man.
I had a similar problem with some oak ply that I bought from my hardwood supplier (NOT HD) and just propped up the ends slightly higher than the amount of warp, and let it sit. No weights, just gravity. 3 days and it was flat, at least flat enough to not have measurable warp.

Do like you always do,,,, get what you always get!!
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post #10 of 16 Old 09-27-2009, 11:44 PM
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Gotta Love ya George

Quote:
Originally Posted by GeorgeC View Post
+1 for TonyB and Cabinetman.G
TonyB says wet it down. Cabinetman says don't wet it, they both get +1.... on a scale from 1 to 10?
Which solution do you really like? Neither? bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #11 of 16 Old 09-28-2009, 12:10 AM Thread Starter
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Quote:
Originally Posted by woodnthings View Post
What kind of plywood? Doug Fir, interior/exterior, Birch or oak hardwood, from Home Depot, Lowes, Lumber yard, plywood distributer, etc. Some of the "cheap" stuff is warped and that's just the way it is. If it's not a critical piece of cabinetry, I like your idea of saw kerfs, but I'd make 3 parallel saw kerfs, just with an 1/8" of the good side, 4 1/2" apart down the length to relieve the stresses, then insert thin strips that fit in the kerfs easily without the glue, but snugly with the glue and then weight the whole piece down and see what happens. It's a simple fix if it works. Of course on the backside of the piece, and use a good glue or epoxy, let them be proud just a tad and plane and sand down flush. They will probably never show running with the grain as suggested, if done with a similiar wood. bill

"OK" why would you run the kirfs the length rather than the width? The bow is realitive to the length like a bow as in archery. If the suspending of the sheet to see if it does straighten does not work I will try my original solution. It is worth a try because I will probably end up getting another sheet of plywood. Has anyone actually tried this?

It seems to me that the manufacture has not let the plywood dry sufficiently in the first place.
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post #12 of 16 Old 09-28-2009, 06:07 AM
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Warp = bow

Without knowing which condition you had I based my response on the one I was most familiar with which is twist, rather than bow. Twist is what I run into most often in plywood from my lumber yard.
And no, I've never used the procedure myself, but I thought your solution would work OK for twist. If over stressing the fibers by weighing it down will correct the bow, man that's your answer! Good luck with it. bill


Wood warping is a deviation from flatness as a result of stresses and shrinkage from the uneven drying of lumber.


The types of wood warping include:
  • bow : a warp along the length of the face
  • crook: a warp along the length of the edge
  • kink: a localized crook, often due to a knot
  • cup: a warp across the width of the face, in which the edges are higher or lower than the center
  • twist: a distortion in which the two ends do not lie on the same plane

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 09-28-2009 at 06:09 AM.
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post #13 of 16 Old 09-28-2009, 02:41 PM
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Quote:
Originally Posted by woodnthings View Post
Wood warping is a deviation from flatness as a result of stresses and shrinkage from the uneven drying of lumber.

Wow, what a revelation! All this time I thought it was the wrath of Mother Nature.






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post #14 of 16 Old 09-28-2009, 02:57 PM
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We can always learn something

even the best of us, C-man .....heh...heh
BTW that definition came from Wikipedia, not me, I just passed it along FYI. "Warp" covers a lot of situations and even a few minds. bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #15 of 16 Old 09-28-2009, 03:09 PM
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1/4" over 56" ????? I don't think I've even seen a piece that flat ! Heck, I could straighten that out just by setting my coffee on it. What are you worried about?

Bud

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post #16 of 16 Old 09-28-2009, 08:02 PM Thread Starter
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Quote:
Originally Posted by TS3660 View Post
1/4" over 56" ????? I don't think I've even seen a piece that flat ! Heck, I could straighten that out just by setting my coffee on it. What are you worried about?


These are the two front doors on a cabinet and when I go to put the hines on the "Bow" will really stand out. It will have a "warped" or "Bowed" what the heck personality.
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