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Best Ripping Width to Avoid Cupping

      A discussion of the workable range of widths for ripped lumber intended for gluing up into panels. February 16, 2014

We rip and re-glue boards every day. I have looked online to see if there is a table by species and thickness that gives optimal widths. We use about 60% cottonwood in our business, but we also use oak, pecan, maple and mahogany. Any ideas were to find information like this?

Forum Responses
(Adhesive Forum)
From contributor M:
What grade of lumber do you use? What size panels are you gluing up? Do you rip first or crosscut?

From Professor Gene Wengert, forum technical advisor:
I believe 2-1/2" is a common average rip width, but variation is noted so that waste at the rip saw is not too high.

From contributor M:
If you are ripping on a straight line rip saw, you can rip random width clears up to 4" and glue up panels. Usually works best after crosscut. If you use a gang rip saw, you make rip pockets in multiple configurations to allow placement of the board to get the best yield out of standard rip widths. Some of the optimizing equipment allows you to run a load of lumber through a virtual process that shows the difference in yield when you change the rip width and sequence of pockets on the arbor. Much study has been done on this subject. Check with gang rip manufacturers like Mereen Johnson for more info.

From the original questioner:
Thanks. I should have been more specific. I wanted validation about how wide we could rip and re-glue into panels (oak and PCN) before cup would become a problem. Some years ago, we ripped everything on straight line rip saws. We made an internal decision back then to have a max width of 2.5" wide so that if there was any taper coming off the saws, we could still press the parts when gluing into panels. We now use moveable blade gang rips and taper is not an issue. I would like to rip the parts at 2.9" to start with and ultimately 3.9" wide. I looked for some literature on this subject, but didn't have any luck.

From Professor Gene Wengert, forum technical advisor:
In order for cup to occur, there must be a moisture content change. So, drying correctly to the appropriate final MC is indeed the biggest key to preventing warp. As many shops and interior environments, especially in the heating season, are close to 30% RH, then 6.0% to 7.0% MC is a great final MC. With no MC change, there will be no cup.

Cupping is worse when the MC change is sudden. So, having a piece made with lumber that is too wet and then having the piece suddenly exposed to the drier environment in an office, etc. will cause more cup than the annual variation in MC.

Cup is also more likely for pieces cut from near the center of the tree (usually No. 2 Common) and pieces that are flatsawn.

For pieces near the center, the bark side shrinks more than the heart side of the piece.

So, do you have a moisture meter to check the MC of many pieces to assure proper drying? If not, expect cup no matter what the width or other processing changes. When you encounter a wet piece, can you get it out of the process? A dry kiln may have 10,000 pieces of lumber, but the operator only measures the MC of 10 or 12 pieces. A few wetter pieces, or even more than a few, can get through unknowingly... or maybe knowingly. The kiln must be properly operated to prevent cup. This means no wets and also no over-dried pieces.

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