Thickness Guidelines With Respect To Cupping

[Raphael Weil]

Hi guys,

I'm a novice woodworker mostly creating panels for art pieces. I mill them at a shop, and I primarily use cherry and maple. The topic of using 12" wide 4/4 came up at the shop (as it's a fairly rare width for a single board), and I was told I'd have a lot of trouble combating cupping in my art pieces. My panels will primarily by 12" by 15" and anywhere between 3/4" thick to 1" thick. I finish all sides with shellac or a poly right now. If these pieces are primarily going to be kept relatively free from extreme humidity, just how likely am I to see cupping with those dimensions?

I suppose question 2 would be back bracing them if in your experience it's likely. One of the more knowledgeable guys at the shop suggested maybe using 1/4" thick brace along the back, screwed in with oversized holes. This would give the wood the ability to move along that axis while still preventing the cup. Is that overkill, or should I plan for this?

I appreciate the advice as it will have a significant impact on my planning.

[Andrew Pitonyak]

You will always have movement. The wider the board the greater the movement, including cupping, twisting, and similar.

I have used large wide boards with no particular issues; for example, a very large shelf about 1" thick. Of course, the board was flat when I started, I finished it. Over the years it has not moved much. Obviously there are issues in general or it would not be common to use bread-board ends when building a wide table. Certainly supports from underneath would help limit / mitigate the tendency to warp.

A big danger is that you sell these to people who do not understand how the process and you do not know their environment. The works may end up in an environment that has hot sun shining on half the product while the other half is in the shade (or some other environment that would encourage wood movement).

[Andrew Hughes]

I agree with Andrew never know where your finished art will be displayed.So cupping down the road is out of your hands.
The more wood you handle the better you will get at picking the well behaved boards.And if your lucky the ones you like the most will have some figure or interesting inclusions.
Wood is both mysterious and amazing.

[Raphael Weil]

Thanks guys.

Do you think the bracing idea I posted could help mitigate this? All told its maybe only 3 minutes more of work per piece and a few dollars more. But if it won't actually help it's time and money wasted.

[Rod Sheridan]

Ralph, using quarter sawn material would reduce the cupping, or eliminate it completely..............Regards, Rod.

[Keith Hankins]

Ok this topic has a lot of variables. The type of wood, how it was milled, and dried, and stability of humidity in the areas it will be.

First, since movement is laterally across the wood not long grain side the biggest factor IMO is how its milled and process for getting it to final dimensions. I work mostly with cherry and other hardwoods, with limited experience with softwoods i.e. pine.

Where twist, or cupping is a concern, I always go with Quarter sawn or Rift sawn. This will have the least possibility of cupping. Flat sawn is the worst as moisture escapes or enters the wood will move against growth rings. This is why with door rails and stiles you see QS is the first choice and Rift sawn would be a second choice.

You can rip larger boards in to smaller ones, and that will help too. Some recommend that if you use flat sawn alternating the rings (up/down) will help, but I have seen recent research published that that is not necessarily true, so that trick is iffy at best.

Bracing the backs can help some, but I found if's she's going cup that will only minimize it. Bread board ends will help a lot but requires more work. It really depends on how its to look.

I know that QS is the most expensive and I found that if you buy wide boards (which you pay a bit more for) you will find that on each side of those wide boards have a bit of QS on the edges. When I make Rail & Stile I will take a wide board take the edges of both sides of the flat sawn board, and use the middle somewhere else that does not require it. If you are not familiar with the diff types of cuts, I included a link that expains it.

http://www.hardwooddistributors.org/blog/postings/what-is-the-difference-between-quarter-sawn-rift-sawn-and-plain-sawn-lumber/

Another factor is air dried .vs. kiln dried. I prefer kiln but Air dried can work if done properly and over a long time. Of course if the kiln operator does not know what they are doing that will not be true.
Anyway sorry to throw a buch at ya, but it's not a simple math formula gets the perfect answer. Wood is a hard medium to work with as its constantly moving but I've loved working with it the past 30+ years.

Finally high humidity or low humidity is not an issue, but if your environment swings in humidity that will be a problem, as wood takes in or expels moisture to reach equilibrium with its environment.

good luck and post some pic's of your pieces we love to see others work. Good luck and cheers.

[Andrew Pitonyak]

Thanks guys.

Do you think the bracing idea I posted could help mitigate this? All told its maybe only 3 minutes more of work per piece and a few dollars more. But if it won't actually help it's time and money wasted.

Yes, I think that it will help. I expect that you would want to brace them across the grain rather than with the grain, but others will likely have a better opinion on that. My gut reaction would be to place one near each end across the grain.

[roger wiegand]

Bracing can help, using quarter sawn wood would be best. How tolerant of joints is your application? You aren't going to find many 12" wide quarter sawn boards.

OTOH, are people going to be laying straightedges across your pieces to check for flatness? All of the real wood around us in all of our furniture is moving all the time and for the most pard we never notice if the builder has paid even the slightest attention to materials and construction methods.

Another approach to reduce but not eliminate cupping is to use a plywood or MDF core and veneer both sides.

[Raphael Weil]

Ralph, using quarter sawn material would reduce the cupping, or eliminate it completely..............Regards, Rod.

Sadly with how hard it is to get 12" wide maple and cherry here, I doubt I'll have my pick.


Bracing can help, using quarter sawn wood would be best. How tolerant of joints is your application? You aren't going to find many 12" wide quarter sawn boards.

OTOH, are people going to be laying straightedges across your pieces to check for flatness? All of the real wood around us in all of our furniture is moving all the time and for the most pard we never notice if the builder has paid even the slightest attention to materials and construction methods.

Another approach to reduce but not eliminate cupping is to use a plywood or MDF core and veneer both sides.

These are literally just art frames. I will be producing art on top, and I suspect some may hang on walls, but many will sit on mantles given the weight of the hard woods. I don't think a single person will check for flatness. I just don't want the art to warp excessively with time. And given how fine some of the detail is, that's a distinct possibility.

Truthfully I guess the best test is to just try and see how much I can warp them, and what different braces might do. Would any of you pick an aluminum brace over wood brace?

Thanks for all the replies btw, it's incredibly appreciated.

[Raphael Weil]

Thanks Keith. By looking at my boards, it looks like flat sawn, kiln dried cherry and maple. I guess my first priority will be to attempt to order quarter-sawn where possible. I'll post some pictures soon :)

[Lee Schierer]

There really are no hard and fast rules. It Depends is the best answer any of us can give.

I personally have made panels as thin as 1/4" that have stayed flat. Where trouble generally occurs is when you try to make two 1/4" panels from a 3/4" piece of material by resawing. In any piece of wood that has been sitting around the shop, the outside of the piece is at a slightly different moisture content than the center of that piece. When you expose more of the center on one side of a piece than the other by resawing or planing you have a moisture differential and this creates stresses in the wood as it equalizes. Those stresses cause the problems. If you change the thickness of a piece, and want less problems, remove equal amounts of material off each surface.

A trick to help thin panels not to cup is to never leave them laying on a flat surface or stacked together without stickering and when you put on the finish apply equal amounts of finish on both sides.

[Larry Copas]

I've tracked moisture content of wood furniture/cabinets for years in our house. Our house is located in Arkansas with hot humid summers. The highest the MC has ever gotten is in the 9-10% range. The lowest it ever gets is in the 6-7% range, depending on how much I run the wood stove. That's really not much of a change. I have a sawmill and sawed all the wood for our cabinets. All of the raised panels are one piece of flat sawed lumber, with some up to 14" wide. I've never had a problem with cupping and I think its due to the small MC swing. Of course my panels are in a frame so it would be hard to see if they cup a very small amount during the year.

[John T Barker]

I didn't read all responses here but of what I read there was no mention of two important factors. First, what causes cupping? One of the main causes is the cells of the stock are dryer on one side than the other. This happens when more finish is applied to one side than the other. Example: your art panels have less finish applied to the back than the front, the front is then exposed to more light, these things combine to dry the front and the panel will cup toward the the front. Good furniture construction uses methods that help prevent this, example: panelled doors. The panel floats freely in a frame and is finished on both sides so the panel should remain very close to flat if not completely flat. Is there a reason you can not do it this way (in a strong frame?) Actually if this method is a possibility than using a plywood panel would be possible? Plywood is not warp resistant by any means but a 1/4" panel in a sturdy frame, if finished on both sides, should not cup.

Quartersawn is a good idea for stability but if the finishing is not controlling the moisture you may still have problems.

Is there a reason these panels have to be one board (and where do you live that it is hard to find 12" wide boards?) Isn't gluing up these panels okay? Are they covered by paint?

Last thing, I don't want to sound like an a**hole here but I don't see how 1/4" would do anything. My 9 y o daughter can bend 1/4" stock with no problem, the hydrostatic pressure in wood will bend it with no problem whatsoever. I see furniture companies shipping their table tops with 2" bracing screwed to the bottom and the tops still flex. Keep this in mind when taking advice in that shop. (Sorry.)

[Raphael Weil]

I didn't read all responses here but of what I read there was no mention of two important factors. First, what causes cupping? One of the main causes is the cells of the stock are dryer on one side than the other. This happens when more finish is applied to one side than the other. Example: your art panels have less finish applied to the back than the front, the front is then exposed to more light, these things combine to dry the front and the panel will cup toward the the front. Good furniture construction uses methods that help prevent this, example: panelled doors. The panel floats freely in a frame and is finished on both sides so the panel should remain very close to flat if not completely flat. Is there a reason you can not do it this way (in a strong frame?) Actually if this method is a possibility than using a plywood panel would be possible? Plywood is not warp resistant by any means but a 1/4" panel in a sturdy frame, if finished on both sides, should not cup.

Quartersawn is a good idea for stability but if the finishing is not controlling the moisture you may still have problems.

Is there a reason these panels have to be one board (and where do you live that it is hard to find 12" wide boards?) Isn't gluing up these panels okay? Are they covered by paint?

Last thing, I don't want to sound like an a**hole here but I don't see how 1/4" would do anything. My 9 y o daughter can bend 1/4" stock with no problem, the hydrostatic pressure in wood will bend it with no problem whatsoever. I see furniture companies shipping their table tops with 2" bracing screwed to the bottom and the tops still flex. Keep this in mind when taking advice in that shop. (Sorry.)

Thanks for the advice, I'll make sure my back is finished to the same degree as the front (I use shellac right now). I definitely can't use plywood for art panels that I engrave. Has to be hardwood. I'm in Ontario (Canada), and it's hard to get 12" wide boards in cherry and maple. I'm making artboard panels in bulk, gluing them up would kill my margins. They are finished with shellac and then engraved with a laser. This is pretty representative of what it will look like:

323969

[Robert Engel]

Ralph, using quarter sawn material would reduce the cupping, or eliminate it completely..............Regards, Rod.
I agree QS would be the best way to minimize.
He would have to do panel glue ups.

[Robert Engel]

On the bracing, yes I think you could do it.
I'm thinking route 3 - 1/2" deep dadoes across middle and 2" from each end.
Embed a piece of very strong wood like hickory.
One screw in middle, one on each end NO GLUE.
The screws on the ends need to be elongated to allow for expansion/contraction.

Another way to do this is to glue thinner pieces (1/4" thick or whatever allows for your carving depth) down to a substrate like plywood and apply an edge.

[John T Barker]

Thanks for the advice, I'll make sure my back is finished to the same degree as the front (I use shellac right now). I definitely can't use plywood for art panels that I engrave. Has to be hardwood. I'm in Ontario (Canada), and it's hard to get 12" wide boards in cherry and maple. I'm making artboard panels in bulk, gluing them up would kill my margins. They are finished with shellac and then engraved with a laser. This is pretty representative of what it will look like:

323969
You said finished with shellac and then engraved with the laser. You are engraving through the shellac? Best to engrave and then finish again or that will cup big time.

How deep is the engraving? I don't play with plywood much but there might be a ply out their with the top layer thicker than most and could handle the depth of the engraving???

I'd go with the frame around that panel, I'd think it would look very nice...as does that carving. Are these commissions? Doesn't 20 Century Fox care that you are using that image?)

I can't understand why you can't get decent wood there but you can always get it mailed to you from a lumber source...thus cutting in to your profit margin. :( Aren't there big trees in Canada? My sister belongs to a ww club in Ottawa, I can ask for a source if you like.

[Raphael Weil]

You said finished with shellac and then engraved with the laser. You are engraving through the shellac? Best to engrave and then finish again or that will cup big time.

How deep is the engraving? I don't play with plywood much but there might be a ply out their with the top layer thicker than most and could handle the depth of the engraving???

I'd go with the frame around that panel, I'd think it would look very nice...as does that carving. Are these commissions? Doesn't 20 Century Fox care that you are using that image?)

I can't understand why you can't get decent wood there but you can always get it mailed to you from a lumber source...thus cutting in to your profit margin. :( Aren't there big trees in Canada? My sister belongs to a ww club in Ottawa, I can ask for a source if you like.

ya you raised an excellent point, I may need to spray a clear finish on at the end to manage the part that got engraved. My brain didn't even think of that.

As for an Ottawa source, that would be amazing. I'm currently trying to do this through a place called the Wood Source. I guess in theory plywood could work. I just don't want it to look cheap is all.