[SteveS]

I started with 3/8" hard maple that had been resawn at the mill. Used a 1/8" upcut tool to cut 1/4" deep figure in it on one side. Simple 2D. Then cut around the perimeter of the figure.

It has cupped, curled or warped (choose the term) so that the part is no longer flat. This happened after setting overnight.

The inside of the warp is the side that had the machining done. Over a 9" part there is 3/8" warp if I hold the opposite end down.

One idea I have is to wet the other side and use clamp to hold it flat. Don't know if that will be permanent or not.

Anyone run into this before? Any tricks or suggestions on flattening it out?

[machineman66]

usually if you soak the wood really good and can clamp it flat to something until dry, it will flatten out fairly good. Maple is pretty stubborn (closed grain) so it may take a time or two. Try using hot water also. GOOD LUCK!

[stevespo]

Sticker it so that air can circulate and let it re-acclimate. It may settle back on it's own. If you leave it flat on a bench or table, the exposed side will gain/lose moisture much more quickly than the covered side. Wetting/clamping may force it flat, but it will probably continue to move after that water evaporates.

If the wood was flat when you brought it home from the mill, it doesn't sound like case hardening or a serious moisture imbalance. I like to make sure my material has time to equilibrate to the shop before I resaw or start milling it. A moisture meter is helpful.

Steve

[ger21]

You need to let it acclimate to your shop conditions for at least week or two before using it. The internal moisture content is different than your shop conditions. Ideally, you'd mill the wood a little thicker than your final thickness, then let it acclimate a few weeks, then mill to final thickness before using it.

Once warped, correcting it is virtually impossible.

[SteveS]

Gerry/Stevespo - Wood was in my garage for over a month prior to use. It was stacked flat without stickers. Didn't know about stickers until researching the cupping problem.

machineman66 - I'm going to try that this morning.

Additional info: The side that cupped was orbital sanded extensively to remove the fuzz from the upcut router bit. I am going to orbital sand the back to see if the surface stresses of the sides are different due to that.

Also, I use a vacuum table to hold the material and it wasn't cupped to any extent that the table didn't get a very good seal. I remember only a 1/16" or less visible gap at one edge.

Will try to post pictures today if the camera cooperates.

Thanks to all for the quick responses.

Gerry - I appreciate that true flatness is probably not practical, but would like to reduce the cupping by more than half. It is wood.

[ger21]

Is your garage heated? Where do you live? The relative humidity needs to be close to where the finished piece will end up.

It is possible that internal stresses from drying are causing the warping, but it's usually a humidity issue.

[SteveS]

Gerry - I'm in Trenton Mi, Exit 32 on I-75. The garage has heating, but it is not heated 24-7. I preheat about an hour before going out to work and only to about 50-60 degrees. The cupping occured within a couple hours after the parts were left in the garage. It was raining the past couple days.

[ger21]

I work in Wayne, not too far from you. You'd be better off keeping the wood in the house this time of year. Most likely a humidity issue.

[SteveS]

This is amazing. Added more value to the parts this morning by putting polyurethane on them and they are now flat as pancakes. Only took an hour.

Poly was only put on the painted side. Back is still bare.

A part that was not poly'd is still cupped. That part was cut from same board about an hour prior to the ones poly'd. All parts stayed in the same room (garage) the whole time.

Picture from the end has a 1/4" drill under it to show the extent of cupping.

Picture from top has the square set at 9".

[ger21]

If it's water based polyurethane it can definately make it warp back. It may or may not stay flat.

[ViperTX]

Steve,

It looks like it warped against the grain...so, it is warped and not cupped (which would be with the grain).

So, I'm suspecting that the wood came from a limb or a part of the tree that was understress.

Paul

[SteveS]

Paul - The picture may not be the best. Grain is parallel to the length of the cupping.

Update - Gerry read the tea leaves well, the poly was water based. Last night I poly'd the rear and edges of the parts. All was flat when I left them for the night. this morning the cupping was back.

The parts that cupped had the least width when cut in that orientation to the grain. Other parts cut from the same board within hours don't have any cupping. It seems the minimal width let the board release in that area. Next time I cut them (plenty of that board left) it will be with the opposite orientation to the grain if that doesn't reduce my yield too much.

History of the boards; I picked two of them out at the mill as 5/4, straight and 9 inches wide. One being cut right now was 8 feet long. The mill did the resaw to give the reduced thickness. They were not planed or sanded at the mill. Mill uses a huge diameter circular saw for the resawing. I did not plane or sand the wood prior to CNC except to get the length needed. Side of board that was smooth went against the CNC table, rougher side faced up for cutting. After cutting I orbital sanded the cut side until smooth.

[twoartistic]

I deal with this allot. If you can cut the design less deep it will help, I like to use a 60deg sign engraving bit, it gives the same basic design effect while removing the minimum material. I agree changing the grain direction will also help. Stress relief cuts on the back of the piece will also help. We paint the back of a piece with gesso, available from art supply stores. Gesso will shrink a little better than poly, it is not enough for the extent of warping you experienced in your first go round, but in combination with the other tips it may be enough.

The bottom line is this, get a board straight, then rip it, and end up with two crooked boards once in a while. I have yet to remove any substantial amount of material from a portion of one side of a board, and not get some cupping. Some woods like African Teak have so much internal stress that I have had panels literally fly apart on the CNC table while cutting.

Here are some doors that I carved, the relief is about 3/4" deep at the deepest point. The design is dished to keep the edges thick, and the design on the back has relief cuts to minimize the cupping. We still had major cupping issues and had to make the grooves in the rails and stiles curved to accept the panels. This is on 1 3/4 inch thick panels that are 4/4 glued face to face to help stabilize the material.

[Me2]

This happens to me quite a lot on the patterns I manufacture. If its not critical I route a slot in the back and screw in a length of flat steel bar.
Keeps them nice and flat no matter how they are stored or the temperature differences.

Cam

[Fiddlemaker5224]

I have had this happen many times over the years.

The problem is in the moisture content of the material. the more material you remove the more area is exposed for moisture evaperation. This causes the uneven contraction resulting in the cupping.

As was decribed before try to keep the material in a controled enviroment while machining.

When cupping occurs you can wet the opposite side with a damp cloth and place under a Heat lamp or set it in the sun and have a cup of coffee while you watch it strighten out.

The heat lamp and the sun accelerates the outgasing of the moisture causing the wood grain (pulp wood) to contract.

When the board is nearly flat clamp it to a flat surface and allow it to dry naturaly in a controled temp and humidity.

You can prove what I am explaining by finding a warped board weting the surface opposit the cupping and set it in the sun. If you let it set in the sun to long it will cup in the opposite direction.

It is amazing how much wood will move as it dries out.

[multiplex]

i realize this is an old thread, but there is alot of good info in here and i had a followup question i hope someone can help out with.

I'm running into a similar problem. 11"x11"x1.5" piece of walnut, glued from two 5.5" wide boards. i have about 60 of these pieces. they are all together inside a carboard box right now, and are all nice and flat. I've been keeping them together thinking that is better for moisture reaction.

i keep these in my house, and take a piece out to the shop as needed. within an hour its back in the house after machining.

i get some slightly cupping on the boards. some worse than others. my questions are:

storage, should they be in a box? or stickered?

should they be stored where i'm going to machine? even though its not the conditions of final usage.

in hindsight - i wish i could rough machine them, let them sit, and then finish them. always something

[ger21]

Ideally, I'd store them in the same environment they'll end up in. Stickering is probably best, but with 1.5" thick, a lot of time is needed for acclimation to humidity changes.

There are possibly two reasons for the cupping.
One is that internal stresses can cause it when material is removed.

The other is that by removing the wood allow the inside of the board to dry out faster, causing the cupping due to shrinkage.

[multiplex]

thanks gerry, just the person i wanted to reply

i had another thought too - right after i cut, if i sprayed the cutout with lacquer to seal the wood do you think that would offer any help?

we did a batch of parts like these over the summer and had good success. but every batch of wood is slightly different. its always a battle. sometimes i prefer man made products!

[ger21]

right after i cut, if i sprayed the cutout with lacquer to seal the wood do you think that would offer any help?

It may, but it'll probably just delay the movement. Try it and see if it works for you.

[zool]

The problem you are experiencing is from the depth of the cuts on the face, while there are no cuts on the back. The cuts on the front of the piece are relieving the stress on the face of the piece, while there is no stress relief on that back.

Go look at door casing, door trim or baseboard. You will note that on the back there has been material removed; on baseboard it is about 3/16" deep and about 2' wide on a 2 1/4" to 3" wide trim.

Try this: After making the design on the front, flip the piece over and route two concentric circles on the back. Make one as large as possible, and the one inside it about half-way between the larger circle and the center of the circles. Make the cuts on the back the same depth as those on the front.