Wood warping after milling

[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.