Wood is cupping as it is being cut.

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Wood is cupping as it is being cut.

Postby SawdustandSmoke » Sat May 17, 2014 5:12 pm

Hello I am having a very persistent problem, when I cut a 3d model in maple it turns out perfect, However when I am cutting walnut or oak I get cupping, This wood has been planed down before it comes to me. I believe my problem is caused by the amount of material I am removing, This would not normally be a big issue however I am gluing these back to back and it is not uncommon to get 1/8 inch gap or more when I try to force them with clamps It cracks.

I hope some day to get a 4th axis and glue the wood together first and then cut, but I don't have the money for that now.
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Re: Wood is cupping as it is being cut.

Postby 4DThinker » Sat May 17, 2014 6:59 pm

Wood expands and contracts as the moisture content changes. On a thick piece the parts closed to the faces may be drier or wetter than the core as moisture enters through the faces and end, and take longer to get to the core. When the air is drier than the wood the outer part dries faster then the core as the water in the core takes longer to make it to the faces/ends. Planing wood it is best to take material off both sides rather than just one side to leave that center moisture an equal distance from all faces.

But now you are removing material from the top face in your 3D cuts and none from the bottom face. The core moisture can now leave the wood through the top, but the bottom will still be wetter. The top starts shrinking in width, making the wood cup as you've noticed.

Maple is more dense/less porous than walnut or oak. The movement of moisture through it is considerably slower than through open grain woods.

Store your wood with spacers between the boards. Both top and bottom need air across them to dry evenly without cupping. Even the boards that are cupping after you have cut them may return to flat after drying evenly on the bottom side. You may just have to be more patient for this to happen with open grain woods.
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Re: Wood is cupping as it is being cut.

Postby SawdustandSmoke » Sat May 17, 2014 10:04 pm

Ill give that some thought, The wood I am getting is kiln dried, What happens is I load flat wood on the table and when I take it out it is cupped on the backside immediately after.

Can you recommend any non exotic wood that is like maple?

The least likely to cup wood I have found is hard maple followed by walnut and oak with a 50% fail rate, with the worst being hickory 100%
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Re: Wood is cupping as it is being cut.

Postby basecircle » Sun May 18, 2014 12:45 am

Hi sawdust.Let me throw this out there.First how wide and how thick is the wood ? 4d is correct on the moisture content.But if you are using a wide board and thick.Look at the growth rings on the end of the board.Rule of thumb the rings should be up.If your using 2 boards the rings are alternated.This keeps the board flat. As you machine the surface in a rough cut Stress is relieved in the board.(YOU REALLY CAN SEE THE CUPPING AFTER THE ROUGH CUT WHEN YOU REMOVE THE CLAMPS)Then when you finish cut the board and remove the clamps there is the cupping.I would cut and joint the edges like you would as if you were making a table top.I would really like to know the size of the pc.you are cutting. let me know Basecircle
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Re: Wood is cupping as it is being cut.

Postby SawdustandSmoke » Sun May 18, 2014 1:38 pm

Here is a picture of the problem, the large models that I cut are about 5.5 wide and .86 tall the wood is usually 6.5 inches by 1 inch thick. The smaller walnut models at 3.5 inches on a 5 inch or less board have very little warpage.

I usually cut 4 at a time on a 36 inch board with clamps on the ends. Im going to add screws to the center and see if that helps much. Thanks for the advice.
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Re: Wood is cupping as it is being cut.

Postby Gc3 » Sun May 18, 2014 4:29 pm

machine a pocket on backside of blank to help minimize the stress
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Re: Wood is cupping as it is being cut.

Postby scottp55 » Sun May 18, 2014 4:52 pm

Glue before cutting and do 2-sided machining?
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Re: Wood is cupping as it is being cut.

Postby SawdustandSmoke » Mon May 19, 2014 1:26 am

That's an idea. How deep of a pocket do you think I need to make? The model is about .86 thick

I have tried to do two sided machining before and not had very much luck with it most of the problem I have is getting the depth right
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Re: Wood is cupping as it is being cut.

Postby FixitMike » Mon May 19, 2014 6:14 am

Quartersawn wood is much less likely to cup. It will be more expensive.
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Re: Wood is cupping as it is being cut.

Postby Leo » Mon May 19, 2014 5:15 pm

Maybe you do need wood - but - can you use a different material?

PVC - HDU ??
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Re: Wood is cupping as it is being cut.

Postby Paul S » Tue May 20, 2014 12:49 am

It sounds like you have your wood milled before you bring it back to your shop...... Have you ever left it sit for a week or so after you get it, prior to machining ? Just a thought ?
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Re: Wood is cupping as it is being cut.

Postby SawdustandSmoke » Tue May 20, 2014 1:20 pm

I think quarter sawn might fix the problem or at least help a lot. Most everyone in my area has small band saw mills so I don't know if I can get it. The maple I buy is already cut on all sides, everything else I have plained down at the mill on one side, Most of it sets for a week or two before I can get it cut.

Im going to try adding more tabs and more clamps on by walnut to see if that helps much.
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Re: Wood is cupping as it is being cut.

Postby SawdustandSmoke » Mon May 26, 2014 7:14 pm

Thanks everyone for the help, increasing and strengthening the tabs has fixed most of the problems
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Re: Wood is cupping as it is being cut.

Postby COWBOY1296 » Tue May 27, 2014 2:26 pm

Welcome to my world.
All 3-d cuttings that i have done warp/cup, some more than others. I work with black walnut, alder and cherry.
One of my first carvings was in a solid piece 18 inches tall and 22 inches long. What a nightmare. It was none of what i listed below.
What i have done since to improve on it.
1 wood has to be kiln dried;
2 rip the boards into 3 inch strips;
3 re-glue the strips back together mix and matching the end grain
After doing all of this they still warp a little.

What i have not done and keep saying that i will is to cut a pocket into the back of the plaque.
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Re: Wood is cupping as it is being cut.

Postby Leo » Tue May 27, 2014 3:02 pm

Even if the wood is kiln dried - there is a moisture difference from the inside to the ourstide skin.

As moisture content in wood increases - the wood expands

As the moisture contend decreases - the wood contracts

Anytime the outer skin is removed - (which is likely drier than the inside) - the inside wood with either dry a little more (most likely) or get a little a little moister.

The result will be - one side (bottom) will most likely contract - pull together - warping the wood.

What turners will do is to Rough out the shape - let the wood air dry (1" of thickness per year) then go back and finish turn.

In the CNC we may tend to be impatient to do that.

But - rough out the shape - leaving 1/8 to 1/4 material on all surfaces - including the back.

They - 6 months to a year later - finish it. Even at that - it is still going to warp.
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