correcting bow

[poolhound]

I have just milled up some stock for a project and after letting it sit for a short while a couple of pieces are now slightly bowed. They are still oversize but are at nominal thickness maybe 1/16 over. I was going to cut them to exact sizes and then run them through the planer to final thickness.

I currently have them all clamped between 2 thick cauls and will see what the look like tomorrow but I am not sure my prayers will be answered.

I seem to recall there is a technique involving wetting down one side and then clamping. Does anybody know the best way to do this?

What might happen if I wet all the surfaces and then reclamped in the cauls?

Help please

[Slik Geek]

A little more information may be helpful. What kind of wood? What was the source? (How dry was it to start with?) How long had it been in your shop before you machined it? Were there knots or unusual grain patterns in the pieces that have bowed?

The "bow" you have observed could be due to the wood not having been "acclimated" to your shop (so that it is still drying). It could also have been due to stresses in the wood that were released by the machining. Overcoming bending due to stresses doesn't seem very likely to me.

I've only attempted corrections like you describe with treated lumber (softwood), and even then I didn't need the precision that you seek. If you sufficiently wet the "short" side of the pieces, the wood should expand and undo the bow... but will it go too far? I don't have the answer that you seek, but with more information, perhaps some good suggestions will be made by others.

[poolhound]

The wood is cedar. It should be very dry my guess is at least a couple of years old - in Arizona that should make it very dry :-) It has been in my shop for 3 weeks - machined it today. All the pieces have interesting grain, thats why I liked it.

Stresses in the wood were definatley released as I noticed some pinching on the TS as I straightend them and cut to rough size.

When you say "short side" do you mean the convex or concave side?

[poolhound]

This mornings update is, I guess, not very surprising. I unclamped them and it didnt do any good. I am going to let them sit for a few days as I cant get back to this project until next weekend.

Any other suggestions in the mean time are welcome.

[ironhat]

Wet the concave side and put it back in the clamps. Recheck it frequently to see if you need to tighten the clamps. Due to your low humidity conditions you could try draping the thing in plastic to keep the water in the wood longer. I'm not famliar with working wood in low humidity conditions. Here in humid PA I've never had to clamp wood for this condition. Best of luck and let us know the outcome.

[cabinetman]

Any solution is hit or miss. I've tried wetting and overbending, and at times get lucky. I haven't found a predictable fix.
.

[poolhound]

Earlier in the week I tried wetting the concave side and leaving them with the convex side up in the sun. This did have a positive effect, however, some have re-aquired their bow.

As they need to be surfaced to finish quality and cut to final dimensions anyway I thought I would leave them in their current state and see how they react to the process. I hope I wont have to take to much off them but we will see.......

[JR]

There was an article in the most recent ShopNotes about using a planer for bowed wood. Recognizing that a planer will normally make two sides parallel, but not take out bow, they suggested sled that would help.

Basically, the sled would have screws in it, with the heads of the screws above the surface of the sled. Adjust the screws to match the bottom of your good wood, where it rises above the surface of the sled. Adjust the screws down into the sled (or remove them) where the surface of your good wood meets the surface of the sled. Then run the sled through the planer.

Once you have a flat side, the planer will work normally.

HTH,
JR

[poolhound]

There was an article in the most recent ShopNotes about using a planer for bowed wood. Recognizing that a planer will normally make two sides parallel, but not take out bow, they suggested sled that would help.

Basically, the sled would have screws in it, with the heads of the screws above the surface of the sled. Adjust the screws to match the bottom of your good wood, where it rises above the surface of the sled. Adjust the screws down into the sled (or remove them) where the surface of your good wood meets the surface of the sled. Then run the sled through the planer.

Once you have a flat side, the planer will work normally.

HTH,
JR

Thats actually how I face joint right now. It works really well. My issue unfortunatley is that this stock is already at nominal thickness for my project. I may have a 1/16 or maybe 3/32 available to take it final thickness (3/4). Once I run all the pieces through the planer and see how much I have to take off and wether it remains stable I will know more. One option will be to take everything down to 5/8, this may work. We will see........

[JR]

My issue unfortunatley is that this stock is already at nominal thickness for my project.

I was afraid you'd say that.

Ok, how about this? Drop back seven yards and punt.

That's all I got.

JR

[poolhound]

Punting on Saturday - care to join?

[cabinetman]

There was an article in the most recent ShopNotes about using a planer for bowed wood. Recognizing that a planer will normally make two sides parallel, but not take out bow, they suggested sled that would help.

Basically, the sled would have screws in it, with the heads of the screws above the surface of the sled. Adjust the screws to match the bottom of your good wood, where it rises above the surface of the sled. Adjust the screws down into the sled (or remove them) where the surface of your good wood meets the surface of the sled. Then run the sled through the planer.

Once you have a flat side, the planer will work normally.

HTH,
JR

If I'm understanding your explanation, that the intent is to achieve a flat board from a bow, has some drawbacks. In the exaggerated drawing below, the dotted line represents the stock after planing, according to your method if I'm correct. Even achieving a flat board from a bow, considering the beginning thickness would have to be great enough to accommodate the desired thickness, internal stresses from the remainder of the board after stock removal would give way for any further movement.

There is also the question concerning the safety of this type of planing procedure.
.

.

[Wood_workur]

I've tried the water method on plywood with no success, but maybe if you used steam, and over clamped it (to allow spring back) it might work. I've got about 20 bf of cedar that has been sitting in my garage for the past year, and it is still perfectly flat, so I'm guessing that when you get it back to a flat board it should remain stable with proper storage.

[JimD]

If you need wide boards with minimal loss of thickness, you can rip the boards in half, joint the edges and glue them back up and eliminate most of the bow. If narrower boards would work, you can rip them oversize out of bowed wood and then rip to final dimensions.

Wood with knots has character but is also very prone to moving around as you cut it. Staying away from having knots on the edge helps.

Jim

[LarryG]

Reading through this thread, there's one thing I'm not clear on. Is this wood bowed (warping along its long face), or is it cupped (warping across its width)?

If the latter, which I'm guessing it is, then the best approach -- for next time -- is to first let the lumber acclimate to your shop, then mill it four-square but slightly oversize, let it sit for a day or so and do its cupping thing (which it very likely will), then mill it to final size and incorporate it into your project immediately. It's nigh unto impossible to mill a rough board and not have it move around on you to some extent, especially if your shop isn't conditioned and is subject to wide temperature and humidity variations.