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Bowing after rip cut???

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Forum topic by pete79 posted 03-24-2011 04:53 PM 3954 views 0 times favorited 18 replies Add to Favorites Watch
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pete79

154 posts in 2602 days


03-24-2011 04:53 PM

Not sure if I’m using the correct terms here, but here it goes….

I made a rip cut in a 9/4 slab of walnut last night to take off a 1” wide strip to be used on a bench I’m making. After the cut, I hand planed all four sides of the piece I had cut off. After a while I noticed that the piece had a slight bow in it. I know that the slab I took the cut from was straight, with no bowing, and my rip fence was straight, producing a straight cut. I also know that the planing did not produce the bowing.

Fortunately, this piece is being sandwiched between two other pieces of wood, so I believe the slight bow shouldn’t be too big of a problem to straighten out.

My concern is that I have two more pieces of the same size to cut for stretchers on the bottom of the bench that will not be sandwiched between anything, and I want these to be dead straight.

What reasons would there be that the piece bowed after I made my cut, and what can I do to prevent this going forward?

-- Life is a one lap race.


18 replies so far

View lewis62's profile

lewis62

73 posts in 2100 days


#1 posted 03-24-2011 05:20 PM

? How was this board dried klin, air, and how long ago. The thicker the board the longer it takes to dry,and then it will still acclimate to its enviroment, gain or lose moisture.I noticed that even on lumber that has been in shop for more than three years there is internal stress that shows up after machining,cup, bow, twist are all possible.If it is a stand alone pc, over cut ,let sit few days ,check condition than machine to final size, and if for glue up that size pc. should be no problem to clamp up.The thicker boards are worse for moving after cutting than the regular 3/4 or 1 inch stock.I have checked with moisture meter the edge of boards, reading 8 %and after cutting off 2 inches got reading of 22% in new exposed edge,hence movement as it dries out. Hope it gives you some info . It is wood , it moves , loses and gains moisture.

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pete79

154 posts in 2602 days


#2 posted 03-24-2011 05:29 PM

Totally forgot that the moisture content could be the culprit. Thanks for the info and tips – I’ll take a couple readings and see what I find out.

-- Life is a one lap race.

View canadianchips's profile

canadianchips

2346 posts in 2459 days


#3 posted 03-24-2011 05:48 PM

Different types of woods react differently when cut. You need to look at the end grain as well to try and guess which part of the tree the piece came from. I have no set formula, I am still learning this myself (after 30 years). A good example is ripping 2×4 into 2×2, the cheaper ones will bow as you cut them. I used to by 1” x 8” solid material to make my own face frames for cabinets. One supplier I used was good, anything I bought from him was usable, my local supplier was a hit and miss (cheaper, but always threw some away).

-- "My mission in life - make everyone smile !"

View sawblade1's profile

sawblade1

754 posts in 2489 days


#4 posted 03-24-2011 06:01 PM

Do you have a Jointer ? One alternative culprit could be it looks straight but as you rip it the bow could be magnified on the other side. IE: 1/16” bow on the rip fence side could be magnified up to 1/8” on the blade side depending on the way you cut. Joint before ripping & it also helps to make sure your jointer is parallel and square if you have one :)

-- Proverbs Ch:3 vs 5,6,7 Trust in the lord with all thine heart and lean not unto your own understanding but in all your ways aknowledge him and he shall direct your path elmerthomas81@neo.rr.com

View TomHintz's profile

TomHintz

207 posts in 2860 days


#5 posted 03-24-2011 06:08 PM

Moisture and all that can influence a board but some just have stress within and when you cut it narrow enough, that stress takes over and the board curves. It’s always best to use dry wood and let it acclimate to the shop before cutting but that bow can still happen, often when you least expect it. I have not found a good way to fix a piece that bowed significantly either. We have tried wetting one side, over correcting it and wetting it, heating it, yelling at it loudly, nothing works very well.

-- Tom Hintz, www.newwoodworker.com

View jayman7's profile

jayman7

218 posts in 2967 days


#6 posted 03-24-2011 06:09 PM

I had some walnut that wasn’t dried properly and had some internal stresses that were released when ripping. It bowed pretty badly actually. At least I have two curved clamping cauls out of it. :D

View wasmithee's profile

wasmithee

58 posts in 2155 days


#7 posted 03-24-2011 06:23 PM

I have seen severe examples of this with wood that has been dried well and acclimated to the shop for over 18 months. The problem lies within the wood itself.

If a tree grows on a slope it can build up stresses into its very fibers by virtue of fighting against gravity as it grows. This pent up ‘stress’ can be released when the wood is cut. This is a good argument for using riven wood since it allows the woodworker to work with the internal grain structure of the wood etc. Even then, some twisted wood is difficult to use as anything beyond firewood. Peter Follansbee wrote a bit about twisted wood on his blog: recent splitting & some answers to some questions

Another problem that can lead to this is case hardening. Case hardening describes lumber or timber that has been improperly kiln-dried. If dried too quickly, wood shrinks much at the surface, compressing its damp interior. This results in unrelieved stress. Case-hardened wood may warp considerably and dangerously when the stress is released by sawing. This is what some of the previous respondents have spoken about.

View CharlieM1958's profile

CharlieM1958

16241 posts in 3680 days


#8 posted 03-24-2011 06:33 PM

I think the bottom line is that when ever you cut a piece off a larger board, you can’t be certain exactly what will happen. It’s just one of those variables that comes with the territory.

-- Charlie M. "Woodworking - patience = firewood"

View pete79's profile

pete79

154 posts in 2602 days


#9 posted 03-24-2011 07:02 PM

Ah the beauty of woodworking and finding a solution to a challenge! I suppose I’ll check the moisture content first. If that checks out ok, I’ll try ripping off a slightly larger piece than I need and see if I can get down to a straight piece from that if it bows again. If that doesn’t work, I suppose I will find a creative way to work a couple curves into the piece – nothing says I have to stick to the original design right?

-- Life is a one lap race.

View stefang's profile

stefang

15512 posts in 2796 days


#10 posted 03-24-2011 08:29 PM

Besides moisture there can be a release of internal tension.

-- Mike, an American living in Norway.

View Al Killian's profile

Al Killian

85 posts in 2087 days


#11 posted 03-24-2011 08:47 PM

In my shop we see lots of boards twist up and bend in different directions. It is something that will happen no matter what you do. The only thing you can do, is cut it oversize then let it still for a few days and what and see how much it moves. From their you can see if it is still usable or if you need to use one of the extras that you cut for that project.

-- Owner of custom millwork shop

View dbray45's profile

dbray45

3178 posts in 2238 days


#12 posted 03-24-2011 08:49 PM

It can also be the wood. Walnut can have a lot of stresses and can move – a lot!. When I cut walnut, I cut it large and let it sit for a day or two before milling to size, especially from a thick piece. Sometimes, you can tell if a piece will do this by the twists and checking. I once had a piece all but explode on the table saw, sending a piece by my ear.

The wonders of woodworking.

-- David in Damascus, MD

View bobkberg's profile

bobkberg

420 posts in 2535 days


#13 posted 03-24-2011 10:09 PM

I’m going with the “internal stresses” mentioned by several people. I usually take a small rip of something that’s got to be perfect to see whether or not it’s going to bend. In some cases (early on in my woodworking time) I have been really surprised to see the internal stress of a board bend the piece I was ripping hard enough to close the kerf, and in one case bind the blade to a stop.

There are two approaches I take for things like that depending on the need and the expected appearance.

1) Over cut the piece by 1/2” (or more if it’s going to be a really long piece) and let it sit – then re-rip it by cutting off the convex side first, and then the concave side. In some cases – although not many, this has just made it curve more.

2) Cut the piece into several thinner/narrower strips and glue them back together clamped as needed – much the way that Glue-Lam beams are made, but on a tinier scale.

-- Bob www.singularengineering.com - A sideline, not how I earn a living

View juniorjock's profile

juniorjock

1930 posts in 3227 days


#14 posted 03-24-2011 10:22 PM

Yeah, what Bob and the others are saying…. Once I was ripping a piece of cedar and just happened to look up toward the tip of the board on the out-feed and noticed that there was about a 1.5” gap there. That’s a lot of stress. But it goes the other way too, and that’s when you can get the kick-back. I’ve seen that too. Look up and see that the board looked like it hadn’t even be cut on the out-feed side. I stopped there as it was binding against the blade…. turned it off, held it tight and waited for the blade to stop. Now, I use a splinter.
- JJ

View ScottN's profile

ScottN

261 posts in 2141 days


#15 posted 03-25-2011 04:58 AM

A couple of pieces here and there are normal but a lot of it has to do with the drying process and more importantly the conditioning of the wood after the drying has been done.
One of my local sawmills does a good job drying lumber except for aspen… It case hardens so bad the wood will split before I finish sawing it. After I cut 10 boards in a row and all the boards bowed from case hardening I took the 600 bdft of aspen back got my money back.Luckily it was still on the trailer.

One of my other sawmills I go to told me aspen, poplar you have to bring the temp up and condition the wood slower during the drying process.

-- New Auburn,WI

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