Cupped wood = bent blade?

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Forum topic by toofy posted 11-27-2014 03:17 PM 1331 views 0 times favorited 8 replies Add to Favorites Watch
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1 post in 1817 days

11-27-2014 03:17 PM

Topic tags/keywords: table saw saw blade question tablesaw milling

Up until now I’ve always worked with either s2s or s4s lumber. Today I decided to take a shot at 4/4 rough soft maple and walnut.

I flattened one edge using a plane and was going to rip a 12 inch long piece into 1.5 inch strips on the table saw. The walnut had a little wobble in it, but I was able to rip it without any problems. The maple had a pretty good cup which I put concave side down so the wood would be stable and ran it through the table saw. It started burning and when I stopped the saw and removed the wood, the blade was bent.

What did I do wrong? Should I have flattened the board before ripping?

8 replies so far

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20634 posts in 2397 days

#1 posted 11-27-2014 03:35 PM

Maple will very often bow as you are cutting it, severely. Never had it bend the blade though. Are you sure the blade is bent? What kind of blade/saw?

-- Bill M. "People change, walnut doesn't" by Gene.

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1222 posts in 2977 days

#2 posted 11-27-2014 03:38 PM

Before ripping the board on the saw, you need to have one flat face and one edge flat and square to the face. The flat face goes down against the table and the square edge goes against the fence.

-- Tom Finnigan - Measures? We don't need no stinking measures! - Hmm, maybe thats why my project pieces don't fit.

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320 posts in 1851 days

#3 posted 11-27-2014 04:02 PM

When you put a cupped board concave face down, it’s possible to actually flatten it by force. That’s why you hand plane the rising edges of the concave side first, not place it concave side down and plane off the hump of the convex side first, as would seem logical. Same reason people get cupped boards coming out of the power planer when it would seem they shouldn’t.

It may be that you were unwittingly exerting leveraged sideways force against your tablesaw plade using the mechanical power of flattening the arc of the cup wood, trapped between fence and blade.

Did the blade bend away from the fence?

I know it sounds kind of far-fetched, but what other explanations are there, if the blade really did get bent? First time I’ve heard of a tablesaw blade getting bent in a single shot, by the way. Teeth, sure, and cracks, and bent over time and abuse, but in one pass?

-- Lao Ma: You are so full of anger and hatred. Xena: Everybody's gotta be full of something.

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117746 posts in 4118 days

#4 posted 11-27-2014 04:09 PM

If you can it’s best to flatten your material first,but I have riped cupped wood many times in the way you described,you never want the cup side up,because it will pinch the blade as the cup is cut. When I cut cupped wood I always hold the side next to the fence flat to keep a consistent cut and prevent kick back. How you bent the blade doing this operation is puzzle to me unless you have an incredibly cheep blade.

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929 posts in 2615 days

#5 posted 11-27-2014 04:28 PM

is the blade permanently bent? the heat from burning wil make the rim expand faster than the center of the blade and it will bow but when it cools down it will return to normal. it can be straightened if the blade is worth it. it is called hammering. large blades are hammered to prestress them to run true.

some other venues:
a blade with less teeth
a blade with more set
use a cheap blade and offset one tooth one way on one side and another the other way on the other side, be sure to set the tooth toward the tip and not away. this will give more clearance. use teeth 180 degrees apart on the circumference. This will help to allow more clearance so the stress built up during drying don’t bring the board together after the blade and stick on the riving knife or the back edge of the blade.

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5974 posts in 2949 days

#6 posted 11-27-2014 04:31 PM

How old is this blade and how much abuse has it seen. Be honest, me I cut 2×4’s and all kinds of construction grade wood with my saws that is what they are supposed to do. But for hardwood and good projects the better blades come out.

Did you cut through the 4×4 all at one time? Cutting through purpleheart 2 inch thick blocks I come up half inch at a time until I get through then come back with a real thin cut all the way across. The heat buildup between that kind of wood while cutting is high, why the rush? Sure the blade can take it, so can my tires if I peel off once, try that everyday and soon you have no tread left.

-- Live to tell the stories, they sound better that way.

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320 posts in 1851 days

#7 posted 11-27-2014 05:33 PM

So, blade’s already hot from previous cuts, gets pinched by either or both tension wood movement and mechanically due to cup; blade bends, doesn’t bend back due to mileage or cheapness.

With a combination of factors, it doesn’t seem so far-fetched.

-- Lao Ma: You are so full of anger and hatred. Xena: Everybody's gotta be full of something.

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5772 posts in 3784 days

#8 posted 11-28-2014 08:21 PM

Possibly, the blade got hot and warped.

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