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Can thin kerf blades result in a crooked cut?

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Forum topic by awsum55 posted 11-11-2018 08:21 AM 1219 views 0 times favorited 15 replies Add to Favorites Watch
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awsum55

361 posts in 712 days


11-11-2018 08:21 AM

I have a WWII 10” 40T blade in my Unisaw. When I tilt the blade to 45º I don’t always get a square cut. I notice this when I butt the pieces together to run some masking tape over the joint for glue up. I’m using a sled to cut the 45º and I seem to get a cut that bows out in the middle (so the joint only touches in the center).

I can’t seem to solve this problem consistently. Sometimes when I put a square on the cut it is actually straight, but not usually. Any suggestions on what might be happening?

-- John D, OP, KS


15 replies so far

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Peteybadboy

522 posts in 2153 days


#1 posted 11-11-2018 10:51 AM

I get some burning with my 45 sled. Minimal bowing as you describe. I use the Forest WW2 thin kerf blade. Is the blade sharp? or maybe needs to be cleaned?

-- Petey

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awsum55

361 posts in 712 days


#2 posted 11-11-2018 01:37 PM

Blade was sharpened by Forest and has very few cuts since then. I also happens with my Freud thin kerf blade which is brand new.

-- John D, OP, KS

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Rayne

1109 posts in 1743 days


#3 posted 11-11-2018 01:59 PM

Is the blade perfectly parallel to the miter slot or perfect 90 in your sled and perfect 45 in front and back of blade? It seems it might be making a compound cut if it’s doing that.

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jmos

902 posts in 2573 days


#4 posted 11-11-2018 02:02 PM

If it is a blade deflection issue, and I’d be surprised, slowing the feed rate should help some.

Are you just trimming, where one side of the blade is in the wood and the other out of the wood, or are both sides in the work. If it’s the former, it’s more possible blade deflection is an issue. Do you have, or could you borrow, a full kerf blade to try and see if the problem goes away?

Is the blade parallel to the miter slots at 90? Is the blade still parallel to the miter slots at 45? The blade can be parallel to the miter slot at 90, but go out when tilted. Is the sled fence at 90 to the blade? Is the work sitting firmly and flush against the sled fence? If you get debris between the work and the fence, the piece might shift as you’re cutting. Or, if the reference edge of the piece (or the sled) isn’t nice and flat, it may shift.

It’s also possible the work is shifting slightly when your cutting. I’ve had this issue with my miter gauge, no matter how well I think I’m holding it, sometimes it moves just a hair, but it’s enough. You might want to try clamping the board to the sled.

-- John

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awsum55

361 posts in 712 days


#5 posted 11-11-2018 03:14 PM

I’ve checked and rechecked several times and everything seems to be square and parallel.

“It’s also possible the work is shifting slightly when your cutting. I’ve had this issue with my miter gauge, no matter how well I think I’m holding it, sometimes it moves just a hair, but it’s enough. You might want to try clamping the board to the sled.”

I was thinking about adding some sandpaper to the miter fence to prevent slipping, but I haven’t done it yet. I’m also going to try clamping the piece down.

“Are you just trimming, where one side of the blade is in the wood and the other out of the wood, or are both sides in the work.”

Yes I’m a miser when it comes to wasting wood. If I had to guess, I bet 90% of my cuts at 45º is where only one side of the blade is in the wood.

Thanks to everyone for the suggestions.

-- John D, OP, KS

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jbay

2893 posts in 1103 days


#6 posted 11-11-2018 03:24 PM

More than likely if everything is set up square and straight, it may be that the board rises up in the center making the cut longer. That’s why it hits in the middle and not the ends. (does the wood have any bow in it?)

Try another cut and make sure you are holding the material flat throughout the whole cut.

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Rayne

1109 posts in 1743 days


#7 posted 11-11-2018 03:32 PM

Are you using hardwood or plywood? Hardwood may be moving (relieving stress) after a cut. If it’s happening on plywood too, then I’m out of ideas. If you haven’t done it on plywood, test it and see if it’s the same result.

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wichman3

76 posts in 825 days


#8 posted 11-11-2018 04:53 PM

I have 23 years experience cutting wood picture frames in a production shop. In my experience these are the top issues:
1. The blade plate is not stiff enough for the material you are cutting. To remedy: slow down your feed rate or make a primary cut about 1/8 ” long then a final trim cut, OR both
2. There is pitch or resin build up on the blade causing the blade to deflect while cutting. Remedy- clean the blade.
3. dull blade
4. warped stock, even mildly warped stock will pull against the blade and cause problems. At one time at the shop we had a molding that was weighted on the side away from the rabbit all the other molding were weighted at the rabbit. To get a tight picture frame I would have to adjust the double chop saw about 1/2 a degree to fix the problem. The point is even small difference will make an impact.

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MrRon

5202 posts in 3447 days


#9 posted 11-11-2018 05:32 PM

I would venture to say, making trim cuts is the culprit. When trimming, the blade is cutting on one side only. That puts a side pressure on the blade causing it to flutter. It also “dulls” one side of the blade more than the other side. A full kerf blade may lessen the side pressure. Now that the teeth on one side of the blade has “dulled”, making a full cut (not trim) will cause the blade to wander to the “dull” side of the blade. I think most cutting problems can be traced back to the blade. Even a new blade may exercise poor cutting, especially cheap blades. The blade may wander just a few thousands of an inch, but it’s enough to give a less than straight cut. I base this on the physics of machining. One must remember too that the machine itself is not perfect; it has it’s own tolerances in it’s bearings and other moving parts. One would think wood being much softer than metal, would not be influenced by a much harder metal blade. Tolerances will always be there. There can never be a zero tolerance. If there was, nothing would move. Heat also is a factor that affects metal. Expansion and contraction are always present, although it is so small, we fail to recognize it. We are merely woodworkers and don’t think about it on a molecular level.

Metalworking is a different story. Tolerances are kept as tight as possible and even then, a perfectly machined piece will still show a deviation from a design dimension. There are specialized machines that can take it to the final dimension, but never 100% perfect. We would never think of trying to work wood to machine tolerances, but it’s something that can’t be completely ignored. When it comes down to a perfect fit, hand working is usually the answer with a plane, scraper or sandpaper wrapped around a block of wood.

I may sound like I’m full of crap and maybe I am, so take this with a grain of salt.

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jbay

2893 posts in 1103 days


#10 posted 11-11-2018 05:57 PM

Ron, what you’re saying has merit, although I don’t think it’s the problem.

I’ve cut hundreds of taped miter joints and have come across all of these mentioned problems.
The solution for me was holding the material flat and tight against the fence.

Another thing, if it were me, I would raise the blade about 1” higher than the material to help put more downward pressure on the cut.
If the blade is too low, or a bit dull, it can have the ability to force the wood up.
If the wood raises and lowers as you’re making the cut, it will not be straight! period.

I would never tell anyone to raise the blade higher than they are comfortable with,
so that’s why I say, if it were me.
ALWAYS ONLY do what you’re comfortable and feel safe doing.

I’m miles away and don’t know what material you’re using, what saw you have, left tilt, right tilt, hp, how big of piece you’re cutting…so on and so on…..
so take any information as in general, some unknown factors could change the procedure.
(hold downs, push sticks, jigs…...) whatever else it may take to keep it safe.

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bilyo

424 posts in 1306 days


#11 posted 11-12-2018 03:16 AM

I’m assuming you have a clean sharp cross cut or combination blade that is properly aligned. Under those circumstances, the only thing that has caused me that problem in the past is the work piece slipping a bit along the fence. When making a cross cut with the blade tilted, the blade tends to pull or push (depending on which side of the blade you are on) the work piece along the miter fence. I’ll wager that you can feel it as you grip real hard to hold it in place. Glue some medium grit (+-150 grit) sandpaper on your fence and it will help prevent this.

With respect, I must disagree with MrRon. When making a cut like this, I almost never make my first cut to the line. I always leave about a blade width of material to “plane” off with a final cut. This way the friction of the blade on the work piece will be less with less tendency of the blade to push/pull the work piece. Thus, a more accurate final cut. BTW, I always use a thin kerf blade.

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Rick Dennington

6305 posts in 3398 days


#12 posted 11-12-2018 03:49 AM

The answer to your question is: No……!!

-- " At my age, happy hour is a crap and a nap".....!!

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hairy

2783 posts in 3736 days


#13 posted 11-12-2018 01:53 PM

The miter sled I made has a stop and a hold down. I get good miters that go together right. I make miters in 2 cuts. The first at 90 degrees for the proper length, the second at 45 for the miter.

http://lumberjocks.com/projects/308986

-- My reality check bounced...

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awsum55

361 posts in 712 days


#14 posted 11-12-2018 05:04 PM

Thanks again for all the suggestions. I think I’ll add some sandpaper and alter my sled so I can use a hold down. The fact that it doesn’t do it all the time really points to something I’m doing, rather than blaming the blade.

-- John D, OP, KS

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pintodeluxe

5798 posts in 3017 days


#15 posted 11-12-2018 06:05 PM

How wide are the boards / panels you are cutting? I would suspect slightly bowed stock more than the blade.
Even when using a sled, I’ll often hold panels down flat with push pads while making the cut.

If we had a video recording of the cut, it would probably be obvious what was happening, but we usually don’t have that luxury.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

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