Plywood rips coming out curved

[tjbnwi]

For plywood do not use the 12 tooth blade.

Unfortunately bad products are out there.

Tom

 

[makpacman]

Ok that makes more sense to me the that the sheet is indeed dead straight.

Out of curiosity, assuming the sheet ended up the exact dimension you were after, have you measured the exact width of the offcut to see if there's any variation along it?

Yes. It was actually the "offcut" I was after and it was perfect along it's entire length as expected. It was meant for a vanity drawer. I moved on to deeper drawers and all those rips came out straight. I have enough stock left to recreate this rip so I think I'll do that and report the result here.

 

[Crazyraceguy]

I think it is a problem with the plywood quality.

Seth

Yup. That is tension in the wood itself. I have had that happen lots of times, when cutting on a giant industrial beam saw that literally cannot cut crooked. I used to cut 4" strips, from an entire sheet, stacked 4 sheets high, on that machine, and a few of them would bow like that.
If you made that same cut, down the middle of the sheet, this would not happen.

Now, I'm not going against the advice of those saying to clamp a block behind the track, because you can get some bowing over that much distance, but it will never be that much.
If this was being caused buy too much toe-in, it wouldn't return back out, on the other end. It would just keep digging further into the side of the rail.

 

[Crazyraceguy]

Great tip — thank you for the photo!

Side question — why did you get the 2700 track, and not the 3000 track? A buddy and I are trying to learn the logical reason.

For some people it is literally about storage space and handling issues. That 300mm makes a difference in a van.
You should see me spinning that 3000 around in my 12 x 24 shop. After using it for years, in a 60k sqft shop, with very high roof.....it's a bit "cozy"

If you don't need to be portable with it, there is no point to the 2700

 

[Crazyraceguy]

Have y’all figured out the clues for such wonkiness?

How would us normies be able to detect bad plywood sheets?

Asian Birch is the usual candidate, though it does "just happen" with the regular "good stuff" too.
The better-quality stuff is more likely to turn into a Pringle, than bow sideways.

 

[Crazyraceguy]

OK, didn't expect my "Pringle problem" to garner much attention so pardon the late response. I'll attempt to answer any questions I've received in response here.

Yes. The track is clamped on both ends and the offcut is precisely the same size from one end to the other. The sheet of ply is left perfectly straight. Only the 3 3/4" offcut exhibits the curve.
During the cut the entire 4'x8' sheet is supported using a custom made interlocking support system that runs fully both directions. there's no bending taking place. although admittedly, the effect is similar to as if something like a bend in the middle occurred.

I'm just using the one perfectly straight 2700mm track. I use the 2700 because I travel with it and it's enough to rip full 8 foot lengths. 3000 is nicer in a large shop but at this point I know well enough how to place the 2700 that it's no bother.

I haven't seen this happen more than a few times to me over the past 5 years. Certainly never twice in two days and never with such a dramatic bend(~1/4" concave) in each board. The first bend happened toward the center of a leftover 3 foot wide by 8 foot long sheet. In other words, the first 4 rips were fine and then the last one warped. And the second time it happened was on the first cut, of a brand new 4x8 sheet. Both were sheets of ply were stored nearly vertically on edge at the jobsite for a week. The flooring is tile but stickers were used and the sheets looked dead flat. There was no discernable curve to them.

I'm in CA so although the weather has been changing over the past week, it's still CA so no rain and not much humidity.

Could the speed 6 blade rotation be problematic? Was I moving too slowly? Or perhaps it has to do with the 42T blade? Should I consider a dedicated 12T rip blade? If it turns out that is the answer, it would in effect mean I transport two saws. One with a rip blade, one with all purpose blade.

Thanks for all the responses. As you can see from my replies, I'm hoping the "It's just bad wood" answers are not what I have to live with because that's going to get expensive quickly.

It's the width of the off-cut. If you did exactly the same, but 8" wide, it wouldn't happen.
The humidity fluxuation is not really a thing with plywood, it's just that initial drying. Plywood is made up of green veneers (especially the rotary-cut type) so the potential for some difference in moisture content exists.

 

[makpacman]

It's the width of the off-cut. If you did exactly the same, but 8" wide, it wouldn't happen.
The humidity fluxuation is not really a thing with plywood, it's just that initial drying. Plywood is made up of green veneers (especially the rotary-cut type) so the potential for some difference in moisture content exists.

Thanks for the info. I did cut a 5.5 inch and 10.25" rip at the same time and both came out fine. I also had a 1 inch rip from that sheet that came out straight. All of that combined made me wonder why the 3.5 inch section was the only one affected. But just chalk it up to the wonders of modern plywood construction I guess.

 

[Windows]

A gradually changing vertical distance between track and material together with a blade that is not perfectly at 90 gets you a horizontal curve.

If the workpiece flexes downwards in the middle more than the track does (or vice versa) and the blade of the saw ends up at a slight angle (either because the track tipped or flexed, or the saw was not fully at 90 to its base), the curve can be significant.

If there is any gap between the track and material, even if a fixed distance, you can also get a curve by gradually changing the tilt of the saw, for example if the bevel screws aren’t tight enough and operator pressure changes the angle of the saw slightly and gradually during a cut.

You probably checked all that though.

I think it’s interesting how a problem with work holding in the vertical plane can produce a curve in the horizontal plane even if not the cause in your case.

 

[Packard]

The off cut is curved. Is the mating cut on the rest of the sheet straight or curved?

If the mating cut is straight, then the plywood is inducing the curve after the cut.