Plywood rips coming out curved

[makpacman]

Does anyone know why plywood rips may come out curved? Initially I thought it may be due to uneven moisture content near the edge of the sheet but a recent batch of cuts toward the center of a sheet left me doubting that.

This image shows a fresh rip off the side of a 12mm sheet after an initial 5mm edge trim. When I flip the piece over, the center of the concave curve is 5mm off the perfectly straight sheet and guide rail.

This happened last night with some 18mm prefinished ply when cutting the same 3.5 inch rip using that material.

Equipment used: TSC55 KEB, clean, newish, stock thin kerf 1.8mm multipuropse 42T blade, cut at speed 6 and a blade depth 2mm deeper than the plywood.

 

[Stevenski]

Is the strip that you have cut off a consistent width all the way down its length? If it's not that would maybe suggest a problem with the track edge but if it is then the material is twisting (but that would be strange for a manufactured sheet good and to happen with two different dimension sheets - if I understood correctly?)

Last year I had a similar issue but I was using two tracks joined together and I had fitted them badly. I understand you are using just one track? No joins?

 

[luvmytoolz]

Firstly, just confirming you're clamping both ends of the track?

If so, to rule out the track line it up again for a consistent distance from the edge at both ends of the sheet, then clamp a long bit of metal or timber across the sheet so it's up against the edge of the track in the middle to stop it from moving. Then do the cut again and see if it's dead straight or curved again.

 

[tjbnwi]

We’ve had problems with certian brands of plywood doing exactly what is shown here, be it cut with a track saw or on the CNC.

Stand the cut piece on edge on the sheet of plywood, one edge will sit corners touching, the other edge will rock.

Tom

 

[P_K]

Clamp both ends and add a support at the center of the rail. This will prevent it flexing laterally as you push through the center of the cut.

 

[Stevenski]

Clamp both ends and add a support at the center of the rail. This will prevent it flexing laterally as you push through the center of the cut.

Great tip, I never thought of that. A very simple, belt&braces best practice just to eliminate the risk. I like it - thanks for that!

 

[MacBoy]

Clamp both ends and add a support at the center of the rail. This will prevent it flexing laterally as you push through the center of the cut.

View attachment 378990

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.

 

[semenza]

I think it is a problem with the plywood quality.

Seth

 

[P_K]

I bought the 2700mm rail locally rather than risking it getting bent in shipping. Was just a matter of what the dealer had in stock, if they had the 3000mm I probably would've gotten it instead.

 

[MacBoy]

We’ve had problems with certian brands of plywood doing exactly what is shown here, be it cut with a track saw or on the CNC.

Stand the cut piece on edge on the sheet of plywood, one edge will sit corners touching, the other edge will rock.

Tom

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

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

 

[tjbnwi]

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

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

No clue why it happens, we just avoid the brands now.

Tom

 

[bobtskutter]

I've been fighting with this this problem for a few years.
I buy plywood from my local builders merchant, it is not premium quality material.
Making a cut thats 8' long creating a board that's 8' x 4inch results in a bent board - the same as the post above. The cut board width is 4inch, but its bent - I put this down to stress in the plywood.
If I trim the 8' x 4" board I can straighten it out, but it's no longer 4".

So now, if I want a piece that's 8' x 4inch, I start with a piece that's 8' x 4.5inch then trim it to 8' x 4inch.

Regards
Bob

 

[P_K]

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

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

The overall quality of domestically produced plywood intended for cabinetry or interior millwork has been on a downward trajectory over the last decade. It doesn't seem to matter who's mill your materials come from, you're just rolling the dice anymore.

 

[Hipplewm]

I've been fighting with this this problem for a few years.
I buy plywood from my local builders merchant, it is not premium quality material.
Making a cut thats 8' long creating a board that's 8' x 4inch results in a bent board - the same as the post above. The cut board width is 4inch, but its bent - I put this down to stress in the plywood.
If I trim the 8' x 4" board I can straighten it out, but it's no longer 4".

So now, if I want a piece that's 8' x 4inch, I start with a piece that's 8' x 4.5inch then trim it to 8' x 4inch.

Regards
Bob

We call those "Pringles" when we do this to metal, especially if you are surface grinding a thin piece of (essentially a planer for metal) but if the part gets hot it will curl up and in extreme cases it looks like a pringle potato chip - unfortunately we can't recover, we just start milling a new piece.

 

[Stevenski]

We call those "Pringles"

 

[Lincoln]

Most likely tension in the sheet being released. Only way to combat this is to oversize your cuts, then trim down. Standard practice, for me, using a track saw or a panel saw.

 

[MacBoy]

Honestly until this report, I’ve always treated plywood as s.t.a.b.l.e and have had exceptional results.

This put me on alert for the next new sheet I get.

 

[luvmytoolz]

I'm also finding this baffling. I've never experienced released tensions in any plywood I've ever used.

Given it's layers glued at 90 deg angles to each other, I'm trying to work out how it could deform upon cutting without delaminating?

 

[makpacman]

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.

 

[luvmytoolz]

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?