My rail cuts concave, why?

[Bob Gerritsen]

Hi all. I keep running into this problem, maybe someone can shed some light?

When cutting particle board finished with melamine with my long Festool rail, the material comes out concave. I've tried with and without the thing behind the saw-blade (sorry can't think of what you call that right now), no difference.
In the photo I'm cutting a relatively narrow strip but the cut before was quite substantial on both sides and had the same problem and it was about as concave as this is.
And now for the kicker: The piece on the left (in the photo that is), the wider one that was on the right side of the Ts55 when cutting is more concave than the narrower piece that was under the rail. That seems to be typical, the piece on the right side of the cut ending up more concave.

The rail seems pretty darn straight, I've tried cutting in a couple passes, very slowly or quicker, no difference. It's beyond me...

Anyone?

Cheers, Bob.

 

[Bob Gerritsen]

Oops, the photo should be rotated once clockwise...

Also, the thickness of the particle board is 19mm or 3/4 inch. The standard saw blade is sharp, I have the same problem with other blades.

 

[Alan m]

i would say it is cheap sheet goods.
we often get sheets moving like that after being cut.
the rail probably isnt bent .
are those pieces in the same orientation as when they were cut
if the are then even if the  railis bent the same shape should be in both sides and they should match up.

 

[sgryd]

Could it be that you have some slop? If you haven't tightened the "non-slop"-screws and if you change hand or grip in the middle of the process you might rotate the saw and the slop gives it room to "eat" wood to the left of the kerf and then to the right of the kerf. Long shot, but maybe..

//michael

 

[waynelang2001]

If your rail is staight and it still does this then I would say its the sheets. I never try to cut the final size i need right off the board. I cut it about 10mm bigger then trim it after. Even in good quality melamine I have seen the board bend after it has been cut off the main board. I think there are some internal stresses at play hear.

 

[semenza]

Like Alan said it could be the quality of the melamine. I have had some pretty wide pieces of plywood bend after cutting. Since it only happens on the melamine , which is slicker than other materials, be sure you are not putting any side force on the saw as you push it down the track. I have had slightly concave and crooked cuts from accidentally doing that.

Seth

 

[PeterK]

In my experience, the longer the rail the easier it is to push it left or right and end up with a curved cut. I try hard to walk the saw down the long cut line keeping pressure down as much as possible. These aluminum rails will flex even with the ribbing. Still it sounds like your issue is more the material than the rail.

 

[Kev]

Draw a straight line a couple of inches from the edge on a sheet of particle board, then confirm it is straight ... then cut to one side of it. You'll most likely see a cut that's a perfectly uniform distance from the line, but now the line is no longer straight !

Now ... take a piece of quality plywood ... repeat  [smile]

 

[Laminator]

I have witnessed this in MDF and Particle board (Melamine).  If the board has stress in it (and many do) it will bow when cut.  Thankfully its not too hard to cut a little oversize to allow another cut to straighten the edge.

 

[Michael Kellough]

Hi all. I keep running into this problem, maybe someone can shed some light?

When cutting particle board finished with melamine with my long Festool rail, the material comes out concave. I've tried with and without the thing behind the saw-blade (sorry can't think of what you call that right now), no difference.
In the photo I'm cutting a relatively narrow strip but the cut before was quite substantial on both sides and had the same problem and it was about as concave as this is.
And now for the kicker: The piece on the left (in the photo that is), the wider one that was on the right side of the Ts55 when cutting is more concave than the narrower piece that was under the rail. That seems to be typical, the piece on the right side of the cut ending up more concave.

The rail seems pretty darn straight, I've tried cutting in a couple passes, very slowly or quicker, no difference. It's beyond me...

Anyone?

Cheers, Bob.

Like the other guys said, material shrinkage.

If the cut edge in both left and right pieces are concave from the same cut it can't be the guide rail can it? It could be bent one way but it can't be bent both ways.

It's probably that the humidity is too low. As the center is cut the sheet shortens to a more relaxed position. If you measure an un-cut sheet carefully you might find that the outside edges are shorter than the middle and when you rip through the middle you allow the middle to "catch-up" with the perimeter.

I've had it happen with ply too. It's not just the present humidity but the difference in the humidity when the product was made until you cut it. Some manufacturers control the assembly humidity better than others. I've always had best results with Columbia products. The bad ones I have to make each cut twice to eliminate the shrunken line. Often you can just butt the two bad cuts together and re-rip to straighten both out at once.

The kicker (the wider half is more concave) is because there was more material that needed to shrink.

 

[Bob Gerritsen]

Thanks all, it makes total sense to say it is the material and I will experiment with some other sorts.

I order the bulk of sheet material I use, also this stuff, pre-cut, funny thing is that stuff is typically dead straight...

Well, like said, nothing left to do than try some other materials. Hope to have time for that soon and will report back.

Cheers! B

 

[KorDes]

I get this all the time with melamine sheets, its worse in 16mm particle boards than 19mm mdf boards. I just move the rail just enough to get a straight cut and trim the cut edge straight. It's a pain in the arse, but theres not a lot you can do to prevent the release in tension of the sheets when you cut them.

If you line up your guide rail to the edge of the cut, you may find it is flush in the middle and the ends will stick out a bit.

 

[Bob Gerritsen]

Well, I had to cut a long strip of mdf the other day and although the dimensions were similar to the pieces in the picture, this time I had no problems. So indeed, particle board with melamine seems to be causing the problem.

What I still don't understand though is how my supplier manages to always supply perfectly square and straight pieces when I order the sheet cut to my cutlist. I know they use a cnc setup but that doesn't magically get rid of stress in the sheets.

In any case, I'm glad I understand this a bit better now.

Cheers all, Bob.

 

[awdriven]

Your supplier may just make two cuts, unless you've watched them prepare orders for you?

 

[Bob Gerritsen]

No, I've never been there to watch it. I will surely ask though, next time the rep comes by.