Rail saw wierdness

[peter-harrison]

I’ve noticed a strange thing with my railsaw. It’s a bit hard to see from the photo but when the two cut pieces are pushed together there’s a 1mm gap in the middle, tapering to nothing at both ends. It’s the same for all cuts. It can’t be the rail slipping as this would look a lot different. The saw is a good one and the blade is in good nick.
Any ideas?

 

[Sporky McGuffin]

Is the board sagging in the middle?

 

[Distinterior]

Is the material you're cutting MFC....?
If so, it could be stress in the panel that, when cut, the 2 boards are springing apart.....

 

[Lefley]

Does it do that with all sheet goods.try a scoring cut then a full depth. Could be blade flexing! Or not perf3ctly tight in rail and when reaching in middle of cut a slight twist in saw causing a bigger cut out.

or the easiest thing is measure you sheet goods after cutting, are the board# the same width, start middle and end, or is middle 1/2 mm smaller. That will tell you if sheet good moving or saw and rail problems.

 

[Sgian Dubh]

Maybe the rail isn't perfectly straight? Slainte.

 

[Lefley]

Maybe the rail isn't perfectly straight? Slainte.

If the rail wasn’t straight he would still be able to push the two boards tight together. after thinking about this it could only be two things , the sheet is bowing after cutting or the blade is wobbling or track not tight to saw whole time. Unless I’m forgetting something.

 

[Inspector]

Even if the rail was a gentle S shape it would push together after the cut. If it isn't meeting evenly when offered it is the material moving after the cut. Try a cut half way through the sheet and see if the starting end closes up. If it does it is the material.

Pete

 

[Sgian Dubh]

If the rail wasn’t straight he would still be able to push the two boards tight together.

Ah, I see what you mean because it's a case of pushing the offcut up against the piece that's been dimensioned. I was thinking of putting the sawn edge of one board against the sawn edge of another piece. For example, the result of using a rail that has a continuous arc in its length, either concave or convex. My mistake. Slainte.

 

[Doug71]

Cut a few mill off each edge and try them together to see if they are straight, if they are straight it means the rail is straight and it was probably tension in the sheet.

 

[peter-harrison]

I have tried a couple of cuts -going very slowly in case it's harmonics in the blade making it wobble. One cut, in a bit of veneered MDF, closed up perfectly, while the other, longways down a sheet of MF MDF, had a 2mm gap. So not very helpful! The gap is tapered like a very squashed rugby ball. The saw fits well on the track with no movement.
If it was tensions in the board, these would show up when I was cutting them on the sawbench- which doesn't happen.
I think I'll start by getting a new blade and seeing what happens

 

[MikeK]

If the rail wasn’t straight he would still be able to push the two boards tight together. after thinking about this it could only be two things , the sheet is bowing after cutting or the blade is wobbling or track not tight to saw whole time. Unless I’m forgetting something.

Even if the rail was a gentle S shape it would push together after the cut. If it isn't meeting evenly when offered it is the material moving after the cut. Try a cut half way through the sheet and see if the starting end closes up. If it does it is the material.

Pete

Shooting from the hip on this, but I think your statements would be true if there was only one cutting edge of the tool in play, such as with a jigsaw or bandsaw. The trailing edge of the circular saw blade can also cut into the piece on the outside of the arc if the rail has an unwanted curve somewhere along its length. This could account for the ends touching and a gap in the middle. I would expect to see similar variance in the anti-splinter strip on the rail.

If the gap in the middle is present with every cut, then you are either consistent in an incorrect technique or there is a problem with the hardware. I would rule out the hardware first by either trying a different rail or verifying the rail you have is straight. The second method will require using something you know is straight and not cut with that saw, which can be placed along the edges of the rail. I would try the factory edge of a new sheet of plywood or MDF. While the factory edge might not be square to the other three sides, it should be straight.

A simpler test of straightness might be to lay the rail on a flat surface and draw a fine pencil line along the back and front edge of the rail. Then rotate the rail 180 degrees end for end and compare the line for each edge. This is an easy way to check a carpenter's square for accuracy.