TS 75 varying kerf

[whyland]

I have a TS75 with the 1900mm (75") rail and have been using it to cut a 5' x 5' x .7" sheet of Baltic birch into 5' long by 5" wide strips.  I noticed that, after completing my cuts, if I pushed the two freshly cut pieces up against the remaining plywood the ends would meet up nicely but there was a noticeable gap along the center of the cut. After cutting a couple of strips, I measured the width of the outer ends and the width of the center of a strip with calipers and found the center width to be 0.01" less than the width at each end.

Procedure:
- Lay sheet on top of two pieces of 1" rigid foam with about a 1/2" gap between them for blade to run through.
-Checked the factory edge of the sheet by eye for straightness (not the best way but it looked straight and when I sight down my problem strips the 0.01" "bow" was very noticeable in comparison).
-Set a combo square to 5" + blade kerf and set the edge of the splinter guard (the splinter guard had previously been cut to match the saw for full length) on the rail up against the end of the combo square at each end of the panel.  Adjust at each end back and forth until splinter guard is snug against end of combo square at each end.
-The rail extended past the sheet about 9" on the starting end of the cut and 6" on the far end. I supported the ends of the overhanging rail with some scraps the same thickness of the panel. My cams are set so the saw doesn't wiggle on the rail but still slides freely.
-I put the kick stop in right at the end of the guide rail and but the saw up against it.  This puts the center of the blade pretty well right over the edge of the plywood so I begin the cut by plunging in then advancing at moderate speed listening for any sound of the saw bogging down and stopping when the blade exits the far edge of the plywood.

Has anyone else had this issue and if so have they found a fix?  I have read that the saw comes set with a slight toe in attitude, could this be causing my issue due to beginning my cut on a plunge or is 0.01" an acceptable deviation over 5'? I would like to use the saw to build cabinet cases so I am trying to work out the kinks in advance.  Any replies/criticism are appreciated.

 

[squall_line]

Is the rail straight?

If so:

Is the rail clamped during the cut?

What is your hand/arm position when pushing the saw?

What is your previous cutting experience before the TS75?

I ask these things in combination because my own experience prior to Festool was with a straightedge clamped to a folding plastic workbench with a standard circular saw.  Because it wasn't a rail saw, I have a tendency to want to push the saw towards the rail to keep it straight and true.  As a result, a longer rail may have a tendency to flex in the middle and produce gaps like what you're seeing.

I need to re-train my body and muscle memory to get my hand and arm behind the saw (even when walking next to the cut), let the saw do the work, and trust that the rail will be straight and true, instead of forcing the saw sideways to keep it from walking away from the rail (which it can't do because it's on top of the rail, not next to it like a straightedge).

 

[whyland]

The foam is set on the concrete floor of my garage and I am crawling on the panel on the guard rail/handle side of the saw.  At times I would have only one hand on the trigger of the saw but mostly I kept my left hand on the forward handle or I tried keeping some pressure on the front left corner of the saw base with my hand.  The rail was not clamped to the piece, so if I had moved the rail due to pressure I would expect that the thickness of the strip at the ends would be different, but ends the ends were measured to within 0.001" of each other.  That said, I don't think I was applying sideways pressure and if it had been only a matter of deflecting the rail, both sides of the cut should have been equally bowed and mated back together without the gap I see in the middle.

 

[mino]

If the width is consistently wrong, the issue can easily be the plywood.

Even plywood can have built-in stresses and once cut into strips there is little preventing the material to "un-stress" by shrinking here and there.

When you need "strips" from a material to be precise, you need to make each edge twice. First cut the strips to approximate width, say about 0.2" more than you need. Then "finish" both sides to make them straight. You need to "finish" both sides as the whole strip will bent once cut.

This is tricky with a tracksaw as you have no easy way to prevent the thin moving against the rail. But if you are just making full-length strips, you can simply screw-through on the ends to some material below.

Even assuming your rail is perfect and your saw cams are tight, doing these types of "finishing" cuts is the only way to get true and equal-width strips.

That said it is mostly not worth it. Wood is not aluminum ... even plywood will again warp with moisture, what was straight when cut will easily deviate 0.02" or more when humidity changes. You need MDF if you a need stress-free material that can be processed like wood can and will not change shape all the time.

I found out when making an ad-hoc "shop rule" from plywood. The resulting strip from 3/4 ply was about 1.5" wide and came out bent by about 0.1" when fresh-cut. Width was also way off too from the shrinkage which caused the bend. After a finish cut it remained OK, but is in a stable humidity area. I still check it once in a while against an engineer's edge.

 

[Roachmill]

If material isn't fully supported on a *flat* surface you can get wonk in the cut - as in the material droops and the rail goes with it resulting in a slightly curved cut. I can think of two things from how you've described your process above:

1) the concrete floor isn't flat - you would not be alone there!
2) the foam is possibly compressing in the middle as you move over your panel.

Or it could be a combination of both... or neither  [big grin]

 

[squall_line]

I will also gently point out that a TS75 for 3/4" plywood sounds like a mismatch between the tool and the job.  Full support of the material may be to blame, as stated above.  And if it's on a concrete floor, is there a dip or droop in the floor for drainage?

 

[whyland]

It is possible that my garage floor isn't as flat as I need it to be for this operation.  I will try finding a flat section and retry.  Internal stress in plywood is also something I hadn't considered.  Thanks for the suggestions.  A dedicated cutting table may be in the future...