denovo said:
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The only way I could get a straight cut was to place the connected rails on the wood, use a string line to make the rails straight and have someone stand on the joint while starting the cut to prevent it from pivoting in the middle.
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What I found the "issue" is also with the Festool anti-slip strips working, a bit too well ...
This means that with the rail being 180mm wide at a 3m and more spans the rail-bendign also comes to play (point 3):
1)
the "old" Festool connectors could not be tightened sufficiently /without denting the rail/, allowing movement. The Makita, TSO or the new self-aligning Festool connectors are much better here, Makita's still allow the tightest connection thanks to being coated with the paint acting like an "adhesive" with the aluminum
2)
the accuraccy of a connected joint depends on the accuracy of the last 200mm of rail form each side of the joint, for those who mess with metrology => this makes two rails with, say, overall deviation of 0.1mm have a "connected" deviation of 0.5mm _even_assuming_a_perfect connection_
3)
the lateral forces from the anti-slip resistance not only can move the connectors (especially the old Festool ones) but can even institue a sufficient bent on the rail, the bending can be to the tune of 2mm (0.1") over the 5m run if one "forces" the rail position /without lifting it/
The way I use to solve these:
1) This one is simple:
Do not use old-style Festool connectors for such long setups, go for Makita (ideal), TSO or new Festool self-aligning set.
2) This is simple for pros, tricky for hobbies:
Use a 3rd (4th) relatively long rail or a straight edge (1400 or more) to "place/align" the rails when connecting.
note: Yes, this has limited effect with self-aligning connectors as they will "want" to miss-align even so slightly, hence why you should really have a set or two of Makita ones for these purposes.
3) This is a procedure, applies also to single rail above 2000mm or so:
Place the rail gently, so anti-slip strips do not "overpower" the stiffness of the rail:
- place the stock on an even undersupport /within means/
- first place the rail "vertically" on its back on the stock
- go to the middle and gently flip it to be flat, this should prevent excessive force on the connectors
(two people may be needed for a 6m run like yours, I had a 2700 + 1400 + 600 at the time so was OK in one person)
- THIS WILL NOT BE ENOUGH for the rail to be straight
- lift one of the rail in the air (such that about 2m lifts off the surface, so the anti-slip strips do not get into play) and gently place it down again, after this, DO NOT ADJUST this end
- repeat with the other end
At the end, to make sure your workflow works, take a laser range finder to validate you got a straight line, adjust as needed (the rail will bend to the tune of 1-2mm so you can "force it" if you have a reference line).
Oh, and to spike the Maffel fans, theirs are even worse. By being narrower, they bends even more when too long and them having no "non-self-aligning" connector option, there is no way to compensate with a straight edge or another rail so the errors add up ...