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I've made a few jigs for the table saw that needed runners. What I found was that my runners were just perfect before I attached them to the jig but after I screwed them on, the screw seems to widen the runners just a tiny bit so that they don't run as smooth. This requires extra work to get them running perfectly again.

I notice people on YouTube gluing and then screwing their runners. I don't see why you'd need to screw them if the glue is doing all the work (considering that on most sleds the runners are not under stress).

I've also noticed that solid wood runners don't show this problem as much as UHMW (ultra high molecular weight polyurethane) ones. I've never tried metal or plywood runners.

Note: I always pre-drill my screw holes.

Has anyone found any problems with just gluing the runners?

  • Have you tried a comparison with screwing the runners on just snug versus as tight as they'll go? The UHMW polyethylene is very slightly compressible and may be swelling as it's tightly pinched by the screw. FWIW if you make runners from wood then glue-only should be fine as long as you do everything right, since glue joints really are stronger than the wood itself. – Graphus Feb 7 '18 at 8:07
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Has anyone found any problems with just gluing the runners?

One problem is that most glue doesn't stick so well to UHMW. Another is that when a glue joint fails, it tends to fail all at once: your sled could be guided by the runner one moment, and not guided the next, and that's bad news at the table saw. Personally, I'd feel a lot better with some mechanical fasteners.

What I found was that my runners were just perfect before I attached them to the jig but after I screwed them on, the screw seems to widen the runners just a tiny bit so that they don't run as smooth.

I can think of a few ways to solve that problem:

  • oversized holes: Fasten the runners from the bottom into the sled using pan head screws. Predrill the runner with holes that are slightly larger than the threads, and use a flat bottomed countersink so that the screw holds the runner to the sled, but there's still a little wiggle room. You can add glue to fix the position of the runner, or you can just tighten the screws enough that friction holds the runner in place. This last option would could let the runner get out of alignment, but it also gives you the ability to adjust the alignment.

  • tapped holes: If you want to fasten the runners from the top side of the sled, drill and countersink the holes in the sled, and then drill and tap holes in the runner so that you can use machine screws. With the threads already cut, the runner material shouldn't expand as you screw in the fastener.

  • adjustable runner: Don't make your runner quite so tight. You've already inadvertently discovered one way to make a runner fit more tightly in the slot, and there are countless others that would let you fine tune the fit of the runner after it's installed instead of trying to fit it perfectly beforehand.

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    A "flat-bottomed countersink" is called a counterbore, and I think Caleb is spot-on with that as the secret to avoiding spreading. @Julian you didn't mention, but I'm assuming you're using flat-head screws. The taper on the head of those will spread whatever you screw them into, how much just depends on how soft it is and how much torque you apply. Using flat-bottomed heads, like pan heads, will eliminate the direct spreading force applied. There will still be some deformation from "squishing" forces, but much less change in dimension I expect. I like the tapped hole idea too :) – scanny Feb 7 '18 at 3:55
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    I have been looking for an excuse to buy a tap and die set. You just gave it to me. Thanks! – Julian Feb 7 '18 at 18:47

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