13% humidity change in 4 hours, sled runner slop?

What kind of glue did you use? With white or yellow glue the wood will expand due to the water in the glue. I would be surprised by the wood shrinking that fast due to a change in the relative humidity in the air but things have surprised me before.
To reduce the expansion of the runner have the radial direction of the wood be the width of the slot, the 0.75 inch dimension. For a normal flat sawn or plain sawn board you want to cut a slice slightly smaller than the slot is deep. You then plane or sand the thickness to match the width of the slot. Also put finish on the runner. I would use shellac and wax.
Another alternative is to use HDPE plastic. I have bought some that is already the right size for the slots in my table saw. I bought some a a woodworking show. I think that I have seen it at Rockler.

Y’know, the other thing that occurred to me was that it was fine until I made the first full cut through the base. I probably built in some tension when I screwed the fences on then it sprung a little when I cut it. I wedged the kerf open as I was squaring the aft fence and it seems fine now. Still a smidge of slop but it cuts finer than I can see with a good combo square and bright light so…
All’s good, Thanks!

Its an issue for everyone.

From now on, I’m going to use Azek for my runners. ;-)

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