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

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Forum topic by Jackdaw posted 07-05-2017 12:46 AM 1088 views 0 times favorited 3 replies Add to Favorites Watch
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18 posts in 2063 days

07-05-2017 12:46 AM

Topic tags/keywords: humidity change sled runners runner slop

So yesterday I made the sled part of a crosscut sled for my tablesaw. I used maple for the runners and made them a tiny bit (one piece of hard thin cardboard, uh, dang it, I’ll go measure….. 0.015”) thinner than the slot and shimmed them to the outside. Then I glued the sled on to them. It slid very nicely after a touch of sandpaper to the runners.
This morning I went out at 6:30 and started putting the fences on. No problem, still sliding nicely. I came in for breakfast and just went back out to square up the aft fence and there was slop in the runners, I could shift the sled a degree, uh yaw-wise, if you get my meaning. So what the heck? I thought the runners shrank but really, enough to make a difference? Not likely. I came in just now and checked the humidity, at 5:30 it was 79%, now, at 9:30 it is 66%. So I wonder if that was enough change. It was certainly fast.
ANYWAY, I guess I’ll see. It will get humid again this evening. Phooey in any case.
What do you folks think? Is that enough to do it? It’s not the hardest maple ever. I’m not too concerned, I might put some anti-slop screws in the side of one of the runners. Or I might just leave it.

3 replies so far

View Mikenln's profile


13 posts in 1778 days

#1 posted 07-05-2017 01:03 AM

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.

View Jackdaw's profile


18 posts in 2063 days

#2 posted 07-05-2017 02:55 AM

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!

View Robert's profile


4432 posts in 2486 days

#3 posted 07-05-2017 12:52 PM

Its an issue for everyone.

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

-- Everything is a prototype thats why its one of a kind!!

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