Tablesaw Jig Question

I was going to make my first jig, a sled for my tablesaw. I bought a piece of 3/4" MDF and some metal rails. Then, I bought a new house and let the MDF sit for a while. I went back to work on the jig and see that the MDF has a slight bow in the middle. I read that many people are using MDF for jigs, but it seems to me that this MDF has bowed awfully easy. I could use some help on either how to deal with MDF bowing or a better material to make jigs from.
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If you put a plastic laminate on both sides it will help with the bow. In addition I have to shelves under my right table extension that are as large as my left and right sleds and a slot for the guide rails to set in. This way the sleds sit perfectly flat while in a storage position.

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Depending upon the orientation, the bow will often straighten out when you mount the front and rear fence on the sled.
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What kind of wood would you recommend to use to make the fences?

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Not to beat this to death, but if it is stable, why not use Baltic Birch for the sled bottom too? Is there some reason that a MDF sled with birch fences is better than all birch? Thanks.

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I _do_ use 1/2" BB for my sled bottoms ... _you_ were the one with the MDF for the bottom. :>)
Also, I prefer 1/2" BB plywood so that I don't lose too much blade cutting height.
Note that BB is also prone to have a bow now and then .... just orient your sled bottom so that attaching the fences takes the bow out.
A good table saw book like Kelly Mehler's has ideas for sleds and sled plans. Also, the meta Inca "miter sliders" come with plans that you can use as a departure to roll you own ... they also have good tips on getting the jig square to the blade, should you need help in that regard.
I have sleds with both hardwood runners and metal runners, and prefer the metal adjustable runners ... but hardwood scrap is cheap. Just don't glue the hardwood runners on and you can change them out when necessary.
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In a word: Rigidity.
With regard to the 1.5" thickness, we were talking about fences, not the bottom of the sled. IME, a 3/4" thick fence on a table saw sled does not provide enough rigidity.
1.5" is certainly not a magic dimension, just the product of doubling what's on hand. Height of the fence is up to you
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