Plywood types

After reading up on table saw jigs (sleds, aux fences, etc), the materials most often recommended seems to be baltic birch, and maple plywood.
I found a source for baltic birch (which comes 5x5). The difference is obvious from the plywood available at the local borgs, but they had other (4x8) hardwood plywoods that seem to have a similar ply structure; how would these compare to the baltic birch?
This same source also had about 8 bins each holding a different kind of "maple plywood". Can anyone clue me in to the differences among the various maple productions?
Also, how much better are these (specialty?) plywoods. For instance, is 1/2" baltic birch just SO much better as a sled base than, say, 1/2" MDF?
For fences (either rip or crosscut), I would think a good plywood would stand up better than hardwood, due to stability. But still some authors recommend using hardwood for fences (most notably on a crosscut sled). Is it all just personal preference, and the choice of material just splitting hairs?
/rick.
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"RickS" <rick --dot-- s --at-- comcast.net> wrote in

OK, Rick. Let's talk philosophy for a moment. Then take what you've learned and make a few of your own decisions.
Sleds and fixtures need to be sturdy, flat and reliable in dimension. Whether you are going to use them all day every day, or pull them off the hook twice a year, the reasons to use a jig all pretty much require that you can count on it for safe, repeatable results. Any material that delivers that in your shop environment, in your use pattern, is a candidate. Beyond those factors, it gets into personal preference.
MDF certainly has a place. Flat, reliable and relatively cheap. Properly sealed, it's dimensionally stable, particularly if moisture and/or humidity isn't a challenge in your shop. One of our stalwarts here called it something like 'the first cousin of cast iron'. (he turned the phrase better, but I'm not going to go look it up for the reference right now.) Join it to the fence(s) properly, and use it in good health. David Marks seems to use truckloads of the stuff for patterns and fixtures.
The last batch of BB I got seemed to have problems with staying flat. I managed to 'encourage it' into square & plumb cabinets, but I wouldn't have used it for a jig, where the twist couldn't be controlled. YMMV. Mine usually does.
Hard woods for fences make a lot of sense, because they are reliable, and easily fastened. I generally clamp a sacrificial face to mine, particularly when I'm using it for mitered cuts. It works like a zero- clearance insert on the table saw, and shows exactly where the blade will cut. I use cabinet ply offcuts, because that's what I have. The vertical grain douglas fir rails on my crosscut sleds all came from a really nice, straight, kiln-dried 2x6 from the lumber yard. Ripped in half, I still have material left. That's what I used, because that's what somebody at the wood club said would work. Doug fir isn't a hardwood; just a hard wood.
Here, finally, is the point. (you mean, he's finally going to make a point?) Most any of these options are in use all over the wReck, most of them safely. They most all of them would work for you. My guess is that what you're trying to build is furniture, or cabinetry, or boxes, or something. But jigs are just a means of getting to the goal. Find something to get it done, with what you know now, with what's easily at hand, and move on to the real goal.
Jigs that don't work can be used to heat the house, come winter.
Have fun. And remember, it's USENET. Opinions abound. Don't trust them all. Even mine. Especially mine.
Patriarch
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And there's always MDO. Especially nice for jigs you want to mark settings on.
--
Where ARE those Iraqi WMDs?

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You most certainly will not find a consensus here. Hopefully you will collect some replies and glean what works for you.
For me, I have had too many problems using any type of plywood for sleds, fences, and zero clearance plates. Plywood often just won't stay flat over time. MDF stays flat pretty well, but I find the edges too soft and flaky. Bang a corner of your MDF sled on the saw table and you have an upset to deal with. All of my current collection of zero clearance inserts are 1/2" baltic birch, and it has taken a couple of years but they just ain't flat enough anymore. Baltic birch might well be my LAST choice for a sled.
I am using more melamine these days. All my current sleds are melamine. I have a couple of clamp on fences for the TS, again melamine. Next time I make zero clearance inserts I'm gonna look for 1/2" melamine, although I will have to find a project first because I will need the scraps to work with. Melamine seems to stay flat no matter what. The corners will chip, a weakness, but not as bad as MDF.
Just one humble correspondents opinion.
BTW, baltic birch is available in 4 x 8 as well as other sizes. Might that be what you saw?
-- Bill Pounds http://www.billpounds.com/woodshop
"RickS" <rick --dot-- s --at-- comcast.net> wrote in message

would
various
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