Self aligning technology for crosscut sled


I am thinking about building a mitre sled for my table saw. I would like to build something that aligns itself to do a 90 degree cut automatically. Not having done this before and not wanting to do a trial and error approach I was trying to work out a way that was self aligning. I was wondering if people would like to comment on feasibility.
What I was thinking of using as the reference point for the cut was the angle between the front edge of the tablesaw top and the mitre slot. That is 90 degrees on my unisaw to within the measurement tolerance of my not very expensive square.
I was thinking using MDO plywood for the sled and either more plywood or hardwood for the front fence.
The idea is that I start off with straight edge on the plywood and make a dado about 0.5" from the lower edge of the fence. I then put the runners in the mitre slots and align the face of the fence with the front of the saw. This gives me a face that should be exactly 90degrees to the mitre slots. Finally I cut off the surplus fence.
I am trying to work out if this would save time or end up with a fence that is slightly out because of something I have overlooked and impossible to adjust.
What do people think?
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That is exactly how I made mine. I built mine so that both runners were in the mitre slots with hot melt glue on top of them and then laid the top on making sure it was square to the leading edge of the table saw. I then cut my end off the crosscut base to make it parallel to the blade. After I checked for 90 degrees I then routed a 3/4" dado 1/4" deep along the front edge of the slide to place my fence. I have had it about 5 years, check it every so often and it is still 90. Of course I have added adjustable stop blocks and a few other jigs to it for other cuts, but works better than anything I have seen commercially. The only thing to make sure is that the front edge of the table saw table is 90 degrees to the mitre slots on the table and that they are parallel to the blade.
Jon

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You can make a sled long enough to cross cut an entire 4x8 sheet. Make and fit a 60" runner from 1/4 Masonite (Duron board is the best I know) then glue it to a 20"x60" piece of plywood using the straight edge of a piece of sheet stock to keep it straight. Posistion the runner so that when you run it through the saw the first time it will cut off a little of the 60" edge of the sled. The runner does not have to be positioned square, just straight. Put on the fence near the away from you end of the sled. Use an acrylic graphics triangle on top your fresh cut to achieve squareness. The acrylic square allows you to see right through the edge making even the slightest error apparent. Hedge your effort and screw down the fence with just 2 screws if it is not right just adjust it.
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On 20 May 2005 16:46:40 -0700, "Phillip Hallam-Baker"

it's fine as a place to start, but before you go fastening it down too well, check it to see if it makes a square cut.
there are a number of ways to get an accurate check of a square cutoff jig like that without relying on an unreliable square. the method where you make sequential cuts on a scrap, rotating it 90 degrees between cuts is easy, cheap and plenty accurate.
somebody here have a better description?
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"Phillip Hallam-Baker" wrote:

<snip>
Piece of cake.
1) Build a sled. (I like 13 ply or 3/4") 2) Cut off the corner of the same sheet of 13 ply, 18" on a side and mount to sled. 3) Futz with it a little bit to get it aligned and you are done.
See Fred Bingham's book, Practical Yacht Joinery for details.
Lew
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