Tom....
have you read Charlie's review...?? It might help some..
https://www.ukworkshop.co.uk/review/review.php?id=21
I think I need to clarify some points for ya...
A large panel cutting jig is a poor man's sliding table; it's no more than a large piece of fairly thin ply (12mm is perfect), a key fixed to its underside to ride in one of the mitre slots, and a back fence. The critical parts are that all 3 pieces are fixed perfectly; the key being square with the back edge of the panel, and the back fence being square with the saw blade. When you build it, the trick is to deliberately oversize the dimension between the right hand edge of the board and the guide key so that it will extend past where the blade will be. It's first pass past the blade removes the oversized section leaving an edge that is perfectly parallel with and flush to the blade. You can then use that fresh cut edge as your reference point. It also provides prefect support for the piece being cut, serving in a similar way as a zero clearance throat plate would in preventing any blow out on the underside of the piece being cut.
One other essential piece of kit while using the panel cutter, is an adjustable roller bearing support; trying to work on a small saw without this makes it impossible to use the panel cutter; there's too much weight overhanging the table.
In use, I'll be honest, it can be a bit of a pain if you're working by yourself. An "assistant" to operate the start button can be useful if you're cutting really big stuff.
To my mind, where it scores hands down over a fixed sliding carriage is that when you're done with it, it can be hung on the wall outa the way.
For small crosscutting work, there's nothing that can touch a cross cutting sled for accuracy. It's construction is similar to the large panel cutter, only this time you attach 2 keys to the underside, one to suit each mitre slot, as it's supposed to straddle the blade. Additionally, you need 2 fences; one at the front just to tie both sides of the base board together, and a taller one at the back to support your stock against. The additional height serves to keep your fingers well away from the saw blade. Now, although you need to remove the guard from the saw to use this sled, you can leave the splitter in place provided it's pretty damn close to being the same width as the saw blade. For safety's sake, there's nothing to stop you fitting a piece of perspex between the 2 fences to effectively box off the blade. This time around, with regard to cut quality, it operates exactly like a zero clearance insert as the slot in the base board is only as wide as a saw kerf; absolutely no blow out on the underside of either the panel or the offcut.
There's an argument that says that with the addition of suitable tapered inserts, a cross cut sled can do away with the likes of a SCMS, although personally I wouldn't wanna put that to the test.
Again, just like the large panel cutter, once you're done with it, you can hang it up (on something substantial) outa the way till it's next needed.
Crosscut sleds needn't be confined to producing square cornered panels; I've seen variants built to produce perfectly mitred corners for picture frames for example; a set up that blows a chop saw outa the water for speed and repeatability.
Provided you take your time, being very fussy with regard to checking that everything is square when you build them, these jigs can earn their keep for years.
