Building a new crosscut sled and have a question or two

[lcm1947]

I've been reading up on making one of these sleds and kind of decided on some ideas but wanted to ask for opinions on a couple of things. I plan on making the base out of 3/4 " MDF. I think this would not have a tendency to warp like plywood. Opinions? I also plan on gluing two pieces of plywood together to use for the fences both front and back. I think plywood this thick glued together would eliminate any chance of warping and the flatness of the ply would more guaranty a straight flat surface then any other kind of wood I could use, plus it's pretty cheap. Opinions? I also plan on using some Deck Mate all purpose exterior screws to attach the fences to the sled bottom. I think these would do a good job but am no expert on screws but it's what I've built all my work benches with and they seem to do a good job. Opinions? Lastly I've seen where some guys use some backers to reinforce the fence but most don't seem to. In your experiences do just screws suffice? Any info here would be helpful as I really can't see them holding the fences on too well especially if you carry the sled around by them as I see myself doing. Anyway oh and one last question. Do you suggest screwing the runners on from below or above the sled. Seems everybody does something different on this and I wonder which is best. I'll be using the plastic stuff for runners by the way not wood. My understanding is that if screwed on from beneath the screws allow you to flatten the runner if you need to tighten up in the miter groove. Oh forgot one really last question. I plan on having the back fence 2" high. My hands are kinda small and if I'm to hold down the material I'm cutting on and push at the same time seems they shouldn't be too high or otherwise I couldn't reach the material. Sorry for the long post but so many questions. Thanks.

[drumpriest]

For your rear fence, mait it HIGH at the cut point (where your hands SHOULDN'T go), and cut a curve in it down the sides so that your hands fit comfy where they'd be holding the wood. Runners are actually available from Incra, made to fit a standard miter slot. I think they come with plans for a cross cut sled. I'm not sure how they'd wear in a BT3k aluminium slot though.

[lcm1947]

drumpriest, I actually already bought the UHMW 34"x3/8" standard miter slot size stuff from Peachtree so already set there. They seem to fit well in my 3100. Good one about the danger area though. I always wondered why almost all the fences had that hump. Thought it was to add strength to the fence. Could be rather important I guess if you weren't paying attention. I'll glue something on there for sure.

[RodKirby]

I actually drilled and tapped the UHMW (1/4-20). I then dropped 1/4-20 pan head bolts thru the top of the sled base - holes counter bored to fit the screw head, and 9/32" thru hole. This gives some wriggle room to align runners. Just be as careful as you can, in locating the holes.

Drop the saw blade down so you can go thru the alignment process.

After you have the sled base sliding smoothly, with no "slop"...
Do the following - IN SEQUENCE! (I'm assuming you're making a 90° miter sled).

1. On the underside of the sled, mark where the screw holes will go for attaching the fences. Predrill/countersink the clearance screw holes.
2. Attach (yellow glue/screw) the back fence (furthest away from you).
3. Set the saw height you want (to not cut right thru the fence).
4. Make a cut about 3/4 of the the way into the sled.
5. Place glue on the underside of the front fence and, using a drafting triangle (or similar), VERY CAREFULLY, align the front fence so it's 90° to the saw blade (not the teeth). This HAS to be accurate!
6. Once you're sure it's aligned, place weights (hammers/bricks) on the fence - check alignment again.
7. Walk away - don't touch it for at least 2 hours!
8. Gently turn the sled over and drill the pilot holes for the screws. TIP: If you use the same drill bit you used for the clearance holes, running BACKWARDS, you get a nice center dimple for the (smaller) pilot holes! Insert the screws.

NOTE: This pic shows how I aligned this fence for my 45° sled, but I used the same technique for my 90° sled

[lcm1947]

Thanks Rod. Beautiful sled by the way. Mine won't quite look that good. I appreciate the help and detail. I got everything except the part where you stated you drilled and tapped the UHMW. Why did you tap the UHMW? The screws heads are in the sled not the UHMW. Right? So why tap the UHMW? I guess I'm not seeing that part.

[RodKirby]

Thanks Rod. Beautiful sled by the way. Mine won't quite look that good. I appreciate the help and detail. I got everything except the part where you stated you drilled and tapped the UHMW. Why did you tap the UHMW? The screws heads are in the sled not the UHMW. Right? So why tap the UHMW? I guess I'm not seeing that part.

I should have said: 1/4-20 pan head BOLTS - not screws. Hence the need for the tapped holes.

Also, on the second sled, I did it in reverse, ie I counter bored/drilled the UHMW and screwed (no bolts) into the underside of the base - much cleaner look!

[lcm1947]

Ok, got it Rod and thank you again. I sure appreciate you taking the time to help me out. I am a very cautious person by nature and really need to fully understand stuff before I do something. Drives my wife and friends and probably a lot of great guys on this forum nuts but that's me. I think I'll do it with screws too. Bolts sure would be strong but I think screws would do fine and like you said nicer looking. Anyway, appreciate it and be seeing you around.