MD ply

[Alan Turner]

I have to make an entire set of jigs for the new shop, and first up is a sled for the TS. My preference is to use MDF becuase it is so flat. I'd like to go with 1/2" becuase of weight issues, and so that I can maximize the blade height. That stuff sure is heavy. And, the 10" TS has a max cutting height of 3.125", so if I can end up with 2.625" above the sled that would be great.

The issue is whether I can inlay a strip of 3mm BB ply in the center section, maybe about 4" wide, which would be fully replaceable so that when I make a bevel cut I can then replace the strip with a fresh one to re-achieve a zero clearance insert effect. Pretty cheap stuff, of course, and pretty accurate in terms of thickness. I guess a side issue is whether, when there is about a 1/2" wide "groove" down the middle from beveled cuts, whether the 3MM BB will be stiff enough so I don't have deflection over the void. I could go with 3/4" MDF, and use a 6mm or 9mm BB insert, but would like to stay at 1/2" for total thickness. I may use carefully trimmed machine screws and t-nuts to attach the runners. I think I will be using alum. runners since I have a pair, with side adjustments for tightness. I bought them for some jig, then never used them, and now can't recall why I made that acquisition. CRS abounds.

The question is whether I can rout the 1/2" MDF to a depth of 3MM and have an accurate and smooth enough surface for the bedding of the 3MM BB ply. Any guidance would be appreciated.

Thanks for the help.

[Scott Parks]

Go for it, make a prototype and see how it works...

[Lee DeRaud]

The issue is whether I can inlay a strip of 3mm BB ply in the center section, maybe about 4" wide, which would be fully replaceable so that when I make a bevel cut I can then replace the strip with a fresh one to re-achieve a zero clearance insert effect. Pretty cheap stuff, of course, and pretty accurate in terms of thickness. I guess a side issue is whether, when there is about a 1/2" wide "groove" down the middle from beveled cuts, whether the 3MM BB will be stiff enough so I don't have deflection over the void. I could go with 3/4" MDF, and use a 6mm or 9mm BB insert, but would like to stay at 1/2" for total thickness.How about this? Main body of the jig in 1/2" MDF, so far so good. Put a 4"-wide inlay of 6mm BB on the bottom side...that one will be permanent. Now your "sacrificial" top inlay can be narrower, maybe 2", and made out of 6mm BB. Using the 6mm, I doubt deflection will be an issue over the 3/8"-1/2" gap it might have to span. You'll need to diddle the depths of the bottom rabbitt or add a shim to get the thickness to work out, but you get the idea...is there any BB or "apple ply" available in true 1/4"?

[Jim Becker]

The sled design will make a difference. Routing out for the replaceable inlay will substantially weaken the 1/2" MDF in that area, but...a design with both a front and rear fence will deal with that issue nicely.

[Jamie Buxton]

If you want to maximize the blade height, why use 1/2" thick material? Instead, use 1/4" plywood -- BB, Appleply, Finnply, or whatever you get in Philly.

One caution with using plywood that thin: run the face grain parallel to the blade. I once built a sled with the other grain orientation, and found that it would bend a bit when I pulled it off the table to start a cut.

Also, why go the trouble making replaceable inserts? Make two sleds. One does cuts with the blade upright, and the other does cuts with the blade beveled over at 45 degrees. If you ever need a bevel which isn't one of those two angles, well, worry about making a new sled for that angle then.

[Alan Turner]

Thanks, guys, but the question I was aiming at seems open.

The sled will be cut in half anyway on the first cut, and will have both strong front and rear fences, so weakening the strength of the MDF would not seem to be an issue. I was trying to see if anyone had tried to do an "inlay" of sorts into MDF, and how that worked.

Will the routed surface just be fluff, or will it be pretty flat and even and thus able to receive the 3mm BB held down by short screws?

[Gary McKown]

Taking a page from Pat Warner's method for making temporary miter slots. Laminate two sheets of, say, ¼" hardboard together, leaving the top with a space for your inserts, which are made from the same material. Might also work with the thin BB over MDF - I wonder how flat 3 mm BB laminated over ½" MDF would remain?

You could use that big, flat assembly table for the glue-ups.:cool:

[Tim Sproul]

Will the routed surface just be fluff, or will it be pretty flat and even and thus able to receive the 3mm BB held down by short screws?

The routed MDF surface will be fine on the assumption that the cutter is sharp and the routing is done well.

I don't know how much purchase screws will have in such a small amount of MDF. Seems like you need to rabbet the replaceable insert with the rabbets facing up and use cap screws in counterbored holes in the MDF sled base so that the screws don't sit proud. Not a whole lotta material to work with....I'd be thinking machine screws but that would also be pretty delicate threads for removing and reinstalling.

[Alan Turner]

Good point, Tim. I think maybe I will coutersink and epoxy in some 10-24 nuts on the bottom, and use short flat head machine screws. I planned to just rip out about 10 or so inserts and have them on hand for a good long while. I could also gang drill them so it wouldn't be a PITA later on. I am thinking I will also let in a rear replaceable plate as the kerf is a pretty accurate alignment tool, and make a bunch of those as well. I did this replaceable insert thing for the center of my drill press table and it has worked well, when the center gets chewed up. Of coudrse that was with 12mm BB plyu, not 3mm. For the rear of the sled I will use either 9 or 12 mm.
Thanks for the info. I'll post pix when it is complete. Working on the outfeed table right now as this is a hugely important safety device also.