How flat must a crosscut sled base be? 1/32" cup OK? Baltic Birch or MDF?

[Erich Weidner]

I had earmarked a leftover piece of 3/4" baltic birch to build a wide crosscut sled (36" capacity). I was going to make this the length of my tablesaw (44")

However I notice a slight cup in the panel. about 1/32" at its worst at the center of the panel.
I see that a lot of the woodworking magazine sites show sleds all built with MDF. I was concerned that an MDF base wouldn't stand up to years of use. But I just checked a 3/4" and 1/2" cutoff which had been leaning against the wall for well over a year and they have remained dead flat... so maybe MDF is the way to go? This is to be a big sled with a thick hardwood rear fence so I wanted to minimize weight if possible.

On that note, could I get away with a 1/2" base, or should I stick with the original plan of 3/4"?

So Dead flat MDF, or slight cupped Baltic Birch? (Of course I'll have to go buy another sheet of MDF... :rolleyes:)

[Prashun Patel]

dead flat mdf. If yr board is cupped, it will cause your target board to tilt relative to the blade, which won't give you a square or perpendicular (or both) cut.

I've made sleds out of 1/4" hardboard, 1/2" ply and 3/4" mdf. I actually prefer the thinner stuff; It allows for a slightly deeper cut.

Also, I wouldn't make the sled 44" long. Make it shorter. You can either just make a longer front fence, or make an auxiliary fence that you can clamp to the short fence if you need to cut longer pieces with a stop.

[Bill Huber]

I agree with Prashun and to me thinner is better. If you use 3/4" ply there is no flex in it at all and if there is a little bow it will be there all the time. If you use thinner stock then if there is a bow it will come out when you lay something on it.

[Erich Weidner]

I've made sleds out of 1/4" hardboard, 1/2" ply and 3/4" mdf. I actually prefer the thinner stuff; It allows for a slightly deeper cut.

Also, I wouldn't make the sled 44" long. Make it shorter. You can either just make a longer front fence, or make an auxiliary fence that you can clamp to the short fence if you need to cut longer pieces with a stop.

Do you think 3/4" MDF then? What length would you suggest? (This is my first crosscut sled, I was trying to make sure it could handle panel width sheet stock that I've already ripped and now need to accurately cross cut. I'm tired of ruining these cuts trying to make due with an extension fence on the miter guage. Just isn't stable enough).

[Erich Weidner]

Thanks Bill.
Since I can't seem to locate any dead flat plywood (even baltic birch). Should I use 1/2" MDF, or do you think 1/2" BB will get pulled flat enough by the hardwood fences and the work pieces themselves? I guess I'm just afraid MDF won't hold up, but I guess it is better to have to rebuild the sled base every now and again, vs not having accurate cuts. (Which is of course the whole point of this exercise).

[JohnT Fitzgerald]

Erich - if that is your use case (solely larger panels), then I would suggest that 1/2" would be fine.

I have seen plans for a TS sled that incorporates T-tracks, for hold-downs and assorted accessories. for doing that (incorporating T-track), i think 3/4" would be a necessity.

[Erich Weidner]

Erich - if that is your use case (solely larger panels), then I would suggest that 1/2" would be fine.

I have seen plans for a TS sled that incorporates T-tracks, for hold-downs and assorted accessories. for doing that (incorporating T-track), i think 3/4" would be a necessity.

Actually I was thinking about adding a T-track for a hold down... more food for thought. I wonder though if routing a channel for a hold down would weaken the MDF to the point it would risk breaking. I guess the solid wood front and rear fences would compensate (he wonders...).

[JohnT Fitzgerald]

Actually I was thinking about adding a T-track for a hold down... more food for thought. I wonder though if routing a channel for a hold down would weaken the MDF to the point it would risk breaking. I guess the solid wood front and rear fences would compensate (he wonders...).

Good point on weakening the MDF (which i why mentioned going to 3/4").

Another option would be to put the T-track on the inside face of the rear fence and use another type of hold-down to hold the piece firlmly on the sled.

[glenn bradley]

Actually I was thinking about adding a T-track for a hold down... more food for thought. I wonder though if routing a channel for a hold down would weaken the MDF to the point it would risk breaking. I guess the solid wood front and rear fences would compensate (he wonders...).

I use 3/4" BB ply. Depending on where your bow is (what direction) a full length front and rear fence could flatten that out. I would get a new piece of material. Sleds aren't that involved to make but still, why put out the effort and not go the short distance more of doing it right. On the sleds I used t-track on I just ran the dados directly over the guide rails and screwed through the track and sled base into the rails. Sleds for large panels would not really benefit from holddowns IMHO due to the material being heavier. You may want to consider multiple sleds. For larger panels, folks often use a large bed with a front fence only. In either case, front fence or both, for a sled scaled to larger panels I would be tempted to use the lighter material; a half sheet of 3/4" MDF is about 45 pounds before you add fences,r ails and your material. I assume 1/2" MDF is about 33% lighter and has a bit of flex once you get past a certain span width. So does ply for that matter ;-)

[Joseph Tarantino]

...... Depending on where your bow is (what direction) a full length front and rear fence could flatten that out....

+1. my only thought is to use 1/2" material, with 1/4" on top of that. by spacing the 1/4" properly in the path of the blade, it provides for a removable insert, so many different blades may be used with the same sled.

[Erich Weidner]

OK, I decided on a 48" length and 40inch "width" (maximum size of panel I can crosscut).
I was out today and picked up a sheet of 3/4" MDF, so that was what I ended up using. I have the front and rear fences installed, and am now ready to add the miter slot runners.

Question is, do I center the sled on the blade, or offset it? I have a right side extension table (36" cut capacity with the fence), so I was thinking I should offset it in that direction so there is much less of the sled on the left side of the left miter slot. with a 48" length overall should I position the left side 8" or 16" from the blade? With either of those ranges, the center of the sled doesn't get too far to the right of the blade to be uncomfortable to push near the center of the sled with my right hand.

Thoughts?

[Jerome Hanby]

Assuming that you will have runners in both miter slots, I'd be tempted to just center it. Be easier, evenly supported, and have about the same stress on each runner so maybe less issue with binding.

[Prashun Patel]

I make my sleds projecting mainly to the left side. The right side overhangs the miterslot by several inches. This prevents me having to move my fence ALL the way to the right to use the sled (my right wing is usually serving as a table, so this is a headache saver for me).

Also, I use the sled for trimming and nibbling the ends off of smaller pieces. Having a short right wing on the sled just allows more control, IMHO. I almost never wish I had more right side space.

I appreciate that you want a 48" sled. To save some weight, remember that only the front fence really needs to be long to support longer stock. You can taper the left side of the sled so that the front is 48" and the rear fence is say, 30"-36". It will lighten the sled marginally without any compromise in service.

[Erich Weidner]

I make my sleds projecting mainly to the left side. The right side overhangs the miterslot by several inches. This prevents me having to move my fence ALL the way to the right to use the sled (my right wing is usually serving as a table, so this is a headache saver for me).

Also, I use the sled for trimming and nibbling the ends off of smaller pieces. Having a short right wing on the sled just allows more control, IMHO. I almost never wish I had more right side space.


Good points. I think I'll set mine up the opposite way.
I keep the extension table clean, so as long as I leave enough space to allow the fence to remain on the table while still using the sled I think I'll be happy. (Obviously still would have to take it off for really long panels)

[Carl Beckett]

I make my sleds projecting mainly to the left side. The right side overhangs the miterslot by several inches. This prevents me having to move my fence ALL the way to the right to use the sled (my right wing is usually serving as a table, so this is a headache saver for me).

Also, I use the sled for trimming and nibbling the ends off of smaller pieces. Having a short right wing on the sled just allows more control, IMHO. I almost never wish I had more right side space.

I appreciate that you want a 48" sled. To save some weight, remember that only the front fence really needs to be long to support longer stock. You can taper the left side of the sled so that the front is 48" and the rear fence is say, 30"-36". It will lighten the sled marginally without any compromise in service.

Same here. I modeled my latest from the fine woodworking article "Ultimate Crosscut Sled" (FWW #199). The entire sled is to the left of the blade. The side tapers off to reduce weight but still provides plenty of support (per Prashuns suggestion) - mine is only about 12" at the top edge - works fine.

[Prashun Patel]

Erich-
Here's something else to consider: Crosscutting anything wider than 12" can be a challenge bkz the sled hangs off the front of the saw. It's hard to register the piece to the fence without infeed support.

For cutting larger panels, I have a sled that only uses the left miterslot, and has only one rear fence (no front fence). The fence ends at the blade; so there's nothing to the right side of the blade.

The piece is cut by pushing it against the rear fence (which is perpendicular to the blade). This allows you to have almost the entire piece hanging off the front of the blade (since yr weight is on the rear fence) and allows a much greater cross cut capacity than a 'dual bounded' sled.

It's surprisingly accurate, and allows you to keep the guard in place (which for me is a big deal bkz mine has dust collection).