More accurate machining when using a sled?

[DickB]

A few weeks ago I was working on a pair of large sign/clocks for a customer:
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These top parts of the sign were machined from 3/4" MDF, 14" x 45". Due to the size, I did not use a sled, although I did adhere to the 7" rule. The parts are carved then cut out. When i machined the first set, the cutouts drifted over 1/4" and ruined the pieces. To finish the project, I redesigned it to eliminate the cutouts.

Today I'm working on another large 44" x 20" two-piece sign, and I need to use the cutout function and it needs to be accurate. I decided to use a big sled. It's built much like others I have made. This one is built from 1" pine boards. It uses my usual thumb screws to hold the work piece in place:
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I was very happy to see that this time the two parts were machined precisely to size and matched up well:
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I attribute the improvement to the sled, although I have no real proof. bergerud has shown that the brass roller can be ignored, he suspects when vibration makes that sensor unreliable. I think the added mass of my sled, along with the 1" rails keeping the cutting bit at least that far away from the brass roller, dampens vibration and keeps the brass roller sensor in play.

I'm curious what others' experience has been and what you think.

BTW since the last conference and on the recommendation of Joe I have been making full-depth cutouts, usually with the 3/16" cutting bit, and I have had no problems.

[cestout]

I make sleds only for something I will repeat many times - like bottle stoppers from 6" x 3" scraps. Almost always I hot glue 3 1/2"+ blocks to each end using the back side of the rip fence on my table saw as a guide. Usually works well, and unless doing a 2 sided carve, the block does not have to be full width. If a 2 sided carve I measure both sides of the board to check the tracking. If there is a tracking problem I have 1" wide stripe of 1/8" MDF that I attach to the edges of the back. Lately I have been using teeny flat head screws at each end, but masking tape will work. If the board is a little bowed, I slip a folded piece of paper under the strip in the middle as a shim. The strips can and should be longer than the board - he doesn't see them, he only looks at the middle for the board for the length measurement.
Clint

[normrichards]

I use a sled on 99% of my projects. For one I use 4 inch ends so I can have my actual board be the projects actual size (usually 1/4 or so bigger just to be safe). Also I use mdf that is cut straight and true and of course thickness is always consistant. I have Al's masking tape on the brass wheel side. This all ensures no problems with carving and no broken bits or slipping of the boards. Issues with my projects is almost non existent with these sleds. This also allows me to use odd shaped, odd thickness, non planed, non straight boards with the sled taking care of the issues caused by these boards. I want to make Burgerunds sled but just have not gotten to it yet.