W O O D U M A K E I T ?
Adjustable Cross Cut Sled
This is my original cross cut sled for my table saw.
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What I like about it is that it's large enough to hold panels up to 36", but sometimes it can be a little too large. My new sled will be a little smaller.
It's also adjustable, which is helpful to account for wood movement. I can easily re-square it by loosening some bolts, adjusting the fence, and tightening them again (as shown in the next photo).
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One thing I don't like about it is that only the front fence is adjustable. That's the fence that I use when cutting large panels, but it can result in tear-out along the back edge. My new cross cut sled will have adjustable fences along the front and back.
I start with a piece of Baltic birch plywood. I've cut it into two pieces. The piece on the left is about xx" x xx" and the piece on the right is about xxx" x xx".
Now I'll move each piece so that it covers the blade and then I'll mark in the position of the miter slot.
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I will cut off the excess wood that is covering the blade after the runners are installed. That will result in a perfect zero clearance opening for the blade.
Using a dado set, I cut the slots for the runners that will go in the miter slots on the table saw. There's no need to worry about making these perfectly parallel to the edge of the boards. This will be taken care of later.
Since I am using wood for the runners, it is important to orient the wood so that the expansion and contraction will be minimized horizontally so that the runners don't get stuck (or too loose) in the miter slots.
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The wood expands less in the radial direction of the grain, so I am going to orient the runners so that the grain is vertical. A good article about this topic can be found here.
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Other solutions to this problem would be to purchase metal runners that are designed for the miter slots, or to cut this material from a regular cutting board made from ultra high density plastic.
After gluing on the runners, I run the two pieces through the table saw. This gives me a zero clearance and guarantees that the edge of the board is parallel to the miter slot and parallel to the blade.
Before going any further, I'll apply a coat of tung oil finish to the surface.
Each fence is made from two thicknesses of Baltic birch plywood that has been glued together. The lengths of the two fences are cut to be the same as the width of the final cross cut sled.
After glue-up, I joint the bottom edge of each fence to ensure that it is flat and smooth.
There's nothing to ensure that the fence will remain straight, so I've glued a straight piece of hard maple to the bottom of each fence.
While the fence is gluing up, I route slots into the surface of the cross cut sled so that I can install T-tracks for various jigs and hold-down clamps.
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I route these just a little longer than the T-tracks so that there's room to insert the head of the 1/4" bolt.
I'm testing the fit by inserting a stop block into the T-track.
I cut a profile along the fence using the band saw.
Then I use a round-over bit on the router table to round over the top edge of the fence. This will make it easier on the hands when using the cross cut sled.
I also cut a 45 degree chamfer along the front edge of the fence. This gives sawdust a place to go so that it does not interfere with the positioning of wood when it's being cut.
I will use 2" aluminum L brackets to attach the fences to the cross cut sled. I cut the L brackets on the miter saw. The aluminum is soft enough that it will not harm the blade.
I drill holes into the L brackets at the drill press.
I attach the L brackets to the cross cut sled using 3/8" bolts with a fender washer, lock washer, and nut.
To determine how much to adjust the fence to make it square to the blade, I use the four cut method (sometimes also referred to as the five cut method).
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Using this method, you cut a large, rectangular piece of wood on all four sides. That way, if the fence is out of square, the error is amplified four times, which makes it easier to measure.
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After cutting each edge of the board, make a final (5th) cut by cutting of a strip of wood that's one or two inches wide.
Make sure you label each end of the strip to know which end is from the front of the sled and which end is from the back.
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Use calipers to measure the difference in width between the front end and the back end of the strip.
When I made this sled, I was using calipers that were almost 50 years old, and only measured to 1/32". I was happy to receive new iGaging IP54 calipers for Father's Day. It measures to 1/128" and is much easier to read, so I look forward to using it on my next projects.
The Math
To determine how much to adjust the fence, it just takes a little math.
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Take the difference between the width of each end of the final strip that you cut (w1 - w2). This is the amplified error, so you need to divide this by 4 to compute the error on a single cut.
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Now compute the error per inch by dividing the error by the length of the cut strip (l1)
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Then multiply this error rate by the length from the pivot point to the adjustment point (l2).
Which way?
If the adjustment is a positive number, then you need to move the adjustment point toward the front of the saw. If it's a negative number, then you need to move the adjustment point toward the back of the saw. This assumes that you are making the adjustment on the left side. If you adjust it on the right side, then those directions are reversed.
These are typically very small adjustments that you'll need to make, so use a feeler gauge to measure the adjustment.