There are three tweaks to the sled to make it perform to a suitable standard. First, the runners may need to be trimmed to be sure the sled slides smoothly. Second, the zero clearance insert should be shimmed to fit perfectly. Finally, the rear fence can be adjusted to get it as close to square as possible.
Tuning the runners
If the sled is not moving across your table saw smoothly, first, check for any glue squeeze out between the runners and the base. The best way to remove any glue residue in a corner is with a rabbeting plane. Unfortunately, not many of us own one. The second best method is using a chisel or card scraper. If you don’t have either of those, sandpaper will also do the job.
Remove all the visible residue and test the sled on the table saw. If it still hangs up, there may be a high spot or two on the runners.
Move the sled back and forth in the miter slots about ten times. Remove the sled and turn it over. Any high spots should show up as shiny or darker spots on the runner. Using a rabbeting plane, chisel, card scraper or sandpaper, trim these areas down and re-test the sled until it moves freely. Be careful about removing too much material at once. This is a balancing act of removing enough material allowing smooth motion vs. removing too much and getting side to side movement in the table saw slot.
Tuning the zero clearance insert
Remember, when building the insert, the goal was to err on the side of thinner so the insert didn’t stand proud of the base or the fence. Lay a straightedge across the zero clearance insert at the face of the rear fence. The insert should be flush with the fence or slightly below. Measure the gap by inserting a feeler gauge in the gap between the straightedge and the insert. Using masking tape, add the correct number of layers of tape to the back side of the insert, taking up the gap without it extending past the surface of the fence. The tape will be approximately 0.005″ thick.
Do the same for the bottom surface of the sled. Measure the gap between the base and the insert, then use masking tape to the bottom of the insert to bring it up just shy or even with the surface.
Be sure to shim the insert an not the sled. The adjustment is a function of the insert and not the sled.
Tuning the fence
The five cut method is used to adjust a fence to a near perfect perpendicular angle to the blade kerf. As far as I know, it was invented by William Ng. He explains it in his YouTube video: “5 Cuts to a “Perfect” Cross-Cut Sled”.
Cutting a piece of material, then rotating it 90 degrees and cutting it again, creates an additive error each time a cut is made.
Start by drawing an arrow pointing away from you on the right side of a board. Rotate the board clockwise 90 degrees, and write a “2” on the right side. Continue rotating and incrementing the number until you write a “5” next to the arrow.
Using the sled, align the arrow to the blade making sure it is pointing away from you. With the board against the fence, cut about 1/4 inch away from the board next to the arrow. Rotate the board clockwise so the cut you just made is against the fence. If the sequence is correct, the side next to the blade should be number “2”. Cut a small amount from side 2, then rotate and repeat until you get to side 5.
When you make the cut on side 5, cut about an inch wide strip from this side. This is the piece you’ll measure with a set of calipers.
Here comes the fun part.
Measure the width of each end of the piece you just cut from your fifth cut. I find it’s easier to manage if I write everything directly on the piece. Next, subtract the width of the near end from the far end (the end the arrow points to). Divide that number by four. Remember the error is additive by the number of cuts, so the actual error is one fourth of your measurement. Divide the result by the length of the piece. This gives you the error per inch of length. The final step is to multiply this number by the length of your fence. This is the total error from one end of your fence to another.
Here’s an example:
If the cut off piece measures: Near end width = 1.136″, Far end width = 1.176″, Length = 10″.
The error per inch = (1.136-1.176)/(10×4) = -0.001″.
On this fence which is 30″ wide, the total error is .030″. Just under 1/32″
Notice the measurement is negative, indicating the far end is wider than the near end. This means the left side of the fence needs to move rearward, so the shim should go on the left end between the fence and the upper base.
In most cases, this is an acceptable error, but since there is a chance to make it better, and it’s not difficult, it’s probably worth the effort.
A hotel key card is about 0.030″ and would be a good shim in this case, but there are tons of items you could use as a shim if you don’t have feeler gauges. For example, blue tape is about 0.005″ thick.
Once you know the error over the length of the fence, remove the screw on the end of the fence needing to move rearward. Insert the correct thickness shim between the fence and the upper base plate to move it backward, then drill a new hole from underneath the fence, and re-mount the fence. Do not use the original hole as it will draw the fence back toward its original and incorrect position.
If desired, repeat the five cut test to confirm the adjustment. Once you’re happy with the fence, turn the sled over and secure it to the base with a few more screws.
When I did the fence tuning, I clamped a straightedge to the fence to maintain flatness until the fence was secured in its final position. I also glued it down which is optional. If you use enough screws to secure the fence across the length of the fence, it will maintain flatness and will be removable in case you need to tweak it later.
