Size the runners for a crosscut sled, tapering jig or box-joint jig so they slide in the miter slots without play. Enter the slot width and depth you measured and the clearance you want; the calculator gives the runner width and depth.
A sled is only as accurate as its runners. Any side-to-side play lets the sled swing as you push it, and the cut wanders by roughly the play multiplied by how far the cut is from the runner. A few thousandths of slop is the difference between a sled that cuts dead square every time and one that needs “a careful push”.
The fit you want is snug but free: the runner slides the full length of the slot under its own weight when waxed, with no rattle. Runner depth matters too. If the runner is as deep as the slot, the sled rides on the runners instead of the table, and dust in the slot makes it rock.
Width is the slot width minus clearance on each side. Depth is either about two-thirds of the slot depth or the slot depth less 0.02 in, depending on which rule you choose; both keep the runner off the bottom of the slot.
runner W = slot W − 2 × clearance · runner D ≈ 0.67 × slot D (or slot D − 0.02)
| Slot width | Clearance per side | Runner width | Depth (⅔ rule) | Depth (−0.02 rule) |
|---|---|---|---|---|
| 0.75 in | 0 in | 0.75 in | 0.251 in | 0.355 in |
| 0.75 in | 0.001 in | 0.748 in | 0.251 in | 0.355 in |
| 0.75 in | 0.002 in | 0.746 in | 0.251 in | 0.355 in |
| 0.75 in | 0.003 in | 0.744 in | 0.251 in | 0.355 in |
| 0.75 in | 0.005 in | 0.74 in | 0.251 in | 0.355 in |
Most North American table saws have ¾ in wide, ⅜ in deep miter slots, but measure yours: some are slightly over or under, and some saws use T-slots that need a different runner shape.
Hardwood runners for a new sled. Slot 0.750 × 0.375 in, 0.001 in clearance per side, ⅔ depth rule: runner 0.748 in wide, about 0.251 in deep. Rip slightly wide, then creep up on the width with the planer or a few passes on the saw.
A little more clearance for UHMW. Plastic runners swell less than wood but are springy, so some builders allow 0.002 in per side and the deeper rule: 0.746 in wide, 0.355 in deep.
An undersize slot. If your slot measures 0.745 in, enter that. With 0.001 in per side the runner is 0.743 in — guessing “¾ minus a hair” from the nominal size would make it too fat.
Binds in humid weather. Wood runners have swollen. Sand or scrape a few thousandths off, seal them with finish, and wax. For a permanent fix, switch to UHMW or quartersawn stock.
Developed play. Wear or shrinkage. Adjustable runners with set screws or small plastic tension discs can take up a few thousandths; otherwise make new runners. A strip of veneer glued to one side is a quick repair.
Snug runners make the sled repeatable; the fence makes it square. The five-cut test magnifies any fence error so you can measure it with calipers. Take a piece of flat plywood about 12 in square, cut one edge, rotate the piece 90° counterclockwise so the fresh edge rides the fence, and repeat for all four edges. Then cut a strip about an inch wide off the first edge.
Measure the strip’s width at both ends. Because four cuts have added up the error, the fence is out by a quarter of the difference over the length of the strip.
fence error over the strip length = (wider end − narrower end) ÷ 4
Example. The strip is 12 in long and measures 1.010 in at one end and 1.000 in at the other. Difference 0.010 in, so the fence is out 0.0025 in over 12 in. If the fence is 24 in long, move its far end 0.005 in. Repeat the test until the difference is under about 0.002 in.
0.001 in per side is a good target for wood, 0.001–0.002 in for UHMW. Zero clearance only works with a material that slides easily and will not swell.
Two runners, one in each slot, guide better on a full-width sled. Small jigs often use one.
Then the sled rides on the runners. Any sawdust in the slot lifts it, and the base can rock on the table.
The calculator works in inches. Divide millimetres by 25.4 before entering.