Step 5: Sled Building Overview
There's a lot of information covered in this step, so take your time to absorb it all. I suggest referring to the images frequently as your read through this.
cross support design considerationsEach cross support (front and back) is made up of three layers of plywood. When you're working out the dimensions for the cross support components, cut out the five middle layer blocks so that the four carriage bolts have enough room to move side to side about 1/4 inch in either direction; it's better to have too much slop here than not enough. Also, remember that the saw blade will cut most of the way through the middle of the thing, so make sure that the cross supports are tall enough to not get completely cut in half.
cross support assemblyThe three layers of the cross supports are held together using screws which come in from both sides. Use lots of screws -- you really want the cross support assemblies to be sturdy. Each screw should fall loosely through oversized holes in the first and second layers of wood, only biting into the third layer deep.
Make sure that you line up the cross support assemblies straight and smooth when you screw them together. Tighten down all of the screws and then back them off a few turns to be sure that all seven blocks of wood are free to rattle around. If this is not the case, you may need to go back and enlarge a few of the oversized screw holes. Align the loose assembly using the surface of your saw as a flat guide. When you do the final tightening down of the screws, go about it in a gradual manner, and (gently) use a hammer along the way to nudge everything flush.
sled plate constructionEach sled plate (right and left) is made up of two layers of plywood. Each layer gets many holes drilled through it before assembly.
linear motionRather than riding down the miter slots on rails (as do most sleds) this design uses ball bearings which run against the inside edge of the miter slots. Attach the bearings to the underside of the sled plates using heavy duty screws with necks that snugly fit the inner diameter of the bearings. The bearings need to spin freely from the underside of the sled plates, so put a couple of narrow washers between each bearing and the underside of the sled. If your miter slots are especially shallow, you may need to countersink each bearing assembly into the underside of the sled, or alternately grind down the screw heads a little bit.
The side-to-side placement of the bearings is dictated by the layout of the miter slots on the saw, however the front-to-back placement of the bearings is purely a matter of preference. Placing the bearings directly underneath the cross supports (rather than closer to the center of the sled) results in the most stability, but reduces the amount of effective sled travel because the sled will slip crookedly if the bearings are extended past the edge of the tabletop. A best-of-both-worlds solution to this dilemma is to mount another bearing (or a stack of washers) in the middle of each sled plate and adjust it so that it almost but not quite touches the miter slot wall. This will prevent the sled from jerking sideways if pushed too far, which in turn prevents the blade from getting all bound up in whichever cross support it happens to be passing through at the time.
thread pointsAs with my machined metal version, this design also has an evenly spaced pattern of threaded holes going through each sled plate. I recommend spacing these hole locations as shown where the distance from any hole to its horizontal or vertical neighbor (but not diagonal) is 1.5 inches. For my example sled (the one that I made to take these pictures) I used press in 10-32 threaded inserts (Servalite 628-E) which I hammered into undersized (6.5mm diameter) holes. Countersinking the threaded inserts without marring up my sled plates was easily accomplished by pounding down on a socket head cap screw which I spun down into each insert before hammering it in place.
Using these threaded inserts was convenient, but they're expensive and hard to find. In retrospect I wish that I had used tee nuts countersunk in between the layers of the sled plates. It would have been a lot more work to drill those countersinks, but I think that it would have been worth it in the end.
Regardless of whether you use tee-nuts (recommended) or threaded inserts, you'll want to get a whole selection of 10-32 threaded socket head cap screws to work with. Get several of each length available, and be sure to get extras in the 1" to 1-1/2" range because they're especially handy. Get a bunch of #10 washers. Get some 1/4" washers as well so that you can step up to clamp over a larger hole. I recommend getting a divider case to hold all of the screws separate from each other and from the washers and other little things that you'll frequently be reaching for as you build up fixtures onto the sled plates.
sled plate assemblyJoin the top and bottom plate halves together using screws which come up into the top plate from underneath. Place a few of these screws in between thread points, and also space them evenly around the perimeter of each plate. Countersink the screw heads so that the sled slides smoothly, and to maximize the amount of bite that each screw takes into the top plate.
blade clearancePlan out your cuts and hole patterns so that the inside edge of each sled plate actually extends into or even slightly past the other side of the saw blade. Once you've screwed the sled plates together and mounted the bearings underneath, trim that extra material off the inside edges of each plate by running them down the miter slots with the saw running. The benefit of doing it this way is that you are guaranteed to get a perfect zero-gap clearance between the inside edges of your sled plates and the saw blade. Getting this plate trimming cut started can be tricky -- lower the blade down so that it's not cutting anything and then hold the sled plate in place (pressed up against the inside edge of the miter slot) as shown in the main image on this page. With the plate firmly held in place like this, slowly raise up the running saw blade to get the cut started.
sled assemblyOnce you've built these four main subassemblies, it's time to mount the cross supports onto the sled plates using eight carriage bolts. Hand tighten the nuts and washers down onto each of the eight carriage bolts, and then switch to a wrench or ratchet to dig the square necks of the carriage bolts into the top layer of the sled plates. Now loosen it all up again in order to more carefully align things. Using a small square to line up the cross supports perpendicular to the cut line between the plates, tighten down the nuts on just the left-hand sled plate. Now push the two plates together (pressing the bearings up against the miter slot walls) and firmly hold them that way as you carefully tighten down the nuts on the right-hand plate.
Experiment setting different tensions between the two sled plates -- you want enough so that there's no side-to-side play, but you don't want so much tension that the sled is difficult to push. Don't worry too much about getting the cross supports lined up at exactly 90 degrees to the cut line -- in the next step I'll show you how to mount a secondary (more accurate) crosscutting fence.