Belt sander build: Redoing the adjustments
My initial intention had been that to switch between normal mode and edge sander mode I would turn the sander 180 degrees and then tip it forward. This way what is normally the front of the belt sander faces up, so that the belt can be removed and the controls are accessible.
But I realized this meant the belt would move right to left, instead of left to right like it normally does on edge sanders. More importantly, the dust collector port would move to the left, which would mean re-routing the hose every time I flipped it.
If I flip the design of the mechanism, the controls would be on the top in edge sander mode, and on the back in regular mode, which would be acceptable.
But the tracking adjustment design I had wasn't very responsive, so this was ALSO an opportunity to change how that works.
The problem with my tracking adjust mechanism is that it has a bit of "play" in it because the tracking screw isn't normally under tension. This means I have to turn it an extra half a turn if I want to adjust in the other direction. I also have to overcome the friction in the tensioning lever (which I'm pointing at with my left hand)
A better design would be to have the tensioner apply the force off-center so the tracking adjust always has force against it. That way, if the knob is turned only a little in either direction, the adjustment changes immediately.
Then, with the new bracket in place, I slid a sanding belt half-way on the rollers, and pushed the idler roller out as far as I could to establish where it normally goes. I used a clamp to fix it in place.
At left, you can see how the new tensioner mechanism works. Tension screw 'T' pulls on lever 'L' turning it clockwise, which pushes the push rod 'R' against the wooden spring 'S', which is connected to the idler bracket 'B'.
Because the tension load is applied asymmetrically, bracket 'B' will want to rotate counter clockwise. The tracking adjust knob will go just left of where the bar clamp is.
The tracking adjust knob will go through the frame of the sander. Because it needs to move side-to-side with tension adjustments, I need to make a slot in the frame, which I'm doing by drilling a series of holes, then squaring out the slot with a rasp.
I also wanted to simplify the design (to make for an easier to build set of plans). I realized, the plywood layer that holds the bracket isn't really necessary. It also blocks access to the wooden spring, if that ever needs servicing. Instead, I can just drill an oversized hole and use a 5/16" bolt and fender washer to hold the bracket in place.
With the plywood layer no longer necessary, the notch where it attached to is now also redundant, so I'm gluing in pieces of plywood to fill in that notch. This is mostly to make it look like my improved design.
Now attaching the idler bracket with a 5/16" bolt. The bolt screws into a 9/32" hole in the plywood (about 0.8 mm smaller than the bolt). I just screwed the bolt straight in the plywood like a wood screw, even though it's a machine screw.
If the belt tracks differently forwards than backwards, that indicates the two shafts for the rollers aren't in the same plane. It's as though the sander was twisted along its length. With it tracking hard right going backwards, that suggests that the idler roller is too high on the right (as seen from here)
But when I tested it again, it appeared that I had way-overcorrected the misalignment, because now it was tracking badly in the opposite direction.
With this shim, tracking was much improved.
I really was surprised that the sander is that sensitive to misaligned rollers, but then again, the crown I put on the drive roller is very subtle, so my ability to fix misalignment with tracking adjustment is limited.
And, I guess I also got lucky that my initial build had the two shafts co-planar probably within about 0.1 mm. But this way of shimming the rollers is a good way to fix being off by a fraction of a millimeter.
Then placing the platen back on the sander and using a center punch to mark the pilot hole locations.
The way the plate is mounted is the same as how I mounted it on my homemade jointer
And with all the alignment issues I had, I also added some metal plates under the belt. That way, if I accidentally track the belt against the back, it will rub against these plates instead of cutting into the wood.