garageFab.cc

Finally, here goes: minirouter and first tests with Dremel on plywood. The router had actually been assembled for weeks - it only took me several evenings to assemble it, but few problems came up and I spent time trying to fix them, then fixes/workarounds resulted in new problems and so on. The same flaws and weak spots mentioned in the previous post still remain in the design. Couplings is one of them, and non-alignable leadnut is the other.

Couplings

The original reason to have couplings was that all-thread rod (10-24 at the time) with plain nuts was used to mount the leadscrew on the bearings; it needed to be connected to 1/4" motor shaft and, if ACME screw was used, it needed to be connected to both ends of ACME screw, like so: motor shaft -> coupling -> all-thread on a leadscrew mount -> coupling -> ACME screw -> coupling -> all thread on a leadscrew mount. When I decided to switch from 10-24 to 1/4" all-thread rod, I did not think about changing this mounting scheme and/or getting rid of couplings.

Instead, I made new couplings from 1/4"ID 1/2"OD aluminum tube and set screws. They were supposed to be stronger and perform better than nylon ones. It turns out that they provide good alignment and grip if the fit is tight, which is the case for motor shaft and ACME screw. However, for all-thread rod the alignment is just not there, and the grip is not very strong - I had them slip a few times when starting/stopping the movement of relatively heavy gantry - and that's with flimsy 60 oz·in steppers. I tried winding some copper wire on all-thread rod - it does help with alignment, but not with the grip. Entire leadscrew, connected to the motor shaft, has three couplings, each with imperfect alignment - it's pretty crooked and it also slips with enough torque applied. The ideal coupling would be a tube having 1/4" bore meeting 1/4" thread midway, but I had tried earlier and could not easily make one with hand tools.

Finally, it occured to me that maybe I should find a way to not need the couplings for ACME screw. I went to McMaster and found that cheapest 1/4" ACME nuts were $1.76 a piece - not too bad. A small hole in the side with the set screw should be good enough to fix the nut on ACME screw. For the motor connection, I found some smallish hub-spider-hub plastic couplings that were also reasonably priced. I want to see if this motor couping, being non-rigid, eliminates or reduces the nasty sound that is made when two or more motors are stepping at the same time. I read that this may be due to stepper resonance, which is apparently cured by a flywheel damper. But I'll try that next.

Lead nuts

Assembling minirouter, I had a problem on Z-axis, with leadnuts (and thus leadscrews) not being parallel to the rails. Trying to fix it took a lot of time. The leadscrews bent and touched the inner walls of another sliding element on the rail, resulting in extra friction. With rigidly misaligned couplings on top of that and tight belt connecting two Z-axis leadscrews, torque required for rotation was extremely uneven. This, in turn, made couplings slip and Z-motor even stalled a couple of times. I repeatedly tried to adjust the leadscrew mounts and couplings to even out the torque, with limited success since the alignment of leadnuts was fixed. This got me seeking ways to get rid of the couplings alltogether. It also made me come back again to the idea of alignable leadnuts - which would require moving leadscrew position from corner to the center of sliding element profile.

Despite these problems I decided to try router in action as it is.

Routing tests

So far I only cut out 1" square and circle. I measured the diameter of the circle at different angles - the variation was within 0.005". So far so good. On the other hand, I quickly discovered what I had read many times - CNC routing with Dremel is like cleaning the floor with a toothbrush. I used high speed cutter (Dremel#194) on plywood at recommended 8 speed setting (18-24 kRPM), and the deepest cut it could take without making ugly sounds was 0.05" at 10 IPM. The sound varied with the direction (along/across the grain) and with hardness of the fibers (darker is harder I think). I tried cutting 1/8" depth really slow (1-3 IPM) but Dremel didn't like it, so I settled on 1/32" passes at 10 IPM which I think should always be doable.

Next up are some scrap plastic, HDPE (got a cutting board) and acrylic. Dremel manual has a table where you can look up what speed should be used with certain bit number on a certain material. It would be nice to also have recommended depth of cut and feedrate there.