garageFab.cc

This weekend I tested the linear drive with ACME leadscrew positioned in the center of sliding element profile. In summary, with good alignment of shaft mounts, it works extremely smooth without any visibly detectable backlash. When shaft mounts are not well aligned, the torque needed for rotation gets uneven unless there is a small axial slack.

To use ACME leadscrew, I needed a way to secure it axially on the shaft mounts. After looking for ACME-threaded flanges and collars, and finding them either too expensive or unavailable for 1/4"-16 ACME, I thought that the cheapest solution is ACME hex nut ($1.76 on McMaster), drilled and tapped for a set screw. The ACME hex nut is pretty big - 1/2" wide on flat side. To avoid the contact with the bearing shell, 10-24 nut is drilled 1/4" and inserted in between. Also, the ACME nut's edge extends long enough that the screw head under it needs to be thinned to about 1/16" - visible on the the second pic. Another thing that needed to be thinned was the end of the ACME screw - it measures exactly 0.250 while the bearing ID is actually 0.249.

Now that I think about it, it is not the cheapest solution. I probably could have used 1/2" round Delrin stock which I have plenty left over, drill it and tap it for ACME and set screws. It would have to be cut at right angle and that's not how it usually works out with the hacksaw, but some filing down could make the surface reasonably perpenicular to the rod axis, and I think it would be strong enough for the forces involved.

A few days ago, the 57BYGH207 steppers I ordered for testing arrived. When I was installing one, I realized that with the way motor mounts are designed, it's impossible to install the lovejoy coupling on a short motor shaft. This hasn't come up before because the shaft on 44A501711 steppers I've been exclusively using is over an inch long. Long story short, I had to modify the motor mount to accomodate that.

The bad news is that 57BYGH207 stepper did not work too well with the RepRap stepper driver. With 16TPI leadscrew, I've been able to get maximum of 2 IPM feedrate, which I think is equivalent of 32 RPM. I guess I didn't think through what the specs meant when I was ordering. The rated voltage is 12V, phase resistance is 30Ω. This is in constrast with 2.3V, 1Ω of the 44A501711 steppers I'm using. I think RepRap stepper driver is too weak for 57BYGH207. If it had 8 leads, it might have worked better with bipolar parallel wiring, but it only has 6. The drivers offered with this stepper are CW-200, at $40/each. It's not too expensive - RepRap stepper driver is about $30 $25. However, I probably won't be able to power these drivers using ATX PSU.

With the existing setup using 44A501711, I can get feedrate up to high 20s (IPM, unloaded) before the motor starts to stall. A resonance damper/flywheel may extend maximum feedrate even more. To make the flywheel, I want to cut out a 3/8" thick 2"diameter HDPE disk with peripheral slots for six 1/4" nuts and install it on the motor shaft (or on a spacer) in half-inch gap between the motor and the motor mount.

The next project is updated version of minirouter with centered leadscrews. It will use all-thread rod, and it will have about 1" height of the work area - I think it will mainly be useful for cutting things out of soft sheet materials.

I'm converting the templates source from Powerpoint to .SVG and will post it up soon.