[grahamcowan]

Before I even got started ordering parts for my plasma table, I made sure that the software and hardware were going to be compatible. After all, why slave over the mechanical part if the electrical part won't do a thing?

The motors that I ended up getting were known to work with my software/hardware combination (which will be explained in greater detail in a later post) and seemed to be pretty beefy. The X-axis drive was a NEMA 34 motor, as well as the dual Y-axis drives. The Z-axis drive was a NEMA 23, only because I really didn't need the power on th Z-axis like I probably would on the X- and Y-axis.

The three big motors are all rated right around 1,000oz-in of torque with a 1/2" shaft. The smaller motor I think is about 250oz-in with a 1/4" shaft.

The driveshafts I started with were the cheapest grade steel (ungraded, actually)I could find on McMaster-Carr, in 6' lengths, Acme threaded to 1/2"-10, Rockwell hardness of 60,000psi. They seemed great considering they were about $12 each, but once I actually put them in and started testing the motors the cheapness stuck out. Talk about floppy. I know that drive shafts have a tendency to bow out and flop at high speeds, but this was absurd. And to top it off, they were ever-so-slightly warped when I got them, which translated into massive problems at high speed.

I went back and searched some more and went ahead and spend the $40 each for Grade B7, 125,000psi Acme threaded rods, 6' long, except I went with 1/2"-8. Less turns for more travel = possibly lower top speed needed.

What a difference they made. They even looked better -- smooth silver finish, not looking like they were forged, and straight as an arrow.

The needle bearings are to support the driveshafts. You really can't have the bearings of the shaft of the stepper motors supporting the weight of the driveshafts -- they'll burn up pretty quick. So I took this into consideration when I designed my motor mount brackets. I found some needle bearings that would accept a 1/2" shaft and drilled a hole to keep them in place. Each driveshaft system and motor has a needle bearing support on each end; one at the motor and one at the far end. There is also a 1/2" shaft coupler at the motor to bolt onto the rod.

The trickiest part of building the motor mount brackets was making sure the needle bearings and motor shafts were aligned. I didn't have my driveshafts yet so I found a piece of stainless 1/2" tubing that actually had a pretty tight fit. I slid the tubing into the first needle bearing and then into the shaft coupler. I rotated the bearing plate against the aforementioned 2" angle and tacked it in place.

Now that I think about it, the motors are somewhat clunky looking mounted where they are sticking out to the side. On the other hand, if I had turned them around to face the other way and used a pulley system, they might have been streamlined better with the overall look of the table, but dealing with pulleys and belts was not something I felt like messing with.

Besides, direct drive has less parts to have to fix/tweak/modify later.