Bart's original design had the Y axis cable carrier going to the front right corner of the work volume.
At several early builders' suggestion (e.g., Ben Jackson
, Bill Meade
) he made the design reversible. The carrier can go either to the front right or the back right corner..
The fixed end of the carrier bolts onto the bottom skin. (That's what I thought I remembered.) There are supposed to be attachment holes both at the front and at the back of the skin. Last night I turned my chassis over and looked for holes in the bottom skin. I did not see any holes at all. I'll double-check next time I'm at the maker space, but I think I'll be drilling new holes. No big deal.
Both sets of holes are clearly present in the drawings. And I'm pretty sure I oriented the bottom skin correctly because the holes marked A and B line up.
Once the Y carrier is installed, I will run the following items through it.
- Air hose
- X motor power: 4 conductors, high current
- X min limit switch: 2 conductors, low current
- Y max limit switch: 2 conductors, low current
- LED strip (see below): 2 conductors, high-ish current plus 2 conductors, low current
That's a total of 6 conductors high current, 6 conductors low current, and a hose. The carrier will be kind of full.
LED strip? Yes. I'm getting old and blind, so I'd like to have a good light on the work piece. I bought a strip of RGB LEDs which I intend to glue onto the bottom surface of the gantry. I got RGB LEDs, individually addressible
, so I'll have yet another software project to play with. At first, I'll just switch them on and leave them on, full brightness. But I have a few ideas for lighting games. The gantry is long enough for 22 LEDs at 60 mA each, so 1.3 A total. A total of four wires is needed, but the other two wires just have to carry a data signal to switch the LEDs.
Here is where I have to ask for advice. Do these wires need to be shielded? Do they need to be twisted pairs?
I thought that for the limit switches, I could just run a 4 conductor ribbon cable. It would bend very well and not take up much space in the carrier. But maybe ribbon cable will pick up too much noise?
The serial signal for the LEDs is more interesting. It's also a 5V low current signal, but it's fairly high speed. (Could be up to 2 MHz; could be lower.) So what do I need? Can I add them to the ribbon cable?
And the motors. Design current is 1.68A. Max voltage is 24V. Full step sine waves could be as high as 30 KHz, but that's probably beyond the motor's mechanical limits. The microstep frequency would be 16× higher, or 480 KHz. Does that need to be shielded? Twisted? Other?
I have some Molex Micro-Fit connectors for the high current leads. I'm thinking about mounting a tiny PC board or perf board on the end of the gantry so all the wires can unplug for disassembly. Any reason that's a bad idea?
I have to decide which way to mount the Y carrier, too.
I'm thinking the most important factor will be electrical noise from the laser and its power supply. If I make an upper shelf of ferrous metal for the power supply and ground it, that should help a lot. The positive lead could be very short, maybe a couple of inches. But the negative lead will run the length of the laser tube and wll be a big antenna in the back of the box. So I'm thinking the front of the box will be less noisy. (But I am not an EE. Is this a valid line of thought?)
If I use the original configuration, loop at the rear, exit toward the front, then I can put the electronics in the front of the box, but the signals to the gantry will be fairly close to the tube. I will also have to run cables for externally accessible Ethernet, USB, or SD card to the back panel. (Maybe USB and SD should be on the front panel?)
If I use the alternate configuration, loop at the front, exit toward the rear, then the controllers will be at the rear where it is noisier, and if I install USB or SD, I'll have to run long cables to the front.
The air hose just doesn't matter. I'm planning to put the air pump wherever it fits inside the chassis and run a second line to the grommet on the back. The line will just stop at the grommet.
I have a feeling I'm overanalyzing this. Is electrical noise a serious concern or will good cabling practice make it irrelevant?