Engraving
CNC version 2.0
I finally finished putting together my second CNC machine. Again this one is put together with surplus parts, but they are much better surplus parts. I have the luxury of three linear slides from Newmark Systems, that have dual profile rails and recirculating ball slides, along with 16TPI precision screws and recirculating ball guides. In short, the new machine is solid as a rock, and has more torque.
LED backlit desk sign
I was messing around a while ago with my CNC and some of the various engraving bits I’ve tried for routing PCB traces. I had a piece of acrylic lying around, so I made myself a sign for my desk at work. I don’t really have that job title, it’s technically “intern” since I’m doing the college thing; but it would be my title if a job title was based on what you did. The description next to my name in the office’s roster is actually “special projects”, but it didn’t sound as interesting.
I used a NUD4001 constant-current LED driver from ON Semiconductor and some OSRAM SideLEDs. The NUD4001 is really quick and easy to implement as it only requires an external resistor to set the current level. The unit is fused, and powered by a 9V AC/DC wall wart. I tried a couple different colors, but this amber looked the best.
As a side note, I really like using ON semiconductor parts. They have excellent datasheets that usually have really good examples.
0.008″ PCB trace isolation on my CNC
Due to a new design that I am working on, I needed to use a component from TI that only came in a 6SC70 package. It’s a boost converter (TPS61221) that operates from an input voltage as low as 0.7V and has 80% or better efficiency in the 1-10mA range. It’s trimmed at 3.3V, which I’m using to run an ATtiny45V. The ATtiny45 comes in a fine-pitch package as well, an 8TSSOP that requires 0.009″ isolation between pads. I haven’t been able to get much better than 0.020″ with the tooling I have, so I have been looking for alternatives.
I found some mechanical engraving bits at Think & Tinker which is actually here in Colorado at Palmer Lake. Their URL descriptor says they offer “Instrument and PCB prototyping equipment”. They also sell carbide engraving bits, and the particular one I selected was the 60° cutter, part number #EM2E8-0625-60V. The bits came in a nice container and are labelled as having a 0.005″ tip. The shipping was also wicked fast. Granted, it’s only a few hours away from my location but I ordered them and got them in the next day.
I’ve only cut one board, but I’m really happy with the results. I successfully achieved 0.008″ isolation between the pads of the devices, and now I can finally route traces inbetween the pads of 0.1″ headers. I couldn’t do that before.
My homebuilt CNC machine
I finally assembled enough surplus parts and scrap material to build a halfway decent CNC machine. I have been looking forward to actually completing this task for some time, but the appropriate parts and materials are prohibitively expensive for the individual on a budget. This spring I managed to complete the project.
Most of the machine is made out of either aluminum or acrylic. These materials are both easy to work with when all you have available is various hand tools and a drill press with a cross slide vise. The motors are NEMA 23 high torque, the threaded rods are 1/2-10 precision ACME and the nuts are anti-backlash. This results in pretty decent X-Y movement. I made the slides from extruded aluminum profiles available at the hardware store and some strips of Teflon.
I made all of the electronics that support the machine. I made the optical home and limit switches, as well as both parallel port interface boards. I also assembled the stepper motor drivers and built power filters/regulators for the input to the stepper motor drives on all three axes. This prevents inductive feedback spikes from the motors ruining the stepper motor control ICs. I need this because I’m running the motors at 48VDC and the controller IC has an absolute maximum voltage rating of 60VDC. I also wanted to keep any switching noise and voltage dips from the switch-mode power supply out of the stepper drives. Each filter has a FET-regulated output that clamps the voltage and then sends it out to some big capacitors to prevent the voltage from dropping.
The parallel interface boards completely isolate the controlling PC from the drives and other electronics. Buffers handle all the appropriate levels and opto-isolators keep the ports safe.
I’ve run it extensively with TurboCNC, but I don’t get any of the limit and home functionality because TurboCNC only supports legacy port addresses, and most parallel port PCI cards can’t map to legacy addresses. This leaves me with only the one on-board parallel port, which is mostly used up by the 3 axis step and direction signals. It works in Mach3 CNC, but I can’t afford to buy that program right now. So I’ve used it in evaluation mode for doing simple text engraving on plastics and acrylic. Oddly enough I couldn’t get it to work at all in Mach2. The pulse train output to the stepper drives was inconsistent enough that the motors would stall. I even tried it on a few different computers, one of which was a brand new XP install.
The machine mostly cranks out PC boards thanks to Eagle CAD and PCB2GCode. I’ve made quite a few since the machine was finished. It does a pretty good job, and holds flatness to a couple thousandths. It’s enough to get routine 0.020″ isolation on traces and pads, and clean 0.012″ trace widths. It’s a bit slow because of the low-buck slides, the motors stall if I speed it up much. It’s not exactly a high precision machine, so I can live with slow.
I’ve got some better parts since I built this one, and I’m planning on building another one with much better accuracy in the near future. This machine won’t handle the fine traces necessary to make boards with the newer components.
Here’s a video of the machine in action:
You can see more videos at youtube: imsolidstate’s CNC machine
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