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.
I’ve received some email and comment requests for the layouts and schematics of the electronics running my CNC machine. I’ve pulled together what I can find for this article.
Power Filter Boards
I made these power filters for a few reasons. One is to protect the stepper motor drives from the inductive spikes that can come off of the motors. They can be pretty big. Another reason was that I wanted to use a switch-mode power supply to run the drives. There’s a lot of good reasons for this:
1. Switch-mode power supplies are cheaper than linear power supplies.
2. Switch-mode power supplies are smaller than linear supplies for equivalent output power.
3. Switch-mode power supplies come in a wide range of voltages, so you can run the stepper motors at the highest voltage possible. (The highest voltage your controller supports) This allows for more torque from any motor because a higher voltage will push more current through the inductance of the motor than a lower voltage. It will also do it quicker, so you should get a little more speed too.
However, I wasn’t sure that the chopper-style stepper driver would be okay with a switch mode supply, as it pretty much shorts out the supply every time the chopper turns on. So I needed a buffer.
I found this article at EETimesAsia by John Betten from TI. I modified the circuit for the voltage levels I wanted to run, and also found a suitable replacement for the FET since I couldn’t find one at the time. Here is the original schematic:
I’m using an IR IRFP9140N in place of Q1. I also replaced D2 with a 56V TVS from ON semi, 1.5KE56A. I used 56V because the LMD18245 motor driver IC I have has a continuous rating of 55V and an absolute max of 60V. I also oversized the output capacitor just to be on the safe side since I had some big ones laying around anyway. They are 22000uF 100V Panasonics. They are overkill, the voltage is rock-solid even when the motors are running at full clip. I wanted to be able to recycle the boards though if I ever upgrade to a bigger machine and have bigger motors. Here’s the layout for my circuit:
Opto-isolated Parallel Interface Board
I designed this parallel interface board after killing a parallel port with a breakout board that I bought off the internet. I think it just pulled too much current from the port. I designed this board to pull the smallest amount of current from the parallel port as possible, while also providing good drive characteristics for outputs. This board is customized to my application, so the voltages and bias might not be appropriate for all. Check to make sure your inputs will work before using the values here. I couldn’t find the schematic, just the layout but it’s not too complicated to figure out if you have the datasheets for the TLP2631 and the SN74LS244N. › Continue reading
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.
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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