February 2011 Archives

Version 1.3 - CNC Update - Faster and more power!

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Some updates since the last time.

After a few attempts to draw an outline PCB, I realized the system was skipping steps - that is not good! I was using type 9 stepping (1/2 step wave drive, 1000, 1100, 0100, etc). I thought maybe this was too fast, so I changed to type 5 - full stepping. The pattern is a single coil at a time (1000, 0100, 0010, 0001). This didn't help. The key was wave driving, type 6, which has 2 coils energized (1100, 0110, 0011, 1001). This cleaned up the stepping, and with a fine tip Sharpie, I was able to draw this PCB! The board was designed in Eagle and the g-code produced by pcb-gcode.

PCB drawing.

Actually milling the board is still a challenge. Bits are breaking, router is wiggling on the weak bridge, etc. Once my bearings arrive, the plan is to overhaul the bridge.

Upgraded driver - Sparkfun had a free day back in January. I managed to score $20, and so I ordered an EasyDriver. This has replaced the rats nest that drove the Z-Axis and free'd up 2 control lines for future considerations.


I also learned how boost the speed by nearly 100%. Thanks to a friend who knows way more than I do about electronics, I increased the driver voltage from 5V to 12V. I had to add current limiting resistors in line to protect the motor. A pair of 15 ohm, 5 Watt resistors per coil did the trick. They run quite hot, so some heatsink will probably be in order. But still I'm happy for the speed! I can now run the X & Y axis at 13" / min on 1/4" 20TPI rod.

Speed resistors

To help with troubleshooting, especially when we rebuild the bridge, I am working on a test controller. A 555 generates pulses into a PIC which puts out the full step wave driver pattern. This will be sent to the driver circuits when the machine is away from the host PC getting rebuilt.

555 / PIC controller

And finally, a big power upgrade. The little rotary tool I'm currently using is pretty limited in the bits it can handle. This week Canadian Tire had their 'Spin Saw' on sale for $69.99 (regular $139.99 - 50% off!). I could not resist. This thing has power. Over 5A of power. 30,000RPM of power. 1/8" or 1/4" collets means I can put the little finishing bits for PCB work, or bigger milling bits for more manly jobs. (insert Tim Allen laugh here).

Spin Saw

Till next time!