Monday, April 13, 2015

CNC control

I have had these bits and pieces laying around for quite a while now and decided to drag them out and have a play around with NEMA17 stepper motors.  Not sure what I am doing yet or going to build, but wanted to get a little experience with them.

These are small NEMA17 motors that I obtained from Adafruit that are inexpensive enough for having a play around with the following specifications:

  • 200 steps per revolution, 1.8 degrees
  • Coil #1: Red & Yellow wire pair. Coil #2 Green & Brown/Gray wire pair.
  • Bipolar stepper, requires 2 full H-bridges!
  • 4-wire, 12 inch leads
  • 42mm/1.65" square body
  • 31mm/1.22" square mounting holes, 3mm metric screws (M3)
  • 5mm diameter drive shaft, 24mm long, with a machined flat
  • 12V rated voltage (you can drive it at a lower voltage, but the torque will drop) at 350mA max current
  • 28 oz*in, 20 N*cm, 2 Kg*cm holding torque per phase
  • 35 ohms per winding
So, you are not going to build a world-class CNC milling machine from these little guys, but they are certainly sufficient for having a go at light-weight, low-precision CNC milling, 3d printing and other hobby projects.

The board is a TinyG v8 board from It uses an embedded microcontroller (Atmel ATxmega192) and 4 stepper motor drivers (TI DRV8811) on a four inch square board.  It accepts G-code from a USB port and interprets it locally on the board to plan motor movements including acceleration planning.  Constant jerk motion equations (3rd order S curves) are used for very smooth motion transitions for lines and arcs.  Step pulse generation uses phase optimized fractional step DDA running at 50 Khz with < 1 us jitter.

The boards are networkable using RS485 for motion peripherals and for networking multiple boards.  Up to 1000 stepper axes can be supported.

The TI stepper drivers handle 2.5 amps per winding so most motors up through NEMA23 and some NEMA34 motors can be handled.

All-in-all it seems to be a really nice package and an easy one for me to get some experience with G-code, stepper motors and the like.  Now to figure out what to build this into.  I like the idea of a CNC router or engraver, but am a bit put off by the expense to build what amounts to a toy.  Obviously more research is going to be needed to find components that will allow cost cutting.

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