Minimal eletronics needed for repstrap?

A couple years ago I built a tabletop Dremel CNC mill which I would like to turn into a RepStrap. The mill has full three-axis motion with 125oz steppers driven by a HobbyCNC board which takes step/dir signals from Mach, EMC, or what-have-you.

I'm looking at buying one of the new pinch-wheel extruder kits, but am a little unclear on what electronic components are required to complete the system. I'm not entirely hostile to SMD stuff but prefer through-hole or assembled since I don't have the tools. I'd prefer to use my existing stepper driver unless the RepRap drivers provide some unique benefit.

Thanks in advance for any advice!

It sounds like your CNC table will be fine - all you need is an extruder you can attach to your CNC head, and the board for controlling the extruder.
I'd guess your existing setup is probably fine, if you can hook up an ardunio/sanguino to your stepper electronics.

It sounds like you've got a good starting point for a repstrap

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Reprapping blog and other rants: [renoirsrants.blogspot.com]
My Reprap: [sites.google.com]

Ah, but Mach and EMC actually controll the steppers. The reprap software on the other hand gets a line of gcode and converts it to step/direction data and runs the stepper.

You would probably need to get either the arduino or sanguino board to be able to use the software as is.

Here is a "block diagram" of my CNC system:

GCode => EMC/Mach =(step/dir)=> HobbyCNC Board => Motors

I assumed I'd need an Arduino to talk with the PC. My understanding is that the Arduino mainboard generates step/dir signals like 99% of other stepper drivers do. If so I could feed this to the HCNC board and save $50 or so on the stepper drivers.

What electronical and mechanical parts are needed to build/run the complete extruder? In earlier RepRap iterations I recall there being a board to drive the extruder motor, another to heat the nozzle and measure temperature, opto home/limit switches, etc.

If you search this site with the site command on Google you will find that there are few others that have taken this approach also.

In my reading it's a matter of coordinating the GCode with turning the extruder on and off with M codes.

I am working with some high school students on using a Prolight 1000 mill that has been collecting dust as a RepStrap.

It has eight inputs/outputs that are controlled via M codes that we building a stepper motor driven extruder for.

We will be using a Sanguino driving stepper and PCM driver boards for the extruder portion.

Since the mill is controlled via a PCI card installed in the host PC we have to go with a separate extruder controller rather then have the ardunio/sanguino drive step/direction lines.

My current thinking is:

Output #8 request heater on/off.
Input #8, heater is on and extruder processor is paying attention to commands.
Output #7 select high or low heat target
Input #7, heater is at target temperature
Output #6 extrude on/off

You're correct - the arduino generates step/dir pulses from the G-Code. It is likely to connect to your HCNC OK.

Current extruders need three things:

Temp sensor - this can be a thermistor plugged into the arduino directly:

High-load heater circuit for the nichrome or wirewound resistor heater

Drive circuit. Either a step/dir, stepper controller, and stepper motor: Or, a dc motor driver board, dc motor and gearbox like a GM3 or GM17.

Ideally, if you use a dc motor, you need a shaft encoder to control the speed accurately. Steppers are more controllable, but are more expensive overall.

MakerBot make a custom extruder board that includes another arduino, stepper driver, and heater output/temp monitoring.

I've got 100k thermistors with a stripboard temp sensor circuit, DC motor (GM3) and driver circuit, and heater driver board from the old RRRF circuits.

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Reprapping blog and other rants: [renoirsrants.blogspot.com]
My Reprap: [sites.google.com]