Okay,
I think I am in trouble here.  I installed a 4th axis stepper motor to drive the extruder.  However in looking at how 3D printers work with the G-code output from a slicer, the extruder is run at fixed speed like a spindle. I got the extruder to work as the C axis and can manually jog it just fine.  I can also enter a feed and distance and it runs fine.  So I thought, okay I need to set up the spindle to be a step and direction type output.  However here is the gotcha.  I have a Xulifeng USB motion card with a Mach 3 plugin doing the motion control. I don't see any way to get step and direction spindle operation with this card.

If Mach 3 could use the A axis as a spindle it would work.  If the motion card could use the 4th axis as the spindle it would work. I don't see any setting to make this happen. Anyne know if this is possible?  This also suggests I am screwed when it comes to using my stepper driven lathe as a lathe. It looks like it can be used for positioning, but not for continuous motion.

I have a smooth stepper card and breakout board, but didn't want to go to the effort of installing them at this time. So the question is can the smoothstepper card use a stepper motor as a spindle drive?  I thought I was ready to try 3D printing, and now I am really stuck.

Not a direct answer to any of your questions but I know you could use a PIC chip and suspect you could use an Arduino or Stamp PC to take the Mach spindle speed output and run a stepper driver with it.
Regards,
Nick

   Where do you get the idea that the extruder speed is constant? I am not sure that is the case. I will have to look at some of my old files.

  OK I looked at a file (a known working file). In my case I use E as the extruder value. My E values are all over the place. The GCode was generated by ReplicatorG 0040 & Skeinforge 50.

I didn't mean to imply that it ran at just one speed. What I meant was it was handled as a continuous running rotary axis not a linear axis.  The E command is a rate not a distance. Also it needs to run at a rate different than the other axis.  It looks to me like using Mach 3 with the parallel port you were able to have a stepper spindle running continuous and the other three or more axis as well. However going back to a parallel port would be a step backwards.

hook up the smooth stepper

Fast,
So you know that smooth stepper will drive a stepper spindle at a constant velocity while using feed rate control for the other 3 axis?

Ok, now I am really confused.  I've been looking at the G-code produced by Simplify3D slicer. I sliced a simple part.  At the beginning of each layer to print it positions the nozzle and then there is a line G1 E1 F1800.  At the end of the layer there is a line G1 E-1 F1800.  What does this code do?  Is it the E (extruder) axis that moves 1 mm at F1800, then back 1mm at the end?  Or is it a rotary axis that starts by E1 and rotates continuously at F1800, then stops when the E-1 is reached? Any help would be appreciated.

Ok, now I am really confused.  I've been looking at the G-code produced by Simplify3D slicer. I sliced a simple part.  At the beginning of each layer to print it positions the nozzle and then there is a line G1 E1 F1800.  At the end of the layer there is a line G1 E-1 F1800.  What does this code do?  Is it the E (extruder) axis that moves 1 mm at F1800, then back 1mm at the end?  Or is it a rotary axis that starts by E1 and rotates continuously at F1800, then stops when the E-1 is reached? Any help would be appreciated.

Gary,

Perhaps ask your question on the Simplify3D forum http://www.forum.simplify3d.com/   

Tweakie.

Tweakie,
I've been asking there, but no one is using Mach 3. In 3d printing they only look at G-code when they have a problem they can't solve with slicer settings. So the level G-Code understanding is no where near what is found here.

If an axis is defined as rotary in Mach 3 how does the G-code for that look? Would +-1 define direction and a feed rate define the speed in some units?

Gary,
I've been trying to set up my cnc for printing too, and as far as I can tell Simplify and Replicator and the other slicing programs define the extruder as "E" axis, so all I did is start a seperate profile in Mach and renamed E to A in the g code, the -1 and +1 you see are a retraction or feed of + or - 1 unit to prevent the plastic from drooling, so you don't get little strings of material every time it lifts the hot end. So far so good, now all I have to do is figure out the PID temperature controller.