Why Manual CNC Is A Bad Idea

Crushed tool length probe switch

Crushed tool length probe switch

Most of my machining involves one-off setups and simple cuts, so I usually type G-Code directly into EMC2’s Axis interface: CNC hits precise locations and makes smoother cuts than I ever could. Most of the time, that works really well.

Occasionally, though, I think one thing and type something else.

Just a typo, happens all the time…

Better, of course, to write a little program and debug it, but then a simple task starts to look a lot like work.

Fortunately, I have a bunch of those switches on hand.



  1. #1 by Kevin Groce on 2011-06-10 - 16:56

    Any sources for good tools to make g-code? Free is possible.

    • #2 by Ed on 2011-06-10 - 18:18

      Truth be known, I’ve been hammering out G-Code by hand for my projects… that’s how simple they are!

      Some useful G-Code generator routines for simple tasks live on the EMC2 wiki. If you already have a drawing, then something from their list of CAM programs may be useful.

    • #3 by smellsofbikes on 2011-06-10 - 21:59

      Inkscape gcodetools will allow you to export 2d drawings as gcode. Eagle and gEDA both have gcode plugins that’ll allow you to export pcb’s directly to your mill.

  2. #4 by smellsofbikes on 2011-06-10 - 17:41

    I’ve been thinking about building a tool length probe switch based on your previous posts, and last night was looking in my junk box. I have a bunch of microswitches with long steel contacts. I was thinking about attaching a line to the steel contact, and driving the z axis down fast until I see an electrical contact, then stepping down until the switch itself trips, to avoid just exactly the situation you ran into here. Any thoughts?

    • #5 by Ed on 2011-06-10 - 18:22

      driving the z axis down fast until I see an electrical contact

      It turns out that the oil in the spindle bearings does a great job of insulating the tool from all the convenient non-rotating parts of the head. Having to remember to attach an alligator clip to the spindle before probing should demonstrate the limits of human memory; I know how it’d (not) work for me…

  3. #6 by david on 2011-06-10 - 18:13

    The steel contact will chip the hell out of your carbide cutters, and even HSS ones it will do no favors. IMHO.

  4. #7 by Jonathan Katz on 2011-06-10 - 21:52

    I managed to break a $300 tapping head on our VMC with a typo like that, happens faster than you can blink often enough.

  5. #8 by Steve Ciciora on 2011-06-13 - 08:50

    I’m amassing a small pile of exhibits for my museum of “Don’t do what I did”… bent 1” drill bit, bent edge finder, I’ve not had the courage to try my Renshaw TP-1 touch probe I bought on ebay after Mach3 dragged it over the surface of the skate board I was digitizing… One of these days (probably after I break something else important to me) I’m going to rig up something in Mach3 that will activate a relay while probing, switching the probe input signal from the e-stop line to the probe input. That way if I accidentally activate whatever I’m probing when doing a G0 it will hopefully stop instead of smashing something!

    – Steve

    • #9 by Ed on 2011-06-13 - 10:37

      my museum of “Don’t do what I did”

      “It was said of him that he never made the same mistake twice, but nobody could remember when he’d made a mistake for the first time.”

      switching the probe input signal from the e-stop line to the probe input

      IIRC, EMC2 has a HAL function that halts motion when the probe closes during anything other than a probe operation. Obviously at least one of these is true:

      • That’s not engaged in my lashup
      • I (once again) remember incorrectly
      • Something (else) went horribly wrong

      I obviously should figure that out…