Plasma: Bolt Ready Holes - Best Practices


I can’t seem to eliminate the divot in my bolt holes. When I do, I end up with a hole that is out of round.

If I go around on the Internet there are many people (specifically some guy named Colt) writing about these recipes about when and where to cut the arc and keep the gas flowing and the like but I cannot replicate it in SheetCAM.

I will grant you SheetCAM isn’t my normal CAM software, I am a Fusion guy so the parameters are probably there and I am just too new to know how or where.

I need to cut #10 bolt holes and bigger in 16GA mild steel and some times thinner. If I could get down to 1/8" or #5 I would be estatic! I have fine cut and regular cut consumables for my Hypertherm 45XP. My machine is run by Mach4 and the OEM for the table is AvidCNC / CNC Router Parts.

Thanks in advance for helping!


the divot at the end of the cut will be caused by 1 of 2 things:

  1. m05 is a non synchronous miscellaneous code (ie it requires the axis to slow down and stop before implementing the turn off torch signal)… result = divot.
  2. plasma has a kerf… if you cut a circle at some point the start and end points will cross… what happens in plasma when a kerf crosses is (if you have thc turned off) is the plasma will overcut the previous cut area given the plasma gas will conduct towards the material… cutting more material away than required… causing a divot.

Jim colt has retired from hypertherm and I’ve not seen him online for a while on the forums he used to frequent. he was seen as the guru of plasma cutting as he had been in the industry for 40+ years. he was biased as he was employed by hypertherm and knew their products very well, but his posts are still very informative.

possible solutions…

  1. use a synchronous macro to turn the torch off fast… without slowing down and stopping … you use m63/m64 with mach4 (or linuxcnc) or M11/M10 with mach3 or M10/M11 with uccnc.
  2. you adjust (lower) the amperage at the end of the cut where the leading part of the kerf crosses / meets up with the trailing start of the kerf from where the start of the cut happened.

take a look at the YouTube clips on hypertherm truehole technology

… ok that is done with a high definition plasma cutter… and it uses a mixes of gasses to aide in the exothermic reaction of plasma cutting by adding oxygen to the cut (look up thermal Lance on youtube or DIY ones).

… another option… more adaptable to air plasma is to use a macro to cut holes… you pass the macro the hole diameter you want and the kerf thickness and the linear feedrate… and the macro cuts the hole … automatically…

also… consider that your machine will probably be limited by physics too… does it reach the feedrate required asap and maintain it throughout the whole hole cut…
and finally consider that I believe the minimum hole diameter you can cut with air plasma is 2-3 X the material thickness… I use 3x material thickness.

Thanks for the reply.

I do have the RS485 kit for my 45XP and I need to install it to get to the amp adjustment. I will have to script it in Mach4 which isn’t hard.

When you talk about macros, you mean in the controller? Skipping sheetcam entirely?