Hole Bump & Divots... Help

[Duraramp]

Good Afternoon,

Hopefully someone can offer some help with my hole quality.

I have a torchmate 3 kit w/ THC on a water table.
I use Nestmaster for the gcode creation
Hypertherm Powermax 1250 w/ machine torch

I am trying to cut a part that has several .750" holes in a .250" sheet of HR steel plate. I have been playing with the settings but can't quite get a good looking hole.

My lastest settings are as follows:
80AMP consumables new condition
Plasma set to 80Amps
.28" pierce
.11 Initial cut height
105 ipm Feedrate
152 voltage (i know it sounds high but thats what i set to get proper standoff.)
using a .250" 90 degree linear lead in with a .10 over travel.
I have a bump and a divot where the lead in finishes and starts the hole.
the finished hole dimensions are as follows.
topside diameter .810" underside diameter .754"
topside diameter at bump .800" underside diameter at bump .725"


The holes are fine for what i need them for but i am more concerned about future projects where I may need a better quality hole.

If anyone has any suggestions it would be most appreciated.

Thanks,

Brandon

[SignTorch Vector Art]

Wouldn't you need to go with a tangent radial lead-in because a 90 degree turn will always leave a divot?

And shouldn't the torch be turned off a half of a kerf width or so before it reaches the start point?

That's my assumption, I'll be interested to, if anyone can clarify their best practices.

[steelfx]

Hopefully, Jim Colt from Hypertherm will jump in here soon, but 80amps is waaaay overkill for .250" HR.

[jimcolt]

Rule of thumb #1: Always use the lowest powered process that your torch can use if you are looking to improve cut quality. For getting the best holes on .250 plate with your Powermax1250....use the 40 Amp shileded consumables at 40 Amps.

Pierce height: .150"
Pierce delay: 1 second
Cut height: 1/16" (do a few test cuts and adjust your arc voltage to whatever it takes for 1/16" steady state cutting)
Cut speed for outside perimeter: 48 ipm
Cut speed for holes under 1" diameter: 29 ipm (this is 60% of 48 ipm)

Use a long lead in that starts the pierce near the center of the hole, ensure that the height control is moving the torch from pierce height to cut height before it gets on the hole contour. Radial lead in is best if your machine has acceleration under 40 miligees (most stepper motor machines) or has any mechanical backlash.

You need to modify your part program so the holes are done at 60% of the (Hypertherm operators manual) optimal cut speed. You also need to modify the program to freeze the height control on all holes...it should freeze at the physical cut height and not allow arc voltage based adjustment.

End of cut should be an overburn.,....meaning stay on the radius of the hole past the lead in kerf....for about .150" to .200".

This will give you better holes. If you email me directly I can send you a presentation on "Best practices for Hole Cutting"

With some cnc machines the above processes are easy...as they have functions that control speed in holes that is automatic, as well as the ability to freeze the height control automatically. On some you must learn to edit the machine code to make holes better.

Jim Colt jim.colt@hypertherm.com