[rahilm]

I have a custom build CNC plasma. It works pretty well for simple art cuts, but I'm really having trouble getting good cut performance for more precise parts, especially for thicker aluminum. For example, in 1/4" 6061 aluminum, I have at least a 1/16" taper on cuts, which is much worse than what I've seen done by other people.

The software I use is Mach3 and SheetCAM. I've set most of the parts to use perpendicular lead-ins and radial lead-outs, which I understand is optimal. All the feedrates I use currently are best quality settings as per the Hypertherm manual. I use a THC system to manage height, and it seems to be working as far as I can tell. I use ohmic sensing to find the top of the material, and that works consistently.

What I'm wondering is what accelerations, speeds for the z-axis, and maybe other SheetCAM settings I need to use to improve performance.

Thank you.

[jimcolt]

There is a natural bevel (gets worse as material gets thinner....also is worse on non ferrous materials) with an air plasma. If it is excessive....then your torch is probably riding too high off the material (torch height too high). Pay attention to the physical cut height.....not the arc voltage. Observe the torch while cutting and adjust the voltage until the physical cut height (from the Hypertherm manual it is .06") is correct. The voltage is not critical, the physical cut height is to ensure best angularity.

Jim Colt Hypertherm

I have a custom build CNC plasma. It works pretty well for simple art cuts, but I'm really having trouble getting good cut performance for more precise parts, especially for thicker aluminum. For example, in 1/4" 6061 aluminum, I have at least a 1/16" taper on cuts, which is much worse than what I've seen done by other people.

The software I use is Mach3 and SheetCAM. I've set most of the parts to use perpendicular lead-ins and radial lead-outs, which I understand is optimal. All the feedrates I use currently are best quality settings as per the Hypertherm manual. I use a THC system to manage height, and it seems to be working as far as I can tell. I use ohmic sensing to find the top of the material, and that works consistently.

What I'm wondering is what accelerations, speeds for the z-axis, and maybe other SheetCAM settings I need to use to improve performance.

Thank you.

[rahilm]

As far as I can tell, cut height is correct. Do you recommend lowering voltage then, until cut quality improves?

[jimcolt]

If you are positive that the height is at .060"....then your height is correct. Higher or lower by more than .010" will affect the cut quality. Normally when users complain about excess bevel...it means the torch is too high off the plate....so that is the reason I suggested that your height may not be correct.

Often, even with a THC....the height (which likely is controlled by arc voltage feedback) can be incorrect. A normal system with full height control capability will:

-Find the surface of the plate (you said yours uses "ohmic contact"), then retract to the pierce height.

-Stay at pierce height (pierce height recommendations are in your plasma system operators manual for each material thickness and power level...if you use a Hypertherm plasma) until the pierce is complete...then index down to a physical cut height (also from the manual).

-The torch should freeze at the cut height....then after a time period should switch to arc voltage feedback control (which watches the arc voltage to maintain the height) (there is a suggested arc voltage in the Hypertherm manual for each material, thickness an power level) for steady state cutting height control.

-If arc voltage is correct you should se the cut height stay at the physical cut height even if the material is severely warped. If the torch gets too high after the arc voltage control (avc) activates....then you should reduce your voltage until the cut height is correct.

The arc voltages listed in the Hypertherm manual are correct under laboratory conditions. In many cases different height control systems, different installations, etc., have different calibration that affects the voltage/height ratio. Often...for your particular machine you may have to adjust the voltage to maintain correct physical height....to a voltage that is different from the suggested voltage on the cut charts.

Physical height is important to control cut edge angularity and consumable life....arc voltage is not critical.


Jim Colt Hypertherm

As far as I can tell, cut height is correct. Do you recommend lowering voltage then, until cut quality improves?

[rahilm]

Hi Jim,
Thanks for all the help. Based on what you said, I spent a bunch of time (and a nice chunk of 1/4 sheet) playing with different settings to see what works. One of the biggest improvements was to just turn off the THC, it seemed to be causing more problems on thick material than it helped (works great for thin material, though, to deal with warping and all). I also had to use feed rates at about 25% of what is in the manual. Is that recommended, or can it have adverse effects?