Beveled Kerf and Elliptical Circles

Quote:

Originally Posted by Shanghyd

Couple of questions about some settings.
I am cutting 3/16 mild steel.
Pierce is .190
Cutting speed is 55 ipm.
Cutting height is variable. The THC is setting it at about .120 even though the gcode is set at .025
#1. I am getting a beveled kerf.
#2 I am getting elliptical circles.

The torch is square so that isn't where the bevel is coming from. I suspect its coming from the torch height. I called Ezouter and received some advice about changing a setting in the post file. The setting was "switch offset". Originally the offset was .150. I changed it to .030. I ran the program and when the torch touched the plate and rose up to pierce height I stopped the machine. I measured the height of the torch off the plate and it was .170. The DRO for the z axis read .216 for a difference of .046 I put the value .076 for the switch offset. I'm not sure this was right. I'm not sure if I should have put .046 or .076. I unloaded the g-code from mach3, went back to Sheetcam, ran the post processor and reloaded the g-code. The cutting height was set at .025 in the Sheetcam settings for cutting height. I ran the program and it cut at .125 Not what I expected.
Any thoughts are appreciated.

SwitchOffset (variable in the POST) only controls the IHS (inital height setting) and the subsequent pierce's. It references the tip of the torch at the top of the material. The offset value is the "overtravel" of the floating holder and it's trip switch (how far you have to move the torch up the slide to trip the swtich) It's usually different on every machine because it's mechanical and dependant on the switch, the switch mounting, etc. You need to detemine the REAL value for that by:

Hit the REF button next to the Z DRO and it should move down to the metal and stop.

Zero the Z DRO

Carefully jog the Z up until you can slip a piece of paper between the tip and material (feeler gage).

Write down the value in the Z DRO

Do it as many times until you get close to the same number.

Put that number into the SwitchOffset = ### and save the POST (make sure it saves with a .post extension and NOT a .txt extension)

It should then touch off, automatically raise to the offset (with the tip sitting just above the top of material, rezero the Z DRO and raise to pierce height. After the THC takes over the arc gap (standoff) is a function of your Target (Preset) Volts. Each volt is about .015 of gap. If you are cutting too high your cut will tend to have a lot of flair. YOu can lower it by moving the target volts down. Do it a volt at a time.

The feedrate of a cut is also a determining factor as well as the cut current. Stick with the numbers in the cut chart but try 10% lower feedrates at a given current setting. Don't use lower cut current on a given nozzle size to try and get better cuts unless the chart calls for it. The direction of a cut also determines which side of the cut is flared

It's a dance between proper Feedrate, proper cut current, proper nozzle size, proper consummables type (NEVER use a drag tip with CNC cutting and a THC) , proper arc gap and having clean air, dry air.

The pierce height being correct is critical to get max life from your consummables and prevent misfires or ugly divets in the start of the cut.

TOM caudle
www.CandCNC.com