Radius Lead ins cutting into shape

[ARironworks]

On curved pieces straight leads ins will often leave a little mark as they transition in, so for some organic art I tried arcs instead of lines. but the end up gouging much more for some reason. It seems like the torchpath scoops into the material nearly an 1/8" before re aligning to where is should be and continuing the cut. I'm left with with a piece where the lead in point looks like a mouse took a nibble. Anyone have recommendations for this?

TM 4800 plasma

[kicktillmonday]

I am certainly no pro but I have noticed the same thing on many of the pieces I cut. I ended up sticking with the straight lead in but I always try to locate it at any sharp corner or direction change at the end of the curve rather than starting on the curve. Some software will give you plenty of options with your lead ins, some may not, hopefully your software gives you flexibility to place the lead in where you want, and at an angle that is a nice soft flow into the part.

[acourtjester]

You may see if you have any slop in the axis movements. And a stated above you can use SheetCam's option to move the lead in/outs to any location with a closed cut, lines you can only have one end or the other with no offset. Another thing in you drawing/DXF file if you keep the line width very narrow I think it helps with tighter movements I try to use like 0.004" stroke width.

[adbuch]

On curved pieces straight leads ins will often leave a little mark as they transition in, so for some organic art I tried arcs instead of lines. but the end up gouging much more for some reason. It seems like the torchpath scoops into the material nearly an 1/8" before re aligning to where is should be and continuing the cut. I'm left with with a piece where the lead in point looks like a mouse took a nibble. Anyone have recommendations for this?

TM 4800 plasma

I agree with Tom that you may have some slop in your table causing an exaggerated defect at your lead in location using your "arc" lead-in . For most applications, using a 90 degree lead-in end (perpendicular to the actual cut path) provides the best result. This "divot" problem can be exacerbated at the end of the cut path where the torch will continue to cut after the "torch off" signal is sent. So it ends up cutting twice at the lead in point.


Per Jim Colt:

"The plasma cutting arc does not instantly extinguish when the cnc calls for the cut to stop. There is an exothermic reaction between the oxygen content in your compressed air and the workpiece that stays on for a number of miliseconds....and this reaction continues to burn the metal. So motion stops, the exothermic reaction continues....and a divot that does not show up on your cut file is developed (as shown in the picture). On more advanced plasma controls (industrial high definition class machines) the software sends the plasma off signal slightly in advance of the end of cut, and ramps down the amperage and cut speed in a co-ordinated manner than eliminates or minimizes the divot. Similar co-ordination can also be done at the beginning of the cut cycle. In the big boy cnc plasma systems this is often transparent to the machine operator (canned in the software). On hobby class and light industrial cnc tables this type of coordinated motion / off signal can occur however it would take some operator manipulation to get it to work as well as the high definition technology works. Many hobby and light industrial cnc softwares do have some functionality such as an overburn (keeping the motion going slightly beyond the point where the plasma arc is extinguished, however every different process power level, cut speed, material type and thickness would need specific moves and timing to provide the best end of cut quality. This type of coordinated motion has been available for a number of years when a Hypertherm high definition plasma is integrated with Hypertherm CAD/CAM software and Hypertherm cnc controls and coupled to a precision cutting machine. Hole cutting (Hypertherm's process is called True Hole Technology, and it uses similar timing, motion and even plasma gas changes real time while holes are being cut, making "bolt ready" holes with virtually no taper or start/stop divots."

One possible solution is to use a lead-out in addition to your lead-in, in combination with setting the "gap at end of loop" to some small positive value.

[TJS]

This is good information. I have never tried a Perpendicular lead out in holes. I think next time I will. My question here is: How do you get the gap.I assume a custom rule or whatever it is called in sheetcam.
I know in command CNC that custom rules conflict with the rules built into it. EX. There is an End Of Cut rule canned.

[cutnweld]

If you enter a negative number in the overcut option in sheetcam I believe it will stop before getting to the end of circle

[TJS]

If you enter a negative number in the overcut option in sheetcam I believe it will stop before getting to the end of circle

Are you answering my question. If so that is pretty cool and you learn something new everyday. I will try that.

[cutnweld]

yup that was for your question TJS

[adbuch]

Thanks cutnweld for you information on doing the "gap at end of loop" with SheetCam. I don't use SheetCam for cnc plasma cutting, but those of use who use Plasmacam Design Edge have this option in the Offset/Convert settings tab.