In a nutshell….here are the things necessary to get the best quality holes, but the mix and condition of your equipment (plasma, torch height control and machine motion capability) certainly has an impact as well:
1. Make your lead ins as long as possible…preferably near the center of holes under 1-1/4” diameter.
2. Ensure your torch height control is operating correctly….pierce height is critical, and the ability to index from pierce height to cut height while still on the lead in is critical as well.
3. Your machine part program (machine code) should have a federate override that cuts all holes under 1-1/4” diameter at a slower speed (a rule of thumb is 60%) than the rest of the part contours.
4. The arc voltage height control should be frozen at cut height on all holes under 1-1/4” diameter. Absolutely critical. This should e done in machine code, and with some types of nesting software this and other functions related to good holes can be automatically set (MTC software can do this)
5.There should be no lead out, rather an overburn (staying on the radius of the hole) should go past the lead in kerf by approx. .200”.
6.If your software and controller (I don’t know what equipment you have!) is capable, the plasma (depending on model, as they all have different arc ramp-down characteristics) should receive an off signal so that the arc starts extinguishing slightly before crossing the lead in kerf. Plasma arcs do not instantly shut off….as there is an exothermic reaction occurring, as well as gas and current ramp down’s that are used to extend consumable life. If all of these shut down delays are allowed to occur after the machine motion has stopped….a rather large ding/divot will be present in the cut.
All of the above will take some experimentation to get the best results….every power level, every thickness and every hole diameter will require slightly different parameters to be perfect, but you should be able to make some pretty good improvements…again, assuming that your machine motion is good (no slop or backlash, good acceleration and de-acceleration), your height control works well, and you have a good plasma system.
If you forward an email to my direct address I can send you a .pdf of a presentation on best practices for hole quality. The bottom line, the best plasma system in the world (or laser or water jet) will not cut nice, round holes, unless all of the systems (height control, cutting machine motion control, cut part program) are properly coordinated and have the ability to tightly control the process. My email: firstname.lastname@example.org
Attached are a few pics of holes cut with my Powermax 45 using above techniques on a machine with a full featured, properly operating height control system.