The plasma arc on many plasma torches has a natural lag while cutting. You can se the lag angle if you look at the lag lines on a piece of steel you have cut. Now think about that lag angle as you cut smaller and smaller hole diameters....as the machine keeps turning left to produce a small hole, the lag angle follows and produces a taper.
Better plasma torches produce an arc with higher energy density....which means the plasma just has a higher velocity and a stiffer, narrower arc. Developing this technology requires engineering with high temperature physics, and that does not come cheap!
The other critical components of getting good hole quality are covered in this description of how to get decent holes with an air plasma:
For best quality on holes:
-I pierce at about 1.5 to 2 times the recommended cut height.....so if cut height is .062", pierce at .125".
-Use a lead in that gets as close as possible to the center of the hole for 2 reasons: 1. There usually is a slag puddle on the top of the plate..if this puddle stays on the radius (contour) of the hole, it will cause the plasma arc to waver and create a divot or ding in the hole. 2. A longer lead in gives the plasma arc time to stabilize (pressure and energy take a while to ramp up), also allows the height control to index down to cut height before it gets to the contour of the hole.
-With an air plasma, it is best to have no lead out. Let the arc shut off right at the 360 degree position on the hole contour. Some software has provisions to keep the arc on for a time period after the motion stops....on steel under 1/2" this usually is not necessary.
- Cut speed on holes should be right at about 60% of the speed used to cut the outside contour of your parts....this will create some low speed dross on the bottom of the hole, but will minimize taper in the hole. Some machines will do this automatically on all holes under a certain diameter, such as 1", while other software may have to have the G Code manipulated to achieve this.
-Ensure that cut height is correct. Usually on holes under 1" diameter it is best to disable arc voltage control....allow the pierce height, allow indexing to cut height, but don't allow arc voltage height correction...as the slower speed used for cutting holes will cause the arc voltage height control to move the torch too close to the plate.
-Try spraying some mig welding anti-spatter spray on the top of the plate before cutting. You may be surprised at the improvement in hole quality. The spray usually makes the top spatter from piercing non -existent,minimizing arc wobble on holes. While you are at it spray a little on the front of the torch to keep spatter off the shield/nozzle. Do not use the dip type, and I recommend the water based spray vs silicone or oil based.
-last, but not least....use the lowest powered consumable set recomended for your material thickness for best hole quality. This will reduce cut speeds but will give you better results. If it is Hypertherm use FineCut consumables for all holes on material thicknesses under 3/16", 40 Amp shielded consumables for thicknesses above 3/16" to 3/8", 60 Amp consumables for thicknesses above 3/8" to 5/8".
The nozzle and shield orifices must be perfectly round, no nicks, dings or craters. Inspect with a 10x eye loupe....if they are not perfect, use these parts for hand cutting or contour cuts that are not as critical. The orifice shapes the arc, the arc shapes the part you are cutting. Piercing too close or too thick can damage a nozzle orifice in one pierce.
So....to get the best precision on holes...you need to use the correct programming techniques, you really need a torch height control system, and of course a good plasma (plasma systems are not all equal)
I have a .pdf presentation on best practices for hole quality. If you provide me your direct email I can send it to you. My email is
[email protected].
Attached are some pics of holes that I cut with my 4 x 4 cnc equipped with a Hypertherm Powermax45 and a full function THC....And yes....you will notice some 1/4"-20 tapped holes in a couple of the pics. When I want to make the highes presicion holes, I program them slightly undersized with the plasma, then drill them out with a cobalt drill bit (the holes are nitride hardened from the nitrogen content in the air plasma jet)...in this case I cut them at .188", drilled to .201", and tapped them easily to 1/4"-20.
Jim Colt