Making good-quality holes with plasma is easier than ever with Hypertherm’s True Hole™ technology, but what if you don't have True Hole? Though not quite as automated, it is entirely possible to make some very nice holes using plasma on a non-True Hole system. Here are some things you can do to get your holes just right.
Pierce height. Set a higher pierce height. The general rule is 1.5 to 2 times the recommended cut height, so if the recommended cut height says .062, pierce at .125".
Lead in location. Set your lead-in as close as possible to the center of the hole. There are two reasons for this. One, the top of the plate usually contains a slag puddle. 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. The second reason is that a longer lead in gives the plasma arc time to stabilize (pressure and energy take a while to ramp up), and also allows the height control to index down to cut height before it gets to the contour of the hole.
Lead-out. If you’re using an air plasma system like a Powermax, it is best to have no lead-out. Just let the arc shut off right on the hole contour. Some software has provisions to keep the arc on for a few seconds after the motion stops but on steel under 1/2" this usually is not necessary.
Cut speed. The advice here: slow down! Cut speed on holes should be about 60% of the speed used to cut the outside contour of your parts. We admit this will create some low speed dross on the bottom of the hole, but the good thing about this is that it 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.
Cut height. If you are cutting a smaller hole (anything under an inch) 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. This is because the slower speed used for cutting holes will cause the arc voltage height control to move the torch too close to the plate.
Use mig welding anti-spatter spray. We suggest that you try spraying the top of the plate before cutting. 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, or silicone or oil based spray. Use a water-based spray instead.
Consumable selection. Use the lowest powered consumable set recommended for your material thickness. Yes, this will reduce your cut speed but its worth it as you will get better results. If you are cutting with a Hypertherm system, use FineCut consumables for all holes on material thicknesses under 3/16", 40 Amp shielded consumables for thicknesses between 3/16" and 3/8", and 60 Amp consumables for thicknesses above 3/8" to 5/8".
Consumable inspection. Last but not least, regularly look at your consumables. The orifice shapes the arc and the arc shapes the part you are cutting. just one pierce too close to the plate or on thick material, can affect the shape of your nozzle. The nozzle and shield orifices must be perfectly round with no nicks, dings or craters. (Remember this from our series of posts on consumable wear?) Inspect with a 10x eye loupe. If the orifices are not perfect, take them off the torch and use them for hand cutting or contour cuts that are not as critical.
What you do think of this list? Do you have additional tips when it comes to cutting holes with plasma?