I'm attempting to squeeze as much precision as possible out of my Torchmate 2X2 + Harbor Freight plasma + manual torch height z-axis. I'm not a professional, so can't justify the more expensive plasma cutters out there, nor an AHVC, nor an air conditioner.
My holes are getting tapered at different rates across the circumference. I'm working with 0.25 CRS. The amperage and air pressure is set at the highest level. The lead-in edge is good, but it tapers in as it works its way around. I've tried feedrates from 10 down to 3 ipm. While watching it from the side, it is clear that the cut isn't piercing equally throughout the travel around the circumference. It veers off towards the middle. I assume that this is because the material is more hot near the middle, thereby melting more quickly and causing the cut to wander into it. I don't think its because of a damaged nozzle because straight cuts look OK.
I thought it might be possible to try a spiral pattern from the center. The idea is to heat the material equally around the circumference, get it close to melting temp, and blow out a clean edge for the last pass. I'd experiment with different speeds to minimize dross. Maybe even change the speed on the last pass?
Any other techniques? Has this been tried before?
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 firstname.lastname@example.org.
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.
Try reversing the cut direction, you may be going the wrong way.
Like Jim I also make the holes slightly undersized and finish them on the drill press. I use a straight flute reamer, it seems to work better than a drill bit.
I don't think your spiral idea will work well. My machine (ESAB) cuts best when the arc is "supported" by metal on both sides.
Plasma cutting is directional.....holes should be cut counter clockwise. If you are cutting a ring, the ID cuts counter clockwise, the OD cuts clockwise.
Actually....the only holes that I normally drill are holes that need the edges to be soft, such as for tapping. The vast majority of holes that I cut for bolt holes are accurate enough without drilling by using my hole cutting techniques, and a Hypertherm plasma system. I cannot make that claim for plasma systems that are older technology, or are not Hyperthem brand as my experience shows me that the cut edge angularity is not adequate to remove the taper in holes....no matter what technique is used!
Brands aside, if you use Jim's guide you will get nicer holes. We use a Hypertherm machine that is considered by many "entry level" (pm1650) and it is a cutting fool. round holes with little taper within .030"
'Whoa, hold on there! I'm not giving you more ammo for your "Hypertherm or nothing" campaign."
As a Thermal user all I can say is I have learned more from Jim Colt's posts on here and other places than I have from the factory guys that represent Thermal Dynamics.
Don't take that the wrong way as they have helped a great deal but all you have to do is a little searching to see that Jim is a wealth of information.
I have two Thermal machines and if Hypertherm would have a buy-back promotion I would probably switch! If for no other reason than the on forum support.
End of rant!
You can certainly use my techniques with any brand and model plasma cutter! You will see varying degrees of improvement based on the design and condition of the plasma. While I have worked with many brands of plasma systems over the years...my expertise is primarily with Hypertherm systems. I'm not here to blatantly advertise, just to offer my knowledge and expertise that may help with your plasma cutting processes regardless of brand.
Mongkol.....I guess I don't understand your question in regrds to the use of anti-spatter spray.