Kerf size and shape: inside dimensions

I’m cutting square holes in 10 ha steel. I’ve found I have to add increased distance to the size of the hole to get what I want. I’m trying to noodle through why this is. I’m new to plasma, but I’m familiar with the problems in 3D printing with inside diameters, especially in round holes. That said, here are some questions for that the plasma gurus:

  1. Perpendicularity of the kerf: is it true that you can’t quite get a totally square kerf? I’m getting about 0.04” difference between the size of the hole at the top surface and the smaller hole at the bottom surface, so each kerf leaves about 0.02” extra material at the bottom edge of the cut. I’ve slowed down from what the cut chart recommends, from 160 in/min to 110 in/min to get to this point. Question: can I get the kerf more perpendiccular than that, or should I be happy with it like this?
  2. when I set the kerf width in CAM, should I measure the result on top surface of the plate?
  3. with those two issues resolved, should I keep notes on each material and thickness and record the resulting difference between top and bottom dimensions and just allow for that in CAD? Is it reasonable to just predict what the kerf angle would be instead?

Thanks for any advice!

Air plasma cutters will always produce a little bit of edge angularity, especially on small holes where the diameter to thickness ratio is high. All of the big name plasma cutter manufacturers are constantly working to reduce cut edge angularity through better torch design.

To get the best angularity possible, be sure to use the lowest amperage nozzle that can still clean cut the thickness you are working with. For example, when i cut sheet metal with my 60 amp Hypertherm, i use a 30 amp nozzle for the best angularity (typically about 3 to 5 degrees).

Also, you should cut holes at about 50% of what the recommended cut speed is.

Which plasma cutter are you using? When in doubt, swap in a brand new nozzle. Good holes require the nozzle orifice to be completely defect free.

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Thanks! Hypertherm Powermax 900 with a Duramax hand torch.

Got it. Like with a laser, there will be some degree of taper in the kerf. Reducible, but not possible to totally eliminate.

I will buy some low amperage consumables. But I’m cutting the 10ga at 55A (max). I will definitely go slower on the internal surfaces and see how that does with dross.

Mainly I’m curious what the conventional (theoretical) “calibration” sequence would be so that I’m not compensating for things in an inefficient way. Do people set the kerf size and then measure what results on the top surface of the part? Then, would it make sense to add an allowance in CAD, depending on material thickness?

I’m not trying to cut with chainsaw, measure with micrometer. I just want to know if I’m thinking about the sequence correctly (or at least conventionally). Hoping for some “common shop practice” nodding or shaking of the heads.

Thanks again @langmuir-daniel!


My personal machine is the powermax 1000 with the duramax torch retrofit. It’s an awesome machine with the best torch design on the market.

Definitely consider dropping down to at least a 45A nozzle for 1/2” down to gauge thicknesses. I always set the plasma cutter 5 amps less than the rating on the nozzle.

For bolt holes, i always cut about .050 larger than bolt so that they go in without any post-drilling. That will vary based on material thickness and diameter.

I never mess with the kerf size, I always program it to .060 for the 45A nozzles.