Tips for Cutting Circles

[jimcolt]

Cutting round holes is a well choreographed combination of x and y motion control, x axis height control, hole program including lead in and lead out shapes....and with a good plasma cutting system using the best consumable combination for the material thickness and type.

1. Adapt a fine tipped pen to your machine torch holder, draw some holes at different sizes and different speeds....look carefully at the drawing to ensure the holes are round. If some...particularly at higher speeds have non round areas at the lead in or lead out areas, then suppect some loose mechanical compenets on the machine. Worn gears, racks, loose motor mounts. sloppy machine motion ways will not allow round holes.

2. Ensure your height control system can accurately locate the surface of the material before each cut, then adjusts to the pierce height, pierces, then quickly indexes accurately to the proper cut height. After the cut height is achieved (within about .005") then the x and y axis should move on the lead in kerf and onto the hole profile. If the cut height is incorrect at the beginning of the hole radius the hole will have severe taper and out of roundness in that area. More noticeable on small holes.

3. Lead ins should be long enough so that the plasma arc has time to stabilize gas pressure, longer is generally better. On any hole under 3/8" the lead in should be at least at the center of the hole.

4. Hole cutting speed should be at approximately 60% of the suggested best cut quality speed for the power level and material thickness.

5. The lowest power consumables should be chosen that will cut the material thickness. If the systems 45, 65 and 85 amp consumables list the abiliy to cut 1/4" steel....know that the 45 amp process will provide the least taper and best edge quality.

6. At the end of the cut there should be an overburn....which means stay on the hole radius for about .150 to .200" past the lead in kerf. If you machine has the ability (or your software) shut off the plasma about .100" before the motion stops.

7. Torch height must freeze at the cut height (no arc voltage control) for all holes under about 1.25"

This is how good holes are cut. Older technology plasma systems...or low cost import plasma systems with one set of consumables for all cutting thicknesses cannot be expected to cut good holes. If the machine has undersized drives and cannot maintain constant speeds, do not expect nice holes. If the height control cannot maintain exact pierce and cut heights, do not expect good holes. if the machine has rough motion or mechanical backlash...do not expect good holes.

Doing all of these will provide holes that look like these, slight taper of about .04" (larger on top than bottom), but round.

Hypertherm Techniques to Improve Small Quality.pdf

Powermax holes samples 001.jpg

Powermax holes samples 002.jpg

[ibewolfer]

great info Jim.....Thank you for all your tips!

[Wadzii]

I'm using sheet cam.. it gives the option for arc, tangent and perpendicular for lead in. Which way have you found to work the best?

[motoguy]

Jim's articles all state that perpendicular lead in and no lead out give best results. I've been using that, but find I still have a divot. I have found that arc lead in, and arc lead out (with a small over cut) gives me the best results.

My table has 35 i/s/s accel (90mg, I think), and rigidity doesn't seem to be an issue, but the arc in/out still leaves the cleanest hole.