a couple of things

[stirling]

looking for pointers to track down why small holes around 12mm or less come out as polygons rather than circles on my cnc table with PM45 machine torch setup.

Also I'm using a PM45 with my own THC and setting power, voltage, feed etc. as per the manual and generally cuts are good - but if I wanted to lower the feedrate (thinking that might help with the circle problems) are there formulae for working out how to change the power/voltage/etc. accordingly.

Thanks

[jimcolt]

In a nutshell….here are the things necessary to get the best quality holes, but the mix and condition of your equipment (plasma, torch height control and machine motion capability) certainly has an impact as well:

1. Make your lead ins as long as possible…preferably near the center of holes under 1-1/4” diameter.
2. Ensure your torch height control is operating correctly….pierce height is critical, and the ability to index from pierce height to cut height while still on the lead in is critical as well.
3. Your machine part program (machine code) should have a federate override that cuts all holes under 1-1/4” diameter at a slower speed (a rule of thumb is 60%) than the rest of the part contours.
4. The arc voltage height control should be frozen at cut height on all holes under 1-1/4” diameter. Absolutely critical. This should e done in machine code, and with some types of nesting software this and other functions related to good holes can be automatically set (MTC software can do this)
5.There should be no lead out, rather an overburn (staying on the radius of the hole) should go past the lead in kerf by approx. .200”.
6.If your software and controller (I don’t know what equipment you have!) is capable, the plasma (depending on model, as they all have different arc ramp-down characteristics) should receive an off signal so that the arc starts extinguishing slightly before crossing the lead in kerf. Plasma arcs do not instantly shut off….as there is an exothermic reaction occurring, as well as gas and current ramp down’s that are used to extend consumable life. If all of these shut down delays are allowed to occur after the machine motion has stopped….a rather large ding/divot will be present in the cut.

All of the above will take some experimentation to get the best results….every power level, every thickness and every hole diameter will require slightly different parameters to be perfect, but you should be able to make some pretty good improvements…again, assuming that your machine motion is good (no slop or backlash, good acceleration and de-acceleration), your height control works well, and you have a good plasma system.

If you forward an email to my direct address I can send you a .pdf of a presentation on best practices for hole quality. The bottom line, the best plasma system in the world (or laser or water jet) will not cut nice, round holes, unless all of the systems (height control, cutting machine motion control, cut part program) are properly coordinated and have the ability to tightly control the process. My email: jim.colt@hypertherm.com

Attached are a few pics of holes cut with my Powermax 45 using above techniques on a machine with a full featured, properly operating height control system.

Jim

Originally Posted by stirling looking for pointers to track down why small holes around 12mm or less come out as polygons rather than circles on my cnc table with PM45 machine torch setup. Also I'm using a PM45 with my own THC and setting power, voltage, feed etc. as per the manual and generally cuts are good - but if I wanted to lower the feedrate (thinking that might help with the circle problems) are there formulae for working out how to change the power/voltage/etc. accordingly. Thanks

[stirling]

Jim

Thanks again for your help. email sent to you as requested.

Meanwhile - another.

Attached is an image of the edge cut quality I'm generally getting. The mild steel is 3mm thick and as you can see I'm getting a bit of a wavy edge. All machine settings were as per the PM45 operators manual. Any suggestions on improvement as ever gratefully received.

Also can I ask again - if I wanted to slow down feedrates generally, is there a formula for figuring out the changes to amps/voltage etc. to those in the hypertherm operator manual?

Ian

[jimcolt]

No....there really is not any type of formula for determining the correct voltages/amperage at lower speeds. The important thing with any Hypertherm plasma is with torch height.....maintaining the recommended physical torch standoff height is more important than arc voltage.....in fact once you get the speed where you like it, adjust the voltage setting on your height control until the torch rides at exactly .062" off the plate....make sure you are using the shield designated for machine cutting.....with the hand cutting shiled this height will not produce a good cut.

From the picture , I would surmise that the torch to work distance is too high.....indicated by the rounded top edge, the angularity, and the wavy cut edge.

If you have the arc voltage adjusted to maintain the correct height, and you decide to slow the feedrate down....the torch height control will actually move the torch closer to the plate. So, when you slow down, increase the arc voltage while cutting at the new speed until you are at the recommended height.

For amperage....the 45 is a bit of a different technology that previous plasma systems. It has a nozzle shiled design that uses a patented "conical flow" technology. It allows one nozzle design to cut a wide range of thicknesses, where other plasma systems need to have a few different nozzle sizes. I leave my 45 at full output power from 1/8" thickness and up. Below 1/8" I reduce amperage to 40, and on materials thinner than 16 gauge to 30 or 35 amps. This allows me to slow the cut speed down to stay withing the capabilities of the cutting machine. Most entry level, stepper driven machines do not contour well at speeds above 100 to 150 ipm.

Jim

[stirling]

Thanks Jim - I'll take away all you've said and see where I get. However - one question...

Originally Posted by jimcolt The important thing with any Hypertherm plasma is with torch height.....maintaining the recommended physical torch standoff height is more important than arc voltage.....in fact once you get the speed where you like it, adjust the voltage setting on your height control until the torch rides at exactly .062" off the plate

This might seem like a daft question - but - how do you know you're exactly at the correct cut height - I mean how do you measure it? It's a bit hot to stick a vernier in there whilst you're cutting....

Ian

[jimcolt]

I am not familiar with your height control.....but with many that I am familiar with you can simply hit the stop button mid cut and the torch will stop cutting and stay at the height that it had ben cutting at.

On some Industrial height controls, and on PlasmaCam DHC2 and Samson machines, the top of the plate is sensed using ohmic contact, which is a very accurate method. The torch tehn retracts to the pierce height (using an encoder feedback for measurement), then after piercing indexes to the cut height (again using encoder feedback), then starts cutting, samples the voltage and locks onto that voltage. These types of torch height controls generally maintain the torch to work distance very accurately.

The other method is just watching the torch to work distance,,,,the human eye can usually judge the height fairly accurately (within less than .010") once you are familiar with watching it operate.

Jim

[stirling]

OK Jim - thanks - I'll go and have a mess and see where I get.