cutting tips for circles

[Little Leonard]

Guys I'm new to the cnc plasma table and just got mine cutting last week still have a few bugs to work out but all is good. Can anyone help me out on cutting round holes, my holes are not round and cuts a a angle any help would be great here's a pic of my first try.

[weldor2005]

You will always have some angularity to your cut, and somewhere in this site....search field.... there was just a long drawn out thread on this very issue. basically out of round holes get better with slower cut speeds.

If you ever want to get away from having to re-drill out the hole afterwards, you need h def. or laser, or water jet.

So except it, and just drill them out.

[GoingCustom]

Slow your cutting speed to about 60%, that will help. But ultimately as stated above, something other than a regular plasma cutter is required. Pretty much anything under 3/4" I'll drill instead of cut.

[beefy]

You should see Jim Colts holes he does with his Hypertherm, it just makes me want to cry His holes seem bolt ready straight off his table.

I've tried the 60% cut speed thing and THC off and I still get heaps of angularity. A plasma table manufacturer here in Australia recommended lowering the normal cut height in an attempt to create more flare out at the bottom of the metal. Nothing to loose by trying it.

When I get round to it I'd like to try lower cut height and go silly with speed reduction, right down to 10% if that's what it takes to get better hole quality.

Keith.

[michmetalman]

I used to get perfect holes on my Torchmate, but I have a few worn out parts that are causing irregular holes I need to get replaced.

[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.

Powermax holes samples 001.jpg

Powermax holes samples 002.jpg

[dgeorgester]

I was also told that the direction of the cut was important due to the swirling of the plasma jet. Can any one confirm this?

[beefy]

Yes, that's true.

It's probably machine specific but on my Hypertherm, if you were looking at the torch travelling away from you the good cut side will be on the right.

So for an internal cut on a circle the torch will be going counter-clockwise.

The jet is swirling so if a different brand of machine had a jet swirling the opposite direction then the cut side could be different.

Keith.

[KM Koncepts]

Hey guys new to the cnc plasma game and have been following this thread. So as your talking about the swirl from plasma cut has a good cut side because of the swirl going in opposite direction, thats why software cuts outside circles one way and inside opposite correct? Also if your getting a bad cut on inside holes you can try reversing cut direction and see if it improves?

[jimcolt]

Beefy is correct. Think of cutting a ring, the inside diameter is cut counter clockwise, OD is cut clockwise. This provides the best edge squareness, and is due to the swirl pattern designed in the plasma torch.

The gas (air) swirl in a plasma torch does a few things, one of which is to help control edge squareness. If the torch was designed with reverse swirl rotation, then you could reverse direction on holes. There are some high end industrial plasma torches that have available reverse (CCW swirl) consumables. These are used in "mirror image" cut situations where two torches cut mirror image parts simultaneously. They are also used when you are cutting a flat plate into strips (bar stock) with two torches simultaneously......the torch on the left (driving away from you) will have standard swirl, the one on the right will have ccw swirl. This insures that both edges of the strip being cut will have the squarest edges.

I mentioned that swirl does a few things. Another very important function of gas swirl in a plasma torch is to increase nozzle life by improving the nozzle bore cooling. The air inside the torch gets superheated, however not all of the molecules are the same temperature, with the coolest being the heaviest. The swirl, properly engineered, slings the coolest air to the outside edges of the nozzle bore....effectively creating an insulating layer of cool gas. This makes the nozzle last longer. To make this work....the consumables must be manufactured of the correct materials and to very strict dimensional tolerances, and the air flow circuitry (pressure and flow) needs to work accurately and correctly. Now we know why Hypertherm genuine consumables last longer than others!

Some Hypertherm torches have a swirl ring designed with a dual swirl pattern, one swirl is designed to increase nozzle life, the other to improve edge squareness. This technology is used in high end oxygen plasma torches as used with high definition class plasma.

Jim Colt

[BrianMick]

Some Hypertherm torches have a swirl ring designed with a dual swirl pattern, one swirl is designed to increase nozzle life, the other to improve edge squareness. This technology is used in high end oxygen plasma torches as used with high definition class plasma.

Jim Coltjimcolt

Posts: 1041
Joined: Mon Jul 20, 2009 9:18 pm
Location: New Hampshire
Private message

Jim, do all high definition torches have the dual swirl? and does this mean you can cut in any direction with this?
I have an hpr400xd does this have dual swirl?
Brian

[jimcolt]

Dual swirl is used in the oxygen consumables for your HPR400xd. Both swirl paterns spind the same direction, but at different rates. The torch is still directional.

Jim Colt

Some Hypertherm torches have a swirl ring designed with a dual swirl pattern, one swirl is designed to increase nozzle life, the other to improve edge squareness. This technology is used in high end oxygen plasma torches as used with high definition class plasma.

Jim Coltjimcolt

Posts: 1041
Joined: Mon Jul 20, 2009 9:18 pm
Location: New Hampshire
Private message

Jim, do all high definition torches have the dual swirl? and does this mean you can cut in any direction with this?
I have an hpr400xd does this have dual swirl?
Brian

[tm america]

Jim Colt you mention continuing the cut around the radius .100 to .200 past the point where the lead in comes into the the cut?Doesn't this cause the consumables to wear faster?I ran mine at an overlap of .02 and did see an improvement.So question is How much does this wear the tip and electrode vs not having an overlap?I found a big difference in my circle quality when i switch from lines to radius for my lead in and lead out.I'm still looking for the perfect balance for the feedrate.If i go slow enough to get really nice circles i end up getting over cuts at the beginning of a sharp turn=90 degree... Maybe i shouldn't say feedrates i played with my accell rates if i run them lower like 50 it makes the torch maintain a constant speed around the circle.If i go higher it will hit full cut speed just after it enters the radius after the lead in..Slow accell rates give me an angle to my 90 degree corners for about 1 inch.If i raise the accell rate it gets almost no defects on a 90 but the circles will not be round