Hole Quality

[slvr98svt]

Ok guys, I am about to pull the trigger on a cnc setup and am most likely going to be going with Torchmate and a Hypertherm Powermax1250. We do a lot of small tabs made out of .100 mild steel with holes that range from .1875" diameter up 2". Now with the material being fairly thin, I suspect I will not see much of a problem with the larger holes, however My question is, how precise with the smaller diameters be?

Thanks Alot guys
-Matt

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[millman52]

Ok guys, I am about to pull the trigger on a cnc setup and am most likely going to be going with Torchmate and a Hypertherm Powermax1250. We do a lot of small tabs made out of .100 mild steel with holes that range from .1875" diameter up 2". Now with the material being fairly thin, I suspect I will not see much of a problem with the larger holes, however My question is, how precise with the smaller diameters be?

Thanks Alot guys
-Matt

I am not currently a plasma user on my CNC table. I am however using Ox/Fuel on thicker plate 1/4" through 1" I can cut down to 1/2" holes that are very near drill quality holes. I can cut smaller diameter holes but get lots of rounding of the top edge from heat. Although the plasma stream tends to wander a bit as the cut process tales place you should be able to burn fairly small holes. Not sure if you can achieve good success on the 3/16" holes. I guess it will depend a bit on the accuracy you need between hole centers on bolt patterns.

[jimcolt]

Good quality holes with plasma are a function of 4 major systems on any cutting machine....the Plasma, the motion capabilty in terms of path following and acceleration, the torch height control, and the part program in terms of lead in design as well as end of cut design (lead out vs overburn)

1. The plasma....an air plasma (such as the Powermax1250) is at the low end of the spectrum as far as cut part quality goes.....compared to industrial high definition oxygen plasma systems....there is no comparison in terms of cut quality, cut edge metalurgy and consumable life (which affects cut quality!)

That being said....if the system is being used as designed and the consumable parts are in good condition....you can still cut some very nice holes with minimal taper and quite round....
2. The cutting machine and accurate motion control. It is relatively easy for a cnc machine to accurately follow a part program. It is considerably more difficult for a cnc to follow a part program while very accurately maintaining a steady speed. If the machine slows dramatically during a cut....the plasma arc will burn a wider kerf, and the arc will tend to wander causing angularity problems on the edge.

CNC machines with properly (inertia matched) drives and well tuned motion control and high acceleration/deacceleration characteristics will cut better holes and fine features with a plasma than machines with lesser capability. Many of the industrial quality machines that are used with high definition class plasmas are rather expensive....and the reason behind that is with the drive capability (very small following error, very tigh mechanical design, very high acceleration).....many machines with superior drive capability cost well over $100,000.

3. Torch height control is (after maintaining cutting speed) the second most important plasma cutting parameter. A good height control first finds the surface of the plate....then indexes to the manufacturers (Plasma) recommended pierce height (usually 1.5 to 2 times the recommended cut height).....fires the torch, then as soon as the material is fully pierced indexes down to the cut height (before moving off the lead in)...then maintains proper torch to work distance within plus or minus .005" of the recommended cut height regardless of any warpage in the material being cut. If the torch is not at the correct height....expect wild angularity changes as well as kerf width changes as the hight fluctuates. If the torch pierces too close (even just once)...expect the plasma nozzle to become damaged.....and all bets for good cut quality are off.

Good torch height control is absolutely necessary for good cut quality.

4. Part programming. For holes.....plasma likes a straight lead in....with adequate length to allow the height control to achieve proper cut height before the cut path is reached....then do not use a lead out (that works well with oxy fuel)...rather use an overburn (overburn is when you extend the radius of the hole past 360 degrees).....and if the controller or part program has the capability...shut the plasma off while in the overburn area...and keep the motion going for at least 20 degrees or so past 360 degrees. (this allows the plasma to extinguish while the torch is still moving....which smooths out the end of the cut. Holes should also be cut at a slower speed as compared to the outside contour of the cut....60% of the cut speed is a good starting point.

So......it is difficult to answer your questions on hole quality and accuracy.....as all of the components of the machine have an effect on this accuracy! The ebest thing is to experiment with your system....taking into account the items mentioned above. Once you get the best quality on a particular hole (by playing with speeds, programming technique, plasma power levels, etc.) then record that data so the next time you cut holes it will be easier.

Jim Colt

[Big John T]

Jim, I always learn something from your posts.

Thanks
John

[skippy]

Hi Matt,
I'm going through the same thing at the moment. I presently subcontract out a job to a laser cutting company. Their work is very good but naturally I'd prefer to buy my own machinery and do it myself if the outlay isn't rediculous. It's a metal cabinet (4 sides plus top and bottom) 1500 x 750 x 660 made in 2mm (59" x 29.5" x 26" x 12 gauge?) sheet, Minimum 50 units at a time. Minimum hole size is 5.0mm (3/16") (for rivets) and I'm wondering if I bought my own plasma cutter whether I could get it to do a good enough cut quality. In fact now that I think about it they do little holes in the cuts in the corners where the returns are to folded. No doubt there is a proper name for these little holes but they are to aid in the folding process so that the metal will fold properly in both directions. Generally these holes are equal to the sheet thickness = 2mm (0.078"/12gauge). Too small for the plasma?

[jimcolt]

If you had a precision cutting machine with a precision torch height control...and a high definition plasma....you probably could cut 3/16" rivet holes with good quality on steel up to about 3/16" thick. As the holes get smaller in diameter it becomes increasingly difficult as most machines cannot maintain the speeds required by plasma for the best quality. If the machine cannot achieve the speed...the plasma will produce a larger kerf and a cut with varying angularity. It is unlikely that you can cut perfectly round holes of 2mm dia. in 2 mm thick plate with plasma.

Jim

[DISCONNECTED]

Quality holes with a conventional plasma setup and a mechanically tight machine.The best I have seen is .5 even that is not drill or punch quality.Anything under .5 is not round anymore.If you need small holes drill or punch.Some people pierce a pilot hole in the metal then drill it out.Watch out though ,plasma leaves a hard edge that is not kind to drill bits.CNC plasma is a neat process but not the best for any kind of precision process.I would take a water jet anyday,much more versitile machine.But price and the speed at which you want to cut can be an issue.

[Gec]

I have Hypertherm 1250 with Burny THC and the smallest hole with good quality is about 8 mm on 5mm steel plate. Problem is on speed, if I put more speed then the hole isn't round...lower speed - more slag, everybody must find posibilities of machine. On 5 mm plate use finecut tips 50A current with lower speed.

[skippy]

thanks for the tips guys
skippy

[jimcolt]

Conventional air plasma should not be compared with industrial oxygen plasma and high definition plasma capabilities. Very fine features and holes can be achieved with high end plasma (which is less than 1/4 the cost of industrial lasers....and far lower in operating cost). Attached are some pics of a 1" plate with 1/2", 3/4", 1", 1-1/4" and 1-1/2" holes cut with a Hypertherm HT2000 oxygen plasma......next a piece of 10 gauge steel with .200" diametr holes cut with a Hypertherm HPR130 Hyperformance plasma, and a piece of 3/8" steel with a 5/16" hole cut...then threaded with a $3.00 craftsmen (Sears) brand tap. Plasma, coupled with good motion control technology can provide very nice...unhardened holes and fine features!

I personally cut each of these samples using standard plasma processes....and would be happy to share the cutting specs with anyone.

Best regards, Jim Colt

[GDuck]

Regarding your comment "rather use an overburn (overburn is when you extend the radius of the hole past 360 degrees).....and if the controller or part program has the capability...shut the plasma off while in the overburn area...and keep the motion going for at least 20 degrees or so past 360 degrees."

I am using the Hypertherm Powermax 65 with the Maverick software. Do you know if this setup has the capability your suggesting?