Smallest size bolt hole

[D.O.T]

Hello Folks,

So here is my first question on this forum.

What is the smallest sized circular bolt hole that a typical industrial grade CNC Plasma Cutting machine could cut?

I am looking to cut 3/16" Diameter bolt holes on a 1/4" stainless steel 304 plate. Will a typical industrial plasma cutter be able to do this or should i be looking into laser cutting instead since they have much smaller kerf widths and higher precision cutting?

[tcaudle]

That is a loaded question. It all depends on a host of factors. Generally i say that you should not try to cut bolt holes smaller in diameter than 2X your material thickness Plasma cutting is generating an arc, vaporizing the metal and blowing out the material with air. Its not a precision machining process. .A lot depends on the machine. other factors are the plasma cutter, the torch, the consumables , the table performance (acceleration), and the operator. If the holes need to be round and accurate I would consider another method. You can peck mark the centers with the plasma and it does not heat harden the material like a full start hole would, then punch it (if that's possible) or drill it . Options 2: Use a plate marker to center punch the small holes and either punch or drill

I may be wrong but I don't think you are going to get constantly round small holes in 1/4" SS with an air plasma. I doubt you want to lose even moderate sized pieces of 1/4 stainless. Lasers can do that if it big enough but you might also look at abrasive water jet. The Plasma Guru (Jim Colt) can give you more details.

Is heat discoloration a problem? Can you sand or grind on it after the cuts? How accurate does the hole diameter have to be? how much flare on the hole sides can you stand? Will the holes be covered?

[D.O.T]

Thanks for the thorough reply tcaudle.

I think from all that you said its quite clear that going with plasma cutting would be unconventional and risky since i am looking for precision cutting of small holes with minimal post processing. My interest is to simply have the plate cut and then given a mirror finish. For this reason i will start looking into laser or waterjet cutting as the alternative options.

[Jaime Branco]

For stainless steel up to 4 mm I use my CNC punching machine is the faster ... For 4 mm up I use my waterjet. For steel I use punching up to 5 mm plasma from 2 to 20 mm and Waterjet from 1mm up to 150mm .
But when I want qualify I always use waterjet or the cnc punching machine...
One important thing if you need a very good cut you must buy the waterjet with special head for cutting bevel elimination.... Like this for example: https://youtu.be/McVnZnIxM0A

[D.O.T]

Well, first of all, i am not a machinist. I am the customer. I came to this forum because there was no better place to get my question answered.

As for my decision, i have actually decided to go back to plasma since i realized that the company doing the cutting for me has an HD Plasma Cutter - which can attain very thin kerf widths and tight tolerances.

So far i have learned about the inside and outside kerf offsets that i need to instruct the operator to use for the holes and the outside outline of the components. But the part that i still cant figure out is the tolerance of the machine. I can instruct the operator to adjust the machines variables, such as reducing speed, so that i can get the maximum tolerance, but the part that i cant figure out is whether the machine automatically adjusts the tool path to compensate for the tolerance or whether i have to adjust it myself in the DWG file. If i have to do it myself then that means that i have to know what the tolerance of the machine is, which i dont believe is information that i can obtain.

For instance if i want my bolt holes to be 3/16" in diameter and the maximum tolerance of the torch is .020", which means the bumps of the outline of the cut grow larger than the holes radius by .010" and grow shorter than the holes radius by .010", then that means that i have to increase the bolt hole radius by .010" so that the radius of the hole reaching the first bump of the outline cut is precisely 3/16" and would thus allow for a proper fit of the bolts into the holes.

So to simplify, does the operator handle the tolerance offset or do i handle it? Better yet, is there such a thing as a tolerance offset? And if there isn't then how do customers cut male and female components that are supposed to fit together perfectly without the existence of tolerance offsets.

My reasoning is there has to be a tolerance offset and the operator should handle it since the software in the Plasma Cutter should be able to take in all the information, such as material type, thickness, speed of cutting, and whatever other variables, and then compute the offset automatically. This would be the only thing that makes sense since its impossible for the customer to know what the offset is.

But the problem is that i cant seem to find any information on tolerance offsets for cutters, whether plasma or laser.

Perhaps i am not understanding how the cutting process works?

Maybe someone can shed some light to my questions or thinking.

[tcaudle]

There you have it D.O.T! all you have to do is spend another 100K+ on waterjet and CNC punch press machines and you are good to go! I saw several combo CNC punch press/ plasma machines at the last Fabtech show. One had an auto material loader to boot. Cost about as much as a moderate new house.

[tcaudle]

You do offsets in the CAM. You simply make the kerf width wider and it widens the toolpath out . You would make it .005 nor kerf width to add .01 to a circle. Since kerf width is a tool parameter you may need to create a new tool with different parameters to make it cut specific hols (or grups of holes differently.

[D.O.T]

tcaudle, i am not the machinist, i am the customer.

nonetheless, so what your saying is that the operator of the machine can handle the tolerance offset by increasing the kerf width rather than offsetting the tool path using the CAM software. So this makes sense then.

The only question is how does the operator know how much to increase the kerf width by to get the precise hole diameter? Does he do it manually or is it done automatically by the machines? Or is it both, some machines need to be done manually, but a high end and new HD plasma can do it automatically?

[Jaime Branco]

In my water jet you make the drawing with the size you want and then when you load the CNC code in the machine she automatic makes the offset based in the nozzle diameter and the wear of the nozzle to obtain the desired size, but if is not correct the operator can adjust....

[tcaudle]

The machine itself cannot do anything (but cut) The magic stuff is all done in software. When you get into the HD systems they have specific software that can do things like the "truehole" technology where they can vary the cut current in conjunction with air pressure and precise timing to do cleaner holes...but you need a system that has the full interface to the HD system (including the gas mixing controls) meaning you need a control system designed expressly for the the HD cutter you buy. Figure that into your costs. In short you will place yourself outside the mid level/light industrial control systems and tables and run with the Big Dogs. Still cheaper than a water jet but a lot ; more than a general CNC plasma table. You ARE the machine owner/operator and not the customer when you start buying machines and doing the cut files in house.

Since plasma (even HD) has a hard time holding .01 tolerance it more of a cut and try to figure out the optimum kerf width. You cut a hole, measure it and increase the kerf width and cut it again. Kerf width IS offset because it cuts (just like a rotary tool) on the circumference of a circle and the actual toolpath is the radius off (offset) the cutting edge of the flame. Diameter of the flame is a function of the torch design , the cut height , the consumables and the age of the consumables.