Machining plasma cut steel

[john_c_kennedy]

Sometimes I like to rough cut parts on my cnc plasma table and then machine them to final specs. It's much faster than machining the part from raw stock. The problem is that the plasma cut edges, holes, slots, etc. are very hard and will ruin hss drills or endmills. Does anyone know a way around this problem? Can plasma cut steel be annealed to help this? Can carbide drills, endmills, etc. be used? Suggestions please.

 

[Rustrp]

Sometimes I like to rough cut parts on my cnc plasma table and then machine them to final specs. It's much faster than machining the part from raw stock. The problem is that the plasma cut edges, holes, slots, etc. are very hard and will ruin hss drills or endmills. Does anyone know a way around this problem? Can plasma cut steel be annealed to help this? Can carbide drills, endmills, etc. be used? Suggestions please.

I think you know you have a lot of ugly material that doesn't fall into a category of something that we would choose to anneal. Even if you could anneal it would it be worth the time. Light sanding or grinding is the fastest, easiest and surefire approach. Your question is ages old, and not just from a machining standpoint. Flame, plasma, laser cut edges on material are really tough on any piece of equipment that's next in line in the production process.

 

[RJSakowski]

I have not dealt with plasma cut steel but with flame cut steel. The steel hardens due to the rapid cooling. Mild steel shouldn't be a problem for plasma cutting as there isn't enough carbon in the steel to create a hardened zone. With flame cutting, carbon can be introduced from the cutting process which can cause localized hardening. Cutting a hardenable steel with either plasma or flame is likely to create localized hardening.

I have usually found it sufficient to grind the immediate area prior to machining. If there is still a problem, I will heat the area hot enough to destroy the hardening and allow it to cool slowly. It is usually soft enough to machine then. In a worst case scenario, I would do a proper anneal.

 

[john_c_kennedy]

For the current project I don't know what kind of steel it is, just a 4ft sq. piece of 1/4 in. hot rolled plate I picked up at the recycler. So I think it's just mild steel. I cut a 4in sq part with several slots and a couple of holes and all the cuts are very hard. The last part I made from 3/8 in. plate (a wrench) had a 1/2 in. hole that I tried to drill out and ruined my drill bit. I finally used an old carbide tipped masonry bit that I sharpened and it drilled it just fine. That's why I asked if a carbide endmill would work? Thanks for the replies so far everyone.

 

[cjtoombs]

I suspect that carbide would probably do the trick, you would just need to start out machining at cutting speeds commensurate with hard steel. It will likely still be hard on the carbide tools, so may be costly from that standpoint.

 

[Rustrp]

So I think it's just mild steel. I cut a 4in sq part with several slots and a couple of holes and all the cuts are very hard.

There's a good possibility the plate is mild steel but since you sourced it at a recycler then you have no way of knowing for sure. The chances are high that it's mild steel. If any of the parts are critical then it would be best to know what you are working with. With that aside, any cutting process that heats the steel to a molten state will affect the physical properties of the piece being cut. If we add into the drilling or machining process the dross that's left clinging to the edges, it's hard on tools, even high quality tools. Prepping the edges prior to machining is much easier than it is for holes so my suggestion would be to cut the hole large enough with the plasma or drill them.

 

[Glenn Brooks]

I had a similar problem flame cutting round 6" wheel blanks out of 1 1-2" steel plate. The edges became really hardened. Even carbide bits wore out and broke apart in my 12" big iron lathe. A big problem with carbide is that it is brittle. So doesn't take to interrupted cuts very well - which often occurs with flame cut roughed out shapes.

I finally resorted to grinding off the hard edges as best I could, then applied the brute force/multiple tool grinding method for cutting through the remaining hard shell. I Don't know if I would even attempt something like this with a mill. Sure fire way to go through a lot of expensive cutters...

Glenn

 

[coherent]

Just another possible option... I use a water table and the metal doesn't get hot at all. Mine is on a CNC plasma machine, but a small metal tub etc would suffice for hand cutting. Just the air blowing through the cut splashes enough water up on the bottom the piece to keep it cool. I have the water level within a 1/2" or so of the metal bottom. You can also have the metal on/in the water. I can cut a piece out of 1/4" plate and pick it up by hand immediately after the cut finishes. You just need to dry it off to ensure it doesn't rust. I haven't done any extensive testing to see how much it helps since the plasma cut itself has to be melting the metal so there is obviously heat generated at the cut itself, but I pierce the metal using the plasma to mark/initiate where I need to drill holes/mounts and in my experience drilling or milling with HSS hasn't been an issue.

 

[epanzella]

I torch cut some circles in 1/4" plate and turning the edge with HSS ruined the cutter in 10 seconds. My last resort (because of the interrupted cut) was indexable carbide and it cut thru it just fine without chipping.

 

[4GSR]

Some of the earlier plasma cutting machines I've dealt with several years ago, we ran them on nitrogen for shielding gas instead of air as used today. That nitrogen would caused the surfaces in the heat affected zone to become nitrided. Luckily, we didn't have to do anymore machining to the surfaces for finish.
I have encountered hard surfaces from torch cut and plasma cut surfaces being hard, even on A-36 plate. If we couldn't bury the cutting tool to cut under and peel off the outer surface, we would take a side grinder and attack the edge and grid off as much as needed to get a good cut.