Plasma cut "rough" hole size then drilling

[pdjcutandfab]

got a question, I need to cut a bunch of 3/16" thick plates for a local home builder, he wants various 9/16" holes in the plates (for mounting), my thought was to plasma cut the holes in (use about a .54" Diameter or so). my problem is that i ran a sample part last night and tryed to "drill out" the hole to the proper size (using a 9/16" Drill bit). I had a bitch of a time trying to "open" the hole up. I assume it was because the plasma cuttin process hardened the edge of the hole? does anyone have any suggestions as to how i solve this problem?

Thanks

Pat @ PDJ Cutting

[Scratch]

I used to think that it hardened the edge too. I make beadlocks for 4X4's and need (24) 3/8" holes drilled in them. I cut them smaller and drill them out. I sell them with me drilling them and with them drilling them. People always complain about how hard they are to drill out. I showed them on my 150 RPM drill press and with a cobalt bit that I can drill them out in seconds. So I don't think it's a hardening issue.... could be.... but it's pretty easy to drill out.

I also do alot of 1/2" drilling out of motor mount holes. Not a problem anymore since I started using cobalt bits and got a slow drill press.
Just use cobalt bits and a slow drill press. You should be fine.

[jeeplogic83]

I have to respectfully disagree with Scratch, the process does harden the edges and using a harder cobalt bit and slow speeds will get through the hardened edge with ease. Also if you cool the bit with a mister you will have an even easier time and longer bit life.

[Scratch]

By all means, don't listen to me. What do I know.... I'm still trying to figure how to keep my shoe laces tight.


... And they're Velcro!

[Scratch]

By the way.... does anyone else here have a hard time with cutting clean holes? I can't get a nice round 3/8" hole to save my life. I always have to cut em smaller and re-drill. How about anyone else....?

[Loyd]

When I first got my Pcam it would cut fairly round holes, now that the machine has "worn in" the holes are not round like they were. I use a step bit for the thinner materials that is titanium coated, works great and countersinks the holes at the same time!
Loyd

[jimcolt]

Air plasma hardens all edges on steel. The hardening is aheat related chemical hardening called "nitriding", and has to do with the molten metal edge and a nitrogen rich atmosphere. Air is 80% nitrogen. This hardening is usually only about .005" thick, and on outer edges can easilly be ground off, but on holes can cause a problem.

I always cut my holes undersize and use a high quality "cobalt" drill. Cheap cobalt drills work for a while, but if you spend a littl more for the higher percentage cobalt, they will last a long time and drill many holes. Titanium coated drills work as well, but if you resharpen them they are no longer titanium coated.

I just cut a plate with a bunch of holes that needed to be tapped 1/4-20. I programmed the part with all the holes to be cut to .188" (dialed in my height control and cut speed so the holes were perfectly round and only a bit of taper) then after cutting I drilled them all on my drill press using a #7 (.201") cobalt drill, this removes the taper, removes the hardening, and makes the hole the right size for 1/4"-20.

There is another whole world of "big boy" plasma systems.....High Definition plasma (Hypertherm HPR130xd, 260xd, 400xd and 800xd) are the big industrial plasmas that use oxygen plasma and oxygen shield with a process called True Hole technology.....they cut round, taper free holes with no lead in ding or divot, and since they are cut with oxygen (no nitrogen present on the cut edge) there is virtually no hardening....you can tap these holes as they come off the machine. Do not use pure oxygen on an air plasma system, it can be quit dangerous as the oxygen can support a fire at the torch.

Here is a picture of the previously mentioned part with small holes:

Jim Colt

IMG_2943.jpg

IMG_2945.jpg

[Loyd]

I sure wish I could get my setup to cut like that. I have a bearing or something going bad it "jumps" so that my edges sometimes looks like serrations! I need to take some time and find the issue....
Loyd

[plasmajoe]

Scratch, I had that problem also for years then I slowed down my speed when I cut holes and had the lead in from center and lead out back to center of the holes. That solved my problems of having "slotted round holes". Yes there is a large amount of dross but it chips off very easily. In 3/8" plate for example I cut it normaly at 165 IPM, but slowing down my speed to 110 IPM the holes come out great. Hope this helps you out. Oh yes also turn off your auto hieght if you have it.

Plasma Joe

[pdjcutandfab]

Then I think the other problem i have it that the drill press that I have is unable to go much slower then 400RPM, so this is the speed that I had it set at to "open up" these holes. My drill press has 4 different pully options and this is the slowest option that I have? i did purchase a brand new 9/16" drill bit and plan on trying it tommorow, hopefully this works or I may have to go to plan B (and I don't have a plan B yet).

Thanks

Pat @ PDJ

[1paakman]

These are all good suggestions... I have one point to add.
I seem to be able to drill much easier when flipping the part over.
It seems like it is less hardened... If that makes any sense.

[coyote577]

Reviving an old subject, by an old guy, I use 3 flute taper reamer, expensive 9/16 $45-50, but it will last for 200-1000 holes depending on metal thickness and grade, and I always use a mister ($.99 @ walmart in the travel size bins.) much faster than drilling in steps. I built a Grizzly for a sand mixer conveyor, 2048 holes 1/2" dia. in 1/2" a36 flat barstock, and too close to the edge to to punch on an iron worker, plasma cut and reamed it took just over 8 hours, including cutting the blank, plasma cut and ream. All together I used 2 sets of consumables (Hypertherm 1000 handheld torch) and 3 reamers. In all we built 12 32000# 75 ft. long diesel powered conveyors over a 3 year period.

[Gamelord]

Rapid Tap - cutting oil is a life saver!!! I slowed my press down and used the Rapid Tap, I can drill about 60-70 holes on one bit, more if I don't hit a chunk of dross or something that chunks out the bit. Then I just re-sharpen on the grinder and go back to drilling. I don't know what is in Rapid Tap...but man, that stuff is like liquid metal dissolver when it comes to drilling. Makes drilling out plasma cut 13mm holes in 5/16" plate like butter! Just take it sloooow and let the drill cut its stuff.

[beefy]

I use Sheetcam and it has a method of piercing hole centres, no actual cutting after that.

If there's not too many holes I replace the gcode at each pierce point so that the torch just gives a momentary blip. This gives a "centre punch" mark instead of a full through pierce, and I've had no problems drilling this with normal high speed drills, no coolant.

While talking about drilling, I very often use lower speeds and flood coolant for general drilling. I've still got a below average quality set of reduced shank Chinese drills, but using them like this they are still going good after a decade. Speed and heat seem to kill many drills, even quality ones. It's true about coolant / lubricant, it often makes the drill "sink" into the steel for some reason. A pedestal drill with a lower speed is definitely a drill bit preserver and a huge stress releiver. Torque, low speed, alignment, clamping and ideally a coolant system. Don't know how I survived without one before.

Another thing I've done is used masonary drills to drill through hardened steel, a tip I picked up off some forum. Some guys use low speed but I used the opposite method. First sharpen the carbide bit with a green stone, then use high speed and flood coolant (not mist). The flood coolant stops the brazing from melting and the carbide tip dropping out, but the high speed and pressure (you really need a pedestal drill) build up heat at the point of contact and anneal the steel. Takes a few seconds of nothing happening then all of a sudden it starts going through.

There's actually dual purpose masonary drills out now. Here in Australia Bosch have some in blue packets, but there are other brands. I keep these ones purely for hardened steel drilling, no masonary work.

Keith.