I surprised myself. It won’t happen again.
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If you can get the drill to bite past the hard layer it works well. But if you’re just enlarging them a little carbide is the way to go. And if you want any hope at precision gotta go with a drill press. Even something like this $36 Drill press adapter will do the trick.
The ream will hold up fine on a few holes but you might find yourself buying more than one for too many holes (depends on the quality of the ream and how hard the nitriding is on the hole).
Carbide burs will open the hole with ease but you’ll have a harder time keeping the precision.
I have started just marking the centers of holes that I need to be dead on size. Then drilling them afterward. But I make mostly 1 off parts. If I was doing more runs of parts I might take the time to dial in my machines hole cutting more.
Whatever way you go Good Luck!
Bruce
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All this time spent and energy that’ll go into doing it, I’d just cut new parts at this point 
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Some of us have more time than $$.
When I was working, I spent lots of money to save me time. Priorities change, certainly.
The question I was asking is how to enlarge a hole, especially one made by a plasma cutter. Considerations about time, using a different process, re-cutting everything, are surely interesting but are a different topic.
As there is no magic procedure, I will try:
- using a die grinder with carbide burr to clear the hardened part of the hole
- using a 7/16 reamer like the one I linked
- using a step drill
- combination of the above
As I said, I cannot use a drill press at this point (because the brackets are already installed etc) so I’ll try all of the above freehand.
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True.
Still, whether one has more time (or less) shouldn’t determine the value of said time.
I would do this work on “paid” time specifically. If the job of enlarging 80 holes (or 75 remaining) took an hour to do, plus a tool purchase, I would cost it out to labor + parts.
Using myself as an example that’s $100/hour. I already have tools, reamers, etc but let’s just throw in a $25 tool, now we’re at $125 that we’re costing ourselves.
If it took $85 of material to cut out again, and maybe 5-10 minutes of cut time, it’s still cheaper having to cut all new brackets again without having to do any work to them.
But, as I type this, the author said they’re already installed so this suggestion is moot. Maybe something to consider in the future, learning experience if you will.
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Ah the “needlemaker 9000’s”. How I loathe thee.
Maybe hit the hole with a round file to scratch/break through the dross layer? Let a bit at least start to get under it?
I’d second the step bit option, TiN coated if you can. A GOOD name brand with the base material being HSS or CoHSS for hardness. Shank at least 3/8. Plenty of lube.
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Looks like our club just got a little more exclusive… I think I may be “the last of the holdouts”
Jim at out last club meeting
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I watch repair work on YouTube a lot. The other day I say a guy repair a shaft the weld was full of porosity and looked like the Tin man " not our tin man" dropped a load on it. I started once to submit a picture with my application.
I didn’t because I didn’t want to stir the pot to much. I thought joking to much you guys may think we are mean
When I need to re cut an error, I place a piece of thin metal over the existing hole, maybe 2"x2" and tape the corners so it doesn’t move. then reprogram the pattern with large holes and re-cut. The thin metal will allow the torch to stay connected. Use the correct amperage and speeds for the original metal. If you have ndexed the sheet to the table, it should work fine.
I do find that if I want exact holes, to just pierce the metal at the center and follow up with annular cutters for an exact size and shape.
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OK I must say, I was probably not able to describe my problem well enough when I asked, but most of the answers on this topic were not really that useful for my specific issue.
I worked on it last night, it was a surprisingly quick job that delivered good results:
- start with an air powered die grinder with a SB-3 Tungsten Carbide Burr ( Amazon.com ) (dry cut)
- this takes about 5-10 seconds, in the first couple of seconds you can really feel the thin harder layer being eroded away. Then it’s about enlarging the hole to the desired size
- finish with a tapered reamer with a cordless drill using first gear ( Amazon.com ). This guy LOVES cutting fluid
The result is that in less than 30 seconds a hole can be made of a surprisingly precision. I did all 80 holes in about one hour.
Final considerations:
- if one plans to have precision holes, maybe because they need to be threaded, he should use a post-processor (a couple of forum users have made one) that only marks the center and then a rigid setup to drill (and ream if necessary), this is very well known
- a good process (optimized CAM, tuned speed, amps and pressure) can help a lot if we want to avoid further machining operations, but that is also obvious
- if the part if still on the plasma table and we determine holes have been cut wrong, it’s very well possible to cut them again, a little knowledge of G code doesn’t hurt, I never had to use a strip of metal, I just touch off on the edge of the base metal.
- for all other cases where the parts have already been removed from the table, maybe half-installed, and only then you realize you need more clearance, I found the technique I described above surprisingly effective both in time efficiency and precision.
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Mano,
Glad it all worked out well for you. If not stated before it’s not stupid if it Works!!!
Once parts are installed the “Right” way to fix it is however you can. Thanks for sharing the outcome of your issue that way we can all add to out skill set for future problems.
Bruce
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Fun fact: I realized (one of) the holes were in fact not even in the right place 
So I pulled all the 20 brackets, TIG filled in the old holes (a relatively fast procedure), used a three-dee printed jig to drill pilot holes in the right spot, and then drilled/reamed the 7/16" holes with the drill press.
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I had an issue with sever tapper on some 3/4" dia. holes in 1/2" A36 plate a while back. I did not want to eat the money wasted as I would have had to buy another sheet of steel. I grabbed my oxy/accy torch and ran round the edges till they were orange hot and let them cool. HSS drill walked right through after that. It is not fast or cheap but if you end up there and need a solution other than “do better next time” this has worked well for me.
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