You sneak up on it.
Use the high speed grinder with cutoff, where you can. You may have to reverse the drill to oppose the grinder depending on what angles you have available. To make inside cavities, you can use burrs or small stones. You obviously can't do all the same things you could do with a boring bar, of course.
You don't have to get a perfect surface. You'll leave ridges. That doesn't matter at this stage; you're just roughing it out. You just want to get your low spots to the right depth by sneaking up on it, and checking with calipers.
Then switch to something like sanding block or file to take down the high spots and get relatively flat/round surfaces and to finish inside corners.
It's more a matter of patience. It's like working with hand tools. You use your eye and you make things fit. You just have to get it right, once, near the end. You need a good drill with very low runout and high speed grinder so that the grinding remains concentric rather than being influenced by imperfections in the surface of the workpiece. (I have a very concentric and heavy and smooth drill that I use for 2 things... drilling holes in concrete and turning small parts. IME, cordless drills are all bad with runout, no matter how many hundreds $ you spent on the name.)
A lot of the times you need to mate a steel surface, it doesn't have to be 100%, either. It may need just 2 or 3 rings or areas of contact, and low spots here and there may not even matter.
Here's one tiny example. The original pin had enough slop to theoretically make the disconnector reset the trigger earlier than usual, not that I ever experienced a problem. It was visibly worn and loose. This hardened tool steel replacement pin has a smooth surface and a pretty crisp inside corner under the head. With calipers, I measure 158 thousandths OD of the shaft everywhere except the last 1/16" inch that measure 157 1/2. As far as I would guess, that bit of taper was intentional. But it was so long ago I made this that I can't say for sure. Notice that in person, the taper is very obvious to the eye, and it's less than a thousandth. I've done much larger and more complex parts, close to half inch stock, but they would take more work to disassemble and/or need to be recalibrated after reassembly.
The other side gives a hint of what I turned it from. You can just make out a hexagonal shape that would be the right size to fit in a hex bit driver. Maybe not so good in the pic, though.
It fits so tight, I had to work the trigger while removing it, and it was a bugger to get back in. It almost completely removed the problem, but there's still too much slop/wear somewhere else.
Another thing I've done a few times is to grind down the shank of a steel bit from 1/4" to 6mm. Either because I could not obtain the bit in 6mm, which is relatively easy to change. Or because I had a 1/4" bit that wouldn't seat straight. That takes more work to get right.
Grinding a morse taper with an angle grinder?
Here, he uses a stone and a jig to true a table saw arbor. I bet it is easier to use a grinder and a cutoff disc to graze off the high spots, first. Then finish with a sanding block. You don't need a rigid setup when using a high speed grinder, and once it's grossly even (but rough and with some minor local unevenness), it will finish straight from there with a file or rigid backed abrasive. If there are low spots that are in a spot where it matters, just touch it up with the grinder to the chase out those low spots and try again.
50 years ago, lots of precision stuff was hand ground with die grinders and eyeballs were much more important. This was a skilled job in demand at one time. And it's really not that hard. With a grinder and a drill/lathe, you can make pins/axles, bushings, punches/dies. You can cut O-ring or circlip grooves in a round. If you are really patient, you could cut gears and finish with a file, ala Clickspring. It might take a lot of time, but compare how many times a decade you might want to invent a reason to use your mill vs the time (and space and sweat equity) to move and setup heavy machining equipment. I'm sure everyone with a mill is just brimming with inventions they make on the spot, daily, rather than using a mill to churn out hundreds/thousands of the same thing day in/out for business and/or to maintain/service some machines they use for business.
Anyone who has followed AvE's adventures with his $400,000 5 axis mill might be curious when he's going to actually use it to make something beyond a grill that doubles as a welding table or a fancy crowbar. I'm pretty sure the plate steel was flat enough to cook a hamburger the way it came from the supplier, and I'm certain you could have drilled and filed and ground a crowbar attachment by hand quicker than you can do it on a mill. Unless you think you're gonna make/sell "a lot" of 'em? $400,000 worth of your special widget.
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