What is Milling with "Optical or Precision Orientation"

What is meant by "optical or precision orientation" for locational position of drilled holes at the following link:

Machinist Drilling Mechanical Tolerance Capabilites Chart - ANSI Size Drilled Hole Tolerance, ISO Metric Drill Sizes - Engineers Edge

Do I have the following right? Looking at the "normal" tolerance, a positional tolerance of .016" could be done on a manual mill if you invented a fixture? A normal tolerance of .005" would require your mill or cnc machine to then have "optical or precision orientation?"

I think they intend that phrase to mean "using a DRO" or, 50 years ago, "using a tubular micrometer+length standards".

I have to say that the "normal tolerances" on that chart look pretty crude to me. 2mm positional accuracy when drilling a hole with manual layout? That seems pretty Harry Homemaker. Same with "normal" fixturing. I'd be pretty disappointed if I didn't get 0.001 relative position control on all features machined on one setting in a fixture. 0.016" seems like hand-file tolerances, not precision mill or CNC tolerances. Jig boring with 0.002" "normal" tolerance? That's like sandlot horseshoes for a jig bore.

So "normal" on that chart must be aimed at slam-it-out high-volume production, not precision work as we know it. The "tight" on that chart is much more like what you'd expect from a skilled machinist or well-maintained CNC machine.

I don't know who came up with that document or how they did it, but it has really loose tolerances. The second table, where your phrase "optical or precision orientation" is used says that a hole located with a center punch is expected to be as much as 0.080" or 2mm off that location. Now, that may, and I say may, apply for a 1" diameter drill that is not very well sharpened, but if a 1/8" drill was that far off, it would not even touch the center of that punch mark. Or do they mean that you can not locate a punch mark with an accuracy of less than about 0.075". That is just plain ridiculous. Even without magnification, a punch mark can be VERY EASILY located under 0.010" of the intended distance and I dare say that most, if not ALL of the people here could have it within 0.005". Now, which of us can not hit that punch mark within another 0.003". That would give you 0.008" total error even without using magnification. That's a full TEN times better than their "spec."

Further down that table they say "Precision milling or CNC machine with optical or precision orientation" would have a normal positional tolerance of 0.005". Good grief! I would trash a Chinese import mill that would not give me 0.002" just using the feed screw and handwheels.

I have no idea what they mean by "optical or precision orientation". But whatever it is, it must really screw up the accuracy of an otherwise good machine.

Unless you are a really poor machinist, I would not pay any attention to that table or to the overall document. Or to anything else that comes from that source.

I have a cheap optical center punch that I pull out for pretty much any hole I have to layout and drill because I've never gotten the hang of getting a center punch dead center to my scribe lines by feel... Does that count?
(Damnit Jim! I'm a welder not a machinist! )

The drill size allowances are what you’d use with plane jane 2 flute twist or spade drills. Add bushings a little better, swap bushings & finish with multi-flute core drill is about the best you can get for just drilling.

The positional allowance from .080” (normal tolerance manual = ironworker + mag-drill on a bridge) to .0005” (tight tolerance jig bore = machine with feedback other than screw or screw with resolver). That isn’t far off in the real world & definitely true for the next bridge you drive over… It’s just reality.

I’ve worked with G&L’s (Fraser) that had inductosyn’s that roughly doubled the price of the machine. It would move each of (4) axis there (.0001”) but by the time you worked through 12’Lx8’Hx8’Zx4’WZ all the .0001” could add up to real problems if you didn’t allow for it in design. Depth is a real bummer for drilling holes & keeping location on the back side.

Good luck, Matt