rleete,
That is a major problem nowadays, everyone thinks that hi tech machining will give perfect results, in a lot of cases yes, but under certain circumstances hand finishing is required, we would call it fitting. Now all these old codgers are gone, the art of fitting is getting very rare to find in industry any more.
I remember visiting Dowty Rotol in the late 60's, and they employed disabled ex military fitters to flatten platens that went into powered controls for aircraft. They used a combination of three things, a 2ft diameter DTI, a polished oblong of glass and the Radio Times newspaper. The glass was placed on the top of the platten and a light shone from one end, this generated internal rays of light, similar to Newton rings, only straight, that if it sat over a high spot would 'bend' the beams of light. The shown high spot on the platen was gently rubbed on the paper and the results were checked again under the glass block. The DTI was the rough check, it was the bending light beams that gave the super accuracy. They reckoned that the Radio Times had the perfect abrasive surface for what they were doing, all others were tried, but gave mediocre results. Those people were working, in my estimation, to millionths of an inch.
These plattens were made to have the hand fitted surfaces sliding against each other, and when up to 2,500psi of hydraulics were applied, like a slide valve in a steam engine, no sideways leakage occured, but still allowed the plattens to ride face to face.
Such skill is a dying art form.
John