How critical is the exact placement of the tonearm

It's hard for me to believe anyone could drill a hole that large in an armboard and get it within .01969 of an inch.... Then get the armboard mounted that precisely... I'm not saying it's not critical, just the nature of the drill bit that size.

I don't believe anyone said the hole had to have that tolerance. There is play within the hole to adjust the arm to that tolerance which can be achieved with micrometers and other measuring tools. The actual drilled hole would have a much greater tolerance. Then again you would be surprised how accurate a drill press can be - let alone plowing it out with a Bridgeport if you wanted to get ridiculous.

Zen,
I design circuit boards. I deal with .001 inch every minute of every day. I have 3 points. 1) one wouldn't want that much "slop" in the hole size. Just tightening the arm would cause it to move more than that. 2) I'd bet you be surprised at how much runout the typical homeowners drill press has 3) at that size, the typical drill bit has a pretty large tolerance.

IMHO, to get the hole that precisely located and sized, one would have to use a CNC controlled drill and a very expensive, high tolerance bit.

Not to mention the fact that you are, usually, dealing with a fiberous material such as wood, or worse yet, mdf.

Just my 2 cents.

A drill bit isn't used to control center to center positioning of a pair of holes (or an array) when close tolerances are required. It is simply used to clear material out of the way in quick fashion for a more accurate method of machining. I wouldn't trust using a drill bit to maintain center to center positioning for tolerances tighter than 0.010" (.25mm) I'm not saying another machinist wouldn't TRY to drill 2 holes within that tolerance but I wouldn't. Sometimes you get lucky but it isn't worth the risk. Do it right the first time.

VERY!!