In article ,
Ken Sterling (Ken Sterling) wrote:

Gang,
Quick question.... possible situation ... need to tap a hole in, say
1/2" plate, through to the other side, but needs to be "perfectly
aligned" and say the plate is mounted on a piece of machinery that
can't be moved to another location and the tapping has to be done by
hand. Possible solution - is it possible, (or has anyone ever done
this), by grinding a "pilot" on the front end of that tap and then
relieving the cutting edges again to look almost like factory just
behind the end of the pilot? Is this a viable concept?
Ken.

Don't know about that, sounds complicated. Might or might not work (only
1/2" for it to work on). Standard method in the labs I've worked in has
been to have a handy block (several inches thick) with various holes
drilled through (on a drill press) which could then be clamped to the
workpiece and used to guide the tap "pretty precisely". Generally
aluminum, so it will bugger itself up rather than the tools it's
guiding, and was cheap/easy to make another one of if needed. Guide the
drill, too, if needed (in a smaller hole, or use bushings). If you have
money to splash about or a deep tool crib, a mag-mount drill might
provide a handy tapping jig which you can take to the machine and drill
the hole with, too.

If you _really_ need "perfect" alignment, threads are probably not the
solution, so I expect that if threads will do, the tap-guiding-block
will get them started "perfectly" enough.

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