Sigh...another question from StudentLand, that strange area where,
after reading the textbook, being shown what to do by the teacher, the
results are STILL all over the place......

Was shown how to use a boring head at school - (always wondered what
they were for) nice big Bridgeport mill, good quality tooling, so
cannot blame anything in that department.

Trying to bore a hole to press fit a 15mm OD bearing for an optical
chopper project I am working on. Learnt how to use the thing on bits
of scrap of the same material as the project - aluminium, cut from the
same bar as the rest. By trial and error, managed to set up the boring
head to get a good result in a piece of the scrap - it is , for me
anyway, virtually impossible to actually measure the diameter of the
hole via the telescopic bore gauge/micrometer method, so crept up on
it till it was a nice fit with the arbor press - ie , went in easily
but strongly retained, not so tight as to wreck the bearings getting
them in or too sloppy so they fell out. (And yes, was taught how to
use the "limits of fit" tables to get a theoretical value, but as I
said it was impossible to get two consecutive measurements of the same
hole the same..AND yes, I did check the zeroing of the micrometer)

OK. Having got it right, did two more practice holes in the scrap, all
went well. Been fooled before by blind luck, so wanted to check it was
consistent - it had taken EONS to size and square the two end bearing
retainer blocks, so didnt want to stuff them up.

(BTW - NEVER buy powder coated aluminium from the scrapyard, the only
way to get it off is to face mill it, and then your left with marks
from the cutter, so HOURS of hand sanding with decreasing grades of
wet and dry paper to get a nice finish - it will be buried in a radio
project, no one will ever see it, but I will know it looks crap)

Put the job pieces in the mill vice, locked the tables, drilled a
14.5mm hole and then, without changing anything in the X-Y dimensions,
used the boring head, Result - holes were undersize...stuffed 2 more
bearings.....(thats ok, had heaps of them from discarded VCR's)

So, whats going on - same methodology used both on the practice pieces
and the job pieces - the only physical difference was the job pieces
were wider than the practice scrap pieces - the vise distorted it
somehow? - it is 15mm thick aluminium plate ...any clues....?

Basically, it took 12 hours to drill two holes. Not good. Unlikely to
get a job as a machinist. Finished my 8 hour day class, the teacher
kindly let me stay on for the 4 hour night class as the setup was a
real pig. (and it was getting a bit tense, as well...) The only good
thing was not having to drive home in 5pm peak traffic....

The regular day shift storeman had cracked it over something (who
knows what) so denied having an adjustable reamer to do it the easy
way. (the so called "correct " reamer was oversize...) The storeman on
night duty said "no worries" and got one out of the store for me,
showed me how to set it up, so I reamed them to the right size at
last.....

And, another query - somewhere, somehow, in internet land, I saw a
home made device to hold a piece of emery cloth with a "flapper" fold
in it for precisely this sort of final "****it this will get it to
size" application. Anyone can point me to the right place, stuffed if
I can find it again...

Grrrr - still finding machining absolutely fascinating, but, bloody
hell, sometimes the learning curve is just about vertical...

Regards,

Andrew VK3BFA.