Last weekend I made an eccentric strap, similar to a connecting rod, for a
small steam engine. The eccentric strap (follower) needed to fit the
eccentric loosely enough to turn with usu. oil film. First, the strap was
made to outside dimensions with the bolt holes drilled and tapped - flats
were left on 2 sides perpendicular to the split lines, which is the normal
profile for an eccentric strap. Then the rough opening was bored to within
a few thousandths of finished size which was to be 0.806". Next, I made a
pair of go-no-go gauges on the lathe, of 0.806" and 0.808". (Wish I had a
set of gauge rods or balls) The strap was repositioned in the mill with the
flats oriented for splitting. It was split with a 0.020" slitting saw
through each side and the sides tightly bolted together. Next, it was
recentered in the mill and bored to near 0.806". Several boring passes were
made until the 0.806" gauge would just barely "go". The 0.808" gauge would
not "go". That yielded a very round hole (after splitting) of very nearly
0.806", maybe 1/2 to 1 thousandth over, but certainly between 0.806" and
0.808". Measuring with test gauges is a lot less "nervous" than the usu.
small hole gauges, at least for me. One thing to note: When very nearly to
finished size, 1 or 2 tenths can be removed by running the boring tool down
into the hole again, maybe more than once.

Bob Swinney
"CAMCOMPCO" wrote in message
oups.com...
Hi all,

Is there a way I can get a mirror like finish on a turned piece of
metal? I know I can use 1000 grit and finer on a piece as it is
chucked and turning in a lather, but how about the bored hole? How
would I get a, say, 1/4" or 3/8" internal surface glass smooth? I am
thinking of some sort of expanding reamer that I could put a 1000 grit
wet/dry sheet around and insert it in the hole as it is turning...seems
like I would just bunch up the paper though ;-)

Anyone ever have to get something "barring" smooth internally....what
did ya use?

Thanks again gang....as always, any help is greatly appreciated

John