I think I know what's happening, but I'd like people's take on this:
I was drilling six 5/16" holes (in a line, spaced 0.4037" apart),
carefully centering the Millrite spindle over the crossed layout lines,
and drilling using a two-flute 5/16" endmill held in a 3/8" R-8 collet,
with the workpiece (mild steel) held in the mill vice, and black sulfur
oil brushed on as the lubricant/coolant. The table X and Y ways are
clamped, and the quill is moved manually to drive the endmill into the
work. As drilling proceeded, I continually brushed the chips away and
replenished the oil.
For most of the holes, it was slow, but no drama and the holes ended up
where they belonged.
For two holes, the table jumped around visibly, and the holes ended up
The holes are all a tight fit on a 5/16" drill rod, but there are signs
of galling in the bores of a few. The endmill is undamaged, and still
My theory is that a chip managed to get in between the side of the mill
and the wall of the bore and welded to the wall, only to be torn loose a
short while later, and that this is what generated the large forces
needed to throw the machine table around. The hole sometimes ended up
displaced because the table ended up somewhere else from intended. This
would be more likely near the beginning of the drilling operation than
the end, and the more wall there is, the more constrained the table is
by the stubbly little endmill captured by the hole it just made.
I suspect that a 1/4" twist drill followed by boring to 5/16" may be
faster and more reliable, and less dramatic.
15 years ago