Re: high-speed spindles
Harbor Freight's 1/4 inch electric die grinder was tried, first for
drilling some small diameter holes in aluminum blocks and then milling solder
paste stencils in 3 & 5 mil brass sheet. In both cases the results showed
excessive runout, unpredicted by stationary measurements. It is a nice tool
for the intended purpose though. Convient to mount also.
"Grinders" have the advantage of self conforming grinding stones - if the
cutter shows runout, it won't do it for long. The Harbor Freight die grinder
showed little runout on a tool's shank close to the collect but, apparently,
the shank(s) was not pointed exactly down the axis and an inch or two down
that axis, ie at the tip of a drill, the runout was noticebly greater.
Milling the brass sheet brought the same conclusions. The diameter of the
endmill used is .015 inch but the width of a resulting slot was 10 or 11 mil
oversize - .026 inch. Much to big a minimum size for a pcboard stencil with
component pads at .025 between centers - and that's considered big; .020, half
a millimeter, between centers is a common now, and headed even smaller.
Porter Cable's laminate trimmer looks good as does (did) the 1/4 die
gringers, but the collet design shows the primary problem. The shank is held
just inside the screw retained collect cap. The collect itself contacts the
shank for a length of 3/8 in. The shank typically fits into the motor shaft
roughly another 1/2 in. with several thousandths clearance - suitable holding
for router work on wood or with grinding stones ala the 1/4 inch die grinders,
but with tiny tool work in metal it doesn't fit.
Vibration is a possible explanation for some of the lack of accuracy with
the die grinder - both within the grinder itself and the equipment it's
mounted on. I tried Harbor Freight's 1/8 in. air powered die grinder with the
thought of reducing vibration, but the runout/rattle was numerous thousanths -
an air bearing maybe. Unfortunantly, Harbor Freight's horse didn't even get
out of the gate - wouldn't rotate. Still, the idea of an air driven spindle is
Another difficulty with using 1/4 inch die grinders for cutting tools 1/8
in. & below is the need of a tool holder, typically a piece of 1/4 inch bar
stock with an 1/8 in hole for tools with 1/8 inch shanks. A little epoxy and
careful holding while curing gets within .001 thou pretty reliably. However,
on an .010 endmill, the size for the next effort, .001 thou is a giant. The
equivilent of .050 runout on a .500 in. endmill.
From here where? Go small. "Smaller tools, smaller runout" seems
reasonable. This leads to 1/8 in. spindles, preferably those packaged with a
driving motor, be it electric or air.
A Dremel grinder next will be the next try. That tool definitely leads to
less vibration and the 1/8 inch collet clears one previous problem. If threads
on the Dremel's foremost part are actually 20 mm/2.5mm as appears likely from
rough measurement, mounting should be simple.
However, there are likely better spindles than a consumer hand held grinder
for this application. If anyone knows of one that fit in along these lines, do
let me know.