On Wednesday 28 March 2007, Roland wrote:
>At 10:35 AM 28/03/2007 -0400, you wrote:
>>over half an hour per pass though. MCS is selling a little air grinder
>>rated at 70k rpms, I wonder how that might work mounted on the side of
>>the head on my micromill for something like that? Has anyone here
>>attempted something along those lines?
>
>When I was working on the Emco mill, 1mm carbide bits had a short,
> costly life, so we wanted to try using tungsten dental bits. This was
> for metal-work, doing fine work on brass patterns for spin casting.
>
>We ran some straight line test cuts in steel with a mock up, and it
> seemed ok, so we removed the milling head entirely and mounted a Pferd
> air-tool. Firstly, the air consumption had a fair size compressor
> running at 50%. Then we found that the spindle on the air-tool is not
> rigid enough. It's fine when you use it manually, since you 'press' as
> required, with visual and audible feedback, as well as being able to
> tilt the tool for a better scallop, but for automated running it was a
> disaster. Especially in cavities, and cutting 'downhill' it tended to
> bite and whip, making horrible squealing noises with a crappy finish.
> Steel or brass yielded similar results. We abandoned that idea, but i'd
> be interested to know if anyone had more success with a bigger tool.
>
>Regards
>Roland Jollivet
>
I wasn't aware the air bearings could be that sloppy. My only experience
with them was 20+ years ago, in quadruplex tape recorders, those ton or
more versions that recorded and played on 2" wide tape all those years
ago. The video head wheel had 4 tips arranged at 90 degree intervals
around the edge of a wheel that was a bit over 2" in diameter, and was
turned at 14,400 rpm by some strong servo amplifiers that could hold its
average position within about a microsecond, and in later machines could
bring that headwheel from stopped to speed and positionaly locked-stepped
to the video in 400 milliseconds. The tape was vacuum sucked into a shoe
whose curve matched that of the head path as it spun. All this was servo
controlled including the position of the vacuum shoe with the intention
of having the tip of the head as it crossed the tape, about .0005" into
the tape.
The air compressors were fairly small, quarter horse per machine, but
those headwheels would often be seen coasting slowly 10 minutes after the
stop button had been pressed. Setting up those servos for proper control
could be a chore, but I don't ever recall seeing the servo's attempting
to correct for the headwheels radial positioning error, which is what the
above description seems to fit best. Axial play could play tricks on the
capstan servo in earlier designs until both the axial play was pretty
well removed from the air bearings and the capstan motors went from old
style synchronous to a printed circuit rotor of a pancake design which
gave the servos the ability to position the tape dead in the middle of
that track across the tape even if the record head of a strange wheel had
one tip as much as 0.010" out of line with the rest.
I guess what I'm saying is that air bearings can be quite precise, but
then that whole assembly on one of those late 60 through early 80's
machines usually had a rebuild it price from 300 to 1800 depending on the
head tip material and length of warranty, but to buy a spare was more
like $3k or more. That, compared to the $100 tool I'm looking at in the
MSC catalog, says the MSC device is pretty sloppily made. Your comment
re the air compressor tends to back it up. The tape heads bearings
needed about 30 psi, and used about 2 cubic foot of air a minute, far
less than your comment would indicate for the device you tried. That
says volumes about the precision build I think. I would expect that the
tip should be able to stay within 0.0005" of where its supposed to be
under most any loading condition that didn't stall it.
But, I have NDI where one might be able to buy such a well built device
either, and I wouldn't expect that $100 one to achieve that level of
accuracy either.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The trouble with eating Italian food is that five or six days later
you're hungry again.
-- George Miller
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