some newbie thoughts

I've been doing model engineering in metal for a few weeks now, and want to
tell you all about it. You'll all probably all know all about all this, but
anyway.
F'r instance, I was just taught how to grind a tool to cut a thread on a
lathe with (not to plunge cut, but to cut on one side). Left side done on
the outside of the wheel, and so hollow, with a very large (apparent) rake
angle. Right side done on the side of the wheel - wow, it can even be with
an apparent negative rake angle.
The left side angle is the thread angle plus the cutting rake suitable to
the material to be cut, and the right side is the thread angle minus the
suitable rake. But don't forget to leave space there so it doesn't contact
the thread side, and screech when you use it.
Then get the angle between those two surfaces to 60 or 56 degrees at the top
surface (never mind the number of other threadssssss that are possible). And
the vertical-ish line where those surfaces meet, if looking at the tool
head-on, should be not-vertical to the top of the tool, but at the cutting
angle.
Then guess a bit at the roundness of the thread tips and bottoms, because
you will never get it right without a chaser, and very lighly round the
vertical line to match. And then grind the top surface to the cutting rake.
wow. Apart from the complex shape, which I am sure both can and has been
done in CADCAM, but wow anyway, and the wholly impressive skill involved in
actually grinding it,
-
there is no accuracy.
It is all only-so-accurate. This much accurate. I'm used to that no in
chemistry, but not in mechanics.
I misse the "w" in now - was that a freudian slip? I miss "p"'s sometimes,
but that's because the "p" key on my keyboard is scummy.*
Now I could do flat, over say 6x10 inches - and I mean atomically flat, by
cleaving a suitable crystal on one plane, and in about ten or twenty years I
think rich people and good shops will be able to afford _flat_ diamond
surface plates - but apart from that, it isn't exact. Ever. Yet.
Sigh, I can't get seem to get used to this stuff, not that easily. The
problems I had with uncertainty theory are almost legendary (if a few people
remembering them 30 years after counts).
Another thing I thought about which might interest you, it might be soon be
possible to make say a 5 inch threadcutting lathe without gears. Any gears.
The head would have a direct drive motor, and so would the a carriage. Power
drive electronics and motor technology is getting quite near that point now.
Maybe some more later, hope I didn't bore anyone.
*
Apple do some good stuff, but their keyboards are shit. The membranes
aren't sealed, and they should be.
That means if you spill water on them - just water, not orange juice, or
coffee, or coke, or beer - they will stop working. I mentioned this to the
seniorest person at Apple I could find - very - and he said "you shouldn't
let drinks get near your keyboard".
That asshole just asks for a new one (keyboard, that is). His 4 million
customers have a more serious problem ...
But you can wash most sticky/splashed/spilled-upon PC keybards in the sink
or bath, leave them to dry, and plug them in (and they'll work). try that
with an Apple keyboard and you end up with.. junk.
Reply to
Peter Fairbrother
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This has been possible for some while now. Even the hobby-level CNC packages (for example, Turbo CNC) make this possible - no fancy drive motor required, you just stick a simple one pulse per rev sensor on the spindle and drive the leadscrew via a suitable stepper motor. The software measures the spindle RPM & generates a stepper drive pulse train to suit.
Regards, Tony
Reply to
Tony Jeffree
But the steppers are geared to the shafts, and the lathe head is geared to its motor ? AIUI the OP was referring to linear drives on the beds and direct drive motors :-)
Reply to
Boo
"Peter Fairbrother" wrote
Welcome to the world of tolerances, one of the biggest steps forward I found was when I started to think about what tolerances I really needed for a part. Up till then I made everything as accurate as possible (Physics/Science background it's ingrained, couldn't help myself) then you realise that in some places is just doesn't matter. I became far more productive and the cost of my out sourced stuffed dropped as well ;-)
Stephen.
Reply to
Stephen Woolhead
The stepper motor (singular) would generally be directly coupled to the leadscrew, but as the sensor reads the lathe spindle speed directly, the spindle motor can be belt driven, direct drive,...whatever.
Reply to
Tony Jeffree

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