Subject: Turning a parabola

From: [email-masked] (MORRISON ROBERT)
Date: Fri, 15 Oct 1993 20:20:45 GMT

Anyone have tricks/ideas on what it takes to turn a parabola
on a lathe?  Certainly if you have a CNC lathe, it would be
easy.  But otherwise?  One idea that comes to mind is to spin
a sheet over a parabolic master, but that just sort of inverts
the problem from turning an inside parabola into that of turning
an outside parabola.  Perhaps some adaptation of a ball-turning
attachment, with a kind of pantograph attachment, but I think
it would make an ellipse and not a parabola.

I don't currently have a specific project in mind.  But a while
back I walked in front of a steam-locomotive headlamp on display
and was impressed by the tight beam.  Probably the reflector had
been spun and polished by the factory, but it got me wondering
how a home shop machinist might be able to make parabolic reflectors,
or just parabolas in general.

Jim Kirkpatrick
[email-masked]

(posting from a co-worker's guest account on a neighboring machine;
I'm really not Robert Morrison, though I work for him!  Our news
system still won't post :-( ).

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From: [email-masked] (Ed Richley)
Date: 16 Oct 93 21:51:45 GMT

Although I don't know how to turn a paraboloid, it's really easy to turn 
a hyperboloid of revolution. If the tool bit is set below center, and fed
by the compound rest set at an angle, that's what you get. With some calculations
and patience, some amount of "precision" can be obtained.

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From: [email-masked] (gary preckshot)
Date: 19 Oct 1993 19:10:26 GMT

In article <[email-masked]>, [email-masked]
(MORRISON ROBERT) wrote:
> on a lathe?

[more stuff...]

If you have a taper turning attachment, you could modify it to follow a
parabolic track, which would work pretty well everywhere except around the
center region.  There, you'd need some sort of assist because taper
attachments convert bed movement to cross-slide movement by a ramp, and at
the center your mechanical advantage would be zero (i.e. it would be easier
to move the bed by moving the slide than the reverse).

Of course, this technique would work equally well with non-parabolic curves
too.

Gary

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From: [email-masked] ()
Date: 19 Oct 93 20:11:38 GMT

Use a radius tool that looks like a "C".  If you don't know what I'm talking
about I could go futher into that.  Replace the bearing section of the tool 
with a setup that will hold the C off center from the rotation axis of the tool.
The deapth of the parabola will be the distance of the offset and the width of 
the cut will be the radius described from the tools ratational axis to the 
cutting edge.

Dave Parsons

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From: [email-masked] (Rob Pauley)
Date: 19 Oct 1993 21:06:48 -0400

Jim, I haven't turned/spun a parabola specifically (though I've thought
of it, since I'm a microwave engineer) but I have turned similar shapes
and spun metal over them.  If some precision is necessary, I cut out a
template from some stiff material and turn the form by hand to match.
Then the metal is spun over it.  For the train lamp, I doubt that an
optical quality paraboloid is necessary, since it's not generating an
image and the light source at the focus is probably not a point source.

-- 
Rob Pauley
[email-masked]   * or *  [email-masked]

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From: [email-masked] (Gary Rosenkranz)
Date: Tue, 19 Oct 93 20:31:00 -0500

Jim,
I have a mfg business and if I were to make a reflector that you
described I would not turn it on a lathe I would have it stamped out on
a press and then polished and plated. This would be the least  expensive
and the way to turn out the most amount of parts. In my opinion.
      Gary

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From: [email-masked] (William F. Kaiser)
Date: 21 Oct 1993 17:30:26 GMT

MORRISON ROBERT ([email-masked]) wrote:
>  [...]

This doesn't contribute anything to this thread, but...

Steam locomotive headlight reflectors were made of glass, silvered on the
back side.

Bill Kaiser
[email-masked]

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From: [email-masked] (Dan Rubesh)
Date: 22 Oct 93 20:29:06 PDT

Jim Kirkpatrick ([email-masked]) inquires..

> it would make an ellipse and not a parabola.

(rest of article omitted)

If you have access to a CAM system or CAM software, you can construct the 
parabola (or whatever other shape you want to turn) as a contour, call out 
your tool diameter as twice the nose radius of your lathe tool, and post 
process for an older point-to-point control (like Acramatic 330) that was 
incapable of circular interpolation. What you'll get is a _bunch_ of xy 
data points (in the case of milling software, xz points if you're using a 
turning program), that you can use every co-ordinate pair, or skip 1-3 
point pairs if you don't need as much accuracy.. Keep in mind that the CAM 
program outputs points that are to the _center_ of the cutter, so you'll 
have to offset your lathe tool accordingly.. Use the x axis dimensions 
as the longitudinal direction, and the y axis dims as the cross slide 
direction.. If your lathe doesn't have DRO or at least a Trav-a-dial, you 
can rig long-travel indicators to keep track of 'slide' movements.. It's a 
pain in the butt, and it's slow, but you can do some pretty intricate 
contours this way.. I've done large radii, parabola, and turbine inlet 
contours with this method.. Use absolute programming, as incremental will 
make you crazy trying to keep track of where you're at in the program. I 
realize that I've just hit the high points in this description, and if you 
want more detail, drop me a line..                                 

All of my tools _subract_ metal.. None of them add..

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