Subject: Elliptical gear update

From: [email-masked] (darylb)
Date: 10 Aug 1995 20:52:35 GMT

I have studied the elliptical turning mechanism in Fine Woodworking's
Lathes and Lathe Techniques. It didn't make sense to me so I made it out of
wood to understand it. It was then that I realized part of the mechanism is
meant to be stationary (not explained in the book). Then it all made
sense.Without going into great detail there are two dovetail sliders at 90
degrees to each other (you mount your work on one). They are controlled by
an offset circular bearing. The offset bearing (stationary) imparts motion
to the sliders at the appropriate rotational angles. Visualize it as having
a chuck move toward you and away from you as the piece is rotating. The one
nice benefit of the attachment as shown is that when you set the offset you
are adjusting the difference between the semi major and semi minor axis of
the ellipse. This is EXACTLY what I need. The wierd thing about this is as
your ellipse size changes so does your eccentricity. For example if you set
the offset for 5" then if you if you turn an elipse that has a major
diameter of (to be ludicrous) 1 mile then the minor axis is 1mile less 5"
(just about perfectly circular. However as you reduce the size you could
end up with an ellipse of 5.5" x 0.5" (highly eccentric). I guess this
means I'll have to stop and measure alot.
I put it in my lathe and set it for max offset (0.750"). The end
(faceplate) was a blurr of spinning and oscillating wood. So I put a pencil
tip into the "faceplate" end then shut off the lathe. Sure enough I had
drawn an ellipse on the "faceplate". It really is a zowie mechanism to
watch and it does work. Now I'm thinking how I could optimize the design
for simplicity and ability to do metalwork. The other thing is how to
index. I tried indexing directly with the unit but the divisions are finer
at the ellipse "tips" and wider along the narrow sides. Also when I index
and draw a line it is not always perpendicular to the side but seems to
point off where a trammel center would be. Therefore I doubt direct
indexing will work. The only other thing I can think of is to calculate the
perimeter of the ellipse then divide that into equal parts. I just
wondering how I would measure those divisions out accurately.
Anyway thats where I'm at.

Cheers
Daryl

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From: [email-masked] (James Kirkpatrick)
Date: 11 Aug 95 08:45:54 MDT

[description of elliptical cutting device]

Now that you've described this, I recall such a mechanism described
in an old Projects in Metal magazine.  If you are interested, I can
try to dig this issue out and copy the article for you.

Jim

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From: [email-masked] (darylb)
Date: 11 Aug 1995 16:01:16 GMT

In article <[email-masked]>, [email-masked] (James

> try to dig this issue out and copy the article for you.

Sure, E-mail me ([email-masked]) and I will give you my address/fax/etc.

Big thanks
Daryl