Re: [CAD_CAM_EDM_DRO] CNC on a lathe

Rotating tool bits? That's a new one on me at the moment. I can see the
sense behind it but I've not seen one that can actually do that available to
the hobbyist. My CNC lathe cuts spheres and tapers with the tool constantly
in the same position.

If you image the stock spinning around, you start the program so the tool
starts the same radius size away from the work as you want to create. It
runs through the program jogging the tool inwards bit by bit until
eventually it just starts to touch the end of radius (The furtherest part
in). From there it carries on jogging in fraction by fraction expanding the
cut it's taking until eventually it's making cuts right across the full
radius you want to turn. The cutting action all takes place on the point of
the tool, maybe a tiny bit to the side of the point sometimes but never
enough to cause a problem.

I don't know how well I've explained that but what I'm trying to say is that
the machine starts miles off from the work and moves in _slowly_ forming the
sphere from the most _extreme_ points first. In the end the tool can pass
the full length of it taking a normal depth of cut. So at the beginning you
have something that looks nothing like what you want. It'll look like a
piece of stock with a bump on the end. After a little while longer the bump
will have a noticable curve. That'll end up as a nice smooth radius. I have
to say here that the steppers on my lathe make a noise like Terminator
himself when they cut a radius, it sounds soooo funky as they cross over!

Tapers are easy and just done in a similar way. You only have a problem when
an angle is created that's steeper than the angle on the point of the tool,
but that's quite unlikely to happen. And even then you can always turn the
toolpost and run it as a seperate program away from the rest at the end.

Regards,
John H.

> Another question for everybody. And for this question I need to
> clarify that I'm NOT a machining wiz - in fact I don't even have my
> own lathe yet.
>
> If a lathe is converted to CNC, I thought that it would make
> machining complex tapers and rounds or spheres much easier. But as I
> looked at the current manual devices that cut these shapes - say the
> spherical ends - the cutter rotates with the device. Which seems to
> make sense because then your cutters' angles are always 'correct' in
> respect to the tangency of your cut surface.
>
> For a CNC lathe, how do you go about cutting a spherical end without
> changing tool bits?
>
> If this is a stupid question, I do apologize.
>
> -Chris
>
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