A few thoughts...
Isn't the Z axis the easiest? Instead of the dual strut you could use a
pantograph, which would keep the Z motion linear instead of radial. It would
still be a sine conversion, but it simplifies the conversion significantly.
The conversion itself is different matter. Others can speak better than I as to
how easy it would be to add something to EMC to provide polar coordinate
outputs, but that is what you need for X and Y.
On the other hand, by the time you build it stiff enough to do anything more
than light duty work it might be cheaper to just use linear ways. For the light
duty work you are describing you can probably get by with steel shafting and
bronze bearings and have the same precision you would get with the pivot system.
Javid
----- Original Message -----
From: Roland Jollivet
To: Enhanced Machine Controller (EMC)
Sent: Tuesday, December 04, 2007 6:50 PM
Subject: [Emc-users] Pivot mill
Hi
I believe that the greatest hurdle in constructing a small CNC machine is the
cost of the linear slides, and I've been trying to think of a way to alleviate
this, and come up with a 'Pivot Mill'. While this is robotic in configuration,
it is for all intensive purposes intended to supplant a conventional 3 axis
machine. (X,Y,Z) Bearings are relatively cheap, and construction of the
mechanics fairly simple.
I have drawn and uploaded a concept picture here, of what I have in mind.
http://www.fotothing.com/CAONgallery/photo/9e62959f5230b6745b40b5285739b62c/
There are still three axis, but they are mathematical functions of each
other, essentially transparent to the user, and where Z is the most complex
interaction. Also notice that the tool will stay vertical during motion due to
the dual strut. The servo's are not shown, and would typically introduce a
further non-linear relationship since they would act indirectly, or at an angle
.
To control something like this, from what I've read here, sounds like a task
for EMC. So, how difficult would it be to implement this inter-relation between
the axis?
As far as the user is concerned, the machine must still execute linear moves
with normal G-code, as in G1 X200.
So the machine has ROTARY AXIS, but executes LINEAR MOTION.
A JOG in the Z axis will cause all three joints to move, but the tool will
only move vertically
Of course, there are always rigidity issues, but it's easy to beef up a pivot
arm, and this is only intended to be of the calibre of 'dremel' type, benchtop,
CNC machines.
There are other appealing factors like the inherent extended reach, for tool
changing, and if the machine were placed centrally on a table, it could serve
four work areas, as quadrants. (assuming here a 360deg. central column)
So, is this worth pursuing? Is the functionality required already a sub-set
of EMC, or is it a huge re-work?
Regards
Roland Jollivet
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