On 4/10/24 13:30, Chris Albertson wrote:
Do you even need a rotory table to cut a spiral?
Now that you ask, I think the answer is probably no. Take a look at the
nurbs command G5.2 where a group of points describes the curve. Then
rotate the points about one end in polar/rectangular space. I haven't
played with that myself, perhaps Andy has a better way?
On Apr 10, 2024, at 12:09 AM, gene heskett <ghesk...@shentel.net> wrote:
On 4/10/24 01:57, John Dammeyer wrote:
A friend and I have been discussing exactly how to write the G-Code to
create a spiral scroll.
His rotary table 90:1 reduction with a 1600 micro-step motor could be set up
to move N steps for each step of the X axis to create the spiral. But that
approach seems clumsy.
Say I wanted to cut a scroll with a 6mm pitch using a 3mm cutter.
Without using G2 or G3 it's really just a triangle isn't it? Move rotary
table distance A and move X axis distance A'. Do it in small enough
increments and you get a spiral. But I feel like I'm missing something
really simple.
Do you need a rotary table to cut a spiral? It is just a series of locations
in (x,y). OK, if you wanted to use only (say) the X and A axis then you should
use polar coordinates, not cartesian. The equation of a spiral on polar
coordinates is very simple. Then you evaluate itat many thousands of points
and at each point write gcode to “cut to” that point. You would not need the
rotary table.
Also why think in micro-steps and worm gear rates, you are using LCNC to do the
kinematics, Use millimeters.
I think this problem shows that in some cases you really can not write the
gcode by hand. FOr continous curves in (x,y) there might be 100,000 or more
lines of code in the file, especially if you don’t do the cut in one pass. You
would nee towrite software to generate the g-code. Or use existing software,
a lot of CAD systems will do this for you
First, a 90/1 is quite high. I have two rotary's, both consisting of a 3NM
3phase stepper/servo I made by combining the 3NM motor with a 5/1 worm. Using a
screw in the worms output hub as a single prox sensor index pulse generator. To
calibrate a complete rev, I measure the steps by starting the count on the 3rd
turn ans stopping the count on the 103rd turn, which gives me a scale*100.
Shift the decimal point 2 places left this becomes the scale for the axis in
the .ini file. All this math in linuxcnc is floating point so I can ask it for
33.333 degrees and it will run to what it thinks is 33.333 degrees. This
stepscale:
STEPSCALE = 22.22222222222 = 1 degree
So one count is about 1/22.22222222222 degrees, probably less than the backlash
in the rvs39 worm, a pretty cheap worm.
Currently to make one of my maple vise screws, starting at 0 degrees its around
60,000 degrees it turns for around 400 mm of screw that y travels. Then I lift
the tool, turn it another 180 degrees, re lower the tool and bring y back to
zero and b=180. Makes a perfect two start buttress thread. The B is turning, in
perfect sync with the Y motion, at something in the 300 to 400 rpm range. That
3NM motor is heating but not dangerously so.
There is no reason you couldn't lay it down to make a C drive, and simultaneously
drive X Z & C to carve an impeller in a quite serviceable scroll.
The versatility of the closed loop stepper/servo, which does EXACTLY what the
TP tells it to do, without a PID in the path, is amazing. I have them rigged to
e-stop linuxcnc in about a millisecond if they make an error, like losing a
step. Tested till the cows come home, has yet to happen working a job. I
haven't hobbed any gears, but it certainly seems accurate enough to do it.
Suggestions?
Thanks
John
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Cheers, Gene Heskett, CET.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
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- Louis D. Brandeis
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Emc-users mailing list
Emc-users@lists.sourceforge.net
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Cheers, Gene Heskett, CET.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
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