Do you even need a rotory table to cut a spiral?
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.
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