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.
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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