On 12/5/22 09:44, Thaddeus Waldner wrote:
I think the design has several other issues:
There appears to be a pretty large imbalance between the motor inertia and axis
inertia. You can fix this by using a reduction on the jackshaft. For reference, some
of the older Stratasys systems have a nearly identical arrangement. They use about
3/4" steel tube as a shaft and a 3:1 belt reduction from the motor to the
shaft. They also use the same reduction on the X axis.
My experiments do not always confirm that, particularly when the gear
ratio is very large, as in a harmonic drive with a 50/1 gear ratio.
There, the max accel is almost fully determined by the motors rotor
inertia. To make a usable B axis for my 6040 mill, I put a 3nm 3 phase
motor on a 5/1 rvs30 worm drive, works great, using a 1NM as the driver
for a 50/1 harmonic, I ran out of motor rpm long before I got a usable
speed out of it.
The bigger issue, in my mind, is the lightweight frame.
Frame "Y" distortion is now very heavily triangulated, X is much less of
a problem so far. And will lose half its weight by the time I'm done.
With double the voltage it should be fine.
Your motors might have around 2NM torque
The factory motors are < 1/3 of that 2nm. Rated .56mnm at 2 amps and 24
volts, but I've a 48V 650 watt supply for the rebuild, so the top speed
will at least double at that same 2 amps. And yes, the whole table
shakes. :) A Cheap used table I may replace with a lower one with a much
stiffer under frame.
which translates to about 45 pound-force on a 3/4” drive pulley. If
you apply 45lbs of force to the side of the printer, it will bend
sideways a lot. Add a 3:1 reduction to it and you get over 100lbf of
side load at the top of a lightweight frame—that is if you actually use
2nm torque during acceleration. For reference, the Stratasys printers I
refer to weigh around 150lbs and still the entire thing sways on its
rubber feet when the axis motion syncs up with the machine resonance.
And they do use stepper motors. This also illustrates why the humble
stepper is still a good fit for lightweight printer frames.
And demands the 2 phase w/o an encoder should not be used in new or
rebuilt designs, they can lose home and not know it, the stepper/servo's
will tell you in a millisecond when home is lost.
Take care & stay well, Thaddeus.
Cheers, Gene Heskett.
--
"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
Genes Web page <http://geneslinuxbox.net:6309/>
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