On 12/5/22 06:59, andy pugh wrote:
On Mon, 5 Dec 2022 at 02:20, Chris Albertson <albertson.ch...@gmail.com>
wrote:
and we don't even know the exact kind of steel in the rod. Without
knowing more it is a toss-up.
You don't need to know, all iron alloys have the same stiffness (to within
a couple of percent).
The shear modulus (G) for steel is 80 and for carbon fibre is around 5.
Steel is a lot stiffer than CF.
Anyway, with a 2Nm motor you might see 1.7 degrees of twist in an 8mm steel
shaft which is 500mm long.
Playing with numbers it would be 3 x less with a steel tube 12mm dia and
1mm wall thickness.
To match the stiffness of the steel bar with a _round_ carbon tube it would
need to be 20mm dia and 1.5mm wall thickness.
https://amesweb.info/Torsion/torsion-of-shaft-calculator.aspx
Note that there is only any torsional load on the shaft when accelerating,
so the extruder will generally not deviate from the path very much, but
will be lagging along the correct path.
(This is true of milling machines too, the f-error is typically
approximately along the path, so has less effect on part geometry than the
raw numbers would indicate)
Thank you Andy. Bearing the difference in torsional rigidity of sq vs
round in mind, I just ordered, be here the 7nth, a 4 pack of 23x25x500mm
round cf tubes, one of which will replace the looooong coupling rod with
the puny motor in the middle of it, across the rear of an Ender5+, some
pulleys I can machine away half the weight of, some belt clamps, and a
10M roll of gt2 belt 6mm wide. I'll put one of the 1NM 3 phase
stepper/servo motors in the inside corner of the frame with the
controller close by, driving one end of this tube. Not the middle.
FWIW, I'm fond of the gyroid pattern infill but its jiggle will be
distorted by the torsion give, so while I've got room for 2" tube, the
biggest I can buy seems to be 1", or 23x25. 2" would probably begin to
affect inertia with its weight, so 1mm wall, 25mm OD may be the best
balance point.
All I need to do is get the step & dir out of an BTT Octopus-pro, which
I'm sure I can do. The belting will replace the x drive belt after I
figure out how to route one run of it thru the square cf tube. Different
piece of the project IOW.
Take care & stay well.
In the realm of motor theory, a chance reading of one of stepperonline's
.pdf's, explains better how they work, and its the same for 2 phase as 3
phase, they cannot lose home unless they can't move the load, AND will
issue an alm contact if they lose a step/home. I've got that rigged in
.hal to stop LCNC in about a millisecond everyplace I'm using them. Sweet.
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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