The final installment of musings about building a large-format 3D printer …
(Continued from yesterday)
Perhaps they saw your blog post?
The old-old (original) high-resistance Kysan motor costs something like $45 and, apart from minor cosmetic differences, looks /exactly/ the same as the old-new low-resistance motor. If you were picking motors and didn’t quite understand why you needed a low-resistance winding, which would you pick? Hence, my insistence on knowing the requirements before plunking down your money.
To be fair, I didn’t understand that problem until the Thing-O-Matic rubbed my nose in it. With all four motors. Vigorously.
So, yeah, I think I had a part in that.
comes back to the same numbers over and over
The new-new leadscrews have something like half the pitch of the old-new and old-old threads; I don’t recall the number offhand. In any event, that gives you twice the number of motor steps per millimeter of motion and roughly twice the lifting force. This is pretty much all good, even though it may reduce the maximum Z axis speed (depends on your settings & suchlike).
When it moves upward by, say, 5 mm and downward by 5 mm, you’re measuring position repeatability. That level of repeatability is pretty much a given (for the M2, anyhow), but it doesn’t involve stiction & suchlike.
Can you move the platform up by 0.01 mm, then down by 0.01 mm, and measure 0.01 mm change after each motion?
Do larger increments track equally well in both directions?
Move upward a few millimeters, then step downward by 0.01 mm per step. Does the measurement increase by 0.01 mm after each step?
Repeat that by moving downward, then upward in 0.01 mm increments.
If the platform moves without backlash & stiction in both directions with those increments, it’s a definite improvement.
I wish I knew more
everything you learned is burned into your head forever
The way to learn more is exactly what you’re doing.
Two things I learned a long time ago:
1. Whenever you have two numbers, divide them and ask whether the ratio makes sense.
2. Whenever you don’t understand a problem, do any part of it you do understand, then look at it again.
Also, write everything down. When you come back later, you won’t remember quite how you got those results.
Which is precisely why I have a blog. I search with Google (site:softsolder.com microstepping) and /wham/ I get a quick refresher on what I was thinking. That’s why I keep link-whoring URLs: that’s my memory out there!
You’ll sometimes find scans of my scrawled notes & doodles. They won’t mean anything to you, but they remind me what I do to get the answers in that blog post.
modern controllers utilize much higher voltage and current bursts
More or less. Microstepping drivers apply a relatively high voltage, far in excess of what the winding can tolerate as a DC voltage, then regulate the current to a value that produces the appropriate waveform.
This may be helpful:
The mass of the bed APPEARS to be cancelling out any magnetic or mechanical stiction.
That can’t be true in both directions: the gravity vector points downward and the results aren’t symmetric. I think you’re reading noise. If the sequences of motions I described don’t produce the results I described, then you’re /definitely/ measuring noise.
From back in the Thing-O-Matic days:
E3D hot end setups vs MakerGear’s?
I’d want that groovemount post in an all-metal socket, though, rather than the traditional plastic, to get solid positioning and tolerance control. Makergear has the right idea with the aluminum V4 heater block mount.
4 thoughts on “3D Printer Design Conversation: Part 5”
A thought on XY axes for a printer: Back in the ’70s, flat bed steppers were developed for wafer probers (and flat bed plotters). The technology is still around, though the prime example (Electroglas 1034X prober) sells for north of $2200 used. These probers are still around, as well as newer and considerably more expensive ones for larger wafers.
They were quite reliable in a clean (ish) room environment, but you need clean dry air for the air suspension for the platten, and particles on the bed would be a disaster. As I recall, the overall platform worked well for 3″ and 4″ wafers. Not sure how far you can push the envelope. Z axis control was never critical, however.
The flat bed stepper might solve some problems, though it would introduce a bunch of new ones. FWIW, I’ve seen flat bed strips for one-axis motion. Still air suspended.
A recent paper on satellite servicing (*) described a 2D air table simulating a zero-G 3D environment. the simulated “satellite” and “service” gadgets floated on air bearings atop an absurdly smooth granite plate, with clearances around 10 µm and gas jet propulsion. I immediately suffered from involuntary hip motions: that would be such fun to play with, I’d never do any actual work!
(*) Towards a standardized grasping and refuelling on-orbit servicing for geo spacecraf
Comments on old posts seem to be disabled – this actually concerns your motor analysis.
Marris Friemannis of Gecko drive fame quotes this rule of thumb (http://www.mechmate.com/forums/showthread.php?t=1618)
32 * √mH Inductance on the motor = Drive Supply Voltage
So in case of 44mH motor, correct voltage would be in excess of 200V, which I choose read as “the motor is junk” :)
In the stuff 3D printers use, single digit mH values at 24V seem to work fine.
Alas, I had to disable comments on older posts to reduce the spam attack surface: 3000+ posts provide far too much opportunity for mischief. I’ll glue a copy on the appropriate page.
Back in the day, L/R stepper drives used crazy-huge external power resistors to reduce the time constant (τ = L/R), with crazy-high voltages producing enough winding current to get the required torque. Microstepping current-chopper drives work much better!
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