Category: Machine Shop

More musings in response to questions about building a large-format 3D printer.

(Continued from yesterday)

make a direct clone of the M2. No thinking required.

The present-day M2 has survived four years of rather fierce Darwininan winnowing, so it’s a much better thought-out product than, ahem, you may think just by looking at it.

To build a one-off duplicate, you’ll spend as much money collecting the parts as you would to just buy another M2 and start printing.

Should you buy cheap parts to save money, without considering the requirements, you’ll get, say, the same Z-axis motor Makergear used on the original M2, the complete faceplant of Thing-O-Matic electronics, or crap from eBay described as being kinda-sorta what you want.

Sometimes crap from eBay can be educational, of course:

https://softsolder.com/2013/01/24/hall-effect-sensors-from-ebay-variations-on-a-specification/

I encourage thinking, particularly with numbers, because it leads to understanding, rather than being surprised by the results.

increase the rigidity of the X and Y axis

In round numbers, deflection varies as the fourth power of length: enlarge a frame member by 50% and it becomes five times bendier. If your design simply scales up the frame, it won’t hold the tolerances required to produce a good object.

https://en.wikipedia.org/wiki/Euler%E2%80%93Bernoulli_beam_theory

If you add more mass (“stiffening”) to the Y axis, then the Z axis motor (probably) can’t accelerate the new load upward with the original firmware settings and the Y axis motor may have trouble, too. Perhaps you should measure the as-built torque to support your design:

https://softsolder.com/2013/07/02/makergear-m2-better-z-axis-motor-calculations/

Reduce the acceleration and lower the print speed? Use bigger motors (if you can find a Z motor with the correct leadscrew) and lose vertical space? Make the frame taller and lose stiffness? Use two Z motors (like the RepRap Mendels) and get overconstrained vertical guides? Try building a kinematic slide and lose positioning accuracy? Your choice!

If your intent is to print more parts at once, buy more M2 printers, which will not only be cheaper, but also give you more throughput, lower the cost of inevitable failures, good redundancy, and generally produce better results. Some of the folks on the forum run a dozen M2s building production parts; they’re not looking for bigger print volumes to wreck more parts at once.

Conversely, if your intent is to learn how to build a printer, then, by all means, think about the design, run the numbers, collect the parts, then proceed. It sounds like a great project with plenty of opportunity for learning; don’t let me discourage you from proceeding!

However, I’ll be singularly unhelpful with specific advice, because I’m not the guy building the printer. You must think carefully about what you want to achieve, figure out how to get there, and make it happen.

To a large extent, searching my blog with appropriate keywords will tell you exactly what I think about 3D printing, generally with numbers to back up the conclusions. Get out your calculator, fire up your pencil, and get started!

(Continues tomorrow)

Wherein I continue dumping my responses to a large-format 3D printer project …

(Continued from yesterday)

What do you mean by 12 hour mean time to failure

In round numbers, the cries of anguish on the M2 forum seem to increase as parts require more than a dozen hours from start to finish; while you can print things that require 48 hours, that’s not the way to bet. There are more ways for things to go wrong than for them to go right, given the rather rickety collection of software & firmware making everything happen, plus the gummy nature of squeezing hot plastic into precise heaps.

Most of the time, it works fine.

much cheaper hardened polished rod system that the taz 6 uses?

Unless they’re doing something non-obvious to make a kinematic assembly, two rods on four hard mounts with four one-degree-of-freedom slides will be severely overconstrained and, I expect, a continuing hunk o’ trouble:

https://softsolder.com/2011/02/04/thing-o-matic-x-and-z-axis-rod-alignment/

FWIW, linear slides don’t eliminate the need for a rigid and well-aligned frame. Even the slab atop an M2 can deform by more than 0.1 mm under belt tension, which is enough to wreck the nozzle-to-platform alignment across the length of the X axis.

“Arduino-class firmware (Marlin, et. al.) is a dead end” Why is that?

Marlin is a dead end: they’re trying to jam hard real time motor control, soft real time command parsing, and non real time UI control into an 8 bit microcontroller teleported from the mid 90s. AVR microncontrollers worked really well up through the Cupcake and have held back printer design & performance ever since.

Which inexpensive all in one board would you go with

Machinekit on a Beaglebone seems to be the least awful of the current alternatives, but I haven’t examined the field recently enough to have a valid opinion. You’ll find plenty of proprietary “solutions” out there, none of which I’d be interested in.

Am I wrong?

I think so, but, then, I may be wrong, too. [grin]

It’s incredibly easy to slap together a bunch of parts that look like they should become a 3D printer. It’s remarkably difficult to engineer a reliable, stable, accurate device that actually produces dependable results.

Mooching design cues and parts from here & there doesn’t get you to the goal; if it did, Kickstarter wouldn’t be a graveyard of cheap 3D printer projects.

design a very rigid system for cheap

If it’s for your personal satisfaction, have at it, but a one-off large-format printer won’t be any cheaper than, say, a Taz 6. Some diligent searching will uncover any number of homebrew printer projects along the lines of what you’re considering; learning from their mistakes will certainly be edifying.

Anything is possible, but if you want to end up with a state of the art machine, you must begin with numbers showing how & why it actually meets the requirements. 3D printing now operates at accuracies, speeds, and controls comparable to CNC machines, with corresponding structural demands. There’s a reason high-end CNC machines aren’t made of sheet metal and don’t use 8 bit microcontrollers.

You might want to start at the beginning of my blog and read through my adventures with the Thing-O-Matic, which will explain why I’m such a curmudgeon …

(Continues tomorrow)

I recently engaged in a wide-ranging email exchange with a guy planning to scratch-build a large-format 3D printer. He figured it would be a straightforward exercise and asked for some advice; I may be more cynical that he expected.

Over the next few days, I’ll dump my side of the conversation so I can refer to it in other contexts. I’ve left his side of the conversation as the short quotes that prompted my replies, but you can probably infer what he was thinking.

He’s well-acquainted with CNC machining and recently added a Makergear M2 to his collection …

I’m hooked.

All of sudden, you realize what you’ve been missing!

In round numbers, I’ve been designing & printing one “thing” every week for the last five years. Granted, my “things” look a lot like brackets, because they go into other shop projects, but 3D printing is how I make nearly all the shapes I formerly bashed from metal.

I loves me my 3D printer!

an open source design with AFFORDABLE, EASILY ACCESSIBLE parts with a build platform of at least 150% X/Y volume of the MakerGear

Some years ago, I had the same general idea. Then I bought an M2 (replacing my Thing-O-Matic), considered LinuxCNC / Machinekit for motion control, and realized there wasn’t much point; I didn’t want to devote far too much time & effort to solving an already solved problem.

A larger build volume doesn’t buy you as much as you think, while imposing far too many hard constraints. Basically, good-resolution extruders run at 2 to 10 mm³/s, so large objects require print times beyond the 12-hour MTTF of the “printing system”: something will go wrong often enough to drive you mad.

Bonus: plastic’s thermal coefficient guarantees bed adhesion problems. Using high-traction materials (PEI / hairspray / whatever) introduces problems in the other direction. There’s a limit to how big you can make things before they either don’t stick or stick too hard.

Some the fundamental design problems that nobody recognizes until far too late in their design:

• nozzle-to-platform accuracy < ±0.05 mm
• XY axis speeds 30 mm/s to 500 mm/s
• Z axis stiction & backlash < 0.1 mm
• filament drive with excellent retraction control / speed
• bed adhesion vs. part removal vs. Z accuracy
• Arduino-class firmware (Marlin, et. al.) is a dead end
• Windows is crap in any part of a machine-control problem

Those are hard requirements. At a minimum, your design must satisfy all of them: miss any one and you’re not in the game. It’s easy to build a cheap and crappy fused-filament 3D printer (see Kickstarter), but exceedingly difficult to build one at the state of the art (see patent litigation).

The M2 descends from the original RepRap design, with the Y axis slinging far too much mass back & forth. That kills nozzle-to-platform accuracy, introduces temperature instability, and soaks up bench space. On the other paw, look at the problems Makerbot (not Makergear) had with their direct-drive extruder on an XY platform; getting that right requires nontrivial engineering

Bowden filament drives have improved, but really can’t provide enough retraction control / speed. Delta printers always use Bowden drives, because they can’t sling a direct-drive extruder with enough XYZ speed & accuracy. Bowden on an XY platform has the worst of both worlds: bad retraction and difficult mechanical design.

I think the M2 occupies a sweet spot in 3D printer design: excellent results without excessive complexity or expense. It’s not perfect, but good enough.

But, then, I’m a known curmudgeon …

(Continues tomorrow)