3D Printer Design Conversation: Part 2

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:


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)

15 thoughts on “3D Printer Design Conversation: Part 2

  1. I can only agree. There was a recent discussion over here on a forum where a guy wanted 0.1mm layer heights and a build volume in meters. There was this long discussion about carbon fiber build platforms etc etc. my immediate thought was, “how many years does he expect it to take to print”? 3D printing is fun, but I still don’t think it’s this paradigm shift that some fanboys get all excited about. I’m lucky enough to have an all metal printer with massive ballscrews for Z, big linear slides on X and Y, and a loadcell based build platform leveler. But that still only has a 200x200x200mm volume and anything over 8 hours I start looking at ways to break it up.

      1. The hotend is mounted on two 5kg load cell strain beam things and the firmware allows you to the have it ‘map’ the bed and create a topographical map. From there it will constantly correct for this topography. It can also be used to watch the force on the hot end from the extruder motor and based on this see if there is a blockage or if there is no more filament (to a degree). See rf1000 or rf2000 from Conrad.de and if your google translate is better than your German, read up on the experimental firmware at rf1000.de.

        1. So how much load do you put on the bed when mapping the topography? The printer I’m modifying at the moment has quite a beefy 10mm aluminum plate supported on 3 triangulated 1605 C7 ballscrews but I can still flex it noticeably by pushing on the corners. I guesstimate around 100g should be safe but that doesn’t leave much resolution on 10kg cell?

          1. I honestly don’t know, but from what I see, they still manage 1-2 gram resolution (it’s read out as digits) and since my bed is a ceramic tile, anything heavy would break it and screw up my hotend. I’d send a photo, but I’m sure ed doesn’t want us using his comments section as a 3D forum ;-). You can email me?

  2. “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:”

    It’s why I am a fan of Delta printers. YMMV, but I have yet to have a bad print off my Deltamaker. Very stiff frame and completely constrained motion. They somehow lucked into a perfect (meaning I’ve had no problems) bed adhesion solution that is simply matte plexiglass and printing at a much higher temp than I see recommended everywhere else. That said, I’ve heard very good things about the Taz.

    1. Deltas are fun to watch, but it’s only recently they’ve gotten Good Enough™ for state-of-the-DIY-art results. The forward-and-reverse kinematic equations are a solved problem, doing the math on an Arduino is possible, but IMO getting the mechanics right and maintaining positioning accuracy over time proved more difficult than anybody expected: none of the tolerances work in your favor.

      1. It is true that the printing artifacts are different that you will see on a cartesian printer, but they are so small as to be less significant than the layering. They produce a lovely moire-like pattern. Before they finished their kickstarter I spent quite a bit of time discussing with the creators how the positional accuracy at all times was a three dimensional problem in a delta and that it varied not just locally but also significantly over the full range of motion so you need quite a bit more positional accuracy in the axes than is required at the print head. That, and good calibration because the axes will never be perpendicular or parallel. And to ditch the printed plastic structure bits for extruded aluminum because plastic would never be stiff enough. They got Good Enough™ by the time they shipped (albeit a year late and without the promised heated build plate and with only PLA capability.) I think a 3-d printer, especially a delta, would be best calibrated by using a touch probe on a 123 block but am way too lazy to ever make that happen.

        1. you need quite a bit more positional accuracy in the axes than is required at the print head

          That’s the heart of the problem for deltas and the thing nobody realized at first: any slop in the motor-to-head linkages eats everything else. Good to hear it’s finally getting the attention it requires!

            1. That’s the first error analysis I’ve seen. I think the positioning errors stack up worse than he assumes: the carriages generally (?) move in opposite directions, so the backlash error signs will be different on all the columns, with none of them near zero error. The distribution definitely isn’t Gaussian and might be bimodal: maximally bad in either direction, all the time!

              If I’m reading the plots correctly, the error looks like a factor of 2 or 3 in multiple column mode (next-to-last plot), so the total of (carriage positioning error + linkage slop) must be accurate within ±0.03 mm to maintain ±0.1 mm across the platform. That seems aggressive, but I admit to not actually running the numbers.

      2. I ran into this today and couldn’t resist sharing it here. It’s obviously possible to do stupid things and still end up with “usable” printer :)

        1. Boggle

          Looks like it’s pooting out a thread the size of my little finger! Which would make sense, if you wanted to get a set of lawn chairs before the heat death of the universe …

          1. I think he said 4mm thread width :)
            Fortunately, all your models are parametric so you just have to drop the point :)

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