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Whosa whatsis
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Whosa whatsis

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Found this teardown of the MBI extruders via +Fabbaloo. I wasn't surprised by much based on what I had already seen/read/reasoned-out, but there were a couple of interesting things. First, a minor surprise was that the encoder wheel is optical rather than magnetic as I had assumed.

Second, the nozzle actually moves up and down during retraction/recovery. This is an idea that I have been working on for a while. The idea is to get the benefits of Z lift without moving the Z axis by pulling the nozzle back slightly in its mounting. There are several types of actuators that could be used for this, but they're not necessary if you can get the nozzle pressure to push/pull the mechanism the way you want it. The normal back-pressure of the nozzle will push the nozzle down toward the platform when you are extruding, so you really just need counteract the weight of the nozzle to lift it (normal retraction will produce a negative pressure, but it may not be enough, and the weight will slowly pull it back down as the nozzle pressure equalizes after the retraction.

The obvious solution to this is to use a spring, but a spring's force increases as it compresses so the spring will want to reach an equilibrium with the nozzle pressure that may not result in the spring being fully-compressed when the nozzle is under pressure, and the spring's force (at least if it's a weak enough one to ensure that the nozzle pressure will completely overcome it even at low speeds) is likely to drop off completely when the nozzle is fully raised. You want it to hold strongly in the raised position and only pull weakly (just enough to lift with no nozzle pressure). You need a bistable mechanism that will hold strongly in the down position when the nozzle is under pressure, and hold strongly in the up position when it is not, but will transition easily between the two with the rapid movement of retraction/recovery.

The solution turns out to be a magnet, which pulls strongly when the assembly moves up and gets close to it, but the magnetic force drops off quickly as the metal moves away so that only a little nozzle pressure must be maintained to keep the assembly separated from the magnet. Apparently, $403 million in stock from Stratasys is enough for MBI to hire someone capable of figuring this out, rather than just staying two years behind everything the Reprap community has been doing.
Matej Rozman's profile photoBenjamin Rockhold's profile photoMike Miller's profile photoCarlos García Saura's profile photo
Well, you can't say they're not moving the state of the art forward. That optoencoder would have saved a BUNCH of my prints. 

ETA: Still, a little janky that every 5th Gen is going to have a vestigal appendix like organ where the filament was supposed to go. 
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Whosa whatsis

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Build later share.
So ya... This is amazing!  I think this is going in my list of CNC projects to do!  +Jason Smith  thanks for sharing!

Pyro Board: 2D Rubens' Tube!
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I still want to see patents abolished, but this is the next best thing.
Billy Zelsnack's profile photoWhosa whatsis's profile photoCamerin hahn's profile photoStephen Baird's profile photo
+Billy Zelsnack that doesn't work... they attack both other lawyers and innocent people and bring nothing to the market
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Oh, tiny microcontroller dev boards that I already have too many of and will probably never get around to using, I wish I knew how to quit you.
Justin Walterich's profile photoCoco Jx's profile photoHaroon Rasheed's profile photo
I got "onboard" this one also.
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The skanbot fits in my tiny car without disassembly, and it will fit even more easily once I get the new, more compact turntable mechanism working. This one is from an old project and is bigger than it needs to be.
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Have him in circles
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Whosa whatsis

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I just thought of a way to unify the two major theories about bridging.

Some people say that they have the best results with bridging when they move across gaps as fast as possible, while others (like me) find the opposite.

It seems that the key to this difference is die swell. Pushing filament out quickly will increase die swell, resulting in a filament that will pull itself tight while becoming thicker as it leaves the nozzle. Pushing filament more slowly by moving slowly with a decreased flow rate will decrease die swell and make it easier to draw out a long, thin strand, that gets thinner as you increase its length.

In either case, the  bridge strands will be round rather than being flattened as a non-bridge layer would, but the first strategy results in thick strands (possibly thicker than the nozzle bore), while the latter results in thinner strands. I've always moved at a slow speed while decreasing bridge flow ratios for bridging with thick layers, and increasing it for bridging very thin layers, but this suggests that the optimum strategy might be to leave the bridge flow ratio alone, and only change the bridging speed. Simply increase the speed to increase die swell if your bridges sag, and decrease bridging speed to decrease die swell and gently draw out the strand of plastic if your bridges are breaking. If the slicer is already using the volumetric flow logic to put out the right amount of material for the bridge layer, this should put you pretty close to the optimum layer volume. Always adhering to one of the above bridging strategies, on the other hand, will result in bridges that span well, but are either over-compressed (swelled plastic on thin layers) or under-compressed (drawn-out plastic on thick layers). This combined strategy should avoid both of these problems while reducing the calibration process to a single variable: speed.
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Ben M
Thanks, I've always thought there was some magic to the bridge flow rate setting but this theory makes a lot of sense. I'm going to give it a try
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Whosa whatsis

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I've found another value that we need to include. I've noticed, specifically when working with smaller-diameter drive gears, that the compressibility of the filament is an important factor. A very compressible filament will need an increased flow ratio because, as the teeth of the drive gear are able to sink deeper into the filament, the filament will be closer to the center of the drive gear. This effectively reduces the diameter of the drive gear, thus increasing the steps/mm. Figuring out how to calculate this increase, and even what measurement to use for the calculation, will take some more research, but this is a value that will be needed for the type of "plug and print" functionality we're trying to make possible, at least with current forms of extruder hardware (though it may be possible to bypass this need by using encoder-equipped extruders).
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But the radius of the hobbled bolt changes with the depth of the bite. Assuming you are using a ~10mm diameter pulley for your extruder and the teeth byte .5mm deeper on ABS than PLA you will need to adjust your flow rate 5% more for ABS (if i am doing the math correctly)

I am hesitant to add this until we find a nice way to correlate a measurable number to a print flow rate. +Whosa whatsis i know you are doing some experimentation on this. we would need to try and decouple this from other information as well to try and make it work without asking the user to preform computations before every print (IE, my material stiffness is X, my harness is Y, my print speed is A, the ambient air temp is..... today happens to be vernal equinox, so my flow-rate is 1.05)
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Maslows_hierarchy[] normally looks like this:

{Physiological needs, Safety needs, Love and belonging, Esteem, Self-actualization}

But some days, it looks like this:

{Coding, Coding, Coding, Coding, Coding}

My guess is that there's a buffer overflow vulnerability that allows it to be overwritten this way.
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Why hasn't anybody made a small motor controller to that uses an encoder and takes step/dir signals in a format compatible with a pololu driver so that we can use servos as drop-in replacements for steppers on printers yet? I would think there should be chips designed to do this, but even if it requires a microcontroller and a separate H-bridge, it shouldn't be difficult...
Brad Hopper's profile photoKevin LaSanders's profile photoMike Creuzer's profile photoOla Sikström's profile photo
+Whosa whatsis Sorry for going on about shaft encoders but I really think they may be useful. A geared dc motor with encoder will cost roughly the same as a stepper, but will be stronger and way lighter (probably closer to 20-50 g, while a stepper often is 200+ g). The most obvious place for this is of course the extruder.

Linear encoders for the axes sounds like a good idea anyway.

Where you actually put the encoder shouldn't really matter though (implementation-wise), so wether you use a linear- or a rotary en coder, you could still use the same driver hardware.

As for the hardware, I'm confident this can be implemented with a cheap ATtiny and an H-bridge driver chip, on the same footprint as an A4988.
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A couple of interesting ideas here. Looks more like a CNC mill's toolpath than a printer's. Shallow-angled nozzles need not apply.
Camerin hahn's profile photoScott Leighton's profile photoWes Brown's profile photoNick Parker's profile photo
+paul mplr that's where I've been looking with all this. Arbitrary layer shape should allow you to mitigate layer interface weaknesses in an appropriate way for your application.

Also, for ninjaflex and similar, you could manipulate flexibility with layer shape.
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Have him in circles
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  • Deezmaker
    2012 - present
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Polymathic autodidact
On a good day, I'm an iconoclastic, autodidactic polymath. The rest of the time I'm just a cynical, dilettantish tinkerer.

Also, I make stuff.
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Designer of Reprap Wallace
  • University of Autodidacticism
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