Dr. Lawlor's Code, Robots, & Things

August 6, 2015

Alaska90: my huge new 3D printer design

Filed under: 3D Printing, Hardware — Dr. Lawlor @ 12:51 am

I started working on a huge new 3D printer design, the “Alaska90”, back in November 2014, but I got busy with school and stalled out.  This summer I finally got a few spare days and finished up the build, which is working well and printing huge things. Main features:

  • 600mm x 500mm x 600mm build volume (24 inch x 20 inch x 24 inch), to construct models as big as a few feet on each side.  By contrast, a typical printer prints about 200mm (8 inches) on a side.
  • Frame is built from 3/4 inch plywood, and parts are secured using 1/4 inch hex head bolts.  By contrast, a typical printer uses 1/4 inch wood or plastic, and tiny M4 screws.
  • Uses heavy 16mm steel rods, so it’s rigid enough to mill wood with a small dremel tool.
  • Support rods are moved inward, to gauss points, to better support the 24 inch x 24 inch bed.

3D rendering of Alaska90 3D printerPhotograph of Alaska90 3D printer The full design details, STL files, and OpenSCAD source code for the Alaska90 printer is on my github.

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October 13, 2014

Extrusion Multiplier: why your 3D printer won’t print

Filed under: 3D Printing — Dr. Lawlor @ 12:54 am

I’ve realized one single parameter, the extrusion rate multiplier, can control whether your 3D printer makes a brittle, stringy object (multiplier too low); a strong, watertight object (multiplier just right); or just jams up and fails halfway through the print (multiplier too high).  This multiplier is called different things in different slicers, but it’s basically the fudge factor that together with filament diameter determines how much plastic squirts out of the extruder during the print.

  • Multiplier too low: not enough plastic, and the top of the object is missing material (not watertight), the layers split apart easily (they’re not thick enough to bond together well), infill is detached from the object perimeter, and the part is overall undersize.  The printer prints, but often parts detach into a pile of spaghetti, and are too weak to use for anything.

    3D printer stringy brittle object

    Rate multiplier too low–not enough plastic for the layers to fuse into one solid part.  25mm / 1 inch diameter part.

  • Just the right amount of plastic, and the part looks great, with a very subtle “waffle” pattern on top.  The printer works reliably, and the part is strong and has correct dimensions.

    3D printer correct look

    Rate multiplier correct: just the right amount of plastic, part fuses correctly.

  • Multiplier too high: too much plastic, and the part will get wider and wider at each layer as excess material squirts out in all directions.  This results in ugly blobby parts that look like an over-iced cake, increases the size of parts and decreases the size of holes, and makes stringing worse as the head plows through excess plastic.  But eventually (1) excess material sticking up catches the extruder head and tears the part off the bed, or (2) the back pressure on the extruder gets too high and the filament shreds under the drive wheel.  Either way, the printer will continue thinking it’s printing happily while it cooks the filament in the hot end into carbonized char, sometimes clogging it up until you tear the hotend apart and clean out the nozzle.

    3D printer malfunction: too much plastic, lumpy ugly part

    Rate multiplier too high: excess plastic is squirting upward and in all directions. Surface is lumpy and ugly.

These three outcomes can be separated by a multiplier difference of only a few percent!

Here’s how to set the extrusion multiplier in your slicer.  You’ll need to re-slice the part to use the new value.

  • In MakerWare, “Create” a new custom filament profile, “Edit Profile”, under extrusion you’ll find “feedstockMultiplier”.
  • In Slic3r, “Filament” tab, “Extrusion Multiplier” (right below Filament Diameter).
  • In Skeinforge, “Craft”, “Dimension”, “Filament”, “Filament Packing Density (ratio)”.

For ABS, everybody recommends 0.85.  Most of my filament seems to work better around 0.88, but it depends on the color.  Filament that has absorbed humidity from the air boils and bubbles during printing, requiring a multiplier as low as 0.80.  PLA squishes less (see nophead describing the complexities of measuring extrusion rate), so needs a larger value, typically 0.95.  This is all assuming you’ve measured the filament diameter with digital calipers and entered it exactly.

With a correct rate multiplier, the print dimensions are correct.

With a correct rate multiplier, the print dimensions are correct.

3D printer lumpy output

With rate multiplier too high, the part is lumpy, holes are too small and the outside is too big.

There are several complexities here:

  • The amount of plastic needed for the first few layers depends almost completely on your platform height and levelness, not on the feed multiplier.  But the effect of not enough plastic (gaps, poor adhesion) and too much plastic (extra squirting out, extruder problems) are the same.
  • If you print at less than 100% infill, you’ve got space for any extra material to go, so you might be able to bump up the multiplier a few percent without any problems.  I usually use 100% infill, because I want stronger parts, so I have no wiggle room!
  • If you use a big layer height, like 0.3mm or bigger, the multiplier isn’t as crucial because there’s lots of space for extra filament to spread into.  At small layer heights, like 0.2 and especially 0.1mm, the multiplier needs to be perfect.
  • If the part curls at all, the corners pushing up acts like the multiplier is too high.  You can compensate somewhat by knocking a few percent off the multiplier, although you may lose watertightness in flat areas.
  • If the filament diameter changes through the spool (cheap eBay filament, I’m looking at you!), one value won’t be perfect everywhere.  For cheap filament I set the multiplier artificially low by a few percent, and ignore the lack of watertightness.

When people show amazing prints from a “tuned” printer, one of the things they tune is the rate multiplier.  Try it!

December 9, 2012

Printrbot Assembly: 2012 Howto

Filed under: 3D Printing, Printrbot — Dr. Lawlor @ 8:34 am

Last month I built up a Printrbot Plus kit, which was a lot of fun and seems to be printing well.  But both the hardware and software for this do-it-yourself 3D printing technology is still pretty immature, so here’s my guide to the current rev of that particular hardware.

The definitive place to start is Tim Stark’s official assembly instructions, a solid photo guide.  It leaves out a few problems with the current kits, though, including:

  1. At step 55, my Y motor stepper’s shaft was too short to reach the tiny setscrew.  I drilled and tapped the plastic pulley for a new lower #6-32 set screw, which still just barely reaches the motor shaft.
    Printrbot Y setscrew
  2. At step 68, while attaching the heated bed to the carriage, you need to insert *something* at least 2-3mm high underneath, or you’ll find the carriage hits the deck before the printhead even gets close to the bed.  I put an aluminum sheet under the bed to even out the heat distribution, which is pretty nonuniform by default.  My under-bed sheet is a little over 2mm thick, cut in a “U” shape to clear the thermistor.  I used the long M3 screws instead of drilling out the PCB for #6-32, but either would work.
    Printrbot bed
  3. At step 104, the 5/16″ black hex head bolt needs the whole stack listed at step 136.  If you wait any longer, you won’t be able to use a hex key to keep the bolt from rotating as you tighten the nylon nut on top.
  4. At step 142, my X carriage belt pulley was just a little too tall to clear the zip ties holding the Z linear bearings.  I sanded and filed the pulley down, and mounted it *very* close to the stepper surface.
    Printrbot X pulley
  5. Many people, including me, got badly crooked gears for the extruder at step 160.  I couldn’t even fit the hex head of the hobbed bolt inside, and cracked the brittle castable material while trying to chisel some clearance.  It’s a chicken-and-egg problem if this is your only printer, but if you can get something working you can print new extruder gears.

Optional improvements:

  1. I dunked all the wood parts in urethane, which makes them look better, keeps screws from backing out, and reduces dimensional changes when humidity varies.
  2. I wanted to protect the extruder and bed wiring, since flexing back and forth repeatedly across sharp zipties will eventually cause wiring faults.  So I spiral-wound grip tape around all the exposed wiring, and ran a little steel wire out from the extruder to guide the extruder wiring into a gentle curve in *front* of the carriage. The sharp corner and stretched wires of step 168 make me wince.
    GEDSC DIGITAL CAMERA
  3. My Y axis belt stretched after installation, resulting in blobby prints.  I tensioned it using a rubber band hooked on a binder clip, pulling sideways to take up belt tension.  Keep it far enough back that it won’t get sucked into the stepper even when the bed is fully forward.
    GEDSC DIGITAL CAMERA

Lots of folks seem to get discouraged when things don’t work straight out of the box, but this is brand-new technology: you’ll need some tools, talent, patience, and creativity.  But the Printrbot is a solid kit, and with a few tweaks makes a reliable printer!

Coating a Printrbot 3D printer in Polyurethane

Filed under: 3D Printing, Printrbot — Tags: , , — Dr. Lawlor @ 3:35 am

One whole generation of do-it-yourself 3D printers (from 2009’s Makerbot Cupcake to 2012’s Printrbot Plus) are made from laser-cut wood, held together with small screws and bolts.  There are good things about this: wood is cheap, light, stiff, and environmentally friendly, and laser cutting is fast and precise.  But wood is fairly weak and splinter-prone, wood structures warp with humidity changes, and the bolts holding everything together tend to vibrate loose when printing.

You can improve each of these drawbacks by dunking your wooden printer assemblies into polyurethane.  Dunking lets the polyurethane soak into the fibers, which strengthens them somewhat.  It reduces the rate moisture can diffuse in, reducing warping with humidity.  And polyurethane soaked onto the threads of the screws keeps them from vibrating loose, but is still removable for servicing.

Last month I built up a Printrbot Plus from a kit, and tried coating the wood.  I dunked each of the major assemblies (the base, bridge, extruder, and printbed) after adding screws and nuts, but before adding any electronics or linear bearings.  Here are the parts ready to go in: assemblies are on the right; loose parts on the left.

Image

Here they are after coating in polyurethane.  I had to brush the poly onto the top deck, since it’s too big to fit in the 1 gallon can, but everything else got dunked on both ends, and then brushed into the middle.  This process was pretty messy, so I wore rubber gloves.

Image

This “quick drying” poly still took about a day to stop being tacky and smelly; the solvent stench is so strong that I wore an activated carbon respirator while dunking and painting.  Total poly consumption was tiny, 50 mL or less, although you need a much bigger container to allow dunking. There doesn’t seem to be any substantial dimensional change, and everything assembled fine.  The poly brought out some beautiful chatoyancy glinting within the wood, an unexpected benefit, and added a nice warm glow to the wood.

This printer has been run hard for several dozen prints and a few cumulative days of continuous printing since then, and haven’t had a screw back out yet!

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