Friday, December 18, 2009

I Trust My Makerbot / Making Model Buildings

Makerbot's much more fun when you trust it enough to leave it alone for 30 minutes.

Last weekend's experimentation made a huge difference; with the cable to the extruder replaced, I'm having every print work fine. The Brio train track was one of the first things I tried, and I was amazed that the first print went perfectly. The Brio track's also interesting because it uses a lattice structure on the lower layers to minimize warping. When there aren't lots of uninterrupted threads of plastic, shrinkage won't cause the part to curl.

With such good luck, I moved on to one of the projects I'd bought the Makerbot for: could I use it to build model buildings for my model railroad? Specifically, I wanted to print out Spanish Revival style building facades. The curves and layers would be difficult to do by hand, and much harder to do repeatedly, but easy to do with the Makerbot.

Here's a quick look at my first print. The print is 90mm (3.5 inches) across and 70 mm (2.75 inches) deep, and was intended to be around 3 mm thick. It's getting close to the maximum width for the Makerbot; the stock build platform is 100 mm square, but the raft for this print - the crosshatched layers of plastic put down first to hold the model to the surface - is 102 mm. I found my machine could move 110 mm, so I cut a new build platform out of Plexiglas to do larger objects. My first two prints for the building facade failed quickly; the pieces warped horribly within a few layers, and usually bumped the nozzle as they moved around. Bad.

But the Brio track hadn't warped. I wondered if I could break up the long side pieces so that the top layer couldn't pull across the piece to cause the warp. Back in SketchUp, I made the piece twice as deep, and cut some wedges out of the thickened back side perpendicular to the warping.

The new design printed perfectly - no warping, and very little problem with the print. I even managed to print four more over the next couple days. Each print took around 35 minutes, and used 10cc of plastic (or about 20c worth). Best of all, these were the first prints I'd done on the Makerbot where I felt like I could walk away for several minutes without worrying if the printer had stalled and was spewing out hot plastic at one spot.

Warping parts is a big deal in the Makerbot community. The warping occurs because the styrene plastic shrinks about 2% as it cools, pulling at the lower levels and causing the part to cup and pull away from the build surface and first "raft" of plastic laid down. In the best case, the parts end up with a rounded base; the lower levels warp, but later levels get affected less by the warping and build up new flat surfaces. In the worst case, the part curls enough to foul the machine. There have been lots of potential solutions. Some think that changing the plastic used will help. Others are experimenting with heating the build platforms to 60 degrees Celsius so less shrinkage and warping occur. My experiences make me wonder if some simple tweaks to the design might be enough.

My printed building fronts are for a 1920's drive-in market. These weren't the drive-your-car-in markets of the fifties, but the predecessors of modern strip malls - inexpensive buildings put up on corner lots along the new roads leading out to the suburbs. The markets had a pretty standard format: one market, one greengrocer, one butcher, a baker. Each was an independent business with an individual space, but together they were just the place to stop for groceries on the way home. Like many 1920's and 1930's buildings, they often used some extra architectural detail to draw in drivers, and Spanish Revival was a popular style back then. Longstreth's book shows pictures of drive-in markets around Los Angeles, but they appeared elsewhere in the U.S., and were certainly seen in the San Francisco and San Jose area that I model. (The old photo, in fact, is of a drive-in market on Telegraph Ave. in Berkeley, CA. It's long-gone, or at least remodeled.)

I'm planning to glue the building fronts together to make an "L" shape to fit the lot space, and my usual model building supplies for the back walls. The Makerbot surfaces are still pretty rough, so I'm planning on using spackle to fill in the rough spots and simular plaster. Some of the detail, such as the sawtooth row above the arch, will stay to simulate some of the details found on the actual buildings.


  1. Thats pretty awesome. Glad you worked out the warping with just a design modification. How are you converting the sketchup file to a STL? A ruby script?

  2. Hi, Charles,

    I'm exporting the SketchUp model as a 3DS file using SketchUp Pro, reading that into Blender, scaling it appropriately, and exporting it as STL, then bringing the STL into skeinforge for conversion to Gcode.

    It looks like there are some other Sketchup -> STL file converters out there, so it might be time for me to try those out instead; this was just the first tool-chain that worked for me.

    As for design mods, I'm hoping to experiment a bit to see how I need to tweak the model to avoid warping. The current notches are v-shaped and about 6 layers deep. I'm curious to see how small I can make them and still avoid the warping.


  3. Thanks for the info on the stl conversion. I had been using a ruby script with my McWire RepStrap until my gen 2 electronics got fried by a PSU failure.

    Cant wait to get a gen3 kit and build a mendel and start printing again. Please keep us posted with your findings.

  4. By the way, I'm now using this SketchUp STL export plugin:


    and exporting straight to STL, then processing the file in Skeinforge. I like not having to scale and convert the object in Blender, but it's harder for me to do scale models in feet and inches, then export to the proper scale.

    For my HO models I was doing this weekend, I was drawing the model in SketchUp using feet and inches, then scaled everything 3.5 times bigger (because HO / 1/87 scale is 3.5 mm to the foot), then exported the STL using "feet" as the units so that my 16 foot wide building became 54 feet wide, and then was treated as 54 mm wide by Skeinforge.

    I suppose that it wouldn't be hard to do the scaling in the STL plugin (for it's just a bit of Ruby program) - just modify it to multiply all numbers by 3.5.