Monday, January 30, 2012

Star-shaped verification model

I developed a set of fit-test models when I was trying to adapt my Wrong Way Nut model to make it suitable for Makerbot-type hobby FDM printers.  This is a series of shafts and disks with a star-shaped cross section to be built in various orientations.

The SD300 built the shapes accurately enough that each disk can fit over any of the shafts all 8 ways.  But when built on a hobby machine without support material I have a hunch the shaft will be distorted.

Will the corners be blunted because the filament has nothing to anchor it, like the yellow line in this drawing?  Or will they extend too far because the unsupported material droops, like the blue line?  Or will both effects cancel each other out, yielding a perfect fit?

I can't really guess how this model would behave on a Makerbot, but I shared the test files here at Thingiverse, just in case a hobbyist wants to test it.

Friday, January 27, 2012

Locked Dovetail Triangle

When I posted my Dovetail Triangle one user suggested adding a locking mechanism while another argued against it.  So let's have it both ways: I built a new dovetail mechanism that has a locking mechanism, shared here at Thingiverse, to complement the one that doesn't have a locking mechanism here.

As before, I aimed to keep overhangs within the buildable range of Makerbots and other hobby printers.  That imposed some interesting design constraints, which forced me to distribute features of the locking mechanism between the base and lid.  But I filleted the internal corners to 0.5mm purely for my own convenience: it's a bit easier to peel unused material out of SD300 models if there aren't sharp internal corners.

At first I built a transparent model and tested it with two black triangular pieces so I could look inside to see how well it was working.  It's hard to see, but the picture below shows how the black triangles slide toward the outside of the puzzle when it's lying flat and thus lock the joint.
The black triangular pieces (dark shadows) slide toward the outer corners.

When it's flipped upside down, the contours in the lid cause the black triangles to slide toward the outside edges, locking the joint like the previous picture.
When flipped over the triangular pieces still slide toward the corners.
It worked, but those triangular pieces just didn't slide very well.  So I tried using round discs instead.  That worked much better because the discs easily rolled around when the puzzle was tipped.

Here's a video showing how the parts work and how the puzzle opens.

I designed the mechanism so it would only unlock when one particular corner of the triangle was pointed downward, because I thought that would be the least intuitive way to hold the puzzle.  As it turns out I'd guessed wrongly about that: testers instinctively hold the puzzle with one of the dovetails facing up (to look at the joint, I guess) and hence they naturally hold it with a corner pointing downward.  So it isn't especially challenging, but it definitely works!

Thursday, January 26, 2012

Return Tool Error

While the SD300 was building a model on Tuesday night it made a distinctive 'click' noise, just once while it was using the Anti-Glue pens.  It didn't sound bad, but it was an unfamiliar sound I'd never heard before.

It never made the click noise again, but a few minutes later it began exhibiting problems putting the Anti-Glue pens away.  Eventually the machine stopped with a "Return Tool Error" message.  In SD300 lingo the Anti-Glue pens are named Tool #4, Tool #5, and Tool #6 so this message means it was unable to return a pen to its holder.

That wasn't too alarming.  Maybe that click noise had been a pen or holder had been knocked loose.  I removed the pen from the plotter and carefully returned it by hand.  The machine resumed building for one more layer but the error happened again as soon as it tried to use the Anti-Glue pens.  I installed a brand new set of Anti-Glue pens, but the error kept coming back.

When the machine had logged 24 consecutive errors I decided to give up.  It was late, so I dashed off an email and went to bed.  By Wednesday morning I already received two helpful replies from the folks at SolidVision, which recently merged with FISHER/UNITECH.  I appreciated the very prompt response but I had to go to work because I only do 3D printing as a hobby, so I didn't have an opportunity to do any troubleshooting until the following evening.

Jason Harris offered me several options, but he suggested the solution might be as simple as squeezing the spring on the affected pen holder.  The pens holders are spring-loaded clips, likely the same ones used in Roland DXY-series pen plotters.  A spring is supposed to ensure an empty pen-holder is narrower than an occupied one.  Could it really be this simple?

Curious, I brought out the calipers and measured the pen holders both with and without pens in them.  The openings on the working pen holders were all about 7.9mm wide, but the malfunctioning pen holder was open about 8.4mm.  That didn't seem like much difference, but I carefully applied pressure to the spring until the bad pen holder measured less than 8mm, then I restarted the machine.

It worked!  Then it worked again!  Just to be sure I repeatedly pressed Pause/Resume to make the machine take and return the pen a dozen times, and the pens returned to the holders every time.

This week my SD300 has logged over a thousand build jobs.  I've occasionally had issues that resulted in canceling a job or damaging a model, but this is the first time the machine itself had exhibited a fault.  In hindsight I consider that a very good record, especially considering how easily the fault was resolved.  Admittedly there are challenges cleaning the support material from some models, but the machine itself has never demanded anything more than clearing a jam or replacing spent consumables.

Wednesday, January 25, 2012

Peppermint Candy puzzle refined

I took another stab at the Peppermint Candy puzzle which had been vulnerable to coming apart in unintended ways.  I carefully refined the curves so each piece would 'reach around' to brace itself against the back side of the puzzle and made the walls thicker so it would be more rigid.

Honestly, I wasn't sure it would suffice.  But it worked!

Here's the side with the ball sticking out.  The cut in the ball is lined up with the cuts in the shell but it won't open in this position.

The other side of the puzzle looks about the same, but it has a white dome instead of an opening so the ball can't be seen from this side.  The parting line forms an S-shape where it passes through the domed part.

Here's a video explanation of the puzzle in detail.

For comparison I ordered a test part built in SLS nylon.  It's stiffer than the part built on the SD300, a good thing, but it lacks the contrasting colors.  But the colors aren't just cosmetic; my testers thought the colors contributed to the challenge of solving the puzzle.

I built the ball pieces in a very specific orientation in order to put the colors exactly where I wanted them.  Here's how the ball looks outside the puzzle shell.

Since the part required a specific build orientation I couldn't really position it for optimal post-processing.  Each piece had a thin edge that would have been particularly vulnerable to chipping during cleaning, so I isolated that area in a little 'box' of peeling cuts.

Most of the layers in the isolated box fell away when the surrounding areas were cleaned, so it didn't make the cleaning work any slower.  Except when I got to the thinnest part, the isolated box allowed me to gently peel away the last bits of material...leaving the thin edge smooth and intact.  As usual, I solvent-dipped the parts to give them greater strength and a slick finish!

Friday, January 20, 2012

MINI Cooper Grocery Bag Hook

MINI offers a "Grocery Bag Hook" accessory that's supposed to prevent shopping bags from sliding around in the trunk.  But bags don't slide around in the tiny trunk of a MINI Cooper, so their accessory seems unnecessary.  What I really wanted was a hook that would keep a shopping bag from sliding around in front of the seats, so I designed this simple hook.

This hook attaches easily to the dealer-supplied cup holder accessory.  Shopping bags slip neatly into the deep slot so they won't slide around.

I put it online at Shapeways and built a sample in their Red Strong & Flexible material.  Don't expect it to be a big seller!

I had replaced the material kit partway through the build, and the tints didn't quite match even though both kits were Amber Transparent material.

Monday, January 9, 2012


A year ago I probably wouldn't have attempted to build something as twisty and complex as Bathsheba's Antichron model.  But someone sent me a reduced-size model of it, and I confidently devised a small set of peeling cuts that should have made it an easy build on the SD300.  Not so easy, as it turned out.

I got it mostly right.  I'd divided the model into four zones in each of the upper and lower sections, so the support material peeled away effortlessly at first.

But when I got near the middle section I found layers of support material permanently wrapped around sculptural features I'd misinterpreted.  If it had been properly planned, there should have been additional cuts to divide the support material but I hadn't instructed the SDMove software to make the proper cuts.

Was it necessary to discard the model just because I hadn't set up the build software properly?  Probably not, but I decided to free the model so from the rest of the material to get a better view of the trouble spots.

The support material was thicker than the walls of the model so I couldn't just remove them by brute force.  So I decided to separate the support layers and snip them one-at-a-time with a small knife.  This was tedious but it would free the model after cutting through dozens of layers on each side.  That would take a while so I put on an old Edward G. Robinson movie (Scarlet Street) while I worked...and it took most of the movie to free the two models I'd built.

Because I'd scaled down the model by 50% the walls were dangerously thin, about 1.4mm.  Somehow I managed to free the first model without any damage, but cracks appeared in the second model while I was cleaning it so I stopped several times and applied Weld-On to strengthen its weak spots.  When I finished cleaning the second model (below right) I dipped it in Weld-On 2004 to heal the cracks and give it a nice sheen.

Friday, January 6, 2012

Test Model vs. "Clean Bubbles"

In Monday's post I concluded that whenever the SD300 has been idle for a week or more that it's wise to either build a Test Model or run the Clean Bubbles maintenance routine to clear bubbles out of the glue delivery lines.

But which method is better?  Is there a difference?

I ran some tests, reviewed the logs, and weighed the glue cartridge: a Test Model takes 15 minutes, uses 8 grams of glue, and consumes some PVC material. The Clean Bubbles routine takes about 1 minute, uses 5 grams of glue, and doesn't consume any other consumables.

So Clean Bubbles is better, faster, and more economical.

From now on, when I want to build a model after leaving my SD300 idle for a week I will 1) first clean the glue trap with a paper towel, 2) run the Clean Bubbles routine in SDMove, and 3) confirm liquid glue has been deposited in the trap.  (Repeat Clean Bubbles if the trap is dry.)

Bubbles in the glue lines tend to resolve themselves, but it's nice to discover how trivially easy it is to prevent the problem in the first place.

Monday, January 2, 2012

Another mystery solved

I think I solved another mystery or two.

Jason Harris at SolidVision told me when I was setting up my SD300 that it might give a false 'replace cutting knife' error around layer 4 or 5 if the build material wasn't stacked tightly at the edge of the build table.  It happened to me once in June 2010 the first time I used my SD300 after a long vacation.

A British SD300 user once told me that he tended to build several batches of models near the end of each month.  His first build job would usually be sticky and the support material would be hard to remove, but all subsequent jobs would be normal.  He worked around the problem by always building the built-in Test Model if he hadn't used the machine that month.

Today I built my first model since 17 December 2011, so my SD300 has been sitting idle for two weeks.  I noticed something strange immediately.  Normally the SD300 starts by bonding a layer of PVC onto the build table with an even layer of SolGL adhesive, but this time the adhesive was spotty.  The adhesive appears here as dark spots underneath the red plastic--it should have been spread under the whole sheet, not just little spots as here.

Then I noticed a shiny droplet of SolGL glue near the edge of the table, on top of the PVC sheet.  I'll bet it dripped there when the SD300 measured the material height with its micrometer, which is attached to the same unit that spreads the glue.

Ordinarily the machine's next step would spread anti-glue (masking fluid) onto this surface with a device like a felt-tip pen, but the droplet of SolGL glue would obviously contaminate the pen so I paused the SD300 and wiped up the glue with a paper towel.

Then I let the SD300 continue building to see what would happen.  The next layer had fewer bubbles than the first, and the following layer didn't have any bubbles at all.  I'm guessing air had seeped into the glue delivery system while the SD300 sat idle, but the air had all been cleared out now.

I wondered if I should cancel the job and start over now that the problem had apparently cleared itself up, but out of curiosity I let it keep running.  Lucky thing, since what happened next was a revelation.

Starting at layer 4 in every job the SD300 automatically verifies the condition of the cutting knife by cutting a test pattern at the edge of the table.  When this build reached layer 4 I heard a loud noise from the media feeder clutch which usually signals a cutting-knife failure.  I watched closely as the SD300 automatically  repeated the knife test a second time: the knife was good but the material was too loose.  The machine raised the table and repeated the test again: the test succeeded this time, but the leverage from the knife dislodged the material so the PVC sheets loosened from the table.

The SD300 started building layer 5 by ironing a new sheet to the top of the model.  Ordinarily it measures the stack with its micrometer to confirm the thickness of the last layer, but this time it started taking repeated measurements over and over.  The red light came on and the SD300 displayed "HEIGHT CHECK ERROR" on the panel.

In my experience, the SD300 tries not to abandon a build if it can possibly continue so I tried pressing the material back onto the table and pressed the continue button.  The SD300 obediently measured the model several times more, apparently trying to get a usable measurement, but stopped again with the same error.

The logs demonstrated the SD300 had made a good-faith effort to continue, but with the material flopping loose from the table it couldn't get a measurement within 446 microns.  This was an error of over 100% because the last layer had only been 188 microns, so there was no sensible way the machine could have compensated.

So the only solution was to cancel the job and start again.  Then it worked just fine, which makes sense in hindsight:
  • The air bubbles in the glue lines had formed while the machine was idle.  The bubbles would have cleared themselves within a few layers, but I could have cleared them manually by using the Clean Bubbles function in the SDMove maintenance utility.
  • The droplet of SolGL glue was also caused by the air in the glue lines.  It could have contaminated the  anti-glue pens, but the contamination would have eventually resolved itself.
  • A height check error within the first few layers of a job probably indicates there were bubbles in the glue lines, but the problem has already resolved itself by the time it's been detected.
All these issues are self-rectifying within a few layers, so I accept my British friend's solution as the simplest: just run a Test Model build job whenever the SD300 has been sitting idle.  A test model uses very little material and it gives the machine an opportunity to work these issues out of its system without needing to understand them.  Or I could just ignore these potential issues, since the worst that seems to occur is that I might have to re-run a job that fails within the first few layers.