Monday, July 26, 2010

Memories of plastic model airplanes



In which your narrator harks back to his youth and fingerprints ruined by trimming plastic flash off of model airplane parts with double edged razor blades.

I have an aversion to infills. Recently, I realised where the aversion came from. During the 1950s you could buy plastic model airplane kits for $1-2. These were made with injection moulded parts that came on little flat plastic Christmas trees. Exacto knives in that era were both expensive, for a child at least, and the blades dulled quickly and were largely beyond the ability of a child to resharpen. As a result, this child nicked paper-thin double edged razors from the medicine cabinet to trim the parts off of the tree and trim the flash off of the parts.

That worked fine, except for the cuts which the blades would make on ones fingertips which eventually grew into permanent scars. That was no big deal in those days.

Having trimmed your parts you glued them together with Testors or Duco cement {ethyl or butyl acetate} and, with a bit of paint, you had a lovely airplane to hang off of your ceiling by a thread and dream about flying.

Early on in the Reprap project, we used Solarbotics gearmotors. When you opened one of the gear boxes of, say, a GM-3 you found that the housing had been made using the same injection moulding process that model airplace manufacturers used. With a working Reprap printer I quickly began to loathe the clumsy, infilled parts that we were designing. They wasted both time {to print} and filament as well. Worse still, one tended to stick the parts together with expensive nuts and bolts.

Why not make parts that fit together like the old model airplanes?

The trick there is that the old model airplanes inevitably had a left side and a right side or a top a bottom that were often simply mirror images of each other. This weekend I was design a finger tip with left and right sides. I had designed the left side and got it working acceptably and was going back to do the right side in Art of Illusion {AoI} when it occurred to me that it would be much simpler to just write a script to swap the sign of the coordinates of the axis that I wanted to mirror around. Writing that script took about five minutes and saved me a lot of time in redesign and reprocessing of the resultant STL since my script operated directly on the Gcode.

Printing the two halves was trivial.



It took a few moments to scrape off the raft and a few drops of cement to finish the part.



The result was an elegant looking part that would have been a right bastard to design and print conventionally.

While I've incorporated this simple mirroring script into my own Slice and Dice STL processing software it would be no big deal to incorporate it into the more widely used STL processing routines available to Reprappers either open source or commercial.

5 comments:

Wade said...

Nice. Have to say though, when gluing larger pieces together, like this:

http://www.thingiverse.com/derivative:2676

having some mating surfaces makes it much easier. Even just some symmetric 3mm holes for feedstock to line the pieces up would help tons.

Most CAD packages have automated mirroring tools as well, but I don't know if any of the open source ones do yet.

Joel said...

Too bad you didn't break the blades in half.

Not to be a feature creep, but I could see the usefulness of a script that can include patterns on the underside that ensure good registration between the two halves of the part, and increase the strength of the glue bond.

There are even model kits that don't require cement. I remember all too well how inferior these were in terms of fit and finish, but I bet there are contexts when snap-together parts would be preferred.

Forrest Higgs said...

I've done a bit of mating surfaces work and hope to do more. :-)

BodgeIt said...

Now how big will you be printing your ultimate Kit could we be looking at some thing like this.?

http://news.bbc.co.uk/1/hi/entertainment/8327914.stm

murd said...

HeeksCAD and NaroCAD both mirror, and netfabb does too.