Percy and the mega-print

With Percy now coated in primer, it is easier to see and comment about how that single big print went. Truthfully, it was two prints because the first one had the cylinders too high. While I wouldn’t consider it a failure, the single big print was not the best decision.

I knew I was in trouble when I started having to add supports for the bell and the overhangs on the steam dome, sand box, stack and headlight. These supports terminated on the top of the tank, or worse, on the front of the cab where they would prove to be difficult to clean up. However, I pressed on, as I usually do, just to see what would happen.

I printed the shell on a 45 degree angle, which was faintly visible on the sides of the tank, and easily sanded off. However, there is noticeable stepping on the back, front and top faces – those that were actually at 45 degrees. This is because the 47 micron pixel size doesn’t divide evenly by the step size, meaning that every so often, an extra layer must be printed to meet 45 degrees. This is something I may be able resolve by changing the layer depth.

I was impressed that the bell printed in place so nicely, and the arm survived a lot of handling before I finally knocked it off while fixing a divot that appeared when the contact cement I used for fixing the weight melted the top of the tank. I was even more impressed that the .015″ handrail stanchions atop the tank lasted through all the handling. I’m pleased with my decision to form the handrails themselves from wire sitting in tiny drops of epoxy atop each stanchion.

Anticipating that I would have to true up some of the flat surfaces, I left the rivets off. Besides, they’re a bit of a nuisance to form in CAD, and installing Archer rivets is fun. I also left the sanding lines off because it would be easier to form them from soft wire than to model them in 3D.

The cab handrails were only .012″, but surprisingly robust. One did break during handling, and I replaced it with .015″ styrene.

There were some notable support failures during printing. The bottom rear of the cab, which was almost the first layer to print, failed at the corners; we have a slight bow there as a result, and the bunker needed extra sanding. The cylinder saddle was also under-supported, and I wound up building it back up with filler. Other bows are harder to explain, like the forward corner of the cab, and the whole back of the tank. These surfaces were well supported.

The back of the tank is only a few scale inches from the front of the cab, and this proved very difficult to coat with primer or paint without depositing too much paint nearby. I left the air compressor off to make painting easier, and in retrospect, I should have done the same with the tank.

Overall, printing a single big piece meant that I was reluctant to spend another 7 hours waiting for a replacement shell, and thus corrected most problems the old fashioned way. As usual, I learned in the process, but I wouldn’t print this model as a single piece again.