Saturday, April 03, 2010

Delta 'bot Axis

It's one thing to design and print plastic parts that have to mate with steel rods. As I am finding it is quite another to design plastic parts that have to mate with other printed plastic parts. During the last week in my spare time I proved out a design for a hexagonal column which I think might accommodate a herringbone rack and a sleeve seating a pinion gear, stepper motor and two guide plates.

Heretofore, I have been butt seating the column sections and then post-tensioning them. While that was okay for demonstrating the design approach, worries about creep said that I needed to have a male/female attachment technology so that I could fit sections of the column together and avoid having them do a lateral creep out of alignment. That proved to be considerably more difficult than I imagined.

This last weekend, I decided to take a different approach and instead of doing a conventional slice and dice of an STL, I decided instead to work with the print roads of my thin-walled column instead. What I did is a radial 45 degree overhang inside the column onto which I printed the male insert. I think it is worth mentioning how I achieved that.

The column was achieved for most of it's length with three concentric print roads of 0.6 mm width. What I needed to do was to build a ledge inside the column. When I began the ledge I widened the innermost print road to 0.8 mm. On the next layer I replaced the 0.8 print road with two 0.6 print roads. This process left me a 0.2 mm miniledge onto which the new innermost print road could adhere. I repeated this process until I had a ledge wide enough to print the male insert onto.

You can see the insert successfully executed here.

I would like to be able to say that that was all an easy exercise. It wasn't.  I did exactly thirty text prints like the one you see before I was satisfied with the seating of the guide strips, herringbone rack and the male/female column connector system.  I also had to chase several heretofore unsuspected bugs in Slice and Dice AND extend the coding somewhat.  All the same, it's done and works so far.

At that point, I decided to see how tall a monolithic column section I could print.  It appears that Rapman three can handle about 225 mm in the z-axis.  I designed a 200 mm column with a 4 mm male connector and am printing it at present.

It is 2120 and I am up to about 50 mm.  I'm doing a 0.25 mm layer every 45 seconds, so I expect to be finished at about 0500 tomorrow morning.

With such a large print I began to encounter xy and z faults.  The xy faults were annoying, but the z faults tended to plunge the hot extruder head into the print.  Upgrading the Rapman firmware to 2.0.8 got rid of the z faults but the xy faults remained.

I reduced the column test print to 55 mm, relocated the Rapman further away from my PC on a table by itself away from the cold window and switched ABS to HDPE because the acrid fumes were getting to be too much when I wasn't able to open the windows.

A thorough cleaning of the extruder with compressed air was also undertaken.  After a few test prints I got the flow rate sorted out for HDPE and was able to successfully print a 55 mm column section in HDPE.

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