Sunday, March 6, 2011

Andromeda CNC out of Australia

I found this site recently. Their work roughly parallels mine in many ways.

CNC Machine Design and Construction Highlights

Everyone should have a CNC machine. Or at least everyone building a UAV should try and get one. They are such useful tools and one can never know unless you have one at your disposal. To create the Project Andromeda aircraft, Damien and I embarked on building a relatively large CNC machine. Large enough to create our wing molds. We agreed on a 2mx1m overall dimension and began planning.
Damien took the helm by drafting up the initial layout of the machine. We visited a local aluminium distributor and Damien promptly drafted their entire catalog into AutoCAD. This step proved to be a lifesaver as we could never have put everything together without the simplicity of selecting stock within AutoCAD. By this stage all our components had arrived. The 2m long X axis ballscrew, the 3 4.5kg motors that would drive the ballscrews and the 5kg spindle. Damien’s initial design was maturing and by this stage all our measurements were in CAD.
It was around this time that my curiosity got the best of me and I managed to dislodge the ball bearings out of the Z axis ballnut/ballscrew assembly while tinkering with it. Little did I know that this would start a saga of misery that would only see closure by the perseverance of Damien in figuring out how to “reball” a ballnut: an apparently impossible task.
The Z axis proved to be the hardest section to design and build. The spindle was very heavy and we needed to keep things close in order to limit the moment arm imposed onto the ballscrew assembly. Damien came up with a very tight design that can be seen below:
The spindle is almost touching the Z axis ballscrew with only a couple of millimeters to spare. Another tight fit is the Y axis ballnut visible near the bottom of the spindle. Its circumference just clears the Z axis ballscrew leaving a tight fit that allows as much movement as possible for the gantry. The picture below shows the clearance at the front of the gantry, which is about 2mm. This allows the Y axis to be sized down as well, increasing the X axis working area.
After the painstaking task of assembling the Z-Y axes, we finally moved on to the X axis which proved to be almost as hard. The process of making sure the 2m long fully supported rails were parallel and perpendicular to each other and the table almost exhausted our patience. This was a necessary evil as we wanted to ensure we could realise as much of the theoretical 10 um accuracy as possible. Once elevated we mounted the Y-Z gantry and turned on the motors for the first time. The authority with which the 80V DC motors moved the gantry was beyond our highest expectations.
The final assembly took a while, and there were some issues with the single sided X axis drive, but these issues were mostly because of the lack of cross bracing between the X axes. Once these issues were alleviated, the construction of the machine looked all but complete. Meanwhile I had made some headway with the motor drive and computer interface circuitry. I will cover these stories in a future article.

No comments:

Post a Comment