January 18, 2014

Now It Looks Like A Robot Arm

In the last month I've more or less finished the hardware side of the robot arm.  For the two fast axes, at least.  I'll deal with the z axis later.

Finishing the arm required a few big machining operations, especially for the elbow joint.  Naturally, I made these parts from big bricks of aluminum billet.  Elbow Part One started out as some 2" square billet, which I faced to size manually on the CNC mill, since the MITERS Bridgeport was temporarily out of commission.  On my first attempt at making this part, I discovered that when plunging into a pocket with a large endmill, the CNC mill's spindle stalls really easily.  Even with .5 mm plunges this occurred.  To resolve this, I manually drilled big pilot holes for the circular contours.  After the first CNC job, the part looked like this:

And after two more CNC jobs and one manual one, it was almost done.  It's a little sad that when you machine something this way 90% of the metal goes into waste chips.  But not nearly sad enough that I won't do it anyway.

I manually added the slot and tapped holes for a clamping mechanism.

When I ordered the pulleys for the belt reductions, I had a different design in mind for the plastic reduction than I ended up using.  In the final assembly, the output plastic pulley was torqued by the linkage it drove, causing it to deflect significantly.  To get a bit more precision, I got a nice aluminum pulley to replace it.  I drilled some big holes in it for moment of inertia reduction, and CNC milled plates for the linkage to attach to.

The second part of the elbow joint was machined manually out of some 2" round stock.

The driving link was made from some unidirectional carbon fiber tube I found lying around MITERS.  It is clamped at each end by a piece of aluminum that passes through the pairs of bearings on the pulley and elbow.  While turning the aluminum clamps, I found that the MITERS lathe turns a pretty significant taper.  To get a tight fit in both the bearings, I had to remove the taper by taking extra small passes off towards the chuck.

Hey, it looks like a robot arm now!  

For the time being I am going to ignore the z axis, and work on assembling the electronics and programming the thing, so that I can have something interesting and moving to display at TechFair.

No comments:

Post a Comment