Tuesday, March 27, 2012

Replacing a broken leg

The final sections of the legs on my walking robots are made from a piece of waterjet cut steel, which the servo attaches to, welded to a short steel tube, containing the homebrew shock absorbers on the feet.  The weld between the servo mount piece and the tube is the weak point in the design.  One of these welds cracked after GenCon 2011.  I never got around to fixing it, instead using some Shapelock plastic to shore up the joint as a temporary patch that held well enough to become permanent.

At Wicked Faire 2012, another leg failed at the same spot, this one cracking badly enough for the shock tube to come completely loose.  I wasn't going to be able to patch this one back together short of re-welding it.

Since I now have access to a 3D printer, I decided to try and see if I could print out a replacement part  instead.  I wanted to keep the existing shock absorber tube, so after a few trial designs I made a two-part plastic assembly that clamps together around the tube and attaches it to the mounting tabs on the servo case.

The mounting frame is still using the same screws to mount to the servo tabs, as I'm still using the rubber mounting blocks to try and spare the servo gears the worst of the impact shocks from walking.

It seems to work, but I'll have to walk it around for a bit to make sure it's as durable as the old steel frames were.  Once I'm confident in the design I'll be printing out parts to rebuild all four legs this way.

This part is close to the maximum size the machine can print - I had to rotate it 45 degrees to fit it on the print bed.  This brings up another problem I've been having with the Makerbot recently.  Large prints, especially parts like this which stretch diagonally across the bed, tend to warp and shrink as they're being printed.  This can result in the part breaking free from the bed mid-print, or merely coming out curled and distorted.  Repeatedly cleaning and resurfacing the print bed, and even using a raft on some parts, didn't seem to help.

After reading some tips on the Makerbot forums, I started to suspect the ambient temperature and drafts.

It probably doesn't help that the printer has been moved to a back closet, next to a large window.  The window is kept closed - actually, it seems to be impossible to open - but it's still somehow cold and drafty in there.  Cold air blowing through the open printer frame can make the part cool prematurely and warp as it's being printed.

The preferred solution to this seems to be to cover the gaping open holes in the Makerbot frame with custom-cut acrylic sheets.  I didn't have those handy, so I've gone for the cheap but immediately available solution of taping paper over the openings instead.

Seems to be working so far.  No warping on this part, although I do have a fairly aggressive raft in place as well.