Saturday, August 9, 2014

Arrbot: Assembling the Body

Now that we've got the body printed and servos modified, it's time for some relatively easy assembly work.  Rewarding too... finally we'll start to have something that looks like a  robot!  In addition to the body and servos, we'll need two treads, two servo arm kits, two front wheels, and two drive (back) wheels.  The drive wheels have two pieces.

Read on to see the step by step instructions.

First, we're going to do the front wheels.  This is what we're shooting for.
Start by removing and cleaning up the 3D printer supports.
The cleaned up pieces should look like this.
We need to sand the axle and inside of the wheel.  This will make it smoother and (on some models) allow the wheel to fit.  I used medium (150 grit) and extra fine (320 grit) sandpaper.  Wrap the sandpaper around the axle and spin the body around.  You want the wheel to spin freely but not be wobbly.
Do the same on the inside of the front wheel. It needs much less because of the direction of the printer "grain".
The servo kit has a one small screw and two larger screws.  We will use the larger screws to attach the front wheels.
If the hole on the front wheel is too tight for the screw, open up the hole with a drill bit or (what I did) with a phillips screwdriver.
Put some dry lube (picture below) on the axle and the inside of the wheel and use the screw to keep the wheel in place.  The wheel should spin freely.
When completed, it should look like this.
Now it's time for the rear drive wheel.  We'll need the two wheel pieces, the small servo screw,  and the long servo arm.
The servo arm fits into the smaller rear wheel piece.
Snip both ends of the servo arm so that it fits inside the wheel. Precision is not important.  Be careful, the snipped off piece of the nylon servo arm will shoot away from the snippers.  Eye safety!
Here's how the snipped servo arm should look when seated into the small rear wheel piece.
With the servo arm in place, start to join the two rear wheel pieces. Important: make sure the gear teeth will be in the middle of the assembled wheel.  If you flip the large piece over, the teeth will be on one edge, which is wrong.
Carefully push the pieces together.  Do it little by little on each side, and be careful not to break the registration pins off the small piece. Tapping it with a hammer, using pliers, and squeezing the pieces together with your fingers all work.  It's tempting, but don't try to bite it together.
Completed!  Notice how the teeth are centered in the middle.
Now it's time to attach the servo.  The servo lead will tail out the rear of the body.  If you put the servo in backwards, the tread will be too loose.
Square up the servo.  It should fit snugly into the brackets.  A tiny bit of hot glue will hold the servo in place, or if your servo has mounting holes you can drill a hole in the body and attach the servo with screws.  If you use hot glue, remember that you can remove the servo in the future by putting a drop or two of rubbing alcohol on the hot glue.
This is how the drive wheel should look mounted on the servo.
When it's lined up, use the small screw to attach the servo to the servo arm.
Now it's time to attach the treads.  Notice that there's a V-shaped indentation along the center of the outside of the bracelet.
Flip the bracelet inside-out, and that V now aligns perfectly with the teeth on the front and back wheels.
Slip the now-inside out bracelet over the front and back wheels. Treads!!
The teeth of the treads should align nicely with the gaps in the tread.  Wiggle the front and back wheels a bit if necessary.
Here's what the wheels and treads should finally look like.
Here's the Liquid Wrench Dry Lube.  It's nice because it doesn't leave any stick residue to pick up dirt, dust, etc.
Finally, put some dual lock or velcro on the top of the body, both sides of the battery pack, and the bottom of the breadboard.  Stack everything up and you're finished with the mechanical assembly!

(And one more shoutout to Rick Winscott at Adafruit for coming up with such an awesomely clever idea!)

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