Grabber Arm

Step 1 of 19. Lay a 2x12 beam down flat. This is one side of the chassis.

Step 2 of 19. Push a shaft through hole 1 and hole 10 of the front row. Count from the end: hole 1 is the second hole along.

Step 3 of 19. Push a wheel onto the end of each shaft. The flat of the shaft has to line up with the square hole in the wheel - turn the wheel until it drops on.

Step 4 of 19. Lock a collar on the outside of each wheel. Press it right up to the wheel. A gap here is what makes a wheel fall off later.

Step 5 of 19. Build a second side frame exactly like the first, and set it down mirrored - wheels facing out. Hold them side by side. They should look like a pair of hands, not like two left hands.

Step 6 of 19. Lay a 1x8 beam across both rails, near the front. Turn it a quarter turn so it runs across the rails, not along them.

Step 7 of 19. Push four connector pins down through the cross beam into the rails underneath. Two pins into each rail. One pin per rail would let the frame pivot - the second pin is what stops it.

Step 8 of 19. Repeat at the back: one cross beam, four more pins.

Step 9 of 19. Sit the brain on the two cross beams, ports facing the back. It should sit flat. If it rocks, a pin underneath is not pushed all the way home.

Step 10 of 19. Stand a 2x4 corner connector on each rail at the front, foot pointing backwards. The tall leg goes up. Both towers lean the same way.

Step 11 of 19. Pin each tower foot down into the rail underneath - two pins each. Two pins, not one. One pin lets the tower rock forwards.

Step 12 of 19. Hold a 1x6 beam against the inside of each tower, reaching forwards, with its end hole level with the top of the tower. Nothing holds these yet. The shaft in the next step is what catches them.

Step 13 of 19. Push one long shaft through the whole lot: tower, arm, arm, tower. Line up all four holes before you push. If it will not go, one arm beam is a hole out.

Step 14 of 19. Lock a collar on each end of the shaft, outside the towers. Leave a little play - the arm has to swing, not grip.

Step 15 of 19. Lay a 1x4 beam down. This is one finger.

Step 16 of 19. Pin a 1x2 beam across the tip, turned a quarter turn so it points inwards. This is the part that actually grips. Both fingers turn in towards each other.

Step 17 of 19. Build a second finger and mirror it, so the two tips face each other. Lay them next to each other first. If both tips point the same way, one of them is built the wrong way round.

Step 18 of 19. Pin each finger down onto the end of an arm beam. The fingers sit on top of the arm, not inside it.

Step 19 of 19. Check it. Push the base along the floor, then swing the arm up and down. Close the two tips together. The arm should move with one finger and stay where you leave it. If it drops, the collars are too tight or too loose.