Intro: Mechanical Mutt
My small friend is always happy to see you and he's a very sensitive fellow. Squeeze his front right leg and he opens his jaw. Squeeze his left leg and he wags his tail. He's used to being small so he looks up at you, although his eyes get all excited if you pick him up.
Step 1: Video
Step 2: Teeth
I found a nail brush in my local store. All I had to do was clean off the varnish and cut it in two to make a splendid set of teeth, where no one's afraid of the bite. The top part will be moved by a thin brass rod with a loop on the end, so I cut a slot in the middle to take the loop. Then I drilled a hole from the side to insert another piece of brass rod which goes through the loop to keep it in place while allowing it to turn freely.
Step 3: Sides & Moving Parts
I made two sides like this from 6 mm plywood. There are two moving parts in each side. The top part is a "shoulder" hinge so that the leg attached to it can move a little. The lower part is pressed in by the movement of the leg and pivots to convert the sideways movement into a lengthwise movement. You can still see the brass rods protruding. When everything is OK, these can be trimmed back with a pair of side cutters.
Step 4: Putting a Spring in His Step
Now I have added a lever pivoted on a screw with a teflon washer underneath. A return spring is held at the back with a small piece of dowel and there is a notch in the lever. You can see that I have attached a leg to the top pivot and when this pushes against the bottom pivot, this pushes the lever to compress the spring and move the top end of the lever forwards to open the dog's mouth.
Step 5: The Essential Cold Nose
Every healthy dog has to have a cold nose, so I cut a part out of a small wooden egg and glued it to the top jaw. This will get glossy black paint to give it its proper healthy shine.
Step 6: Ears & Eyes
Between the ears cut from 3 mm plywood, there is a brass rod which very loosely holds two small wooden balls. I cut the heads off of two brass screws and glued them to the bottom of the balls. This provides the "dolls eyes" effect, keeping the pupils fairly constantly at about 45 degrees to the horizontal. A small piece of brass tube between the eyes keeps them a little way apart.
I used expoxy resin to fix a small brass hinge to hold the upper jaw to the body.
Now you can also see the brass rod attached to the lever which pushes the jaw upwards.
Step 7: What a Wag
The left and right sides basically have the same mechanism. A piece of brass rod hinges the tail and the second lever acts diagonally through the body to wag the tail. I removed the two return springs so that you can see roughly what's going on.
Step 8: Back Legs and Lower Jaw
The back legs and the lower jaw are glued in their fixed positions. A lick of paint and we're done.