- Cut and square up the aluminum block to size.
- On the top (side that will eventually face the sky) and bottom (side that will face the spin, should be opposite of top) of the block locate the center. This can be done by drawing a line from corner to corner. Then drill small pilot (1/8 inch works well) in the center of the block, starting from the top and extending through the block. If you have access, it is faster, more accurate, and safer to drill all holes in the block with a drill press. Now unclamp your block and check the bottom to see if the hole you just drilled pierced the center of the "x". If the hole is more then a 1/16 inch off center, get or make a new block and start over.
- With the pilot hole for the left/right pivot completed, it is time to make a pilot hole for the up/down pivot screws. On the left and right sides of the block mark the holes about 1/2 inch from the bottom and centered from side to side. If all goes right, when the drill comes out the fare side of the block it will hit your second layout mark and have intersected the vertical (first) pilot hole.
- The size of the next hole will be determined by the outer diameter of the bearings that you have chosen. Drill a hole in the top and bottom of the block (use the pilot hole as the center) just large enough for the bearing to tightly fit into (I used 1/2 inch) and only deep enough so that the bearings will be flush with the surface of the block. Note that is a deviation from my first mechanism in that I no longer see a need to recess the vertical screw head in to the block
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- Now that the bearing seats are created, we need to enlarge the vertical pilot hole to allow the pivot screw to freely pass. However, if you drill to large a hole you may compromise the bearing seats. I used a 9/32 inch drill bit.
- With the vertical and most difficult shaft completed, lets now finish the horizontal. Select a properly sized drill bit for taping a 1/4 - 28 thread into (I used 7/32), and use it to drill through the block along the horizontal pilot hole (completely through the block). Next tap the horizontal holes in the block. The threads should extend completely into the vertical shaft. The reasoning behind drilling and taping into the vertical shaft and not stopping the holes short of the vertical shaft is that all taps I have seen will not cut threads to the absolute bottom of a hole (My 1/4 inch taps will only cut threads to about 1/8 inch from the bottom the a hole). And I figure if I did not pierce the vertical shaft I would have left a minimum of 1/16 inch between the bottom of the horizontal hole and the vertical one, shorting the potential depth of the screws by 3/16 inch. Example: Width of block 1 inch less 5/16 inch vertical shaft less 6/16 inch (3/16 times 2) levees at best 2/16 inch on each side to hold the screws. Compared to through taping which leaves 5/16 inch for griping each screw.
- We need to now drill a hole for taping 2-56 treads to accept the treaded shaft of the ball half of one of the ball joints (note black ball in above picture). If your ball joints have a different thread, adjust to fit your joints. The hole should be on the right side about 1/4 inch back from the front and 3/8 inch down from the top. Carefully cut the threads in this hole when you are done drilling it.
- On the left side we need to drill and tap a hole to accept a 4-40 screw, which is used to attach the spring which helps balance the weight of the skull. The hole should be about 3/8 inch down from the top and 3/8 inch back from the front. Tap the hole after it is drilled.
- Next drill and tap (4) 4-40 holes in the corners of the top of the block to allow for the connection of the left/right servo mounting bracket
- Congratulations, you have now finished machining the block
- We now need to cut and drill the left/right servo mounting bracket. Cut the bracket out of a piece of 1/4 inch plastic or some other suitable material. Fit your servo and drill holes that line up the servos mounting tabs. Thread and screw down the servo. With the servo securely mounted, test fit the bracket to the block and mark the bracket for the 1/4 inch screw head, and drill a hole large enough (about 1/2 inch) to allow the screw head to freely move in. refit the bracket to the block and mark the bracket for the four holes that were previously drilled in the top. This is easiest done if the bracket is made from clear or semi transparent plastic. Drill the bracket to accept 4-40 screws and attached the bracket to the block with four 4-40 screws. If you messed up the threads or one of the mounting holes do not worry, two screws in opposite corners are sufficient.
- Assemble (exploded view of mechanism)the vertical pivot by inserting the bearings into their seats and sliding the 1/4 x 1 1/2 inch screw through the bearings and vertical shaft. Screw on the 1/4 inch threaded coupling to the screw, only tight enough to remove any play (up/down movement) from the screw. About a 1/4 inch down from the top of the threaded coupling drill a hole (4-40 taping size) through the coupling and about 3/4 into the screw. Remove the coupling from the screw and tap the hole in the coupling to 4-40. Now take a 4-40 screw and screw a nut all the on, and cut the screw so that only about 1/16 to 3/32 inch remains after the nut. Then slightly file down the thread extending beyond the nut with a slight taper, unscrew the nut. Reattach the threaded coupling to the block assemble and align the holes we drilled earlier. Next insert and tighten the 4-40 pin we just made into the hole.
- It is now time to assemble the horizontal pivot. Take the 1/4 -28 x 1 1/4 inch screw and insert it through the eye of a rod end, and screw on two nuts. Next screw in the 1/4 inch screw into the horizontal hole in the block. This screw should not touch or interfere with the vertical screw, hence Do not tighten this screw down! Spin and tighten one the jam nuts on the screw to the block. The other nut should be moved twisted toward the end rod, to help keep the rod end at the outer extremity of the screw. Repeat this step for the other side.
- Attach the large servo horn to the servo. Take about 2 inches of steel (very stiff) rod and bend it to form the linkage between the small and large servo horns. Attach the linkage and screw down the large servo horn to the servo.
- This completes the physical Pivot Mechanism
- After the Pivot Mechanism is mounted in the skull, securely mount the up/down servo to the skull. The servo should be positioned so that the servo horn and the right side of the block are in a vertical plain. Attach the second ball joint to the up/down servo horn.
- Make a linkage by taking a short ( about an inch) threaded rod and screwing on each end the cup half of our two ball joints. Adjust the linkage for length (threading the cups more or less onto the rod) and snap the ball joints together.
- The skull is attached to the body of the skeleton by cutting and rethreading (to 1/4 - 28) the 5/16 inch bar that runs through the spin of the bucky and screwing on the open end of the threaded coupling. Before threading on the coupling I screwed on a nut which can then be tightened against the threaded coupling to lock the coupling and prevent turning.
- This completes the construction and attachment of the Pivot Mechanism.
Take the small servo horn and attach it onto the threaded side of one of the 4-40 standoffs, and tighten it down with a nut. Now tightly (consider soldering in addition) screw an additional stand-off into the first. The second stand-off should nicely fit (very, very little if not zero slop) into the allen head of the vertical 1/4 inch screw which should be close to flush with the top of the servo bracket. If the fit is good, clue in the stand-off to the screw head. The glue is only here to keep the stand-off from popping out, the good fit of the hex ship is what provided the rotational strength.
