The Crawling Mummy’s Hand

Material List

2 - Pieces of 3/16” (5mm) dowel cut 66mm long
6 – M3x10 socket head cap screws
4 – M3x16 socket head cap screws
10 – M3 nuts
4 - M4 nuts
1 – 15 RPM or 30 RPM, 3 volt, GA12-N20 gear motor with an M4 threaded shaft
1 – 3 AAA cell battery holder with switch (part number SBH431-1AS)
Cyanoacrylate Adhesive (Super Glue)
Muslin (unbleached cotton fabric for wrapping the Mummy’s Hand since it looks old)

Procuring Parts

The screw hardware can be obtained through a search on Amazon or E-Bay. 

The 3/16” dowel size I used for the drive axles is available from a number of lumber stores.

I obtained the battery holder with switch from DigiKey. But an internet search for the part number results in a number of sources.

The best source for the gear motor is either E-Bay or AliExpress.  I found the Amazon search parameters to be broad and imprecise.  A careful internet search is needed since there are hundreds of possible combinations of voltages, speeds and shaft configurations.  I recommend the 15RPM motors which will result in a deliberate crawling speed plus they have more torque, the 30RPM motors will move faster but have lower torque and can stall with the mechanical drag of the final fabric wrapping.

Printing instructions

I printed the parts in ABS using a 0.25mm Z step with 2 vertical shells and 4 top and 4 bottom Horizontal shells. The infill was 20% honeycomb.  The Drive_Shaft_Assembly_Jig.STL file and the Fingers.STL files don’t need support turned on while printing.  The other files need support turned on for the nut traps.

The adhesion pads in the STL files are needed for printing with ABS.  They may not be needed for other materials but do provide reassurance for a successful print.  They are easily trimmed off after printing.

Assembly

Cut off the 0.5mm thick adhesion pads from the prints.  Punch out the support fill from the nut traps and remove any stray plastic filaments inside.  Seat the M3 nuts by inserting a M3 screw from the backside and by turning the screw, pull the nut down into the trap.  Trim off the 0.5mm adhesion pads and supports from the fingers. This doesn’t need to be precise since the fingers will be covered.

Assemble the frame half's using 4 of the M3x10 screws.  Check that the M4 threaded motor shaft turns freely in the center hole.  Then check that the 3/16” (5mm) dowels turn freely in their slots.  I found that if the corner of the frame lifted slightly while printing, the dowels can be tight.  If so, drill out the axle holes using a 13/64” (5.5mm) drill.  The 9mm wide drive gears may also need to be drilled out to clean up the axle holes.  Also, after seating the M3 nuts in the drive discs (ovals) run the drill through those axle holes so won’t bind when you assemble the axles.

The “Drive_Shaft_Assembly_Jig” sets the position of the drive discs 180 degrees apart on the ends of the drive shafts and positions the drive gears at the correct orientation for the front and rear axles.  Slide one of the thicker (9mm wide) gears onto the center of a 66mm long dowel.  Apply super glue to each end of the dowel and slide a drive disc onto each end with the M3 nut toward the center.  Press the axle into the jig with a drive disc cupped into the two end pockets and TWO teeth from the gear inside the bottom square hole.  When the super glue for the drive discs has set (polymerized), slide the gear to one side, coat the axle with super glue and slide the gear back to the other side into the glue.  When assembling the other axle, glue the gear into the opposite side.  The axle should not protrude from the drive discs.  If it does, trim it off flush.

Thread two of the M4 nuts onto the M4 motor shaft up close to the gear box.  Add the 6mm thick gear and two more of the M4 nuts.  Now you can cut off the excess M4 shaft length at about 32mm from the gear box with a hack saw, grinder or a file.  This can mess up the threads so, that’s why the nuts and gear were installed first.  Now put a wrench on the two nuts next to the gear box and tighten the nut on the other side to clamp the gear so it can’t turn.  When the clamping nuts are tight, jam the outside nuts against the inside nuts so they can’t loosen.

Take the frame apart and install the motor with its drive gear in the center and the axles on the outside.  Set one axle so a drive disc points up while the other axle drive disc points down on each side of the frame.  You may need to manually turn the motor gear for everything to line up. This is important, if the two axles aren’t 180 degrees out of phase, the mechanism will jam when finally assembled.

Note: the five splines sticking out of the bottom frame are for attaching the middle finger and they indicate the front of the hand. Select a crank with the splines on both sides of the center pivot.  Insert an M3x16mm screw through a bushing.  A daub of grease or Vaseline petroleum jelly on the outside of the bushing can reduce drag.  Insert the bushing and screw through this crank.  This screws into the right front drive disc.  (If the screw doesn’t catch the M3 nut in the nut trap it can be pulled back into place using an M3x10 screw.)  A matching crank, bushing and screw attaches to the left rear drive disc.  The other two cranks, with the splines on one side of the center pivot, attach to the other drive discs.  When installed the cranks should rotate freely.  Note: the assembly photo's for the direction of the splines where the fingers will attach .

Turn the right front and left rear cranks so the three splines are pointing up on the top of the frame.  Apply super glue to the splines and glue on the top crossbar. Make sure the joints are square before the glue polymerizes, then repeat with the bottom crossbar.  Note: that the nuts in the traps for the battery clamp face toward the bottom of the frame.

Insert the AAA batteries into the battery holder and clamp it in place on the bottom crossbar using the battery clamp and two M3x10 screws.  The switch should be in back facing down. Slide the battery holder to the front since it helps with the walking balance.  Thread the wires through the round wire holder at the back of the bottom frame and route them around to the motor.  The RED wire solders to the (+) terminal on the motor and the black wire solders to the other terminal.  Momentarily turn on the switch to make sure everything rotates smoothly.

Glue the spline into the slot in the end of the middle finger.  When the glue has set, glue the middle finger into the spline at the front of the bottom frame with the finger curving down (toward the battery side).  The first and ring fingers are mirror copies.  They glue into the splines on the front cranks (curving down).  The thumb and little finger glue into the splines on the rear cranks.  If the thumb and little finger are pointing backwards it helps the balance (Note: the difference in the 15 RPM and 30 RPM walking clip, the thumb and little finger in the 30RPM hand point forward, while the 15RPM hand has them pointing backwards).  When gluing on the fingers make sure no glue gets into the rotating joints!!!!

Wrapping the Mummy’s Hand

Cut strips of the muslin about ¾” (19mm) wide for wrapping the mummy’s fingers.  Put a drop of super glue on the inside of a fingertip and hold the end of the cloth strip into the glue.  Then put a drop of water on the spot where the glue has wet the cloth.  This immediately polymerizes (hardens) the glue. Spiral wrap the finger and tack the end of the strip to the base of the finger with another drop of superglue.

Wrap a strip of muslin about 2” (50mm) wide sideways around the hand after tacking it onto the top cross piece.  Leave it loose enough for the walking mechanism to operate.  Tack the end to the cross piece after testing the hand’s operation.

Finally a 2” (50mm) wide strip is tacked to the top cross piece and wrapped length ways around the hand between the fingers.  The end isn’t tacked down for easy removal when changing batteries and drags behind the hand as it walks.

Enjoy!  Post a picture when you build one.

A mummy’s hand crawling around inside the candy bowl could make a lot of excitement when trick-or-treat visitors come to the door on Halloween eve.