Iron MSE Horn Wiggle Mechanism

In the summer of 2023 I built a mashup of a life-sized MSE-6 Mouse Droid and the VTuber Ironmouse (possibly also life-sized), giving her the name Iron MSE.  The base of the droid is my customized version of Michael Baddeley's printed mouse droid files (available from his Patreon).  She debuted at Dragon Con 2023.

The Mouse Droid Builders Club forum (which requires a login) hosts the build thread for this droid – further information on the use of these parts in my droid can be found there.

Cosmetic parts

• Horn Base and Horn Tip.  Need 2 each.  Mine are printed in white TPU.  I glued them together and covered the seam with a ribbon.  Both parts are keyed and are ultimately press fit onto the Horn Pivot Plates via the Horn Key.  You can glue the Horn Base to the plate if you want, but I didn't as they stay on pretty well.

Structural Parts

• Horn Pivot Mount.  These attach to the sides of the droid in cutouts I made in the part before printing, using screws in each corner.  Need 2 of these.

Mechanical Parts

• Horn Pivot Platform.  Attached to the Horn Pivot Mount.  Need 2.
• Horn Key. Orients the horn correctly.  I just use a friction fit for the horn base
• Linkage - Connects the servo to the Horn Pivot Platform

Assembly

Each servo linkage's assembly begins with a large paperclip, straightened except for a 90 degree bend near the Linkage end, forming an L.  

After installation, the long leg of the L must be long enough to extend past the centerline of the droid to reach the servo horn, as you'll see shortly.  Mine extend around 2 inches past the end of the plastic linkage, and that's more than enough.  (The upper linkage may or may not need to be trimmed a bit shorter than the lower one after installation, to avoid excess paperclip hitting the roof of the droid's shell.  Don't trim it until after installation, as you might need some excess length for adjustments)

The short leg of the L goes through a small hole through the height of the plastic Linkage bar (which you might have to drill out so the paperclip will fit) located on the squared end.  It is glued in place with CA glue.  Bend any paperclip that sticks out the opposite side 90 degrees again, so it's parallel with the plastic linkage (forming the paperclip into a sort of S shape as shown on the upper Linkage in the photo above).  Then place a short section of heat shrink tubing around the connection between the plastic linkage and the paperclip to secure it, and shrink it down.

For each of the two linkages you'll need a swivel standoff compatible with your servo horn, as it screws into the servo horn, and compatible with your paperclip, as the paperclip passes through the swivel.  This swivel part allows the linkage to compensate for the servo horn's rotation.  The parts I used were Hobbypark D1.3mm Adjustable Pushrod Connector Linkage Stoppers 

On either side of the swivel are two halves of a spring from a cheap ballpoint pen. These try to keep the swivel centered on a particular point, while also allowing the linkage to absorb the force of your droid's horns getting pushed or pulled.  The endpoints of the springs are set by two retainers per linkage, which each have tiny grub screws in them to lock onto the paperclip.  The parts I used are Du-Bro 595 1/16" Plated Brass Dura-Collars

Before proceeding, push a Horn Key into each Horn Pivot Plate (so the key protrudes from the flat side of the part).  Once you get a good fit, glue it into place.  (Note: The next two photos show an older test model for the Horn Pivot Plate; the hole for the Horn Key is upside down.  You can see the correct part in the final assembly photo.)

The large hole at the round end of the linkage attaches to the Horn Pivot Platform with an M3 screw.  The goal is that the linkage is able to rotate around the screw.  If the screw is tight when threaded into the plastic then you probably don't actually need a nut on this screw, but if you use one, use a nylon locknut or some threadlock (so it doesn't vibrate off) and don't over tighten it.

The Horn Pivot platform attaches to the Horn Pivot Plate with two shorter M3 screws that also act as pivots.   Those screws are inserted from the outside edges of the Horn Pivot Mounts and into the two posts on the sides.  

Both screws must be installed before the Horn Pivot Mounts are attached to the droid, as the droid's shell blocks these screws.  No nuts are needed here for the same reason, though I left space for some anyway.

The process of adjusting the linkages is pretty straightforward.  Use a servo tester to center the servo while the retainers are loose, then put the (exterior) horn at roughly in the middle of its travel (you may require an assistant or tape to secure it there temporarily).  Move the retainer furthest from the plastic linkage bar toward the swivel so that the spring between then is under a small amount of compression, and secure the grub screw.  Then move the other retainer toward the swivel until that spring is also under roughly the same amount of compression, and secure its grub screw as well.

The photo to the right shows the final assembly in my droid.  The camera was looking into the droid's top shell, with the front of the droid toward the top of the image.  You can see how the Horn Pivot Mounts are attached to the walls here (I left little tabs in the cutouts I made in the shell, and the screws in the corners go into them)

Take careful note: the Horn Pivot Plates are arranged so the Linkages are attached ABOVE the pair of M3 screws connecting each plate to its Horn Pivot Mounts, i.e. nearest the top of the droid -- not below those screws.  This is so the mechanism pulls the plate inward to lift the weight of the horns, rather than pushing it outward to do that. This way gravity helps rather than fights the movement.  In the above photo, the mechanism is pulled in slightly, lifting the exterior horns.

The principle of this mechanism is that by attaching the linkages on opposite ends of the servo horn, rotating the servo one way pulls both linkages in, and rotating it the other way pushes them both back out.

The idea is for the axis of the servo motor to fall more or less directly in between the screws that attach the Linkages to the Horn Pivot Plates so each Linkage is roughly the same length.  This helps keep the horns in sync.