I have managed it by drilling a 5mm through axis of a Lego gear then push fit and retain with the horn screw. It works. Job done

Here are a few suggestions:

Adapters: Consider looking for or 3D printing adapters specifically designed to fit the SG90 spline shaft on one side and the 5mm (or desired) shaft on the other side. There are online platforms where you might find designs created by others to address this issue. Websites like Thingiverse or GrabCAD often have user-contributed designs for various components.

Creative Use of Materials: If you have access to a 3D printer, you can design your custom adapter. Alternatively, you can explore creative solutions using materials like epoxy putty or moldable plastic. By molding a custom adapter around the spline shaft and the gear shaft, you can create a snug fit.

Craftsmanship: If you have basic tools, you can attempt to manually craft an adapter. For instance, you could take a small brass tube with an inner diameter of 5mm, cut a longitudinal slit, and then press or clamp it onto the 4.7mm spline. The slight difference in diameter might allow for a tight fit.

Local Workshops or Hackerspaces: If you're part of a makerspace or have access to local workshops or tool libraries, you might find tools and expertise to help you create a custom solution. People in such communities often have experience solving similar problems and might have innovative suggestions.

Online Communities: Consider posting your problem on forums or online communities related to robotics, DIY projects, or 3D printing. People in these communities are often passionate about problem-solving and might provide unique solutions or direct you to specific products or designs.

Source: Emirates ID Official- Latest Emirates ID Info & Services Hub

The SG90 servo motor features a shaft connector that allows easy attachment of external components. This connector enables precise motion control by converting electrical signals into rotational movement, making it widely used in robotics, RC vehicles, and automation projects for its compact size and affordability.

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I  am waiting for a collection of nylon gears to turn up and I will try a system of CA glue to a blank onto the spline.  I will first , have prepared a 5mm center hole, then try scoring a keyway down the id and roughing the inner surface.  At least it will provide some inner key and I will use the baking soda trick for cure .  Try some si oil on the spline so its removable See how durable this is?

We used laser cut plywood parts for the OttO Bot LC. But using the horns and attach them to the wooden parts worked better in the end for the ankles. The servo shaft is pretty fragile. A press-fit with wood only works well if pulled in place with the screw so no pressure on the shaft is needed.

Im waiting for some Lego gears to turn up so I can make a !:2 speed up set. The object being that I can turn a 180 deg movement into a "near" 360 deg turn so I can make a type of clock face indicator with pos ctrl (I dont look for full rotating action available in the other form of the sg series as it just behaves like a speed ctrl motor.

I really want to be able to use the common nylon gears from EB if I can crack to mech connection problem.

I wonder if using a different poti is a better solution for that.

FF  "I wonder if using a different poti is a better solution for that."  what is poti?

Poti is short for potentiometer. It is a variable resistor that measures the position of the servo. There are instructions on the web on how to remove the mechanical end stop and replace the poti so the servo can have a wider range of motion without gearing it up. Not sure if this is a route for you, though,  if you are not that much into electronics.

FF the conventional acronym for a potentiometer is a "pot".  Have seen a vid on the tear down of the sg90.  This pot when roughly selected for halfway 90deg position then ca glued is apparenly prone to drift and a prefered solution is to snip out the pot and replace it with two 5k resistors  AFAIK.

cancelled

mfz as i said  ---------  metal horns what you see on EB are T25 versions 25 teeth for the much larger 25kg motors also 5x cost.  too big for the sg90

RG Im amazed you managed to print the internal spline within a 4.6mm ID?  FYI an equivalent gear calc produces a 0.2 mod with a 4.6mm od and 21 teeth.  IMHO the PRC maker deliberately crippled the design (ie It could have had 22 teeth and an od of 5mm and the collar could have been 5mm long. Even the horns have a non standard collar ring of 6.7mm od it could have been 8mm for convenience.  Also you cant buy spare packs of horns even on Alibaba. There is no full cross piece for a horn, only a partial one, so trying to attached a gear to the shaft is still made difficult.  The plastic is nylon ABS so difficult to use adhesive.  Superglue may work? or some kind of HF welding?  This is my common experience with PRC stuff - deliberately crippled for no good reason other than to make things difficult.  Ive got a cunning plan which I will post in a week..

FWIW it would be great to produce a brass spline broaching tool to make the suitable hole for the motor. The idea is to heat up the broach with a little gas lighter or a soldering iron and punch it through a disc blank of thermoplastic with a 4mm pilot hole centre drilled.  Im sure that would work but I cant work to that level of watchmaking precision.  If you had a dividing head for 21 teeth you could make a brass rod of 4.6mm and scribe the splines notches horizontally with a vee shaped tool.  Drill a 2mm hole down centre axis for alignment than part off say 5mm piece and mount on suitable carrier rod say 8mm.  It can then be held in a drill press , heated to say 150C and plunged into a disc blank with 4mm center hole.  That would cause a revolution.  I would gladly pay for such a little tool

MFZ the sg90 spline have 21 teeth, what you see on EB are T25 versions 25 teeth for the much larger 25kg motors also 5x cost. FYI the sg90 is 4.6mm od with a 2mm collar length.

I've had success FDM 3d-printing parts that fit onto that splined shaft. Admittedly I had to use a 0.2mm nozzle to get anything approaching splines to be printed, but it worked.

Would attaching the gear/pulley/toothed belt pulley to the horns with the holes in the horns be viable? I think metal circular horns with 4 threaded holes can be found online.