Matching 3D Printed Part to RC Servo Horn

I recently got a 3D printer, and want to make robotic mechanisms, using mainly RC servo motors for motion. So the first challenge for me was how to reliably "interface" a servo to a 3D printed part. I scoured the web, and could not find anything about it. So I decided to find my own solution.

It took quite a few experiments before I arrived at my solution. At first I tried making a hole in my 3D part that would form an interference fit with the servo's nylon spline. The was clearly not going to work - only a small amount or torque was enough to break the friction fit and allow it to spin..

Then I tried the hole idea, plus using ABS slurry to try and soften the part and mould it to the spline. That failed as well.

So, while I was trying to avoid using the servo's horn (the white part in the picture above), I finally decided I had to. The horn supplied with the servo is the only part that interfaces reliably to the spline. I was going to have to interface to the horn, not directly to the spline.

So here's how I did it.

I am using Autocad Inventor Student edition. I started by carefully measuring the horn. I can't guarantee all servos come with identical horns. In fact, I can guarantee they don't. You will have to measure your own, so for a good result be prepared to do a few test prints. A digital calliper is handy for this work, even a cheap one from China.

The picture above shows my design sketch. The part is 3.7mm thick. It has a 2mm deep recess for the "arm" of the horn, and a through hole for the cylindrical section that fits on the servo spline. The recess is formed from two circles on 14.2mm centres. The large one is 7.5mm and the small one is 4.8mm. I made the whole thing 12mm wide to give 2mm of meat around the large hole.

The part was printed in the orientation shown in the picture below, i.e. the lower surface in the photo on the build plate. You can see a hint of the red ABS juice I use on the build plate. Printing in ABS, you tend to get a the first layer or two spreading out, and higher layers shrinking. This gives a bit of a rim around the bottom edge. I find adding a 1mm chamfer to the outer bottom edge compensates quite well. For the through hole I used a 0.5mm chamfer - when I found the best diameter to use I wound up with essentially a snap fit between the horn and my part.

Be prepared to do a few test pieces. Unless you have a $50,000 printer, it won't be deadly accurate, and your design will need to be adjusted to get a good result. Using a CAD package like Inventor is an advantage at this stage; it allows you to go back and adjust single dimensions without re-doing the whole drawing.

Print out a sample and see how well the horn fits into it. Ideally you will get a friction fit with the through hole. That will eliminate radial play. Getting a tight fit to the arm or the horn is not as important because we will later glue the arm in place with a void filling glue.

Do the test with a small part to save time and material. You can add whatever the servo is actually turning later. (The picture above does have bits of the works. By the time I wrote this instructable my test pieces had gone to the ABS glue jar).

The horn gets glued into the printed part. I use a thick slurry made by dissolving scrap ABS in acetone. In fact, I keep two jars. One is thick and sludgy for use as a filling glue, the other is thin "ABS juice" for use on my print bed.

Caution: Acetone is nasty stuff. It stinks the house out, which really ticks of spouses or parents, it is not particularly healthy to breath, and it is highly flammable. Use it in a well ventilated area or outdoors. I keep my bulk container in the garden shed, just in case of fire.

If you are printing in PLA I'd suggest using a void filling adhesive such as Araldite (epoxy) or perhaps hot melt glue. The horn is made (I think) of nylon, which doesn't glue easily (comments welcome!), so it's really a matter of filling up the space.

1. Test fit the horn to your part and remove again
2. Use a tooth pick to liberally fill the recess with glue. Keep the glue away from the through hole.
3. Insert the horn. It's good if the glue pops up through the tiny holes in the horn arm. It's bad if you get glue in the hole in the horn that fits the servo spline.
4. Cover the horn arm with glue.
5. Leave it all to dry/set thoroughly. Set it in a warm well ventilated place and do something else for a couple of hours.

Picture: "Here's one I prepared earlier" :-)

Once the glue is well set, you can fit the assembly to your servo. That involves pressing the horn onto the spline. Press on the horn, not on your 3D part. It can take quite a bit of force to get the horn onto the spline, and you don't want to rely on the strength of your glue to withstand that much force.

Happy printing - and servoing!

BTW: The microcontroller in the background is a SPLat EC1. I co-designed the SPLat system in 1995. The EC1 can control 3 servos with stupidly easy programming, much easier than Arduino. Example

If you think an instructable on a basic technique is worthy of your vote, please vote for this in the 3D printing contest (The orange Vote! button top right on this page). TIA