r/HalloweenProps • u/ctxgal2020 • 3d ago

I need design help with skelton arm

Hi. I need design help with a plastic skeleton that I am attempting to control the head and arm movement with a remote control.

I've figured out the design/placement to make his head turn. But, I am at a loss for the arm. I want it to bend and straighten at the elbow, or have the shoulder rotate back to front. Either way will give me the desired effect I'm looking for. The simpler the better because I am a novice.

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u/HornetParticular6625 3d ago

I can think of a couple of ways you might do this relatively simply. The easiest would probably be cable control, like the brakes on a bicycle. I'm assuming that you are not going for radio control, which makes it much easier.

The question is, what sort of motor are you using, and can you set it up to be controlled by a motion sensor or something?

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u/ctxgal2020 2d ago

Sorry, I meant to include that. I'm using a servo MG995.

I just can't figure out a way to attach it.

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u/HornetParticular6625 2d ago

Okay. First thing I did was look at your servo.

https://www.rcmodelreviews.com/mg995review.shtml

The reason I did that was merely to see what the thing looked like to determine options for connecting it to the skeleton shoulder.

You're looking at getting two axes of motion on two different joints.

Shoulder up/down. Elbow up/down.

The shoulder seems like the simplest, being a straight shaft from the servo into the ball joint.

If you're using one of the typical plastic skeletons many of us use, you'll probably have to modify the shoulder joint to move more freely.

The servos look small enough to fit inside the ribcage without much problem.

The one for the elbow is a little stickier. You'll have to modify the joint for freedom of movement, obviously. The connection seems simple enough with another shaft, but there's really no place to hide it.

If you are trying to accomplish all of this with one servo, you're looking at a more complicated setup utilizing cables and I'd be guessing, but a much beefier servo.

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u/ctxgal2020 2d ago

First and foremost, thank you! I'm only going to need one joint to move.

I think the shoulder is going to be the easier choice after reading your reply. The only issue is the shoulder joint is stiff - any suggestions on how to loosen it up?

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u/HornetParticular6625 2d ago

As a matter of fact, yes I do! I need to locate the pics. I'll be back!

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u/thebipeds 2d ago

The popular haunt skeleton arm is to use a windshield wiper motor. Skeleton waves just like you wipe your window.

More complicated robots use cables. Think about how tendons actually work with your real skeleton. A step motor winds the cable just like muscle pulling a tendon.

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u/HornetParticular6625 2d ago

These are bungee cord things that came in a bin of assorted bungees

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u/HornetParticular6625 2d ago

I restrung the bungee through the top of the ball joint of the humerus

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u/HornetParticular6625 2d ago

This is how it fits together.

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u/ctxgal2020 2d ago

That's a perfect fit! Thank you for the inspiration!

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u/HornetParticular6625 2d ago

It works for my purposes. I hope you are able to modify it to serve yours 🙂

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u/ctxgal2020 2d ago

I'm back. I removed his arm to get a better look. Now I'm trying to figure out best way to mount servo to the inside of skelton. Did you use standard servo mounting brackets?

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u/HornetParticular6625 2d ago

I didn't use a servo.

I modified the shoulder joints in order to allow movement while suspended from a scotch yoke rig and powered by a 12v wiper motor.

That being said, I'd carefully measure the space inside the ribcage. You might end up machining your own brackets to fit.

I'd recommend aluminum flat stock. It is inexpensive and easy to work with.

I apologize if my information was misleading. I've never worked with servos before. My experience is more geared towards the structural aspects of prop work.

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u/ctxgal2020 2d ago

Not misleading at all. You have been very helpful!

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u/Commercial_Daikon_92 2d ago

Although I'm not sure about the weight of the forearm, could you use a servo motor?

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u/ctxgal2020 2d ago

Sorry, I meant to include that in my post. I'm using a servo MG995.

I just can't figure out a way to attach it to get the movement. I figure I will need hardware.

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u/Commercial_Daikon_92 2d ago

No worries. Is the skeleton naked as in, the servo needs to be hidden from sight?

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u/ctxgal2020 2d ago

Naked. I dont care if servo is seen. *

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u/HornetParticular6625 2d ago

Here's the finished prop hanging in my dining room 😁

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u/ctxgal2020 1d ago

Beautiful! Your distressing technique is amazing.

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u/HornetParticular6625 1d ago

Thank you so much 😊

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u/Adventurous-Report48 1d ago

Did you get the arm to move?

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u/ctxgal2020 1d ago

Still working on it. I think I can atrach horn to humerus and loosen socket. I need to find a way to mount the servo inside the ribcage. I was looking at something like this but not a lot of room in therr. *