The Asymmetric Spinner

Overview

Asymmetric spinners are pretty great because they allow you to get the most bite for your buck. Symmetric weapons will, by their nature, have at least 2 impactors. This means that for every 1 rotation, your impactor will [ideally] hit twice. This isn't great if you want to maximize all of your power into one, deep-cutting hit. There's a lot of other advantages (or disadvantages) too, but I won't get into that yet.

Ironically, the disc I made in this example doesn't have much bite, but is designed for more consistent, controllable impacts, while minimizing risks of getting caught on the floor with a low-profile impacting "tooth."

Low-profile teeth and deep-cutting scythes are both legitimate approaches. Respectively, they're comparable to short, controlled jabs, and wild roundhouse punches.

Here's how you do it...

Forgive me for skipping a few steps. At this point, I have:

Created a sketch
Created a circular construct
Created an arc with a "tooth" protruding for my impactor
Created a hub-mounting pattern (don't worry about making this yet... come up with a shape first.)

What I want to do is counterbalance the tooth such that the center of mass is at the center of the hub pattern I made. I can do this by:

Removing mass on or near the impactor
Adding mass diametrically opposed to the tooth
Moving the hub nearer to the measured center of mass

As it turns out, that last one is pretty easy, especially if you do a simple trigonometry trick.

I have now removed the hub-mounting pattern. Observe the "center of mass" X and Y coordinates. arctan(0.194/0.222) = 41.1493 degrees. That means that, relative to the X-axis, my center of mass exists somewhere along a line at 41.1493 degrees. Let me put that in my sketch.

And now I can create my hub pattern, at an undetermined point along this line.

Note that the dimensions I have chosen are entirely arbitrary.

And since I know the coordinates of the center of mass relative to the origin, I can find the length of the hypotenuse, which I can then use as a dimension to constrain my hub-mounting pattern.

And since the hub pattern is radially symmetric about the center of mass, I've pinned the proverbial tail on the proverbial donkey.

The next step is to do some weight reduction. So long as the changes you make are symmetric about all axes in the plane you're working in, the center of mass shouldn't change. Use that weight reduction to hit your target MOI, and you're golden. Just don't thin it out too much, or else you run the risk of your weapon shattering like my heart when she told me I'm not husband material.

This isn't the best use of your space, but to prove my point, you could do a pattern like this and keep the center of mass the same. Just don't forget to fillet your corners for machining.

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