How to Cut 5.25” Circle in 8” Steel Sphere

Question

I am working on a project where I need to mount a speaker inside a pre-fabricated 8” steel sphere. The cutout diameter for my speaker is 5.25”.

What is the cleanest way to cut the 5.25” hole in to the 8” steel sphere?

I have access to a drill press, a Dremel, a hand drill, and some files. Open to buying other tools for the project if they’re not super expensive, but don’t really have access to any water jet / laser cutting thing.

Here are my steel sphere and speaker components for reference. Thanks!

https://www.kingmetals.com/Mobile/Catalog/ItemContent.aspx?ItemNumber=2338

https://www.parts-express.com/Tang-Band-W5-1138SMF-5-1-4-Paper-Cone-Subwoofer-Speaker-264-917?quantity=1

Answer 1

Two alternatives:

1. Hole saw.

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   They will look generally like this, but the cutting edge may vary depending on what is required to cut the steel (how hard it is). You will probably be fine with a hole saw rated for steel pipe. 5.25" is a commonly available size in bi-metal hole saws. It needs a cutting depth of more than 1-1/8" because the curvature of the sphere will put the pilot hole 1" inside the hole saw before it contacts the surface to be cut, and the sphere wall is about 1/8" thick.

2. Rotary tool circle cutting jig. You might need to make one for this task. It's an arm with a mounting attachment for the rotary tool at one end, and an adjustable pivot point. You can buy these for cutting circles on flat material; the arm is usually rigid. For this task, you need an arm that will bend around the sphere so that the cutting bit can do its work close to the collet. Because of the curvature, you would need to set the radius roughly 1/4" longer.