Looking for some brilliant ideas on how to go about carving a 3 foot foam sphere.
This is for a project I am planning to work on next year and want to get started with planning as there are many aspects I need to learn. one of which is the foam sphere. I don’t want to share what it is as I don’t 'want to distract from this topic. I might later though.
The base of it would most likely start with foam rings stacked up. but how to carve it down to a sphere. the inside does not need to be clean, but hollow.
things I’ve thought of all ready.
1 - use a rasp and eye ball it. probably won’t end up as a good sphere.
2 - use a rasp along with a template / gauge of some sort so I can check it as I go.
3 - build a router sled. something like the image below but on a radius from top to bottom. rotate the sphere as I go. I see thing getting complicated quick.
Slice the sphere as an stl then export each slice and cut that on the cnc, glue it then rasp the final… Or ball nose bit the outside as a carve and then glue and rasp. Just some ideas.
Thanks. which make sense on a CNC forum.
not sure why I didn’t think of it.
I wonder how far I could take that. I could carve the portion of radius on the outside edge for each ring too.
Would also work to have some way of keying them together so they are aligned as much as possible. would reduce the amount of hand sanding.
Almost Friday, so… What kind of fidelity/resolution do you need? Already considered extruding via spray or foam beads/aggregate plus binder? Seems to work for concrete.
If slicing hollow sphere into layers, then something like deepnest.io will help with efficient layout and reduce waste. Under cuts will be PITA but sounds like uncut undercuts can be hidden inside the sphere cavity?
Indeed, I used prusaslicer to cut it into layers (with the “cut” button in the main toolbar on the left side), this also allows you to add connectors at the same time.
You can then export this back to stl.
I did notice that while the extending part of the connector exported fine, the holes went missing (you also get a boolean operation warning from prusaslicer at the time of export).
Sorry, my native is not english
make for each square a stl. (just for visability I made the final result blue)
first hollow out the square, then turn the square over and mill the outside,
do this for all the squares and glue the parts of the spheres together. for a full sphere do it twice and glue the 2 halfs together. make the thickness off the squares the max your mill can handle.
ready
Thinking of adapting a lathe tool to this activity. Ball turning Jig - Like the Perfect Sphere tool. It assumes the stock is being spun along one axis, so the cutter is in a pivot arm that moves the cutter in an arc 90 degrees from the axis of rotation.
Creative idea. though I would need better resolution than that.
correct.
dowels would be a good idea for keying.
If i use the CNC I’m thinking cutting half circle rings that would nest together better (maybe). Carve the outside of the rings for the radius of the sphere at that ring.
Each ring could be keyed to set on top with the seams 90 degrees to the bottom. So each ring would be like bricks.
This is how a made a similar project. Just milled it out 7,5 cm in 10 cm of styrofoam and cut the rest away with the hotwire. The 7.5 cm is because my routerbit was not longer
By now you’ve probably started cutting in 70mm layers!
If you are chasing a nice finish - why not make a concave sanding disk to match the diameter of the sphere? Foam is easy to sand, keeping a perfect radius on a sphere not so much.
It’s difficult to see how these work from the pictures because the radii for instrument work is huge, but this is what I mean.
I really think the best use of a CNC for this is to make a jig.
The idea of directly carving slices and gluing them together slightly horrifies me. Lots of chances to get that really wrong.
To me the questions are:
Is this a one time thing? Will you only ever need the one size, or one sphere? If you need more than one, will they need to be the same size, or is some variation desired/acceptable?
How thick does the shell need to be?
what kind of foam? If it’s the kind made up from those tiny little balls pressed together, that suggests a different kind of cutting tool…
The idea of the mess made by hogging out large amounts of foam is frightening. I would be finding bits of foam in my shop for years.
I’ve looked through and it seems like there are still outstanding questions of how accurate you need it, how thick does the wall need to be and how many will you make?
Another option would be something like this:
Scaled up it’s going to be pretty hilarious but given that it’s foam, I’d give it a shot! I made one for doing spheres up to ~200mm OD that references the 20mm dog holes on my workbench for easy setup and it’s pretty compact. Scaling that by a factor of 5 would get you something that’s definitely oddball looking but shouldn’t take any longer to actually construct than the small one I threw together in an hour out of scrap. Given that it’s only cutting foam you could probably mow through the cutting pretty quickly as well. I’ve since updated mine to be driven from a stepper motor via a GT2 belt and an Aliexpress adjustable speed driver.
For doing the hollowing out the setup gets reversed and the pivot is placed ‘behind’ the router instead of in front.
That’d get you 2 hemispheres that are damn near perfectly spherical on the outside and inside, although maybe not with perfectly uniform wall thickness unless you’re super careful about getting the setup dialed in when switching from carving the outside to the inside.
This also assumes you’re starting with a solid and carving out, I don’t think it’d work for rounding off a stack of rings.
Edit: @SupraGuy Lol, apparently we were thinking the same thing at the same time.
Edit: Another thought, does it have to be carved out of a block of foam or could it be something like a 3D printed mould that gets filled with spray foam?
It’s not about cnc, but the intention is to make a sphere.
Often we can think too hard about using a machine, when there is another solution, so it’s ok to make conversation like this even if it’s not about CNC.
or a CNC form in the shape of a hexagon (think soccer ball) that can be used to mould heated foam shapes and then glue assembled - I think we are getting somewhere!
