Hello! I am fairly new to Rhino so please bear with me. I have created a SubD model for a furniture piece I need to fabricate. The final piece will me made out of cast cement, so I need to make a mold of this model. To do so, I want to CNC mill the model out of high density foam like the 1" thick Pink Panther insulation foam sold at Home Depot. There is a local company who will work with me to mill the foam, but they only have a 3-axis mill, and not a 5-axis mill. My current model is a 1"thick offest SubD, and when I split this into 1" sections along the z-axis the resulting parts have undercuts which the 3-axis mill canāt handle. So, I think I need to sacrifice the inside contours of my model to create parts without undercuts, which is ok with me if need be. The modelās outside surface contour is whatās most important for my mold-making. My question is how can I create the parts I need for the CNC mill from my SubD model? I have tried several attempts and canāt figure it out. Any help is much appreciated!
Earthstar_1_For Mill.3dm (19.2 MB)
Earthstar_Sections.3dm (10.4 MB)
Example:
Earthstar_1_For Mill.3dm (9.9 MB)
with 3 axis stuff, you can leave tabs on the part connecting it to a āframeā and then simply flip it and run the other sideā¦ takes more time than a 5 axis, but totally doable. I used to do very complex stuff (characters, cars, etc) on a 3 axis when I was doing toy stuff.
thanks for taking the time! I could see this being useful as a ready to pour one-time mold milled from foam. The sections would need to be cut from both sides by flipping the workpiece as @theoutside mentions to deal with undercuts on a 3-axis mill.
Iām looking to get the inverse of your example model, and eliminate any undercuts. I think it requires filling in the inner volume of my model, making a solid rectangle tangent to the outer SubD surface to fill the inside of that surfaceās volume. Then, when sliced into sections Iāll have the outer SubD surface contour, but where the legs end to expose an inner void, there will be a flat surface filling that void instead. I can cut out the arch areas that are now filled after the sections are milled. Thatās not a big deal. I just donāt know how to get my model to that point without doing a bunch of rebuilding part-by-part.
I hope this makes sense.
you can make a negative by boolean subtracting it from a cube, you just need to add a small pipe to connect it to the "outside of the cube, so the boolean can figure out what to do- see image
you may decide that slices are not the best way to split this mold, you may want to split it in 3rds and make a core that pulls out the bottomā¦ donāt limit yourself to simple flat slab thinking, a multi part mold with a parting line that travels along the outer edge of your part may be the way to go.
once you get that designed, you can make the mold out of flat slabs and glue it up.
Example:
Make a hexagonal blank for the external shape of the part, assembled on a threaded rod, and process it in 6 positions on a 3-axis milling machine.
After that, cut the resulting part along Z, into a layer of 2 inches, on the resulting tiles you can process the internal shape without undercutting, glue the tiles together and you will get a finished form for pouring.
Use an āundercutting cutterā and CAM that can enable āundercutsā in 3-axis.
https://www.harveytool.com/products/specialty-profiles/undercutting-end-mills/300-degrees
Smart! I think this could work. Iāve massaged the SubD to eliminate undercuts from the original model. Itās probably a better design now for it.
And I have decided to cut the foam as a one-time mold as you all are thinking, instead of a āpositiveā replica of my model as I originally intended.
Part of my trouble with this is working with the CNC tech. They are a FAB Lab tech at a University that offers the lab equipment for hire. But they arenāt offering me much flexibility in terms of ways to cut my parts. There arenāt any other CNC mills in my area to work with.
The most recent file I sent them is attached here: āsections.ā The tech says they want polysurfaces for 3D mills only, and all 2D cuts as curves. Hopefully this works.
I went with what you and Kyle were suggesting at first.
I will suggest your hexagonal approach if this doesnāt work. Thanks so much!
Earthstar_V2.3dm (18.7 MB)
Earthstar_Sections_Rhino5_v2.3dm (6.1 MB)
Good to know. Thank you!
Thanks Kyle! I went with what you and Inju are suggesting. See details in my reply to Injuās hexagonal approach. I took the time to massage the subD to eliminate undercuts. The design is probably better for it.