So... I recently wanted to mill something that was 11 inches in height, but only had 3-inch-long endmills available.
This is how I did it. There’s no magic, but a couple of tips and tricks that might be useful.
Spoiler Alert: I ended up milling 4 separate pieces and gluing them back together.
Read this as a how-to, or as an intro to CAM... I'm mostly interested in documenting it now so I don't forget how I did this stuff.
I had a design I wanted to mill out of MDF, but it was taller than the endmills I had available.
First thing I did was calculate how many sections I needed to split my model into, and roughly how tall those sections should be. Given that my model was about 11 inches tall (280mm), and my longest ball nose endmill was 3 inches long, I divided my model into 4 sections of roughly 2.77 inches each.
Before splitting the model, I added some registering holes through the whole thing, so I could easily put the final piece back together using some support wooden dowels. I used 5 holes, 3 that go all the way through, and 2 that only go through 3/4 of my design.
The diameter of the holes were selected based on the drill bits available. I initially wanted 1/8” holes, but didn’t have any 1/8” bits that were longer than 2 inches. I ended up using a 7/16” bit because those are pretty long, and I had 7/16” wood dowels available.
Actually splitting the model should be easy to do in any CAD tool. I have my design in Fusion 360, and breaking up a solid body can be done using offset planes.
Planes can be constructed by using the Construct -> Offset Plane menu option, and entering the offset amount. Once those planes are created, the Modify -> Split Body tool can be used to actually split the body into its 4 shallower parts.
I then aligned all the pieces vertically (using the Modify -> Align tool), and arranged them in a way that seemed area-efficient. The overall area for my model is about 730mm x 500mm, or 29” x 20”.
The model should now be ready for CAM’ing!