Week 7: Computer Controlled Machining
Group Assignment (link): 1) do your lab's safety training 2) test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine
Individual Assignment: make (design+mill+assemble) something big (~meter-scale) extra credit: don't use fasteners or glue and include curved surfaces
What is a CNC Machine?
CNC stands for Computer Numerical Control. A CNC machine is a type of machine that can cut different material according to specific computational directions given to it by a computer program. Technically, both laser cutters and 3D printers may be considered as CNC machines as well since they also operated and move according to numerical controls determined by software. Less formally, CNC machines generally refer to machines that are larger and cutting wood or other materials with drill bits.
The CNC Machine at Fab Lab Kerala:
Here is a picture of the large CNC Machine (with computer). It's called a Shop Bot.
Here's another view also showing the computer with software.
Behind the computer are three switches, to control the power for:
- The machine movement in 3 directions
- The spindle, which holds the bit which cuts the material
- The vacuum or dust collector
We'll say more about our machine and other things in the group assignment page for this week.
Design & Laser-Cut Prototype
Back to Fusion 360...
Here is first draft of my project, which is also related to my final project:
Only problem is that parameter for thickness doesn't work well:
So I went back and did the project again. This time I was very careful with the parameters from the beginning:
All the measurements defined in terms of parameters and all sketches constrained:
Here is the final version. I can change any parameters and nothing bad happens.
Now it's time to export. We arrange and export the sketch as '.dxf'. In regards to spacing we ensure there is space for the tool head. As we will be using a 6 mm bit, then we leave a spacing of 15 mm (6+6+3), 6 for each side plus an additional 3 mm as a buffer.
Few important notes:
- To find 'Arrange' go to 'Solid'->'Modify'->'Arrange'
- Make sure only the components you want to arrange are selected. (if you drag box over the area you may inadvertly include edges or other)
- The arrange envelope has 2 tabs: object and envelope. Envelope creates the space; make sure its dimensions can include all components.
- To export, right click on the arrangement in the browser, and select 'Export..' from there.
Now we are cutting in the laser cutter as a test before cutting a full scale version in plywood.
Here is the assembled model from cardboard. Actually it fits pretty well. It is 1/4 scale. Good job.
Just a few issues:
- Forgot to include kerf on the vertical edes. Kerf need to be everywhere!
- The mouth is fine for laser-cutting, but for CNC will not be able to cut the sharp corners.
- The door on the back will be a problem.
Redesign & Kerf-Bending
First we redesign the mouth. The top corners with circles, like an exaggerated dog-bone.
Next we consider to use kerf-bending to make the back door, as in the following example from Sreyas:
This kerf-bending technique cuts entirely through the wood, in contrast to some techniques that cut a groove 90% or so through the wood.
Here's my design:
CNC Cutting
Setting up for printing: we screw the board to the sacrificial layer and calibrate in all 3 directions, positing the spindle with bit in the corner:
We will use the software "Vcarve" a.k.a. "Vectric":
Here's an example, if we wanted to cut a heptagon:
And here's the 3D view for that example project:
Always before starting to cut, we'll make an air rectangle around the border to ensure that we don't go over the screws.
To cut our project we first import in the .dxf format which we exported from Fusion.
Here we can see the cutting process. We are using a single flute 6mm endmill cutter, with a feedrate of 20mm/second.
The endmill cutter actually passed through each part 3 times, each time deeper than before.
We set a depth of 4mm for cutting, and as the board is 12 mm thick that's why it required 3 passes.
Sanding and Processing
Cutting tabs: it was easier with the chisel and hammer than with the knife. But the tabs generally worked well.
Sanding: We used a power sander, but for the small places that didn't work so we cut sandpaper to fit on a small handheld 3D-printed holder.
Fitting it together and final result: it fit perfectly, just a bit tight but with a hammer it forced in and cannot easily come out.
Hero Shots + Analysis
The CNC week gave the perfect opportunity to make a prototype of the structure for final project. This is the first photo with myself and my replacement. It's not completely bad.
Pros:
- Very strong.
- Mouth is sufficiently big to dispense candy.
Cons:
- Very blocky; better to have a rounded head.
- Eyes may be too big for the camera.
- Kerfed door is nice, but not secure.
- Rico thinks the mouth is creepy?