Computer-Controlled Machining

“Make Something BIG” Week.

Assignments

Our tasks for this week are:

• Group assignment:
  • Do your lab’s safety training
  • Test runout, alignment, speeds, feeds, materials, and toolpaths for your machine
• Individual assignment: Make (design+mill+assemble) Something BIG (~meter-scale)
  • EC: Don’t use glue/fasteners

Group Assignment

The group assignment for this week is documented on our group webpage.

As a group we completed the safety training and followed the workflow for using the CNC (fixturing stock, changing the tool, zeroing). This included preparing the CAM files using Rhino CAM to make a cut test array to compare how different parameters changed the resulting cuts. For our 15mm thick plywood test stock, we varied spindle speed (18000 vs 20000 RPM), feed rate (3000-5000), cut style (climb vs conventional), and Down Cut vs Up Cut endmill. We included bridges/tabs in the file to prevent the pieces from flying out during the protocol.

Individual Assignment

I struggled this week with having grand aspirations for a project (a chair that could transform into a ladder? the frame of a skin on frame boat? a giant heart made with a waffle pattern? a waffle-grid Womb chair? A bench that made use of a living hinge for the curvature of the back rest?) but not actually having a lot of time between work commitments.

In the end, I decided to just keep it simple, so I made a stool. And I love it!

Design

Time was ticking so in the end I took inspiration from a few flat pack stool designs I found online and designed my own stool in Fusion 360.

My stool has three pieces: a seat and two legs pieces. I used 15mm thick plywood. It’s press fit construction, and I didn’t account for any kerf in my design because I wanted the joints to be quite snug (and they were! It’s perfect, where I can get them to slide together but they don’t come apart). I opted to make pockets to connect the base to the seat rather than cutting all the way through, and made use of the Boolean/combine tool in Fusion360 to make the interlocking joints.

Flat Packing the parts for CNC milling

Next I positioned the parts on the “stock”:

Adding Dogbones

I found a Fusion360 plugin to add Dogbones to the pieces (this is necessary in CNC-milled parts due to the tool having a radius).

Once up and running, it was easy to select the edges where I needed to incorporate dogbones so my pieces would sit nicely together.

CAM

Although Fusion does have its own CAM workflow, I was running out of time, would have needed to watch a lot of tutorials to figure out how to do it, and our lab has experts in RhinoCAM. So I exported my flat-packed design as a DXFs and imported into Rhino where I could take advantage of the RhinoCAM workflow.

RhinoCAM

Steps:

1. Set up the Stock Measure the actual dimensions of the stock you’ll use, especially the height.

   

2. Set up the Tool For this job, the same 6mm flat endmill can be used for each of the machining operations. We can set the feeds and speeds one time in the tool set up and then import them for each machining operation rather than needing to enter it three times. Feeds & Speeds are determined by the cutting tool and material being cut.

   

   

3. Add Machining Operations

   

3a. Screws: Engraving | In order to fixture the stock to the bed of the machine (both for safety and for accuracy), but in a way that won’t interfere with the cuts (if the highspeed spinning endmill made contact with a fixturing screw during the job, that would be Bad), we actually make a separate Machining Operation to Engrave where the screw holes should go. I added a bunch of points on the stock, distributed around the three pieces which would become my stool.

3b. Pocketing 2 1/2 Axis Pocketing | Since I have a section of my design which is pocketed (material is removed from the surface but not all the way through the thickness of the stock), I need a separate Machining Operation called Pocketing. I left the default settings for the most part but below I’ve included screenshots of the various settings with a brief description, as well highlighting a few changes I did make:

3c. Profile: 2 1/2 Axis Profiling | Outcuts come last! And importantly, with CNC milling you don’t want the endmill to remove ALL the material when you’re cutting the outline, or your piece which is no longer attached to the rest of the stock will get caught by the endmill and thrown from the machine bed. This is bad. We use a features called Bridges, where a small band of material at the bottom layer is retained to leave a bridge between the piece and the stock. That way once the operation is completely, you can go in manually with a chisel and hammer to sever the bridge and remove your part from the stock in a controlled and safe manner. Excess material from the bridges can be carefully filed and/or sanded away to leave a nice finish. Bridge settings are in the Advanced Cut Parameters tab.

4. Postprocessing. There’s a Fab Lab BCN-specific post-processing operation that needs to be applied to the files when they’re created.

   

Here are the resulting three files:

1. Screws | .nc file
2. Pocketing | .nc file
3. Profile | .nc file

Cutting

I used a 6mm DownCut endmill tool for my 15mm thick plywood. The CNC is called the Raptor X-SL

Once I ran the ‘Screws’ file and fixtured down my stock (and re-zeroed the Z, of course), I was ready to send my files to the machine.

Some quick filing/sanding to remove the residual material from the bridges:

Hero Shots!

Reflections

I’m quite pleased with the result, and I’ve been using it at the lab everyday! I like the height I designed it with and it helps me maintain good posture while I’m on my laptop.

Files

Fusion360 Design file for my stool | F3D file