8. Computer controlled machining

This week is all about making big things. I decided to overhaul a small table-stool thing we’ve had in our office/break room for several years now. Originally made by a student at the school who left it behind. We’ve been using it as a couch-height table for eating lunch. The original has always been a bit unstable and is now almost falling apart.

Hero Shot!

Design

I started off by doing rough sketches on the material I wanted to use. Four different plates of Plywood. Two 18mm thick and one 12mm.

Basing this off the original table’s height of 60cm. I adjusted my version 2CM higher and made the legs wider and longer to hopefully reduce wobble.

Fusion

I drew it up in Fusion. Main thing here was making sure every part was it’s own component to simplify assembly and making changes as I was working.

Here’s the full component tree.

I didn’t do this completely parametric as It’s a one-off design and it’s simple enough that if I need to make changes or parametrize it I can do that quickly.

I only defined parameters for the material thickness for each plate I was using. I initially thought I’d have to use four plates but ended up only using three.

Here’s the completed Fusion Assembly:

Note the lack of Dogbones. This was semi-intentional as I wanted to play around with post-processing. I’ve used the Shopbot for hundreds of hours and I’ve always had the machine do as much of the post-processing work as possible. This time I wanted to play around with making the joints as smooth and tight as possible.

Here is the .f3d file for the table.

Vcarve.

I laid out the parts in Vcarve. Making sure to set the material size as I measured it. Leaving enough space to screw the material down into the shopbot bed. For this file I set the Z zero position to the Machine Surface or the bottom of the material. Material thickness can vary but the machine bed is flat relative to the frame. Since I´m not doing any engraving and only cutting through the material I do this to prevent cutting excessively into the machine bed. I had to do three separate files. One for each plate I used. Here are the layouts for each:

Foot rest, Crossbeam and Feet:

Legs:

Tabletop:

Each file has it’s own setup. The first one is 19mm thick, The second was 20 and the third was 12. Doing this parametrically in Fusion saved a lot of time as I could switch the plates around easily to maximize material utilization.

Here’s the Toolpath setup for the TableTop file:

The one on the left is the Downcut so it starts at the material surface and goes down 3mm. The one on the right starts at 3mm down and goes to 12.2mm, I defined this as 12.2-3. (I am bad at mental arithmetic). The one on the right is the upcut going the whole way through the material. I made sure to add Tabs to the design and I always name my toolpaths as clearly as possible. I usually do “Order of cut” - “Bit used”- “Operation” - Description”. In this case I did forget to label the second toolpath as 6mm. But I wasn’t intending on using anything other than 6mm bits so it worked out.

The feeds and speeds were calculated using the Fablab Speeds and Feeds Calculator. The site has excellent descriptions and guidelines on the Details pages for each input.

I’m running the following settings:

Here are the design files: |Filename|Type|Link| |—|—|—| |FootRestCrossBeamFeet|.crv|Link| |Legs|.crv|Link| |Tabletop|.crv|Link|

Cutting

I didn’t get enough images or footage of these steps but I’ll recount them by memory. I’ll expand on this section later.

I warm up the shopbot using the warm-up routine. Shortcut C+5. I move it to the position I want to zero on the machine bed by opening the movement window. Shortcut K to open the window and Arrow keys and Page up, Page down to move the head.

I run the zero using zero plate routine. Shortcut C+2

After zeroing the Z axis to the machine bed I move the head to the X Y position I want to zero to

I use the keyboard shortcut Z+X and Z+Y to zero the X and Y axis.

After triple checking my zero position and the material position paying special attention to the screws I run the file. Making sure to have the RPM window open before starting the job. Shortcut T+R. Without the RPM window the software doesn’t send the control box the desired RPM for the cutting head meaning it’ll default to the previous cut’s RPM. This can be troublesome accounting for feeds and speeds.

After cutting I use a drill to remove the screws. Flip the piece over then use an oscillating saw to remove the tabs.

Here’s a timelapse from the group project:

Post Processing

Now it was time to fit all of these pieces together. The tricky part here is I have square pegs and round holes. I decided to square the hole and round the pegs.

I did this using multiple tools but primarily a hand router, chisels and a dremel. To smooth out the edges I used the hand router with a 3mm bit. That approximately matches the curve left by the 6mm bit in the corners. The hand router couldn’t reach into all the corners so some places I needed to use a chisel to round things out or square an angle.

You can see the marks where the hand router couldn’t reach the corner. This meant I had to use the chisel to dig a small semi-dogbone into the corner.

Finally I use a dremel to fix some of the tolerances. A few of the pieces were a little too tight even after rounding. I wanted this to be tight enough to be done without gluing by hammering everything together.

After hammering everything together I had some pieces that got damaged and splintered thanks to how tight everything was

I filled these in with a mix of sawdust from the shopbot and wood glue.

Here’s me dremelling out one of the pieces:

and here’s hammering everything together:

Here’s the final piece, I’m really happy with it and took it home with me to try it out.

I gave everything a once-over with some 240 grit sandpaper to smooth things out.

I took it home with me and added some feet underneath to not scratch my floor.

and I also tested using it as a mini-desk in the TV room.