Sander van Vliet

@Fablab Amsterdam

Computer-Controlled Machining

Week 7 (9 - 16 march)


This week we learnt more about Computer-Controlled Machining. A.k.a CNC Machining: a process that involves the use of computers to control machine tools like mills, routers, lathes and grinders. CNC stands for Computer Numerical Control.
Under CNC Machining, machine tools function through numerical control. A computer program is customized for an object and the machines are programmed with CNC machining language (called G-code) that essentially controls all features like feed rate, coordination, location and speeds. With CNC machining, the computer can control exact positioning and velocity. (source)
We were introduced to the different types of machines available like 3-, 4- or 5 axis machines, which stock/materials can be used and what kind of tools they can use like drill bits and milling bits. Among other information about software and file formats used for controlling these machines. And last but not least ofcourse the importance of safety precausions when dealing with high speed- and power tools which most CNC machines are.



Our local instructor Mickael gave us a four hour lesson on using the shopbot. There are quite some things to keep in mind while using it so I made a lot of notes.

Make something big (using the big milling machine)
For this assignment we were supplied with a sheet of 9mm plywood, or.. actually we went to the local shop together and got it ourselves.

In Fablab Amsterdam we have a Shopbot. The working area is 2440 x 1220mm so our sheet of plywood can be on there entirely.
Some more specifics about the shopbot milling machine
-Maximum Part Size: 2440 x 1220 x 150mm
-Software used: Partworks 2D & 3D
-Acceptable file formats: DXF, STL
-Mechanical Resolution: 0.015mm | 0.0006"
-Position Accuracy: +- 0.127mm | 0.005"
-You can cut and carve: wood, plastic, styrofoam and many more, but no metal.

An idea, and the design
In our living room there are quite some toys lying around and although we do have some storage space for them, we could use some more easy accessible storage options. This was a nice opportunity to create some and stackable crates seemed to be a good idea!
I started drawing on paper to get some ideas of shape and dimensions and how much would come out of one sheet of 1220x2500mm. I also checked and measured what dimensions would work in our living room.
Then I created a drawing in Rhinoceros keeping the following requirements in mind: stackable, pressfit (no glue, screws etc needed) and nice looking and eventually came to a design where I used pockets and holes in the sides. Pockets are formed when you cut less deep then the material thickness; a bit like what the laser cutter does when engraving.
crate source files - rhinoceros

Machining it
I had drawn test parts to check which opening size would work best to make the parts pressfit nicely without falling apart or being damaged. Although I had joined all lines in Rhino, in PartworksV2 I got the message that some lines were not joined. Luckily there is an easy fix for this (thanks Eva!). I selected all lines and did Edit - Join lines to correct this. I added tabs to hold the pieces in place while they are being cut, zero'ed the xy and z posisions and ran an air job (test run above the material to check if it will do what you expect). It looked fine so I ran the test parts job, but the test parts only just stuck to the rest of the material and moved while they were milled out so the tabs should have been a bit bigger.

I opened the corresponding (to the chosen test piece dimensions) Rhino drawing file and saved it as .dxf file and opened that in Partworks which is the software we use to create the toolpaths. A toolpath holds all information that the Shopbot needs to perform its job like: milling bit type and size, cut depth, pass depth (layer thickness on z-axis), feed rate (xy speed) and plunge rate (z speed). I changed to a 5mm milling bit, zero'ed the Z again and started the first part of the job. BUT apparently I had forgotten to put in tabs for all the holes that would be cut out because I heard some strange noises and notices that pieces came out (and they were damaged). A local instructor came in because he noticed the noises and we stopped the job.

I edited the toolpath, added tabs for all the holes and finished this toolpath. After that I changed to the 3mm milling bit to be able to mill the small dogbone corners and finished the job.
Below on the left you can see how the milling had been fully completed. In the right photo I have removed all the parts.

Assembling it
Now I needed to remove the tabs and sandpaper the parts lightly. That actually took quite some time because of the many parts and especially the many holes!
Although I designed holes in (2 of the 4) the sides of each crate, I found the pieces still in the holes, being held by the tabs nice looking so I decided to only remove the "hole pieces" from one crate and leave them in the other two crates! Below you can see that process including the assembly of the crates, and eventually stacking the completed crates!

And then the crates in their new natural environment: