Week 7: Computer-Controlled Machining¶
*The final result with its two prototypes stacked up on one another like the Town Musicians of Bremen.
Group Assignment¶
Safety training¶
• Verify clearance of tool path before cutting
• Ensure that the collet is in good condition and the bit is fully inserted
• Ensure continuous air filtration, and dust vacuuming
First things first: eye and ear protection! Debris can come flying unexpectedly, whereas continuous noise can result in permanent hearing loss.
After having these considered we proceeded with machine kill-switch operations.
Types of bits¶
Bits come in various shapes and sizes, and from the first glance they can all seem the same. But it’s simpler than it seems: there are drill, and mills. Drill bits are desined to make holes – vertical cuts, and mills cut sideways, making horizontal cuts.
The grooves on the bits can vary also: clockwises, anti-clockwise, bidirectional. When changing the bits, you should first consider the side of the plywood in order to have smooth finishing. With drill bits being upcut, downcut, bidirectional.
To attach the bits onto the CNC machine we use collets, a sleeve put around spindle and tightened so as to grip it. And just like drill bits and end mills, collets come in different sizes, to ensure compatible fit with bit diameter.
Machine calibration¶
First, if changing the end mill, you need a pair of spanners. In our shop, the one which has teeth to grip on the spindle, is attached on the power block’s keys. After changing the mill you must clean the collet with compressed air.
Securing workpieces¶
It is important to have the material tightly secured onto the cutting bed, also called sacrifice layer. For edges we used quick action clamps. Whereas for areas that are not reachable by clamps, we use screws and simple wood scraps. The first scrap piece should match the thickness of the material being cut, and is placed beside it. The second scrap piece is then laid across both — bridging the material and the first scrap, and is fastened down with a screw into the bed, clamping the material in place.
Tests¶
As a first test we made square and circle shaped pockets, then matching-size cutouts to see if they fit into one another. So not only had we used two cutting methods, but also placed dogbones and tabs to ensure accurate fit.
Tab: a small bridge of uncut material that holds the workpiece in place during cutting
Individual Assignment¶
Design thinking.
It was quite a challange for me to come up with a product idea for the week’s individual assignment. I don’t like wasting material, nor I like having things that are not usable, or practical. Another drawback I had was not to limit myself to a simple piece of furniture, not by its feature, but by its category – a table or a chair.
Why this?
So, I looked into objects that are visually appealing, and passed down generationally – a rocking horse. As I happen to have a niece and a nephew, this was the perfect idea to design, mill, and assemble. Though I was hesitant between two options, a boxy modern one, and a simple stool like one, I ended up going for the modern one as the sides of large plywood look rich!
Step 1: designing¶
I always start designing with a pen and paper!
To begin with the proportions and sizing, I looked into child body-proportions aged 2-4. After having some options and features sketched out I transfered to FreeCAD. I first made a list of variables to make the model parametric. As for the rocking motion, which is dependent on the base, the arc, I deceded to leave that for the testing phase.
Step 2: testing¶
Test 1¶
My initial test was a sticky note cut out. In such situations the adhesive part is helpful to quickly assemble extremely rapid prototypes. But then of course I cut out the parts on the laser cutter, using a 3mm plywood.
Test 2.1¶
I actually was not setisfied with the arc angle, the rocking motion was very short, and it would rather oscillate back and forth than rock smoothly.
Test 2.2¶
The second laser cutout prototype had a bigger central angle. This had helped to regulate the quick rocking, and had become more gentle. The difference is more noticable in the photo of the remainings from the first cut, and the silhouette of the second cut.
Step 3: milling¶
As the toy is aimed for 2-4 year old children I went with 12mm thick plywood. This would ensure both heft for balance, calm rocking, withstand weight, and years of abuse from children. From my point of view any thicker or thiner would not work out as good as it has!
CNC Operations & Toolpaths¶
In the VCarve app, connected to the CNC, I set the toolpath outward the stroke. After, I set the dogbones for the joins, and also rounded the outer edges with normal dogbone.
The only parts that differ in thickness are the wedeges. These were designed to be 18mm thick, though I have cut 12mm wedges also. So for the initial wedges I would need to place a new thicker plywood.
The size of the material was 1520 x 1520mm, but I managed to squeeze the skeches into 900 x 1520mm.
Dogbones & Tabs¶
In my initial model the corners of the silhouette were quite sharp. I was planning to round them by hand, but when I was adding the dogbones, I realized that it would be a faster, and more precise result which I would not be able to replicate with consistency. The outer corners were rounded using “normal” dogbone.
Step 4: assembling¶
Before proseefding to assebly, I of course had to rigorously sand all the surfaces and edges down, especially when the product is designed for kids. Though I am not new to working with wood, this took me well over 4 hours to complete.
I regretfully skipped the step of staining and finishing the wood, despite me liking the natural color of the wood, but I would want to apply some finishing. One thing that set me back was the time constraint, and the tight fit of the joints. Disassebling the horse would only damamge the outer layers of the plywood, but still, sanding had helped significantly to avoid lodging splinters.
Surfacing¶
For surfacing, I used P320 grit sandpaper. I liked working with a triangular, iron-like sanding machine. But as the shape of the P320 grit was round, I cut it to fit the shape of the triangular machine.
I then attached the 4 same-sized panels together with clamps to sand their sides quicker. After, I individually sanded the larger surfaces.
Assembling¶
The precision on the ShopBot is sometimes both curious, and surprizing. It’s always just a tiny bit bigger than you set it to be. Just like in the group assignment tests of squares and circles the parts and joints for the horse were a miniscule amount larger. And so you can see in the images below. Although I had already sanded the joints down more signifucuntly, first with a lower grit to take some material off, then with a finer grit to make the plywood smooth. But apparently it wasn’t enough, so with some joints, I cut thin pieces off with a stationery knife.
Eventually, while the fit was still tight, I put some clamps and slowly adjusted the tightness over the whole parameter of the horse.
Experiment¶
Trick 1:
While working, I noticed in our lab’s kitchen a container of almonds and remembered a trick – rubbing walnuts on wooden furniture can minimize scratches and scuffs. This was achieved due to the oils found in the nuts.
Trick 2:
I had once taken a wooden spoon making class, and recall closing the pores on wet wood by applying force with blunt tools.
Combining these two tricks together helped achieve an interesting soft finish on the plywood, which you can see in the images. The darker yellow one is the almond-finish, and the more pale one is simply sanded with no finishing.
P.S. I did not torture myself by finishing the whole toy with almonds…
Conclusion¶
By far this week was one of my favorites! I like working with wood, and I like how computers help achieve near-perfect results. While cutting the shapes with a CNC felt “non-traditional” and somewhat deceiving, I still got to spend time on sanding the wood for hours.
Other than that, I also liked working on the digital model. Testing with the laser cutter was less promising than the real thing. And while dogbones are a compromise, I am looking forward to designing and milling again!