< back

Week 4: Computer-Controlled Machining

Hanging Nest aka MAKE SOMETHING BIG!

October 8, 2015

What a great prompt! I was both eager, excited and nervous to get started on this week's project. While browsing CNC-milling projects from previous years, I noticed that many, many people made chairs or tables. I don't really need any more furniture in my apartment, so thought I'd go the route of creating something less practical but hopefully awesome: a hanging nest chair/bed! I've always been obsessed with these whimsical furniture installations and thought it would be a worthy and fun challenge to try to make one. I perused Etsy and the internet at large for some design ideas:

After a thorough ShopBot training with Tom in the morning, several of us decided that we wanted to invest in nicer material for our projects, so we mounted an expedition to Home Depot with the Media Lab van, ending up with some construction supplies and several boards of 4' x 8' x .5" sande plywood. As I suspect will be a recurring theme for this week, the adventure took much longer than expected but was a fun bonding opportunity and a nice chance to brainstorm project ideas.

Next up: doing battle with Rhino to design something substantially more complex and three-dimensional than anything I've attempted so far. Time spent in Rhino was definitely the limiting factor on this project, though much of that time was actually spent thinking about the different pieces I would need to model and how they would fit together. After a few quick paper sketches, I decided to go for a "waffle" construction. Several people recommended I use 123D Make for waffle construction, but I found this program not only buggy (e.g. it took forever for STL files to load and would crash often) but rather heavy-handed. For example, I wanted my horizontal rib pieces to nest within the vertical rib pieces, and 123D Make didn't have an (obvious) way to do that. I decided to push through with Rhino. (N.B. Many people also suggested that I use Solidworks. I don't have a good setup yet for Solidworks, but one is in the works and I do hope to familiarize myself with Solidworks before the end of the semester).

After quite a bit of time playing around in Rhino, I came up with a way to craft a "bespoke" rib system, using the following steps (partially adapted from Ani Liu):

Before actually cutting all my pieces, I decided to mill a test joint to further familiarize myself with the process and test the sizing of my pieces. With supervision and help from Kim, Sands and Eric, I setup the Shopbot for a test cut on the sande plywood. I used the .25" router bit at a speed of 120 ipm. I set the cut depth at .52" and the pass depth at .26", as had been discussed in our training with Tom. I used a 6.35 mm radius to create a T-bone fillet around the corners of one of my slots, but found this radius to be excessive. I will try with a smaller radius when I do my actual cuts. With these settings plugged into Partworks (as well as other standard ones discussed in the training...10k spindle speed, 50 ipm plunge rate), I exported my toolpath (one profile toolpath that cut outside using conventional cutting), and set up the rest of the Shopbot. It's a long process, but here are the steps briefly:

After going through this whole process (~15 minutes), the test pieces cut very quickly! They came out pretty nicely. I needed to sand down the edges a bit, but was generally satisfied with the cuts. The test cuts showed me that I needed to model wider slots (mine were .44" and the fit was very tight so I made the new ones .54") and also that the wood could break/ bend in places where it was very thin. I also decided to make smaller T-bone fillets next time around. With these lessons in mind, I went back to my model and adjusted the slot width. In my v2 model, I also made fewer contours (and thus fewer ribs) so that I could actually have enough stock to cut everything. Even with about half as many ribs as before, it was a squeeze to get everything on two-and-a-half boards of plywood (big thank you to Miguel for donating his extra stock to me!!). With a test cut under my belt and a design I feel reasonably good about, I will approach my final cuts tomorrow afternoon!

When I came back in the morning, Tom and John (politely!) informed me that they thought my pieces were too big and that I should try to scale everything down a bit. John also helped me figure out how to get my pieces from Rhino to Partworks at the proper scale (there actually does not appear to be a good way to do this...I ended up importing them and then manually scaling them up to the correct size). Back in Rhino, I was able to shrink my pieces along one axis so that I could make my pieces smaller without affecting the slot width. I brought my designs back into Partworks, and created two toolpaths for each sheet so that smaller and interior pieces would get cut out first to minimize shifting of the stock piece. I realize I could also have achieved this by setting the cutting order within a single toolpath, but the dual toolpath solution was simple enough and worked well. I also added T-bone fillets to each internal corner in my piece. I was initially having trouble adding these corner fillets, and learned that I had to go into "Node Edit" mode in Partworks for clean up the node structure at each corner so that the fillet program wouldn't get confused (thanks to Will Langford for helping me out with this!)

With the help of my Shopbot buddy extraordinaire Sands Fish, I followed the Shopbot procedure described above to cut three sheets worth of parts. My pieces all came out very well - a few rough edges aside, all the pieces were intact, nobody got hurt, and we didn't break anything. I should amend my earlier statement about Rhino being the limiting time factor though...the CNC milling process seemed to mysteriously devour time. Sands and I cut four sheets between the two of us, and we were using the Shopbot for almost four hours. Between fixturing the piece, zeroing the machine, the actual cutting, and the clean-up, it was a very time-consuming (yet fun!) process. Many thanks to Sands for being so selfless with his time and expertise!

The process didn't end with milling, though! After milling, most of my pieces (and some more than others, depending on the specific sheet and orientation of the cut relative to the wood grain) needed some serious sanding to smooth out rough edges. After sanding a few pieces manually, I used the Orbital hand-held sander to sand the rest of my pieces. Even with a power-tool, sanding took me over an hour since I had so many pieces (20 pieces!). It was hard to sand the edges of the slots themselves, but I was able to smooth out the outside edges of the pieces quite well. I also used a steel file and chisel to remove the extra gnarly wood debris from some of the internal slots.

Finally, I was ready to put all the pieces together! This part was a little nerve-wracking because I wasn't sure how (or if!) my pieces would fit together. The nest came together pretty nicely, all things considered. I laid out all my pieces in order of how they would be attached, and then inserted the horizontal ribs into the vertical ribs, starting from the top down. The slot size of .54" turned out to be a little loose, but extra tolerance made assembly easier. As I was putting the piece together, I noticed a few oddities about the shape of the nest and the way the pieces fit together. For example, with most of the joints, the two ribs would touch at the end of their respective slots; however, for the smallest vertical rib, it is barely touching the horizontal ribs. This makes me think that perhaps I forgot the scale this particular vertical rib. I also realized that I cut a few extra slots in the vertical ribs that did not end up housing a horizontal rib. I'm sure that it would have been easier to identify these problems during the design process if I'd used a more assembly-focused CAD program. This also would have been a great project for a cardboard prototype that would have helped identify these construction issues...

Problems aside, I was pretty proud of making this nest over the course of just a few days! Using the Shopbot is an involved process and I feel reasonably comfortable with it now that I've spent 5+ hours using the machine. I think the shape of the nest is really beautiful, and it's big enough for a (small) person to sit inside! I will try to hang the nest somewhere in my lab sometime this week, space pending. I'm not sure it's structurally sound enough to support the weight of a human while it's hanging, but I think it could be used for books and/ or pets. This is actually an open question and I will try to post an update here later this week!

Design Files