Introduction: Wood Cup
I'd like to share the process associated with trying to realizing a simple sketch of a wood cup. I have seen many pictures of fantastic 3D milled shapes and wanted to see if I could realize something too. It started off innocently enough as an inquiry into 3D computer milling on my Shopbot (a computer milling machine). I had made several 2D "flat packed" projects (e.g. cabinets and boxes) but really had very little intuition about the difference between this and something more three dimensional. It turns out the software we use with these computer milling tables is probably as important as the table itself and the ability to mill both sides of something affords a much larger universe of possibilities than just milling one side. This new exercise quickly led me into the dark woods of new software. To make the sketch, and other future ones like it, I was going to have to learn Fusion 360. I was also going to need to get familiar with the technical challenges associated with "flip milling". If I could understand these two aspects of computer milling (i.e. the new software and "the flip") I would be in a better place to realize whimsical sketches. A cup is the product of this initial investigation.
There has been a lot written and said about how a 3D printer "doesn't care" if an object is complex. I wanted to see if this Fusion 360/Shopbot combo could accomplish something similar for woodworking, a decidedly less synthetic process. I started out by making some conceptual sketches and then set to modeling a cup in Fusion.
Because I have a relatively simple Shopbot setup there isn't a lot of depth to work with in the Z direction. I've often thought about the wisdom of not getting a deeper setup that could handle thicker material but after this project, I've decided a lot can be done with a relatively short drop. A typical Shopbot PRSstandard setup should get you at least 3 1/2" of cutting depth. This turns out to be pretty usable. Among other things, a 4x chunk of wood from the local lumberyard will work fine. On this project,it was possible to "stack" two interlocking pieces to get the height I needed. Milling with long bits puts a lot of strain on your milling tolerances and my 1/2" end mill did fine. I think if the bit was any longer it would require an upgrade in more than just the gantry height. A four of five inch overall bit length gets put through a lot when it is milling at its extremity. A lot of mechanical advantage is put on the spindle and at some point inaccuracies will start to develop. When I first started working with these bits I saw this first hand. Now I see the 1/2" bit cutting into three or 3 1/2" of material as an acceptable practice with the proper combination of roughing and finishing passes.
Step 1: Drawing Up the Design
Fusion 360 can be a little tricky in the beginning. The whole concept of "sketching" in this program, like Revit, is a big shift from earlier, less parametric, CAD programs. The Fusion 360 forums at Autodesk are a tremendous help. There are many knowledgable people to help shepard you through the process and the simulation aspects is great. After a bit of communication with the forum, the post processing to Shopbot has been updated and goes off without a hitch. If past posts to the forum are any indication, this has not always been the case. It is great to see that Fusion 360 and Shopbot are starting to speak the same language.
Step 2: Making the Stock
This cup has has two parts. The bottom is essentially a simple extrude and did not require a flip. As a consequence, the stock associated with this piece could be less substantial.
To understand why a piece needs to be flipped it can help imagining the sun directly above your piece. If the sun would generate shadows on the piece itself, this means your milling head can't "see" certain certain features of the body. The top piece had a taper to it that placed certain areas that needed to be milled out of reach from above. Taking an relatively organic shape and giving it a geometry that can be easily clamped and precisely situated is the basic challenge of flip milling. I recommend having enough waste material on the sides and bottom of your piece to allow you a stable base for the piece when it is flipped.
I suppose one can create complicated scaffolding for flipped piece of material but I was hard pressed to come up with a more efficient way to do this. It is my present sense, as ironic as this sounds, that some waste material needs to be generated in the name of efficiency. Once the top of piece is milled from underneath, you need to undertake the precise exercise of milling the top side in precise alignment with the underside. I laminated seven 1/2" layers of latvian birch together to make a "log" of stock for the prototype and any subsequent attempts. This was stock left over from my two previous instructables and therefore, if the milling process does allows one to redeem the waste from other projects, there is a silver lining to this waste of the waste.
The Shopbot table makes an excellent place to do this laminating. I used aliphatic resin (aka yellow wood glue) and a bunch of clamps. My strategy on how to do this best is evolving, but my current recommendation would be to drill a pilot hole as part of the first toolpath that goes all the way through the material in the middle of the piece. Once the piece is flipped, mill a similar concentric ring or hole with a "pause" inserted into the g code before continuing the routine. This will allow you to assess the deviation of the centers with a micrometer if there is any. Knowing how to do this, and how to shift the x and y zero accordingly, will save you a ton of toile with work holding fussiness.
Step 3: Placing Stock That Will Be Flipped
There is just a lot of measuring. To precisely locate the stock that needs to be flipped, start off by placing three pieces of scrap plywood on the table and then milling a rectangular recess that exactly matches the width of stock. Initially I gave the recess two hundredths of an inch of play. This turned out to be unnecessary. After matching the width of the recess exactly to the width of the stock, it fit snug. This might be because a 1/2" dia. x 4" bit that is spinning at 18,000 rpms tends to oscillate at its tip a bit more than shorter bits. Perhaps this generates a little more removal but I'm guessing here. Previous projects that had a shorter bit seemed to require more gapping. The second picture here shows the beginning of the milling process for the top half of the cup. I started with the underside of this piece to facilitate work flow.
Step 4: Steps to Make the Flip Work Better...
I recommend having enough waste material on the sides and bottom of your piece to allow you a stable base for the piece even after the top has been milled. You now need to undertake the precise excercise of milling the flipped piece in precise alignment with the top of the underside. I laminated seven 1/2" layers of latvian birch together to make a "log" of stock for the prototype and any subsequent attempts. This was just stock left over from my previous instructables. The shopbot table makes an excellent place to do this laminating. I used aliphatic resin and a bunch of clamps. My strategy on how to do this best is evolving but my current recommendation would be to drill a pilot hole as part of the first toolpath that goes all the way through the material. Once the piece is flipped, mill a similar concentric ring or hole with a "pause" inserted into the g code before continuing the routine. This will allow you to assess the deviation of the centers if there is any. Knowing how to do this, and how to shift the x and y zero will save you a ton of work holding fussiness.
Step 5: Extricating the Top Piece From the Stock
Figuring out a way to hold the cup after the flip was a big challenge. The pieces needed to be immovable for the milling process and yet extractable after it was all done. I've attached a sketch that will hopefully explain the strategy better than I can put into words. After chopping the cup stock off the end of the larger stock "log", I put the cup stock on my router table and separated it from the surrounding stock with a flush trim router bit. Working carefully with the table saw, the extra material that now existed at the top of the cup could be cut off.
Step 6: Assembling the Two Pieces
I've attached the Shopbot files I used for this process. If you would like the Fusion 360 file please let me know and I would be happy to send you the link. At this point you should have two pieces that are ready to friction fit together. With a little glue, sanding and elbow grease the two pieces should go together seamlessly and can be finished with a sealer appropriate to the application. I hope you've enjoyed this How-To and will post other flip mill projects. The possibilities for it seem to be great.
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