I'm Brett Slatkin and this is where I write about programming and related topics. Check out my favorite posts if you're new to this site. You can also contact me here or view my projects.

04 August 2014

From plastic 3D print to milled aluminum — Making a bicycle gear shifter

Earlier this year I spent 30 days designing and 3D-printing a bicycle gear shifter for the Shimano 8 speed internal hub. The goal was to make one for my Mission Bicycle. I wrote about the experience here. After posting that story I worked with the team to revise the design. Here's the result as a plastic prototype. I've been pedaling around with this for the past few months.

My next goal was to find a machinist to turn this part into 6061 aluminum for strength and durability. I searched around the SF Bay Area and found a few machine shops. A couple folks I contacted never responded. One replied but declined because they couldn't open my 3D design files. Another met me in person, but wasn't excited about the project (his primary business is building things that go into space!).

Finally, a friend of mine connected me to someone whose day job is running a machine shop. For the sake of privacy I'll refer to him as "the machinist". It's important to note that there's no way any of what follows would have happened without their work. I'm grateful for all of their effort on the project. I've just been along for the ride.

Design for manufacturing

The machinist also had problems opening my CAD files (from 123D Design — $10/month). The issue is the DWG files exported by one company's software can't always be imported by another company's software. There are tools out there for format conversions (like Teigha) but they aren't good enough. Professionals seem to use Solidworks (MSRP: $5000) over Autodesk. It's the Cadillac of CAD. I can't afford it.

Luckily, I found a free tool by the makers of Solidworks called Draftsight that can be used to open and annotate the DWG files from Autodesk products. As long as it could be opened in Draftsight it could also be opened in Solidworks. When it didn't work I knew something in my model was funky and needed to be fixed.

Once the machinist could open my files we went through another cycle of design tweaking. There were a lot of things I thought were simple to mill out of aluminum that turned out to be impossible. Machining is really an art form like blacksmithing. It's full of arcane knowledge. The only way to learn is to actually use the tools and gain experience from your mistakes. Here's the final design for manufacturing, which looks better than the original.

Milling the part

With the final design in hand I bought all of the parts to build and assemble a prototype. I got the screws, washers, and ball springs from McMaster-Carr. I bought aluminum from TCI Aluminum. Machinists call raw uncut aluminum blocks "blanks". A blank is the starting material that is cut down to the final part by the CNC machine. Here's what my blanks looked like.

You normally buy 12 foot long bars of aluminum that best fit the profile of your part. Then you cut it yourself to size. Here's what that process looks like. It's quite a saw.

Here is the inside of the Haas VF-2SSYT 3-axis mill (MSRP: $72,000 — as configured $100,000+) that we used.

The control panel on the right lets you modify the program before or while it runs. What you see in the middle on a spindle is the business end of the machine. Here's a picture of the 3 inch diameter facing endmill. It's hardened steel and extremely heavy. It has carbide inserts so it doesn't wear down too quickly.

While the mill is doing its work it needs to change tools for particular cuts. To do this it has an enormous cache of end mills that can be changed automatically by the machine. This lets the machine cut, drill, face, and tap without human intervention. The machine uses a robot arm and compressed air fittings to swap the bits. It takes less than a second to swap (insane!). Here's a picture of the revolving cache, about 10 feet off the ground.

The mill is a dumb machine like a computer. It runs programs in G-Code, which describe where to move the cutting bits in XYZ coordinates. The mill does little to ensure the G-Code is valid. If the G-Code said to bash the bit into the vise the machine would do it. It's similar to how a bad set of instructions can crash a computer program.

The G-Code comes from CAD software that's the machinist's equivalent of a compiler (Mastercam and HSMWorks are popular tools). The CAD lets you build a set of "operations". An operation is all of the various cuts you need to make in one side of an aluminum blank. This can include multiple passes by many different types of cutting tools. The software doesn't automatically convert your 3D model into operations. You need to manually look at the outlines of the part and come up with a series of "tool paths" for the cutting blades to follow. The hope is the cuts will result in your design by subtracting from the aluminum blank. Machinists call this conversion process "programming".

One of the coolest things the design software can do is simulate the mill in action. It's like a highly sophisticated interactive debugger. Here's what it looks like.

Once you have the G-Code you put it on a USB stick and upload it to the mill. Then you put the blank in the machine and clamp it down with a vise. You bang it with a dead blow hammer to make sure it's seated properly. Then you use the mill control panel and it's 1000 buttons to tell the machine to measure the XYZ coordinate of one of the blank's corners. This tells the mill where to start cutting so you get exactly the physical object that you expect. Here's one blank in the vise.

You can see aluminum shavings all over the inside of the mill. Those are the "chips" from the end mill. There's also this weird fluid on everything. That's lubricating coolant to ensure the end mills don't overheat while they're spinning at 8000+ RPM and cutting. When the machine's actually going you can't see anything because coolant is everywhere. But the cutting sound is extremely loud. Here's a video of the machine making a cut.

The bike shifter ended up being 7 operations in all. 3 were for the base that goes over the steering tube. 4 were for the knob that pulls the shifter cable. The design also required 3 different pairs of "soft jaws", which are essentially jigs that hold the aluminum blanks in place to handle mating surfaces that aren't square.

I was surprised by how important it is to hold the aluminum blank properly in the mill. It significantly affects how you choose the order of your operations. Each operation must leave on enough "fixture stock" to let you grip the part from the opposing side until everything is square or fits in a soft jaw. Here you can see each step in cutting the shifter's base piece and the various fixtures.

The shifter knob provided an additional challenge because it was important that all of the cuts for the center hole and ball springs were concentric. Here's what the blank looked like after each operation for the knob.

Built and biking

At last! It's all done. Here's the shifter actually mounted on my bicycle.

And here's what it looks like from the top when you're riding it.

For the next month we'll try these prototypes and see how they feel. We'll probably make some design revisions and update the G-Code programs accordingly. We'll get some anodized in black. Once that's done I'm hoping we'll do a manufacturing run at a local machine shop. I'd like to start putting these on Mission Bikes and selling them to anyone who likes the Nexus 8 hub. Let me know if you're interested in getting one!
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