Jim Yurchenco on the Ribcage

Source: Interview with Jim Yurchenco, 25 May 2000.

The Apple Mouse Ribcage

Pang: Can you tell me how you came up with the design for the ribcage?

Jim Sachs talks about the importance of the ribcage.

Relationships Within the Mouse

Yurchenco: Well, ultimately it was a game of "Where do you put in 3D space certain components that have to be related to each other in certain relationships?" You start out and you know you have to have a ball and two encoders that have to be on the horizontal axis of the ball, and 90 degrees apart from each other. You needed an idler wheel that was at 45 degrees to those encoders. So you have these four objects in space that you have to package.

There are also a bunch of relationships that are established that driven by the resolution you want on the mouse-- how many dots per inch from tracking that you get. You start out with a certain size for the ball and encoder wheel, so there's a geometric relationship between the diameter of the ball and the diameter of the rollers on the encoder shaft. You also have the number of slots on the encoder wheel, and the size of the beam that the LED is emitting that passes through these slots and is going to the phototransistors.

The physical process is that you put the known objects into a drawing, and then you start building structure around. Eventually you end up with something that holds all these things in place. Then you have to say, okay, how am I going to make this structure?

Injection Molding

We chose injection molding process because it's a very repeatable process, it gives you parts that are good enough precision for the kinds of things we needed to do here, and it's very low cost. Once you build the tool, the cost is a small amount of plastic resin, plus a small amount of time in an injection molding machine. That allows you build very, very low-cost parts. The parts are durable enough and reliable enough for the application, for the usage they're going to see, the environment they'll be in, and the lifetime of the part; and they're lightweight.

Jim Yurchenco and Rickson Sun discuss the virtues of injection molding in a second interview.

So they meet all the criteria you need to build this thing. It's why you see so much injection molded plastic in so many consumer products: it's very versatile process, and it allows you to create shapes that are complex that you can't get any other way with a low-cost process. So in some ways it's almost obvious. It's like "Why is a wheel round?" Because it works that way. In this case it's something a designer almost doesn't question, because it's such an obvious choice. If you remember The Graduate, and the advice the older man gave Dustin Hoffman: "Plastics." Well, there's some truth to that.

The Challenge of Mold Design

So injection molded plastic allows you do a lot of things. Now, the down side of IMP, as with any process of this nature, is you have to build a mold that can create the shapes. That mold is basically a block of steel, in which you cut out holes that you fill with a molten plastic that then hardens into the shape you want. All well and good. However, there are many shapes that are very difficult to create in a mold for technical reasons, and in the end you have to be able to get that part out of the mold. You need to be able to open the tool, be able to eject the part, so you can close it and make another one. And that requires design features in the part that you wouldn't normally need for functionality of the part, but you need for manufacturability of the part.

That's the interesting task for a designer of a plastic part. You can build a plastic part of whatever shape and size and complexity they want, but how do you mold it? Melding the two-- the functional design of the part, and the manufacturing design of the part-- is where the skill of a designer comes in. That's where being able to manipulate shapes, and being able to visualize how things are going to happen, becomes a great benefit in being able to design a part that's manufacturable. So you spend a lot of time moving lines and surfaces around to enable yourself not only to have them functional, but to be able to make them in high volume.

And then there's a lot of technical details in a molded part, like controlling the tolerances. If you have tight tolerances, where you place them in the part has an effect on how they'll come out, and the length of various features has an effect on what those tolerances are. So there are these other aspects of a design that come into play when you're designing and injection-molded part that makes it a much more difficult process than it sounds at first. You don't just make a shape and fill it with plastic.

The challenge in the case of the ribcage was that there were a lot of very small features that had to be crammed into a very small space, and building a mold to do that was complex. Nobody had actually done this before, so it was never completely clear that it would work when you put it together, so there was this element of risk. And you can model and make samples all you want, but you never know whether it's really going to work until you mold the part. Fortunately, we had a good toolmaker, Vic Renden at Micromold. He built a beautiful tool, and the parts worked right out of the chute-- first shots out of the mold worked.

Jim Yurchenco and Rickson Sun talk about the skill required to make the tool in a second interview.

Since then the technology has improved, people's understanding of the technology has improved, it's better now that it was however many years ago it was. Better designers have attacked the mouse since me and refined it, so current mice are even simpler, although you can still see their roots in the ribcage. Most of those parts are integrated right into the base of the mouse; it's no longer a stand-alone part. It takes more complicated tooling, which costs more, and they take longer to build; but it saves money, and right now mice are commodity products, so you get rid of a part, you save money.


Document created on 28 November 2000;