GD&T: Home Button Placement Problem (Apple)

The interviewer drew a top and side view of the “home screen”, circular button on the front of an iPhone. If the critical features of the button are the diameter and the position/width of an anti-rotational feature (which was drawn in one of the views), where would you place your datums and critical dimensions. Why? How do you define the datums to reduce the gap and offset between the button and the glass, and the tilt of the icon with respect to the glass?


Admittedly, I’m not the best at GD&T but I will take a stab at the above prompt!

For a basic understanding, let’s zoom out first and understand what Geometric Dimensioning & Tolerancing is all about. When you think about part design, and the intended manufacturer that will actually build said part, GD&T is the language of love (per say) that is spoken to communicate design intent, engineering tolerances, and control the variation in a manufacturing process.

TLDR; how to build a part that exactly matches the on-paper plans in 3-D CAD.

I’ve attached a useful resource that does a great job of explaining GD&T (GD&T Symbols | GD&T Basics).

In the context of above problem, your datums refer to a central plane, axis, or point that dimensional tolerances/features reference back to. Think of it as an anchor point for all dimensions made!

If I were to think of important datums here, one would be from a top view of the home screen referencing the top edge, and using that to locate the home button in X-Y dimensions. For the gap and offset between the button and the glass, it would be from a side view referencing the bottom edge to locate the Z height (variation where the button sits up/down).

A good way to also ensure the offset between the button and the glass is located at the same height is to call out a flatness specification.

Thinking more about the physical interaction of the button and the glass paneling, critical dimensions to label as CTF (critical-to-function) would be the radial dimensions of the glass cut-out and outer diameter of the button. In order to facilitate a smooth button press, you’ll want a clearance fit with tight tolerances to prevent gaps in your device.

Here is a useful wall-chart as well for explaining the various symbols, four fundamental elements to keep track of, and which tools are used to track certain features.
GD&T Wall Chart.pdf (2.0 MB)