Anatomy of a push button tailcap

Things are often more complicated than they seem. The tailcap might seem straightforward, but it's been a challenge to get just right. There are lots of dimensional tolerances and lots of machining operations on this one little piece. I'm also integrating two products from two different manufacturers: the rubber tailcap boot and the actual switch.

Different stages of assembly from left to right:
cap, boot, o-rings/spacers, and switch

A couple of fiber washers and an o-ring are needed to give the switch the proper "stand-off" from the little rubber nub inside the switch boot. The correct clearance is +/- 0.015 inches. Thats about the thickness of five sheets of paper. I am surprised at how sensitive the clearance is.

Read on after the jump for some more detailed shots.

These are all of the parts (in order) that are required to make a simple tailcap. Let's put 'em together.

 First, in goes the rubber cap. I have a variety of colors on hand: orange, black, and glow in the dark green.

Next up is a little fiber washer that I'm using as a spacer. You can't buy o-rings in the exact dimension that I need.

Now in goes the actual o-ring. The rubber button actually seals the cap from water intrusion. The o-ring itself serves two purposes no related to creating a seal...which is the normal use for an o-ring. One, it acts as a spacer to give me clearance. Two, I've selected a relatively soft o-ring material so that it compresses slightly when I screw in the switch. This compression acts like a lock washer and should keep the switch from coming loose. If it does come loose, it's a simple matter to screw it back in by hand.

I suppose we are getting a little redundant but here is the second fiber spacer. I'm using two spacers for a specific engineering reason. If the o-ring was in direct contact with the rubber cap and the metal face of the switch, friction would make the o-ring twist and deform as the switch is screwed in. The fiber spacers reduce the friction and allow the components to be fastened together while maintaining even pressure on the o-ring.

Interestingly, the threads on the switch component are metric. All of the other threads on the light are standard national threads. I'm using a single point thread mill because it allows me to machine any thread pitch (within a range) with the same tool. I won't belabor the point but even selecting a threading tool is a series of compromises between thread form, machining speed, tool life, and number of tool slots in the tool changer.

Viola, a completed tailcap switch assembly! Simple right?