limited production :: hand crafted :: high performance :: rechargeable :: premium flashlights

This is my blog about creating a startup LED flashlight business. I'm a designer, fabricator, and strategist and I'm passionate about making ideas real. I believe that products are about people, that they should be built to last, deliver real value, and that we need to do a better job than we have in the recent past.

Most of my career has been contract or freelance work and I've crafted products and strategies for both big international companies and startups. I also used to work in the "industry" fabricating special effects for film and TV, along with the occasional hot rod. Bottom line, I love making things.

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Sunday, February 20, 2011

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?


  1. jason this is great! how do i get one?
    - hh

  2. What is the resistance on that switch? Also, have you considered using Mcclickies?

  3. My Fluke meter reads about .2 Ohms for the McClicky switch and it bounces between .2 and .3 Ohms for the Romisen switch.

    I considered the McClicky and it is a much taller switch assembly. I'd have to add about 1/2 to the length of the light. I also think the contact spring rate is too heavy, but that's personal preference. I also prefer the all metal body on the Romisen switch.

  4. Great post about the clicky. How much current are these good for? I didn't think they could handle the 2.8A you put through them.

  5. Hi Ash, you are right. I've switched over to "McClicky" switches because they do a better job with high current...and they are the gold standard for flashlight switches ;)

  6. Very nice. That's my preference, but by your comments above I thought you had ruled them out. Now I know better.

  7. Yep, there were a few challenges to overcome. In the end the light is about 1/4" longer. I also have to machine another part (retaining ring) to make it work. The McClicky is much more capable of handling almost 3 Amps of current though. I still think the Romisen is a nice switch for lower power applications, and it's easier to work with because it's metal and lower profile.

    Thanks for checking back!