Hi! This will be my second update for the fingerprint skeleton key project. In this update I will explain what the slider mechanism is, about which I talked in my first blog (Fingerprint Skeleton Key - Concept - Design for a Cause Challenge - Blog Post #1 )

, as well as start making it. The first idea was 3D printing a lot of the parts for it, like the special key trays, but I will have to change my plans, since the makerspace is closing its doors for the summer until the 27th of August, so for now, I will stick with hand making the parts. First off, let's start with the idea and design of the mechanism.




Idea and Design

Let's start with the idea, I haven't done any 3D models because I wouldn't be able to print anything so I just did some sketches on paper. The mechanism consists of 3 parts, one being the key holder which will end up holding all of the keys, the tray holding the key, and the last one being the key picking mechanism. In this blog I will be covering the first part, the key holder contraption. The way I imagined this to work is to make a "sandwich" structure out of pieces of metal separated by nuts and washers. In the gaps where the nuts washers are, we will have the keys with their tray. The tray is used to hold one of the keys and make it possible to work with other components in the build. In other words, since the tray is connection between the key picking mechanism and storage, for now all I can do is make a concept for it, but keeping in mind some of the things that will have to stay the same in the final version as well. Firstly let's kick off with the key storage.


Key Storage

(Just a quick disclaimer, this was completely free hand drawn with not perfectly accurate proportions). This was the first kind of design I had in mind, simple metal sheets with nuts and washers between them, the trays would be out of the same thin metal as the sheets so 2 washers with a nut (4 mm) are a perfect distance for the tray and key to slide freely. I will be experimenting with thin sheets in the beginning, If i see that there are issues I will be switching to thicker metal later on. As for the tray, I need it to be as an extension to the key, the hole in the end of it is for a small spring which will hold the key in place when not used. The tray I will be making in this blog will be a simple flat one, while the final version will be with slightly altered design so the key picker mechanism could work easier with it. Now it's time to try and make this!




The first thing I will be making is the tray for the key, from there I will continue with the key holder mechanism. For now I will with 0.5 mm thick metal sheets (the only thing I could find at home) but will upgrade to something else. This is where I would like to ask for some help. I've looked around and there a few options for metal sheets around here, I can either cut it off something existing (I am currently cutting pieces off of a flat rain gutter piece), and I can also order some sheets. The options I have for ordering are: Aluminium sheets which are either 0.5 mm, 1 mm, or 1.5 mm thick, 0.3 mm thick brass sheets and 0.6 mm thick copper sheets. I am for the most part working with a dremmel, hand saw, and tin snips, I am leaning towards the 1 mm thick aluminium sheets. I would like sticking to metal so I can keep it as thin as possible, if anyone has any experience or advice, I would love to hear it! Let's start with building the tray.


First Attempt

For the first attempt I will only be using the sheet metal that I've talked about. I want to see if the longer pieces will show some serious flex due to how thin the metal is as well as how easy it is to work with. For cutting the metal I'll be using tin snips and just filing down the rough edges afterwards.



To make this I outline the key on a piece of paper and went from there, to that I added an arm and some small extensions which will wrap around the key to hold it in place. I cut out the shape and with some double sided sticky tape made a stencil i can reuse and more easily use on the metal. One side of the metal is painted which made it easy to scribe some lines into it. I design the shape with the idea of having stoppers on it. It was pretty tedious cutting this with thin snips, so I had to abandon some of the features for now.

{gallery} Tray



Key Holder

This is the second part of the build, I used the exactly same technique as for the tray, drew some outlines on a piece of paper, stuck that to some double sided sticky tape, and in the end used that as a stencil do scribe the shapes I need to cut out of metal. While cutting due to the thickness of the snips themselves, the metal on both sides of them started to flex. I tried hammering it down flat after that which worked. The only thing left was to drill holes into the metal pieces. Here is a gallery of how the build and assembly went!



{gallery} Key Holder Assembly

Shape: These are the first 2 shapes I cut out, the one on the bottom is the first attempt, and it just didn't work out, while I continued later experimenting with the one on the top. The 4 holes on the top were meant to be used like "guides" to guide the tray, but of course, since I couldn't cut the sheets that accurate I had to scrap that idea, and stuck with the 2 top holes, the 2 holes on the bottom of the plate are meant to be used to stop the key from going out further, but added a couple of millimeters to both sides, so I will revise that in the next attempt.

Cut out pieces: These are the 2 metal sheets and the tray with the key all together. I cut out the second sheet using the first one as a stencil, but again, couldn't get accurate enough, one more problem that will show up is when I put in the screws. To drill out the holes in the second one, to be as precise as possible, I put one on top of the other and marked where to drill that way, but got holes that didn't match perfectly. This caused warping since the metal is so thin, which will be visible in some of the next photos. If I try this with the same metal again, I will for sure drill the holes through both at the same time while clamping the 2 pieces.

Assembly 1: The bottom view of the assembly

Assembly 2: Top view of the assembly

Assembly 3: Side view of the assembly, this is where the bend can be seen.


As I said, I abandoned the 4 screw rail idea, so to keep everything going smoothly a piece would need to go on either side of the assembly, to keep the keys straight, since the plates are cut to the width of the key(tray). Even with that much flex the key slid pretty good without much hassle, here is a video that shows the movement of the key inside the mechanism.



On one of the sides I am holding a piece of the metal to act as a guide for the key, though one on each side would be better. All in all, the key has pretty good movement and doesn't get stuck on anything. The only concern I have with this is that I don't think the key comes out enough, which is something I will have to look at in the next build.



This is just a short update where I wanted to see how the metal I had would work for this kind of thing. For the second attempt at making the mechanism, I will be going with plywood sheets, since they don't have any flex at that size. The plywood model will be to make sure everything works before I order and make it out of metal later on. Now it's also time to get to know the Arduino MKR1000, as well as trying out the RFID module, which I will cover in one of the next updates. Thank you for reading the blog, hope you liked it!