DA OTF Hidden Blade

This is the re-make of the hidden blase models from the Assassin's Creed video game. This version is a dual-action out-the-front model which means that it uses the same finger motion to alternate between spring-launching and spring-retracting the blade. The majority of the pieces were 3D printed with the only exceptions being those I had to re-make due to my own failure to properly design the files on the first go. It was a good experience though as this is the first time I have used 3D printing. I have some experience with the software side, but I have never been able to afford the actual printing myself.

There are two videos included to help explain the mechanism. I would suggest building this out of balsa wood before 3D printing so that you better understand where things might go wrong and can adjust for them yourself if needed. It is a cool mechanism and fun to build.

The materials used are minimal since most of this was 3D printed. However, some of the pieces are easier to use common items for. These are listed below:

• 2 Pins / needles / tiny nails
• Rubber band
• Balsa wood (any modelling wood will do)
• X-acto blade
• super-glue (I used JET modeling glue)

Since the files that had printed were not 100% perfectly designed, I had to modify them a little bit with the balsa wood. This is what the extra modelling wood is for.

The total cost for the above-mentioned materials is fairly cheap if you have the blade already. It was about $4 for the glue, $2 for the wood, and I found the rubber bands and pins sitting around my house. 

The 3D printing was done through a friend of my family, so I cannot give an estimated cost using other vendors. Images of each of the CAD pieces is provided on this and other steps.

It is a lot easier to visualize the assembly with the pieces in front of you. They all fit together smoothly when done right. I have included a short video explanation of how the mechanism works.

The first step to assembly is to pin in the paddles into the base case. The holes in the paddles is already printed into the piece. If they are too small for your pins, you may need to use a micro Dremel bit to bore them through. If you need to, it is easy to cut one out of the modeling wood. You will need to use 2 paddles; one for each end of the case.

Once the pins hold the paddles in place, slide the blade into the case as well. It should glide very smoothly (it should feel like it floats) through the track. If it doesn't feel this smooth, use some very fine sandpaper to smooth it down.

Also, in case you are following my 3D printed parts, I must point out here that I printed the mirror image of the blade that I needed. To correct this, either mirror the blade I have shown or carve by hand the proper shape and build up and missing material by glueing an appropriately shaped piece of wood to the blade where you want to add material.

Make sure the paddles fit snuggly against the blade with maybe 1-2 mm of open space between the paddle and the edges of the blade's butt. If they don't, you may have to re-make the paddles. I give this advice in case you are making the mechanism by hand instead of using 3D printed parts.

I am going to break this step into multiple parts so the pictures appear more relevant. 

Here is the video I mentioned above.

This next step is the most important piece in the mechanism, it is what actuates the blade in and out.

Grab the two sliders (the pieces with what looks like shark-fins on it) and the shifter (that is what I am going to call the large piece in the pictures for this step). First off, cut out the little pieces that I have shown in the 3D printed model because they prevent us from inserting the sliders onto the track. We will have to re-glue the pieces that hold the sliders down later.

The two sliders fit in along the top of the shifter (along the track printed into the structure). See the second picture to see what this looks like. Finally, you must glue down the wooden pieces seen in the second picture. These will keep the sliders in place as they are pulled back to launch the blade (to be explained in the video). 

Place a rubber band between the shark fins on the sliders. This will tension them so that they can spring the blade into place when actuated.

You should be able to slide the shifter forward and backwards along the tracks in the case at this point and tyhe blade should launch out and in smoothly. The final mechanism now is to get the blade to actuate both in and out with the same movement. The natural action for this is to use a circle which rotates 180 degrees with each pull of your finger. So each 180 degree rotation brings the shifter forwards and then the next actuation brings it backwards. In this piece of the mechanism, the circle piece that I had printed has a knob on the bottom to shift the shifter with each 180 rotation. It is what converts the rotary motion into linear motion. On the top side of the wheel, there is an 's' shape (the two fins). As can be seen in the 2nd picture, the wheel has a layer cut out to allow the string to be pulled through the wheel to turn it. The string has a knot in it that catches with the teeth of the wheel. One end of the string is tied to a spring (could be replaced with a rubber band) and the spring is connected to the case. The spring pulls the string back over the teeth of the wheel after each pull. The wheel acts as a ratchet. The other end of the string is then tied to the ring. To aid in the ratchet ability, I modified it to include a tine (the longer stick) that gently brushes along the top of the wheel. The tine ratchets over 2 ramps spaced at 180 degrees (see last few pictures). In this way, a rotation (looking down) clockwise is permitted, but counterclockwise rotation is stopped at the tine by the ramps.