Salvaged mechanics

I think that the funniest and most creative part of all this project was building the mechanics.
I mean, I could have bought specific components. For a project like mine, if I really needed to buy every single screw, my budget would have exploded.
I have started walking around the flat knocking on everything to hear how it sounded, and I finally run into a wooden box.

The drumbox

This box gave me the inspiration of building something like a cajon.
The box had an opening lid fixed with two hinges. Two precious hinges 🙂 I saved them for later, and I glued the opening. After that the glue was dry, I found that the box sounded too opaque when knocking on it. I have then cut a big hole in the back with the hacksaw. Much better.

The sticks for Kick and Snare

Mechanics for kick and snare drum

The strenght of the return effect can be changed by making some more turns of elastic around the screw. Screwing or unscrewing the screw adjust the travel length of the stick, varying the loudness. The felt pad silences everything. This is the “snare” drum, closer to the border and with a hard tip. Behind that, in the middle, there´s the “kick” drum, with a felt pad on the tip in order to give more bass frequencies to the sound.

That was tricky. There were some problems to solve.
I wanted to use the two “pull-type” solenoid for that.
Available materials:

  • wood from broken Ikea bed;
  • hinges saved from the box lid;
  • various screws and nails for wood;;
  • some elastics;
  • lots of felt pads (the ones you stick under the chair legs to avoid scratching the floor). Really the heart of the project 🙂

The problems to solve:

The solenoid doesn´t do anything but pullling the spindle. That means that you have to pull it back in the right position every time, and limit its travel. It´s all about a balance of forces: that´s why I have tried to provide an adjustable mechanics.

  1. Convert the short usable travel of the rod (about 5mm) in order to give enough speed to the stick. Archimedes‘ lever will help us. I made the stick end just in the middle of the box, where the sound is louder. And I have provided the longest possible ratio for the lever, by putting the hinge at the other end and the force (the solenoid) between the hinge and the tip of the stick, as close as possible to the solenoid itself. That gives the maximum possible speed ratio between the solenoid and the stick.
  2. Provide an adjustable travel length: the loudness, thus the impact speed of the stick, depends on many factors:
    1. It is proportional to the force applied to the solenoid. This is proportional to the current passing through it. We will modulate this with PWM;
    2. The weight/momentum of the system. I know my mechanics is not very slim, and it doesn´t allow rapid fills, but that´s it 🙂
    3. The travel length of the stick. The stick accelerates under the force of the solenoid. Approximating: the impact speed is v=\tfrac{1}{2}\cdot a \cdot t^2. The acceleration a is proportional to the force provided to the spindle, which is proportional to the current passing through it. That means, the longer the travel, the longer the time of the acceleration, the greater the speed. Of course, the speed is limited also by other effects/frictions. But I have noticed that in my case the distance is the dominating factor. The drawback is that more travel time means less repetition speed and more delay.
  3. Provide a spring return: in both models of solenoid, the rod won´t go back to its original position. I have used elastic bands. More force equals less volume and less speed of the stick.
  4. Provide different sounds for the two “drums”. A low tone for the kick and a high one for the snare. I have used a “cajon” technique. When you hit a cajon in the center you have a bass kick-drum-like sound, while the areas close to the border produce a snare-like one. That´s how I have positioned my sticks. In order to improve this sound difference, I have put a felt pad under the kick drum (like the ones that go under the chairs legs). For the “snare” I have used one of those hard plastic caps you put on the heads of the self-threading screws to “hide” them in the wood, in cheap furniture.

The Hi-Hat

This is something I am really proud of.


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