🥁 Cam Drummer: step-by-step build

Crank the handle and two drumsticks play a beat all by themselves.

How can a machine play music? With cams! A cam is a bump on a turning shaft. Each time the bump comes round it lifts a drumstick, and when the bump slides away — tak! — the stick drops onto the drum. One shaft carries two different cams, so the two sticks play different rhythms: BOOM… tak-tak, BOOM… tak-tak. Music boxes, old wind-up toys and even car engines are full of cams.

Level 3 of 5 (medium)
About 30 minutes
18 steps, 57 parts
Powered by: Hand-crank
Series: Mechanical Wonders

What you will learn

Cams: A bumpy wheel that lifts a follower up and down.
Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

⚙️ Gear Friends: Three gears holding hands — spin one and they all dance. (level 1 of 5, about 8 minutes, 6 steps)
🌬️ Bevel-Gear Windmill: Turn the handwheel at the bottom — the sails spin at the top! (level 3 of 5, about 25 minutes, 16 steps)

Parts you need

1 × Open frame 5×11
8 × Black pin (grippy)
6 × Split pin block
8 × Red axle-pin
3 × Open frame 5×7
2 × Axle 8
1 × Medium gear (24 teeth)
6 × Bush
1 × Knob wheel
5 × Half bush
6 × Blue long pin
2 × Thin beam 4
1 × Beam 7
1 × Axle 3
1 × Big gear (36 teeth)
2 × Beam 15
2 × Tan axle-pin
1 × Hand crank

Steps

Lay the white 5×11 frame flat. This is the drum machine's table.
Push black pins up into the 1st, 3rd, 4th and 6th holes of the back long side.
Press a split pin block onto each pair of pins and push a red axle pin into each block's cross hole, pointing out the back.
Stand the grey 5×7 frame up behind the blocks and press its bottom row onto the two red pins. This side wall holds the cam shaft and the drumstick hinge.
Front wall feet: push black pins up into the 1st, 3rd, 4th and 6th holes of the front long side.
Press a split pin block onto each pair and push a red axle pin into each, pointing out the front.
Cam shaft: hold the 24-tooth gear in front of the wall and push an 8 axle from the front through it into the 3rd hole of the wall's right edge. Push two red axle-pins into the gear, one at the top and one at the bottom. These bumps are the cams!
Slide a bush, the knob wheel and another bush onto the front of the cam shaft, and put a half bush on its back end. The two bushes hug the knob wheel so it cannot slide.
Drum: push two blue long pins down through the 1st and 2nd holes of one long side of the red 5×7 frame, and slide the two middle holes of a thin 4-hole beam onto their ends underneath. The frame is the drum skin.
Do the same on the other long side: two blue long pins and a thin 4-hole beam.
Cymbal stand: push blue long pins up into the 5th and 7th holes of the drum's back side and slide two split pin blocks down onto them, one on top of the other.
Push a red axle pin into each block, pointing back, and press a 7-hole beam, standing up, onto both.
Push a 3 axle through the top of the stand and put a half bush on its back end. Slide the big gear on its front, right against the stand, as a cymbal, and lock it with a half bush on the tip.
Set the drum on the front end of the table: its four blue pins go into the 9th and 10th holes of the table's long sides.
Drumsticks: hold the blue 15-hole stick and the yellow one in front of it, and push an 8 axle from the front through their end holes, a bush, and the 5th hole of the wall's left edge. Lay both sticks down so their far ends rest on the drum.
Push a tan axle-pin into the far end of each stick and put a bush on each as the stick's head: behind the blue stick, in front of the yellow one.
Slide a bush onto the stick axle right in front of the yellow stick, so the sticks cannot slide off their cams. Then slide the second 5×7 frame onto both axles and press its bottom row onto the red pins.
Put the red crank on the front end of the cam shaft, and a half bush on each end of the stick axle.

Play time

Crank slowly, then fast. Pull one red bump out — what happens to the beat? Move it to a different hole and invent your own rhythm!

Real-world inspiration

Cams in music boxes and automata — https://en.wikipedia.org/wiki/Cam