I made stress testing machine for biomechanics about two years ago, but it’s still in use regularly, and quite interesting how cheaply it was made.
I’ve originally posted it here:
I, and a colleague once needed a stress testing machine with 1-2 micrometer positioning accuracy and capable of measuring forces as low as 100 micro Newtons for our PhD theses (we were testing mechanical properties of tissues for pharmacologic experiments). In the lab we only have large machines in 100 kN - 6000 kN range. When I said that we could assemble one for about 1k$, no one really believed, but we got small grant for it.
The machine had a large subwoofer speaker as an actuator, controlled in closed loop (with linear encoder) using modified audio amplifier and CNC controller. To isolate it from external vibration we embedded a bicycle tube in machine table. We assembled the machine in my flat, and when we proved that it works correctly, suddenly there was a lot interest in it 
Here’s the machine:
The the subwoofer inside it:
And a crossection showing a bicycle tube inside:
The stepper motor on machine does absolutely nothing
