What is the most useful tool you ever made?

Being a cabinetmaker, I make my own planes (buying them are very expensive). Below is a photo of a router plane I made for my daughter, she uses it daily!!

Awesome. Professional grade and beautiful. What woods were used?

I used maple and walnut. I made the blade from an Allen key, good hard steel.

Probably the hoe or whatever preceded it.  The day we started breaking up the earth to plant seeds was the first step on the path to civilization.

Whoops I better learn to read more carefully. LOL.  Somehow missed the word 'you'

I was going to say - you are way older than I thought.....

LOL

One of the most useful tools I made was a special probe for a digital multimeter. It consisted of a 1" length of 15mm copper tube (usually used for plumbing) with a length of insulated wire soldered to one end of it. The other end of the wire connected to the positive terminal on the meter, which was set to 20 volts DC (anything above 15 volts would do).

The copper tube was wrapped in conductive foam, as used for packaging of integrated circuits.

With the negative terminal of the multimeter connected to a convenient 'earth' at the rear of my car, and with the rear screen demister powered up, I was able to slide the copper tube along each element of the rear screen demister. On a good element the meter would read the voltage along the element, showing 12 volts at one end and zero at the other. And 6 volts at the centre point. Sliding the probe along the element was a bit like playing a hawaiian guitar but not as musical.

On an element with a break in it the meter would display 12 volts from the 12 volt end up to the break and then zero all the way to the other end. So not only had I isolated a broken element, I also knew where the break was! This came in extremely useful when talking to a salesman about the defects in a second-hand car I was on the verge of buying.

"The conductive foam?", I hear you ask. It was there just to stop the copper tube scratching the glass, or causing more breaks while 'scanning'. Yes, it is resistive, but the resistance is so low that it was completely swamped by the resistance of the digital meter. If I'd used the AVO Mk.2 it probably would not have worked very well.

I still have that probe, it has proved very useful on many occasions since. Ok, once...

Not sure what to call it, but one day I accidentally 'made a tool' that detects the internal RF switches in test equipment.  I was prototyping an automatic level control circuit and connected it to a network analyzer to see how flat its response was over frequency.  It was nice and flat for the most part but I was able to see a series of bumps.  I also manually swept my circuit with an RF signal generator and observed its output on a spectrum analyzer.  Again I saw little bumps.  This time they occured at frequencies that were nice round numbers.  That's when it occurred to me that these little bumps must be happening as a result of the equipment switching to another chain of its internal circuitry.  RF equipment can be very expensive so we generally aren't allowed to open it up.  This accidental tool was able to provide insights into the internal operation of test equipment without the need to crack it open.

I repurposed a tool.

Problem: How to identify which VHF transmitting antenna was bad on a 10X10 platform with over 20 antennas mounted around the edge. VSWR meter readings indicated one antenna had failed. The drawing that identified the antenna locations was not accurate. Sending riggers up the tower to replace antennas until we got the correct one wasn't an option. Replacing antennas required unbolting them from the tower and opening connectors and then resealing them after repair. This took time and sometime introduced problems.

The site had a six foot grounding rod with a twenty foot cable attached used for discharging old tube transmitters before working on them. The tube transmitters were long gone but the rod sat in the corner of the building. I replaced the ring at one end of the cable with the clamp from a set of booster cable. I had a rigger go up the tower and connect the clamp to the metal tower frame and then touch each antennas as I did VSWR readings. Grounding the antenna produced abnormal VSWR readings allowing us to accurately identify all 20 antennas.

The rigger crew chief refused to give me back the grounding rod when the replacement was finished. He had worked in the industry for twenty years. Most people guessed at the antenna until they got it right. That pissed him off especially if he was doing the work at 40 below. The grounding rod solution eliminated the guess work. He kept the device on his truck until he retired. Never guessing at the location of a bad antenna again.