Industrial torque meters cost a lot ($4500). I'll show you how to build a low buck version with an Arduino.
Why? Because I want to test power tools to see if the actual specs live up to the marketing specs.
The reason this works is Load Cells are cheap on-line. They work by measuring the deflection in an aluminum beam. Normally that deflection is detected by an electrical device called a Wheatstone Bridge. When the Wheatstone Bridge stretches or contracts it changes the resistance, which is then read by a micro-controller.
The trick here is the Wheatstone Bridge doesn't care if it is stretched by a simple weight or by torque. All we need to do is calibrate the software to change the common, dirt-cheap load cell to a rare and very expensive torque meter.
Step 1: Required Parts
1- Load cell 50kg rating. More or less depending on the torque you need to measure. I found that the 50kg load cell is good for about 81 Nm (60 lbs ft)
2- Load cell amplifier. I got mine from Sparkfun for $10 you could make your own for less, but these guys built an excellent tutorial and breakout, so it's well worth the price. https://www.sparkfun.com/products/13230
3- Arduino Uno or your preferred flavour of microcontroller.
4- Software to run the Arduino: IDE, example sketch from Sparkfun and library to run the HX711 load cell amp.
Thank you to both Bodge for his excellent HX711 library and Nathan Seidle (Sparkfun's CEO!?) for his excellent sketches.
Step 2: Machine the Load Cell for Conversion to Measuring Torque
Drill and tap a 1/2" NC hole in one end of the load cell.
Install Bolt in the end of the load cell.
Test torque reading using a known torque. In this case I use a torque wrench set for 40 lbs ft.
The Arduino sketch that comes with the load cell amplifier makes it easy to change from measuring weight to measuring torque. You just need to run the calibration sketch and adjust the calibration values on the fly.
Step 3: Test Your Tool!
Open the serial window in the Arduino IDE and tighten the bolt using your drill. The load cell will measure all the torque values and the you can scroll back through the serial window to determine the maximum torque that your tool outputs.
I tested the veracity of the Milwaukee Drill torque claim of 425 in lbs and my test results indicate that it will output a maximum of 480 in lbs. So the claim is correct!
Here are the resources for the load cell amp.
The free program that they have made for this is excellent.
If you are not familiar with Arduino, it is an amazing device that is designed with non-technical types in mind and there is a HUGE community around it.
Step 4: Bonus Challenge! (and a Small Caveat)
You might be able to get one load cell to measure different ranges of torque by drilling and tapping additional holes in the end of the beam. If you locate the holes in different positions, you'll get different values. All you need to do is calibrate each threaded hole individually.
It is important to note: If you apply too much torque you will permanently deform the beam. You'll see that the values don't return to zero when the torque is removed. This means you will have to re-calibrate!