The mechanical torque wrenches themselves can be as accurate BUT with a competent operator. I guarantee you the majority of mechanics are not as accurate as their torque wrench allows them to be. Just the other day I was at someone swapping my tires, first he didn't want to use a torque wrench at all but I had mine in the trunk and said to just put the nuts on and that I'll tighten the wheels. He felt insulted and continued to take his own torque wrench out of the closet. It was a Unior, so quite a decent one (I think they're ~400€ new). He turned it till it clicked, then continue to click it 10 more times on every wheel. I cringed at every click. It's only accurate the first time, everything more is idiocity.
Exactly.
I damaged my car in Germany not long ago, hundreds of kilometres from my home in another country, and had to go looking for a local mechanic with no knowledge of the landscape. The first guy I found was tightening lug nuts in that style: four clicks per nut (screw, really). Swing, click, click-click-click.
That’s at best useless and worst harmful. You should stop at the first click. And you should be swinging the wrench smoothly and slowly at the point of the click: about 20 degrees a second.
What’s more, that final swing should be large: ideally a quarter of a turn. So it should take about four seconds. Studies have shown a lot of mechanics swing too fast and overshoot the free movement beyond the click, slamming the momentum of the wrench handle into the fastener after it falls over the centre of the click.
You might bump into the click unexpectedly toward the beginning of the final swing. In that case, back off (with another tool) and adopt a wrench position and body position that allow you to approach the target torque slowly over a quarter-turn with a good grip on the centre of the handle, applying force purely tangentially to the arc of the swing at the moment of the click.
And if you encounter stick-slip (creaking, etc.), abort the attempt and clean and lubricate the threads or replace the fastener. Accurate torque measurement is not possible in the presence of stick-slip.
Another couple of points too often ignored in the haste of a busy professional environment:
First, unlike beam-type or electronic torque wrenches, click-type wrenches should be exercised (clicked) a few times before the first use of the day. This is ideally done at max torque, but target torque for your application gives most of the benefit. This breaks up corrosion and replenishes lubricant at the sliding surfaces of the mechanism, returning the wrench as closely as possible to its calibrated state.
Second, screw in the target torque from below. That’s how the wrench was calibrated. Hysteresis will harm accuracy if you approach target torque from above.
Third, it follows from the above that you can’t check the torque of a fastener without turning it. Sticking friction (stiction) may prevent a bolt from turning at its target torque even if its
tension is much lower than it should be (i.e. it has backed out). To check the torque, start again. Break the fastener loose and then set the torque again with the long, slow, smooth final swing. (If you need to check whether a fastener has worked loose, give it a witness mark and compare before and after re-torquing it.)
As is often the case, the tool matters less than how it’s used. Of course nice tools are nice to use.