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There's such tool called impact driver (like this one for example). The manual says it uses a special mechanism to produce a series of torque impulses to fasten/loosen nuts and screws.

The tool has only two adjustments - the forward/reverse switch and the On/Off switch. The deeper the On/Off switch is pressed the faster the tool motor rotates.

There's no clutch to adjust the torque value after which the clutch would disconnect the motor. How do I control the torque without a clutch?

Suppose I need to assemble a section of fence that will require say one thousand screw/nut connections with nuts of the same size (that's not an imaginary task - I did such job without an impact driver). Obviosly I need to tighten all nuts to more or less the same torque. If I overtighten a nut it can just break or cut the screw or bend the parts it connects - that's bad. If I undertighten a nut it will soon loosen and fall out - that;s bad as well.

So let's pretend I use nuts that need to be tightened to approximately 70 Newton-meters and the driver I use is said to produce torque up to 100 Newton-meters.

And of course I need to drive those nuts fast - otherwise why would I use a power tool instead of a screwdriver and a usual wrench? So obviously I can't time the periods of applying torque or count impacts or whatever else - I need some reliable way to achieve more or less the same right torque for all nuts.

Please note: I don't care of achieving exactly the right torque. I only care of achieving more or less the required torque - so that the nuts don't loosen on themselves and parts are not damaged while the nuts are being tightened. The primary concern is that the driver works very fast, so if I hold the switch pressed for just too long I can easily overtighten the nut and cause unintended damage.

How do I achieve roughly the same torque repeatedly for multiple nuts when using an impact driver?

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I have the impact driver you link to, and I have to say I love that thing. –  aphoria Oct 15 '10 at 17:37
    
@aphoria: How do you achieve needed torque with it? –  sharptooth Oct 18 '10 at 9:08
    
I don't really use it in situations that require such tight tolerances. The answers from MarkD and Mike Powell describe really well how I use the tool. –  aphoria Oct 18 '10 at 12:54
    
@aphoria: I guess just about everyone misunderstood my concern. My primary concern is not about precisely the right torque, it's about roughly the right torque. So I edited the question with clarifications. –  sharptooth Oct 18 '10 at 13:05
    
That's where Mike Powell's answer comes in..."Don't slow yourself down unnecessarily by underestimating your own ability to judge how much torque you've applied, and how much is enough for your application." –  aphoria Oct 18 '10 at 17:18

4 Answers 4

up vote 2 down vote accepted

Usually, around the home anyway, there are other signs to let you know when you've applied enough torque. MarkD noted a common one -- when you see the screw head just go below the surface of the wood. In your fencing example, if you're tightening lag screws or nuts with washers, you'll see the washer start to sink into the wood long before you've applied enough torque to shear off or strip the screw or bolt.

It's a great tool for this kind of work, and it's super fast. Don't slow yourself down unnecessarily by underestimating your own ability to judge how much torque you've applied, and how much is enough for your application.

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I would get all the bolts driven, quickly, but not tightly using the torque driver. Don't cinch them down. Then walk along and use a hand held torque wrench on each.

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I see. But this can be done with a "usual" (non-impact) power driver. And that driver also has a clutch. This makes using an impact driver pointless. –  sharptooth Oct 14 '10 at 14:13
    
A hand drill with a clutch likely cannot deliver as much torque as does the Bosch tool. This is why I use an air powered driver to put on and remove the lug nuts on my tractor (and my car) - it can budge the nuts that my drill would fail to move. However, if I needed to tighten a nut that required careful tolerances on the torque, I would always use a hand tool that is designed to yield a specified amount of torque. –  user558 Oct 14 '10 at 14:23
    
Impact drivers are hardly pointless because you don't have some torque limits; they make carpentry so much easier (where one seldom cares about the actual torque). Short of some (very expensive) manufacturing tools, the clutch on every driver I've used is so speed-dependent it's almost worthless. If you want a precise torque, a torque wrench is the way to go. –  Nick T Oct 14 '10 at 17:47
    
Okay, great you mentioned the car. Let's pretend you want to replace a wheel. Every other time I go by some car service I hear recognizable series of impacts - guys definitely use impact drivers for replacing wheels. How do you actually tighten the lug nuts? What's the procedure for that? –  sharptooth Oct 15 '10 at 5:54
    
The good ones are only using the impact wrench to remove the wheels, or to spin the lug nuts on before applying final torque with a torque wrench. –  Mike Powell Oct 15 '10 at 16:06

"How do I achieve the same torque repeatedly for multiple nuts when using an impact driver?"

Short of manually using a torque wrench like woodchips recommends, you don't. The point of an impact driver is not to apply a consistent torque. It is to apply more torque than can be obtained from a traditional driver. One of the more common uses is to loosen seized up, or over-tightened bolts/screws. I also use mine quite a bit when driving screws into wood when the final torque doesn't matter (e.g.- I just need the screw head to be slightly below the surface of the wood.. rough work).

In other words, if your need is consistent torque, an impact driver is not the tool for the job. A traditional clutched driver is going to be more appropriate.

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Assuming the torque can in fact be adjusted you can use a regular hand-help torque wrench to get a pretty close calibration. It's a matter of clamping the later, which has been set to the desired reading, in some way. Start with the impact driver set somewhat lower than the desired setting and connect the two using whatever socket extension joiner you have available. Now gradually adjust the impact driver till it makes the torque wrench clicks or does whatever it does when it's up to its setting.

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The key point of the question is that there's nothing on the impact driver to directly adjust torque. –  sharptooth Oct 19 '10 at 13:31
    
You could use the same method to "calibrate" your wrist. i.e. get a feel for it. Once you know what that torque feels like in your hands and wrists you should be able to replicate an approximation of it. –  John Gardeniers Oct 19 '10 at 20:46
    
Calibrating a wrist won't work in case of tightening hundreds of nuts - hands will become fatigue and also the driver has a far offset handle, so that one really has quite weak control of exact torque. It might work if one needs to tighten several nuts, but it won't with hundreds - whatever one's calibrated for nut number one his hands will forget when he reaches nut number one hundred. –  sharptooth Oct 20 '10 at 5:37

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