Best torque screwdriver
- Thread starter nun_hunter
- Start date
I don't know if either of you read the article I linked to above...
Action Screw Torque
But he shows that the calibration or accuracy of the torque wrench is the least of your worries if you are trying to precisely replicate the clamping force...
Alan
"Some thoughtful shooters have taken to using a torque wrench to tighten their action screws prior to shooting. If you tighten them the same right before shooting, the theory goes, you'll have the same clamping force and better accuracy. If only that were true.
You can easily calculate the approximate clamping force a screw generates:
Clamping Force = Torque / ( Friction Constant * Thread Diameter )
or, more succinctly,
F = T / kd
The friction constant (k) is an empirically derived number that typically ranges between 0.1 and 0.5. Precise, clean, well lubricated threads will have a number closer to 0.1. Rough, dirty threads will be higher. A rule of thumb for regular dry screws is to use 0.2.
For example, a torque of 40 in-lb on a 1/4" screw with clean dry threads (k = 0.2) will exert 800 pounds of clamping force!
But did you notice how variable that pesky friction constant was? Simply adding a decent bolt lube to the screw can take your friction constant from 0.2 to 0.1, which doubles the clamping force. What this means in practice is that torque is a bad way to precisely regulate a screw's clamping force. A rule of thumb is that you should be able to get within 25% or so with a torque wrench, which is better than nothing, but not exactly precise.
Snip...
But what about the inconsistent clamping force problem? I've shown you why torque wrenches are not all that helpful. So what is? Well, one of the best way to measure the clamping force of the screw is to measure the length of the screw under load (that is, when it's tightened) and compare it to the unstressed length. You will see that the bolt actually stretches a little. That stretch is very consistently proportional to the clamping force. But how do you measure such a thing? You can't exactly whip out the calipers and measure the screw inside your rifle.
You can, however, do something similar. If you use precision, high quality screws, and precisely machined, clean pillars you will be able to use a very simple technique. Just tighten the screw to the point where the screw head just touches the pillar and there is no wiggle or play. At this point, there is no bolt stretch or clamping force to speak of. Next, turn the bolt head until you have your desired clamping force . The angle through which you turned the screw head will be proportional to the bolt stretch, which will be proportional to the clamping force. You can use a torque wrench to figure out how far you need to go, but once you've found the right angle to torque to - use that, not the torque wrench.
It's not perfect - this method is a little fiddly and requires precisely machined parts, but is generally considered a more accurate method than using a torque wrench."
Alan,
It's totally impractical to measure the fitted, tightened length of an installed bolt on a riffle, given both ends need to be accessible.
My limited experience is with building engines. Torque setting are often given 3 ways, with engine oil applied to threads (given weight) graphite grease, and figures to be used with a stretch gauge, where one is usable, typically connecting rod bolts.
The other critical input, is that of "experience", and "feel".
These days, I rarely assemble engines, and don't have to tighten bolts on my rifles very often either, mainly just the barrel screws on the Blaser R8, where accuracy appears to be pretty insensitive, based on torque.
Most of us do any of this so rarely, that "experience" doesn't come into it, so using a tool "consistently" should give repeatable results, which is what we look for.
It's totally impractical to measure the fitted, tightened length of an installed bolt on a riffle, given both ends need to be accessible.
My limited experience is with building engines. Torque setting are often given 3 ways, with engine oil applied to threads (given weight) graphite grease, and figures to be used with a stretch gauge, where one is usable, typically connecting rod bolts.
The other critical input, is that of "experience", and "feel".
These days, I rarely assemble engines, and don't have to tighten bolts on my rifles very often either, mainly just the barrel screws on the Blaser R8, where accuracy appears to be pretty insensitive, based on torque.
Most of us do any of this so rarely, that "experience" doesn't come into it, so using a tool "consistently" should give repeatable results, which is what we look for.
I quite agree, "feel" and "experience" are key!
His suggested "taking up the slack and then giving a quarter turn" or similar seems quite a good way of getting the clamp force reasonably even should the circumstances allow.
I have rebuilt a few infernal combustion engines over the years and latterly have been using my lovely Snap-On torque wrench...unfortunately I made the worst mistake using this finest of tools...I was putting the cylinder head on to my little Fergy 35 and managed to misread the manual for the torque figures and used those for the 4 cylinder engine 90-105 ft/lb and not the 3 cylinder Perkins @ 55-60 ft/lb...pretty well double! Luckily none sheared or stripped before I realised "this ain't right" and luckily it did manage to get the hay in that season, but I have since replaced the studs and nuts at £100+
Alan
The point for using a torque screwdriver is as said, to ensure those who don't have the experience, don't muck it up. The old adage, tight enough is good enough, too tight is fxxxed, is not ideal. I have seen some scopes screwed on incredibly badly, as in Racheted right down so the scope wouldn't zoom in or out.
it is a good way of ensuring consistency. I tend to use an Allen key, but I have worked with tools all my life so know not to stick an extension bar on and swing as much as I can. But I would like to use a torque screw driver and may get one at some point when I run out of better things to spend my money on.
regarding the 1% accuracy from Sealey? I will ask them. I am seeing them in a couple of days.
it is a good way of ensuring consistency. I tend to use an Allen key, but I have worked with tools all my life so know not to stick an extension bar on and swing as much as I can. But I would like to use a torque screw driver and may get one at some point when I run out of better things to spend my money on.
regarding the 1% accuracy from Sealey? I will ask them. I am seeing them in a couple of days.
I quite agree, "feel" and "experience" are key!
His suggested "taking up the slack and then giving a quarter turn" or similar seems quite a good way of getting the clamp force reasonably even should the circumstances allow.
I have rebuilt a few infernal combustion engines over the years and latterly have been using my lovely Snap-On torque wrench...unfortunately I made the worst mistake using this finest of tools...I was putting the cylinder head on to my little Fergy 35 and managed to misread the manual for the torque figures and used those for the 4 cylinder engine 90-105 ft/lb and not the 3 cylinder Perkins @ 55-60 ft/lb...pretty well double! Luckily none sheared or stripped before I realised "this ain't right" and luckily it did manage to get the hay in that season, but I have since replaced the studs and nuts at £100+
Alan
Using angles is good, but they can also be deceptive. A combination of angle, and torque works well, but to be honest, on the rifle, I'll go with the tool
I made a torque wrench when I was at school which worked okay...it was a simple pointer running down the length of the tool steel bar reading off a scale. Calibrated/marked by gripping the square drive in a vice and hanging the lifting weights from the Gym off the bar 12" from the centre. Still got it somewhere.
It worked fine on all the old A and B series BMC engines I worked on as a Yoof. The advantage over the click type is that you got the analogue progressive read out as well as the feel.
A number of times using the the click type where you can only feel the difference, presumably down to momentum/inertia, you find that by backing off and then starting again it clicks immediately without any actual further tightening.
Alan
It worked fine on all the old A and B series BMC engines I worked on as a Yoof. The advantage over the click type is that you got the analogue progressive read out as well as the feel.
A number of times using the the click type where you can only feel the difference, presumably down to momentum/inertia, you find that by backing off and then starting again it clicks immediately without any actual further tightening.
Alan
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