I purchased a new torque driver for use in the CTN lab. It is the Wiha TorqueVario 2.0-7.0 Nm (model 28655), pictured below. This is the highest value driver in their variable torque driver selection with a range that is appropriate for tightening 1/4-20 bolts on the table. We already have the much lower range Adjustable TorqueVario 15 - 80 In/Oz (model 28501), but this range is only appropriate for very low torque applications like fastening PBS and mirrors. In the future we might like to get a mid-range driver to cover the whole range.
I also purchased a selection of Phillips and flat head driver blades to go with the driver heads because they were relatively cheap. All the Wiha blades are exchangeable between their torque tools, they will likely come in handy for a range of precision applications.
So far I've only tested the new driver qualitatively.
The range of torques applied by humans in the lab varies widely and there isn't a lot of (good) advice out there on the optimal value. Over torquing leads to deformation of the table, this can misalign optics in the short term and that then undergo a very slow relaxation over a long period of time. In the worst cases the table or opto-mechanical components go past their elastic limit. An under tensioned bolt is obviously bad. Without a strong rigid connection to the table the optic mount may be free to move in a number of lower frequency modes that otherwise wouldn't be allowed (bad). Rana has recommended an applied torque of ~5-6 pound-inch (6.8-8.1 Nm), this is supposed to be just below the limit of most aluminum and steal plates before they go from elastic to plastic deformation.
Here is an except from LIGOX chat channel from Rana:
Most of the time, it doesn't matter. You can use whatever seems right to you. However, in situations where precision matters, you have to consider what the requirement on the fastening is: e.g. when clamping a 1/4" thick base to an optical table, we use 1/4-20 screws because that's what the table is tapped for. The screw length should be chosen so as to use all the threads in the table.
But, how much torque should be used?
If too much is used, the aluminum base will be deformed so that it is no longer in the elastic regime. Once there is significant plastic deformation, there will be slow mis-alignment of the mount.
Washers increase the total force which can be used, since it reduces the pressure on the soft aluminum given a fixed force. For the usual set of Thorlabs hardware we have the correct torque is ~5-6 ft. lbs [6.8-8.1 Nm]. Similar numbers can be found for other cases by considering what materials are being used.
Washers increase the total force which can be used, since it reduces the pressure on the soft aluminum given a fixed force. For the usual set of Thorlabs hardware we have the correct torque is ~5-6 ft. lbs [
. Similar numbers can be found for other cases by considering what materials are being used.
I tested a few different values of torque on the table for a 1/4-20 bolts (with washers) directly on the table in the ATF lab. I used the south table in the ATF lab as many of the tapped holes in the CTN lab have been damaged from over-torquing and contaminants in the treads.
A dozen 1/4-20 bolts (with washers) were fastened with identical torque values starting at 2 Nm (see second picture). Between each torque cycle I undid the bolt under test with a regular ball driver to get a feel for force used. From their I incremented the applied torque by 0.5 Nm on each tightening cycling working up to 7 Nm. When undoing a bolt there is a kind of a 'crack'. This is the point at which the fastener goes from vertical contact friction to loose thread-only friction.
I found that the 'soft crack' point was 2.0 - 2.5 Nm. The transition to 'hard crack' (an audible click) occurs at about 3.5 Nm. However, interestingly, the variability between 3.5 Nm tightened bolts seemed to higher; 3 out of 12 bolts gave a softish crack. Its likely that the particulars of washer-table-bolt surfaces may change the crack point. I found that a torque of 4.0 Nm gave a guaranteed hard crack without seeming qualitatively excessive. The transition between regimes was a rapid one and above 4.0 Nm the friction hold was about the same, giving about the same 'crack'.
I found the recommended 6.8-8.0 Nm was very tight. A value of 7 Nm required a very strong grip on the driver, this is the kind of torque that might only easily be applied using a T-handle driver or a long allen key. A value of 7 Nm seemed unreasonably high compared to what is usually used, this is at the upper range I've seen in various labs.
My initial recommendation is for 3.5-4.0 Nm tightening of regular bolts for most mounts. My usual peronsal choice is for a soft crack at around 2.5 Nm.
Wiha sells fixed value torque drivers in increments of 0.5 Nm (see Wiha EasyTorque), these fit the standard blades and are reasonably priced. They also sell fixed value Wing Handles that have a compact profile. We may want to do some scientifically rigorous tests of various post-fork and base-dogclamp combinations to see what the best objective torque value is.