These torque limiting screwdrivers prevent over-torquing by releasing a clutch at the set force. The drivers are used and in varying levels of condition. Some are adjustable and some are preset. Most are useful in the ranges needed for the CZ 452, are more useful on scope rings. Unless noted all are sold calibrated at 20 lb f inch and any problems with with linearity above and below that point will be noted in the descriptions. There are a few that work in a lower range and in that case the calibration will be noted. I am happy to calibrate them to any setting you specify at no extra charge. My calibration tool is accurate by all tests I have used but it is not currently calibrated to a NIST traceable standard.
The preset drivers are designed to set and forget. They can be set to different settings but not by a user and not the way an adustable can be set. Except as noted they are set to 20 lbf in. Some of the smaller ones are set to 12 lbf in because their range is restricted. 12 lbf in is useful for scope ring screws. Weaver-style / Picatinny cross slot screws can be torqued much higher but you should contact the manufacturer for specific guidelines.
The adjustables can be set to any setting in the range of the driver. They are more versatile than preset drivers but you need adjust the driver for the application. Some people have found that varying the torque on the action screws can affect accuracy and the adjustable drivers are great for tuning the gun this way. I can make no guarantees as to the accuracy of the calibration at this time as my analyzer has not been calibrated to a NIST traceable standard recently. However all the calibrated tools I have worked with (20 or so) agreed with the figures reported by my analyzer.Using a consistent torque on the action screws will ensure that the gun has consistent tension on all parts and hence consistent accuracy. It will also ensure that the screws are not over-tightened. Over-tightening can damage the stock and lead to loss of accuracy. Under tightening can leave the gun loose in the stock or prone to loosening.
There are some sources out there for recommended torque settings. You must take care to ensure that you know what the right setting is for your application. Each maker of guns and rings might or might not publish recommended values and the recommendations might vary widely. Setting between 20 and 25 lb f in are commonly used on the CZ 452 action and barrel screws. The force is sufficient for firm contact between gun and stock and it does not unduly stress the wood. For guns where the screw contacts wood directly 10-15 lb f in might be enough to crack the stock. With a pillar bedded rifle you might go to 60 lb f in. So you must be aware of the limits for your particular application.
The values for scope rings varies widely depending on what part of the ring is being tightened. A survey of several manufacturer websites finds a wide range of recommendations for the scope clamps (ring to scope) and mount clamp (ring to gun). 12 lb f in is a resonable torque for the ring-to-scope clamp screws for most manufacturers. Part of the problem is "what are the limits for the fastener?" and "what are the limits for the application?" - where you want the fastener's maximum torque to be less than the application requirements. Overtightening is an issue - it is possible to crush the scope tube by applying excessive force. You can also easily damage the screw by applying excessive force. Some head styles can handle more torque than others - for example Torx head screws handle much more force than Allen head screws, Phillips more than flat, etc.
Using an ordinary screwdriver most people can apply a maximum of about 35-40 lb f in of torque. Using an Allen key it is possible to apply much more than that with less effort as the arm multiplies the torque applied. The force needed to fasten (make fast, keep from moving) anything may be much less than the force that could be applied with the tools at hand. This is particularly true of the small fasteners used on guns. With a torque limiting driver you can set a maximum force to be applied. In general you should resist the urge to go much beyond the point where the screw stops turning. A little more (1/8 to 1/4 turn or so) and the fastening pressure is applied. Any more than that does nothing for the security of the fastening and actually makes the screw more vulnerable to breakage as it is under excess stress.
With the small brass set screws used on my low Weaver adapters the force I recommend it 5 lb f in. The socket strips at between 7 and 8 lb f in. At 5 lb f in the force applied to the receiver is about 150 lbs per screw for a total of 300 lbs. The tip of the screw is about 0.007 square inches for a resulting pressure of 19000 lbs / sq in per screw.
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