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Posted: 10/30/2010 4:54:52 PM EDT
| I'm about to start acquiring the tools for building a couple of AR's. Since I would like to build from scratch(no complete uppers,etc) I'm told a torque wrench is essential If I'm going to do it right. I've seen comments about the lack of calibration on the less expensive torque wrenches but I don't want to tie all my tool/fixture money up in a torque wrench. What do you experienced builders use? |
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ANY torque wrench should be calibrated before use and about annually. I bought a Harbor Freight click-type torque wrench for doing barrel installations, and I have checked its accuracy more than once (it's a simple test to do*). But here's the deal: even a Craftsman wrench can drift out of calibration, and I'm not sure that Sears will replace a Craftsman torque wrench just because it's out of cal calibration is not covered under warranty. In fact, the only way to demonstrate that a wrench is not calibrated is TO CALIBRATE it. Calibrating a torque wrench, if you can find a lab that will do it, can cost $75 or more, so it's a judgment call on whether to spend over $100 on a Craftsman wrench that will still need regular calibration. On the other hand, a Harbor Freight wrench that is no longer accurate is effectively disposable-just spend another $20-$25 and replace it. (And test the new one fresh out of the box, because if it's off you can get it replaced as "defective.")
*How to check the accuracy of a click-style torque wrench: 1) Secure the square socket drive fitting in a solid vise-make sure to avoid the ball detent with the vise jaws-set the wrench handle horizontal or parallel with the floor. 2) Set the wrench to the desired lowest torque setting. 3) At exactly 12" from the center of the drive fitting, suspend a weight that's LESS THAN the selected weight –– the wrench should not "click." 4) Add weight slowly, and note the weight at which the wrench "clicks." Write down both the selected torque and the actual weight that caused the wrench to click. 5) Set the wrench to the desired highest torque setting and repeat steps 3 and 4. 6) Compare both sets of values. Note how close both sets are. To test a beam-type torque wrench, simply apply the weight at exactly 12" from the center of the drive fitting and compare the weight with the reading on the scale. This of course doesn't ADJUST the calibration of a wrench, but it will allow you to evaluate whether or not the wrench is within a given tolerance. Edited to add new information. |
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30 foot pounds of torque isn't much - a 3/8 box end wrench can develop that and break off clutch bolts on a flywheel. If anything, the upper limit is more important, to prevent stripping the threads when aligning a sprocket to fit the gas tube. 80 foot pounds is about what most lug nuts should get - if the tire shop actually bothers to set up the air gun properly.
The torque value that the wrench is set for is not the actual pressure. Using antiseize and a wrench adds to the actual stress by quite a lot. It's just an indicated value that was chosen to give the optimum results without damage. The tool set was specifically designed to assist an armorer of any age or gender in getting repairs done to as many as 350 weapons under their care - all of which were fully assembled from day one. That's why the instructions don't always fit, an upper from any other source other than Colt or FN means it's not guaranteed in conformance with the TDP - which are the very specific measurements that give exactly the right results. That means being more of a gunsmith in assembly, knowing what to do when it won't turn to a sprocket properly. What torque wrench to use is much less important at that point than how to properly relieve excess material. Mechanics know a good tool will help get the job done, but a good mechanic can still do it even with poor tools improvised on the spot. Case in point, there are many posts - with pictures - of wood blocks deckscrewed to tables as a lower vise block in the mag well, blocks made from scrap 2x4 for the upper, a plumbers strap wrench to tighten the barrel nut, etc. Pin punches aren't necessary for the lower, a pair of vice grips does well and is less likely to scratch the finish or break off a trigger ear. Alternatives abound. An armorer's tool set is exactly what the government makes it, a set of old school tools picked by tradition bound servicemen to do a job they learned on Garands. There are better choices. |
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