IF the lead of the work produced does not correspond to the nut into which it is fitted, do not condemn the die, but measure the lead of both the work and the taps with a scale, providing you can get both in length of 4 or 6 inches. It is practically impossible to make taps that will lead accurately on account of varying results in hardening. This element of uncertainty is eliminated in this die, as explained on the two preceding pages. The error in lead of taps is usually so great that it is plainly visible on 1 or 1 1/2 inches of length. A scale placed on tops of teeth will show at the even inches the error, and at the 1 1/2-inch graduations if the pitch is an even number to the inch.
Fifty per cent of the taps now in use should be discarded When you order new taps ask the maker to select taps of good lead, and if necessary pay an extra price for getting the cream. It will be worth it if you want good work. Measure the diameter of the taps and see that there are no burrs or fins in bottom of thread to spoil shape of thread in the work.
The so called thread gauges, in the form of a circular nut, though nicely finished and hardened pieces of steel with an internal thread, are very misleading.
All that has been said in the foregoing regarding the impossibility of making correct lead dies is equally true of these gauges. Furthermore, such gauges wear in directions for which an adjustment cannot be made. A more unreliable gauge could hardly be invented.
The three distinct dimensions of a screw thread should be measured separately. The shape and lead should be measured when the die is made; in other words, the die should cut a correct shape and lead; then the third dimension, the diameter, should be measured when the die commences on a lot of screws, and occasionally thereafter. The thread may be measured by the ordinary micrometer, snap or ring gauge, taking the diameter at the top of the thread.
As the die becomes badly worn, the lead should be measured occasionally. This can be done by cutting a thread 6 or 12 inches long and measuring it with a good scale, remembering that all scales may "look alike" and yet not be the same in length; hence get a good scale.
The various forms of screw lead-measuring devices may be used with economy of time and material, but such gauges should be handled with special care and occasionally compared by the foregoing method.