The Drawings.— These have been used since very early times, and in many cases to day are the only standards; models not being in universal use. Even with models, they are always an iuvalaable aid for shop reference; while since the invention of cheap duplicating processes such as blue-prints, photostats, etc., their frequent use is actively encouraged. They mostly show each part in detail, give all re quired dimensions and usually the tolerances. Great care must 'be exercised in placing dimensions on them. These should be shown between the points intended to hold certain relations to each other. In no case should the location of any point be given with limits from more than one point in the same straight line; otherwise the tolerances will be mislead ing, as it is impossible to control the variation between more than two such points at once. Wherever possible, a working point or surface should be established in each plane, and all points located from this only; much expense and confusion will be eliminated. Whenever the drawing itself does not give sufficient in formation, notes should be added.
The Gauges are measuring instruments of ' fixed dimensions; and where tolerances are established, show two sizes, maximum and differing by the smallest amount at tainable in practice. For example, the gauge for the size of a hole would consist of two cylinders, the smaller just entering the hole. and the larger not. The wear on gauges is considerable, and they should be and carefully inspected; preferably by com parison of measurements with a duplicate set of master gauges, themselves carefully checked against the model, and standards of the allow able variations from it. Their design depends upon their purpose. Wherever possible, they should admit of ready repair or correction. It is often possible so to construct them that the parts which control the gauging sizes will re ceive no wear, thus keeping the maintenance cost very low and insuring the duplication of the original sizes.
Owing to the inevitable factor of error above noted, it is impossible to make any two gauges exactly alike; and to make them within 1/100,000 inch of each other would cost a hundred times as much as within 1/10,000. With this variation, however slight,. and with work going through several successive inspec tions, much difficulty will be met unless proper precautions are taken. No matter how closely several sets of gauges are made to each other; after very short use a variation develops, due to difference in amount of wear and in methods of handling them. When working to gauges, the work will always follow the gauges. The
setting of a tool is not changed, a tap or a reamer is not replaced, as long as the work passes the gauges used. So if they wear ex cessively and are not set right, the work gets farther and farther away from its original standard, and discovery is perhaps made by loss and discredit. The following practice, if strictly adhered to, will almost eliminate these difficulties, and reduce the entire problem to whether the' of operator, machine and tools is suitable to produce duplicate work: First, a set of master gauges, duplicates to the working gauges, and checked closely to whatever standard is used. These must under no circumstances be employed to gauge a piece of work; only as standards to check the work ing gauges.
Second, a positive limit of error for gauges, governed by the ' tolerance 'on the given part' and the cost of the gauge; all to be inside the total tolerance allowed on the part. For ex ample, a maximum plug gauge could be smaller but never larger than the master, a minimum larger but never smaller.
Third, the sets of gauges should be sorted into according to requirements of in spection. Assurninv a shop inspection and a final inspection, three groups would be needed; one for the operators, one for shop inspectors, one for final inspectors. Again assuming that the final inspectors will use two sets constantly, the shop inspectors three, and the operators four, nine sets will be in use at the same time. All gauges should be graded according to size; the two of each type closest to the masters will be sent to the final inspectors; the next three, varying a little more inside the limits, to the shop inspectors; the last- four, with the greatest but still allowable variation, to the operators. Each operator should hold his work to the gauges furnished him, regardless of any variance from those of other operators.
Fourth, all gauges in use should be checked periodically against the masters, and discarded, repaired, or regraded according to condition, re of which job they were returned from. he frequency of this will be determined by practice, as a capable gauge man will soon determine how many pieces a gauge will meas ure in given hands -without losing its size. Some can be used only on a few hundred pieces before wearing appreciably, others will measure several thousand. Work manufactured con tinuously needs a duplicate outfit. All gauges of any one type, or the duplicates of any one master, to be used during the same period, should be checked at the same time, to permit of proper grading and comparison.