TODAY'S inspection procedures are much more rigorous, as well as more rapid, than those of 25 years ago. The development of improved standards, refinements in old methods of inspecting parts, and the application of entirely new principles and tests to the science of precision have all contributed to this progress.
To contrast the highly scientific and systematic inspection procedures of World War II with the crude and slow inspections of World War I, Mr. Snider compares the methods applied to the crankshaft of the 1918 Liberty engine with those applied to the 1943 Allison crankshaft - an aircraft crankshaft being used because it covers almost all types of measurements employed in checking machined parts.
During the period under consideration, steel composition has not changed appreciably, and yet, results obtained have improved tremendously. This progress has been due largely to more careful selection of raw materials by the transverse fatigue test and the improvements in finished heat treatment.
Poorly controlled only a few years ago, today's heat-treatment results have been tied to finer and more definite units of measurement. Checking case depth and surface hardness through the use of better-machines has contributed to uniformity and durability of the final product.
Grain size is one of the items that have become more important than in 1918, fine-grain steels now being specified because of better life characteristics.
Inspections start with the forging billets and continue at frequent intervals in the fabrication of the crankshaft to the final visual and dimensional check of the finished part. Checking is done at the machine where possible, but other measurements impractical to check there are made at inspection stations. Precision devices formerly available only to the toolroom or laboratory are now being used regularly at these stations.
New developments in precision instruments that are used in checking the crankshaft include master steel block gages, magnetic inspection, and electric gages. Several other valuable developments are discussed by Mr. Snider, although they are not used in crankshaft inspection.
The production of interchangeable parts in commercial quantities has also been possible because of the improvements in every branch of machine tool engineering. As the means of measurement has improved, so has the degree of accuracy of production increased.