Rapid Checking and Inspections of Equipment in Mass-Production Work

Posted By Richard Barker on 06 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No2. All of  the information contained within this article is valid as of 1940. The article discusses methods employed during inspections when parts are mass produced.

During the various forging and machining operations employed in the mass-production of a motor-car, a strict check is kept on every part at each stage of its progress through the works. To take only two instances, at the Vauxhall works there is one inspector for every ten men in the factory; while at the Ford works more than 3,000 of the measurements carried out on parts during production are to within 5-10,000ths of an inch. Equally specialised are the many tests applied at other factories in order to ensure that materials and parts will give satisfactory results in service.

The manner in which the successive tests depend on one another is demonstrated during the assembly of pistons, rings, gudgeon pins and connecting rods. The diameters of the piston and of the cylinder bore are accurately checked by means of a pneumatic micrometer. The piston rings are checked for circularity by inserting them in a steel ring gauge behind which is a narrow ring of bright light. As the gauge is rotated, failure of the ring to make contact with the gauge at any point will be revealed by light shining between the ring and the interior of the gauge.

Piston Ring Diameter

Next, the diameter of the ring is checked by pushing it up a tapered bore until the gap closes. The distance that the ring can be pushed up the bore indicates its size, to within limits as close as .005 inch. The position of the ring in the bore indicates the piston to which it can be fitted. As a check on the strength of the ring it is usually mounted on edge in a measuring machine in which a known weight is applied to the edge of the ring through a plunger, when the gap should just close. Special designs of ring, into which a pressure pattern is built during machining, arc checked by measuring the outward pressure at a number of points around the circumference of the ring, when the pressures recorded should fall within the area of a graph obtained by similarly measuring a master ring.

A  careful process of selection then takes place. The gudgeon pins must be an exact fit, while the f pistons, gudgeon pins and rings must -balance by pairs for weight. After the connecting rods are fitted the complete assemblies are again weighed and balanced to within two drams, so that sets can be selected from a graded batch when assembling the engine. Again the pistons, after fitting the connecting rods and gudgeon pins, are checked in jigs in order to make sure the perfect alignment is obtained.

Crankshaft Balance

In order to obtain a smooth-running engine, crankshaft balancing is the next essential. Every crankshaft must first be balanced statically—that is, while it is at rest. When the crankshaft is rotating, however, further out-of-balance forces may develop, and consequently the shaft must be tested in a dynamic balancer. In one type, the crankshaft rotates freely on rollers, and the fluctuation in its weight caused by any lack of balance is shown by the vibration of needles on dials which record the weight carried by the bearings at each end. The operator is able to adjust counterbalance weights until the needles become stationary, and thus knows not only the extent of the adjustment necessary to balance the shaft, but the position and weight of any balanced mass which needs correction.

A more advanced form of dynamic balancer is equipped with a dial in marked degrees of crankshaft rotation; around this moves a pointer which is coupled electrically to the bearings carrying the shaft, and which rotates at the same speed as the crankshaft. By an ingenious electrical mechanism the pointer indicates by a series of sparks the exact point at which the crankshaft is out of balance, the sparks appearing against the graduated dial. A perfectly balanced crankshaft will spark evenly all round the dial.

Naturally, the other rotating parts must be balanced with equal care. The clutch is balanced while rotating at speed in a machine which is similar to that used for the crankshaft, and the unwanted weight quickly located and metal removed by drilling. The flywheel, on the other hand, is frequently mounted on a vertical spindle carried by a pivoted bar, so that if there is more weight on one side of the flywheel than the other, the bar tends to tilt to that side on its pivot. The tilt is indicated by a highly sensitive spirit level, along the outside of which a weight can be slid so as to give the position and extent of any out-of-balance mass. A drill incorporated in the machine lines up with the heaviest point in the rim of the flywheel during this test, thus allowing the surplus metal to be removed.

Camshaft Inspection

An important test which affects the power output of the engine vitally is the final inspection of the camshaft, during which it is verified that the various diameters are accurate to within half a thousandth of an inch, that the cams are correctly located in relation to each other, and that the lift of each cam is correct. The latter two operations are carried out by mounting the shaft between centres, with one cam beneath a small spring-loaded tappet which moves a needle around the face of a calibrated gauge, thus indicating the lift of the cam. Meanwhile, a graduated timing disc is attached to the end of the shaft, so that it can be ascertained that the inlet and exhaust cams will open and close the valves at the correct points, as indicated by markings on the timing disc.

The strength of the valve springs is measured by compressing the spring between two plungers, one of which rests on the platform of a weighing machine. The pressure exerted by the compressed spring must not vary more than one or two pounds on either side of its designed strength, which may be about 40 lb.

One of the most important series of tests is the gauging of the gearboxes; often from 50 to 100 separate dimensions have to be verified. At the Austin works over 30 plug gauges, alignment bars, and graduated eccentric sleeve gauges are used during the inspection, which takes fully 20 minutes in the case of each gearbox. Meanwhile, the gears themselves are checked before assembly: The gear-testing laboratory provides a check on production methods. Occasionally a gear is taken from the batches which have been passed by the shop inspectors, and is sent to the laboratory, where it is subjected to elaborate testing instruments, among which are those which enable the gear teeth to be tested for concentricity, tooth spacing, involute tooth form, helix angle, and meshing. Tools such as gear cutters and hobs are also examined in the gear-testing laboratory before being passed to the shops for use.

Rapid Tests

These elaborate testing methods, however, would be too slow for the bulk of the output. Consequently, a fixture which is used extensively for checking each gear or cluster of gears before assembly takes the form of a fixed head carrying a power-driven spindle on which the gear to be tested is mounted. This gear meshes with a master gear on a moving carriage. Errors in the design of the gear teeth will cause either changes in velocity or variations in the distance between the centres of the two gears.

Consequently, it is a simple matter to arrange for movement of the carriage carrying the master gear to be shown on a dial graduated in thousandths of an inch, so that a defective gear running in mesh with the master gear can be quickly detected by undue flickering of the needle.

In a more elaborate version of this tester the movements of the flexibly mounted spindle carrying the master gear are transferred through a multiplying mechanism, which magnifies any errors 200 times, to a pen which traces a line on a moving chart. If the gear is eccentric the curve produced will swing to one side of the centre line on the chart during half a revolution, and then back to the centre line. Badly-formed teeth will produce sudden humps in the curve; the magnification and sensitivity of the tester is such that roughness of the contacting surfaces of the gear teeth is revealed by the graph.

In order to test the meshing of the gears, they are run in contact with one another, or with a master gear, while marking compound is applied to the teeth, so that the extent of contact can be quickly seen. At the same time, excessive noise can be detected while the gears are running. After the gears have been assembled in the gearbox, a final test consists of coupling the gearbox to an electric motor and running it under load with each gear engaged in turn, so that unwanted noise can be detected.

Checking Contours

The contours of gear-wheel teeth, screw threads, and similar parts can also be verified by means of an optical projector which throws a magnified image of the part on a screen. If a similarly enlarged drawing of an ideal part is pinned to the screen, the two can be compared and any inaccuracies detected in a moment. When dealing with screw threads and small pinions, the Zeiss microscope, as it is termed, is used. The shadows of larger parts, such as gearbox pinions, hobs or gear cutters are thrown on to a table in a darkened cubicle by a larger projector working on a similar principle.

Hardness Tests

Many parts have to be tested for hardness as well as accuracy, and in the majority of instances an instrument incorporating a diamond point is used. In one type the part is placed on the table of the machine and a plunger carrying the pointed diamond is pressed down on to the metal by a known weight. In effect, the diamond "pricks" the surface of the metal, and the dial gives an extremely sensitive reading of the depth of penetration, although the indentation is barely visible to the naked eye. Tappets, valves and gudgeon pins are typical parts tested in this way, when accuracy combined with speed is essential.

In the laboratory, where time is not .such a vital element, the depth of the impression caused by the diamond point is not recorded on a dial, but is examined by means of a powerful microscope which forms part of the machine. The impression made by the diamond appears as a dark square in the centre of the field of the microscope, while two adjustable knife-edges throw dark shadows on each side. These shadows are adjusted until they touch the corners of the indentation, enabling its extent to be measured optically with extreme accuracy.

Among the more drastic tests regularly carried out in the average factory are those which determine the resistance of parts to hammer blows, crushing and stretching. In the first instance, the component is gripped by the jaws of a vice in the path of a swinging hammer. The distance through which the hammer swings after breaking the sample is a measure of the toughness of the material under test. Other machines are capable of crushing even the largest components, including cylinder block castings, applying a load of over 100 tons. By rearranging the machine it is used for tensile strength tests, in which the parts are stretched to breaking point.

Road springs are tested for deflection in machines which apply a weight far greater than that for which they were designed. In some cases, as at the Ford works, samples are taken at random and placed on a machine which continuously deflects them under a heavy load thousands of times an hour for 40 hours. All materials, such as leather, hood cloth and trimming material, are tested to destruction by taking representative samples from each consignment, gripping one end in a clamp attached to a spring balance, which may read up to 10 cwt., and stretching the material until it tears.