Instruments For Measurement and Inspection

Posted By Richard Barker on 12 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No26. The article’s contents are accurate as of 1940. This particular article looks at precision tools used in Engineering at the time.

The pressing need of an ever-increasing production of munitions, guns, aeroplanes, tanks and other equipment for the fighting forces has emphasised more than ever before the importance of efficient inspection equipment to ensure an uninterrupted flow of components in the workshops. At the machine, in the tool stores, the inspection department and the measuring room, inspection must be both rapid and accurate to maintain the quality and quantity of the products.

To control accuracy in any manufacturing plant the range of inspection equipment required comprises reference standards, length and diameter measuring machines, screw-thread measuring machines, projectors, optical comparators and general-purpose equipment, such as protractors, micrometers, squares, plugs, rings and screw gauges.

Increased Efficiency

The efficiency of inspection methods has been considerably increased by the introduction of instruments incorporating optical systems which not only provide more accurate results, but eliminate troubles due to wear of mechanical parts. Optical projection is now largely used for the inspection of screw threads, hobs, templates, gear teeth and other profiles. The Dekkor, enables this projection to be done quickly and accurately. The projector is supplied for use either in a dark room or with a special cabinet for use in daylight. With the daylight cabinet the projector is a self-contained unit mounted on casters so that it can be easily moved. The cabinet contains the reflecting system, the image being projected on to a translucent screen at a convenient height for the operator.

The daylight cabinet is an ideal adjunct to a thread-grinding machine. The operator can quickly check the work and make necessary corrections without loss of time. The projection equipment is mounted on the top of the cabinet. The work-holder is provided with male and female centres and also a clamp for holding templates and other flat work. Screw vertical and sliding horizontal adjustments are provided and the work-holder base can be rotated for setting to the helix angle of screw threads. A scale is provided for this purpose. Diameters, lengths and pitch of screw threads can be measured by the micrometers fitted to the vertical and longitudinal motions of the work-holder. Thus, by means of slip gauges, the full traverse in either direction can be utilised.

Interchangeable Illumination Systems

Two interchangeable systems of illumination are available. The lamphouses are quickly mounted on an arm which is adjustable for different angles of illumination, a scale and index being provided. The standard lamp and lens give parallel light for the projection of all the usual types of screw threads. A patented lamphouse and lens is used for deep threads and large helix angles.

This illuminating system gives an inclined strip of light which continuously varies in the angle at which it impinges on the screw. By means of a simple adjustment with index and scale, the illumination can be varied to correspond with the helix angle within the height of the screw thread.

Two magnifications are available: 50 to 1, and 25 to 1, the fields of view being .3in. x .25in., and .6in. x .48in. respectively. The maximum size of work admitted between centres is 8in. by 3in. diameter, using the 50, to 1 magnification, and 6in. diameter using 25 to 1 magnification.

Glass templates of all standard screw threads and a protractor can be supplied. These are placed on a projection screen. When work of irregular form has to be checked, a suitably enlarged drawing of the profile can be made on translucent material, such as, Kodatrace, and placed on the .screen with. a piece of clear glass on top. Photographic records can be made by means of a printing frame substituted for the projection screen.

Checking Gauges

Comparator instruments are now largely used for checking gauges and for the rapid inspection of mass-produced components. In some instances these instruments are a highly accurate comparator for work up to 6in. in length or diameter.

The measuring head is adjustable for height by means of a knurled nut engaging a steep thread on the vertical column. Fine adjustment is effected by a knurled collar below the measuring head, and the final precision zero adjustment is provided by a lever which moves the scale itself. The measuring range of the scale is .002, the smallest graduation being -.00005. Readings can therefore be readily estimated to .00001in.

The circular work-table is grooved and lapped flat to a high degree of precision. A hole in the centre of the table will accommodate smaller types of tables for measuring shoulder work, flat strip or wire. Side and back stops, vee bearings, centres for turned parts and other holding and registering devices are fitted to an adjustable bracket at the bottom of the vertical column.

High-Precision Measurement

For high-precision measurement and inspection in the measuring room, the Galileo contour-measuring projector, has a wide field of application. It will deal rapidly and accurately with screw threads, giving information on pitch, form, diameters, angle of thread and position of angle relative to the axis.

Profiles of gauges, form tools, gear teeth and similar parts can be checked by projection against an enlarged outline of the respective profiles. Angular measurement can be approximated to 1 minute by means of a protractor. Inspection of surfaces by episcopic projection can be carried out and permanent photographic records of any work, dealt with can be made. Two systems of illumination are provided, one from above for inspection and measurement of profiles, and one from below for the inspection of surfaces and contours of dies and press tools, the intensity of illumination enabling the instrument to be used in daylight.

Measuring Range

The instrument .can be provided with a plain work-table. and stage glass, measurements being made on the projection screen. Alternatively, a table with longitudinal and cross movements (4in. by 2in.) controlled by micrometer screws reading to .0001in. is fitted. The measuring range of the. micrometer is 1in. For measurements above, 1in. gauge  blocks are used between the micrometer anvils and the slide.

Adjustable centres can be mounted on the table for the inspection of cylindrical work. For thread gauges the centre support can be adjusted to accommodate the helix angle of the thread. Another-type of comparator for checking and measurements by direct reading to .00001in. and by estimation to .000001in. is an instrument which is arranged for wall mounting and involves the use of a highly sensitive spirit level.

The end gauges to be compared are wrung on to an accurately finished circular plate spigoted into a cast-iron base plate, which can be levelled by three adjustable feet. The bracket carrying the spirit level is mounted on a vertical support on which it can be roughly adjusted to accommodate the gauges. Final adjustment to bring the level into the measuring position is effected by a knurled screw.

The spirit level is carried loosely in a tube. On the base of the level is a block carrying two ball contacts which project through the tube. When lowered into the measuring position the balls make contact with the gauges and further lowering raises the level off the screws on which it is normally supported. In this position it rests freely on the gauges and is prevented from moving sideways by a pin, in the top of the level, which moves between two guides.

The scale on the level is graduated in hundred-thousandths of an inch and a mirror is provided to facilitate readings. Differences in the lengths of gauges can be readily estimated to .000001in. End standards up to 40in. long can be dealt with. In use, the level is lowered on to the gauges and a reading is taken on the scale. The level is then raised, the table rotated through 180 deg., and the level again lowered. A further reading is taken and the mean of the two is the difference in length of the two gauges.

Measuring in Millionths

Since the whole inspection system in any works is based on reference gauges, which wear in use, an instrument of the finest precision is required for checking them periodically. This can be done rapidly on the Eden Millionth comparator which will check slip gauges up to 1 in.

The instrument is made to the design of the National Physical Laboratory and is certified by them. Slip gauges to be compared are inserted between two anvils, one of which is adjustable to the capacity of the gauge and the other free to move. Three ball contacts are mounted in the form of a triangle on the adjustable anvil and a single ball on the other. The moving anvil operates a lever, at the end of which is a ring, carrying a strand of spider's web.

A beam of light is projected vertically through the ring. A magnified shadow of the web is reflected on to a scale of 4in. long, one inch representing .00005in. Combined mechanical and optical systems provide an enlargement of the anvil movement of about 20,000 to 1. Direct readings in units of .00001in. can be made and millionths can be readily estimated.

Where accurate measurements of diameters of screw or plain cylindrical plug gauges are required, a diameter-measuring machine will enable readings to be taken directly to .0001in. The measuring head is mounted on a carriage having longitudinal and cross movements. This head carries a micrometer of special design, which is graduated to .0001in.

At the other end of the slide is a mechanical indicator which provides a movable anvil, incorporating a system of levers giving a magnification of the anvil movement of about 250 to 1. Effective and core diameters of screw threads are measured by means of cylinders and core prisms suspended from supports on the top of the carriage.

(This article and the illustrations which accompany it are reproduced by courtesy of Alfred Herbert, Ltd.)