Practical Notes on Plug Gauges - Part 2

Kitmondo 09 May 2014

This article first appeared in Practical Engineering 1940 Vol1 No20. The information is therefore correct as of 1940. The article provides practical notes on developments in manufacturing at the time.

It will have been realised that gauge wear is extremely important, and one of the nonsensical things that is still prevalent is to gauge a case-hardened hole with a case-hardened gauge of practically equal hardness. Obviously, if there is any grinding grit present it will cut the gauge as much as it cuts the hole and therefore it is essential that the gauge should be very much more wear-resisting than the hole it is gauging.

Chromium Plating

Chromium-plated gauges have a higher surface energy than case-hardened gauges, by which is meant that they are more slippery and subject to less abrasion. This is shown by their increased "life" which is approximately five times longer than that of the hardened gauge, together with the fact that they are not affected by ordinary atmospheric, conditions. The cost of a chromium-plated gauge is only a few shillings more than that of a hardened gauge, so that the hardened gauge must cost less than one-fifth of the cost of the chrome gauge to be a financial proposition. This, of course, is impossible, so that although the chrome gauge is initially more expensive its cost over a period is lower.

Stellite and Tungsten Carbide Gauges

Gauges made from these materials have a still longer life, and it would be premature to state the actual increase in life as experiments are still being carried out. But from the available data it seems that at least fourteen times the amount of extra life over a hardened gauge may be expected. Such gauges will probably cost from four to five times as much as ordinary case-hardened gauges, but here again, in the long run, maintenance cost will be lower. With regard to these long-wearing gauges there is one point that must not be overlooked ; that is that the gauge-wear allowance can be reduced to the absolute minimum. It will be appreciated that the smaller this is, the greater the tool life, and if it is possible to use five reamers on a batch of parts instead of nine, the cost of four reamers will be saved. This will, in all probability, more than offset the extra cost of the gauge and show a good profit.

The "Pilot" Plug Gauge

Mention has previously been made of the difficulty of getting the ordinary plug gauge into a "size and size" hole without jamming under production conditions. If this were possible a further extremely valuable H-tenths would be available for tool wear. The "Pilot" plug gauge, shown last week, simply and efficiently overcomes this difficulty and it has been stated that if it were not for the "Pilot" gauge in use on an aeroplane engine part with a 2-10ths limit, it would be almost impossible for the part to be made.

If the ordinary type of gauge is not square with the hole it jams across the hypotenuse of the triangle. This can be amply demonstrated by examining a plug gauge that has jammed; a bright ring, known as the pressure ring, will be seen about 1/16in. back from the entering end of the gauge, at a point where gauges are normally scrapped for being undersize. As the hypotenuse of a triangle is longer than the base, which in this case is the outside diameter of the plug, a very powerful wedging actions occurs firmly gripping the gauge so that, more often than not, it is necessary to knock the gauge out, with the subsequent damage to the gauge and the hole.

Absence of Pressure Ring

The pressure ring mentioned above is never allowed to exist on the "Pilot" gauge for where this would normally occur a groove is provided as shown in Fig. 5. If the gauge is not square with the hole it pivots about the very narrow land, which in effect amounts to a section of a ball, and the edge of the hole strikes the chamfer lifting the gauge into line and it falls in. This is shown in Fig. 6. This means, and it has been proved by very careful tests, that the "Pilot" gauge will enter a "size and size" hole without jamming and without hesitation, and thus enable the shops to have the benefit of the valuable 1 1/2-tenths normally denied them.

Fully Approved

The "Pilot" gauge is used in practically every aircraft, machine-tool and engineering firm throughout the country, and has received the approval of the Air Ministry, Ministry of Supply, Admiralty and the National Physical Laboratory. It is the subject of world patents, and further particulars can be obtained from the "Pilot" Plug Gauge Company Limited, Queen's Road, Coventry.