Practical Notes on Plug Gauges

Kitmondo 06 May 2014

This article first appeared in Practical Engineering 1940 Vol1 No19. The information is therefore correct as of 1940. The article provides practical notes on developments in manufacturing at the time.

Limit plug gauges have been known for years, and tolerance, or the allowable manufacturing difference in size between two working parts, is a descendant from the days when it was left to the turner to decide how much "oil clearance" should be left between the shaft and its bearing. This was usually accomplished by placing the shaft in the hole and then inserting a feeler between the two.

For those days, this was a satisfactory method of overcoming the many practical difficulties encountered in turning the shaft and measuring it accurately, but as the bearing was made to suit the shaft this did not matter very much.

With the advent of quantity production, however, this state of affairs could no longer be allowed to persist, as it became absolutely essential that every shaft should be capable of being assembled rapidly and without fitting into any mating component taken at random, whilst still maintaining the maximum working efficiency of the assembled product.

The Limit Caliper Gauge

In order to accomplish this, improvements were made in machines and the more robust and accurate adjustable limit caliper gauge was introduced. The advantages of this are that the hardened anvils can be adjusted for wear and also readjusted to suit another component with a different tolerance. Additionally, once the tolerance has been set by a competent person and the instrument sealed, it is to all intents and purposes foolproof; unlike the micrometer, which is always subject to the human element.

The adoption of these methods enabled the shaft to be made within a known specified tolerance and it was now necessary to find something which would also control accurately and speedily the size of the hole. There are several ways of doing this, some of them very expensive, but under ordinary workshop conditions the double-ended limit plug gauge has the following advantages:

(1)    It is inexpensive.

(2)    It can be checked for size with a high degree of accuracy by comparatively unskilled labour.

(3)    It is foolproof and cannot be tampered with.

(4)    It does represent the part that is going into the hole so that if the gauge will enter, the component itself will also assemble.

Double-Ended Limit Plug Gauge

This consists of two cylinders of hardened steel made to the closest possible accuracy, having a common handle between them. The longer end of the two is known as the "Go" end and the other the "No Go" end. The "Go" end is made to the minimum diameter of the hole, plus a small wear allowance, which will be explained later, and the "No Go" end to the maximum diameter. The difference in diameter between the "Go" and the "No Go" ends is known as the "limit." In order to show this clearly an exaggerated example will be given.

Supposing that it is necessary to produce a hole which must not be smaller than lin. diameter and not greater than 1 1/4in. diameter, or,  in other words, the hole has a tolerance of 1/4in. The "Go" end of the plug would be made lin. in diameter, neglecting the wear allowance, and the "No Go" 1 1/4in. diameter. It follows, therefore, that if the "Go" end enters the hole, the hole must be larger than lin. in diameter, and if the "No Go" end will not enter, the hole must be smaller than 1 1/4in. diameter, showing that the hole is correct and falls within the tolerance set. In practice, tolerances usually vary from 5 thousandths of an inch downward, and aeroplane engine parts are actually being produced to an accuracy of 2-10ths of a "thou."

Wear Allowance

In the above example, reference was made to "wear allowance" which is a small increase in diameter on the "Go" end of the plug gauge only, for if this were not allowed and the gauge were made to the minimum diameter of the hole, as set by the tolerance, the "Go" end could only be used once in the hole, when wear would take place and it would be undersize. This wear allowance is extremely important and is often a bone of contention between the shops and those responsible for the purchase and maintenance of gauges in the factory. It will be seen that the larger this is made the longer will the gauges last before they have worn undersize, thus keeping the gauge expense account down. This, however, has serious repercussions in the shop for, obviously, the larger the wear allowance the less the actual machine shop tolerance for tool wear becomes, the seriousness of which will be explained later. The allowance to be made for gauge wear varies with the tolerance as drawn up by a well-known motor manufacturer.

Ten Per Cent Allowance

Another simpler method which has the approval of the Air Ministry, is to make the wear allowance of 10 per cent, of the tolerance. Under this arrangement let us assume that the hole being produced has a "thou" tolerance, or, in other words, 10 tenths ; then the "Go" end of the gauge will be the minimum size of the hole, plus one tenth wear allowance. Similarly, if the tolerance on the hole is half a "thou," the wear allowance would be half a tenth.

No mention has so far been made of manufacturing tolerance on the gauge itself, for it will be realised that although gauges can, and are, being made to a very high degree of accuracy it is impossible to get them "dead on" without putting up their cost enormously. Gauges whose wear allowance is below half a tenth should not be used on the machine, but sent to the inspection bench. The "No Go" end of the gauge would be acceptable if it is "size"; that is, the largest size of the hole as set by the tolerance to size minus one tenth. This size to minus one tenth on the "No Go" end is primarily intended to cover the manufacturing error in the gauge itself, for it can hardly be claimed as a "wear allowance" since theoretically the "No Go" end of the gauge should never enter a hole to cause this. In practice the “No Go" does enter on one or two unfortunate occasions, but the wear attributable to this is negligible.

Manufacturing Tolerance

From the above it will be seen that there are two important factors controlling the new gauge. "Wear allowance" and the manufacturing tolerance in the making of the gauge contribute to reduce the actual tolerance available to the man on the machine, and an example will probably show the vital necessity of keeping these to a minimum.

It will be assumed that the hole being produced has a half  "thou" tolerance, or, in other words, five-tenths. From the first line of the table it will be seen that the "Go" end will be-the minimum size of the hole as set by the tolerance, plus one-tenth for wear allowance, and the " No Go" end can be the maximum size of the hole minus one-tenth for gauge-manufacturing purposes, making an actual machine shop tolerance available of five-tenths minus two-tenths, i.e., three-tenths.

This assumes that the "Go" end of the gauge can be entered into a "size and size " hole under production conditions, but where an operator has to extend his arm over the machine, this is found to be a practical impossibility. It follows, therefore, that a certain clearance between the "Go" end of the gauge and the hole must also be taken into consideration and this will vary with the diameter of the hole; the larger the diameter the greater the clearance. This one-tenths, plus the two-tenths already mentioned, leaves the operator with only 1 1/2-tenths for tool wear, or in other words the gauging system has robbed the shops of more than two-thirds of the drawing tolerance and every effort must be made to get this tool-wear allowance increased.

With the ordinary plug gauge, the 1 1/2-tenths required for the operator to enter the gauge must remain unless a simple method can be found for helping him, and therefore the wear allowance is the only item which can be reduced, plus a more accurate gauge on which the "No Go" end is either "size" or down a maximum of a quarter-of-a-tenth at the most, instead of the one-tenth normally allowed. Here it may be recalled that under the 10 per cent method of computation the "wear allowance" would be half-a-tenth in the above example, giving a more satisfactory tool life.

For it becomes apparent that although a large wear allowance on the gauge keeps down gauge-maintenance costs, the tool costs can mount to such proportions that it defeats its own object of lowering the ultimate cost of the product. Somewhere there must be a happy medium where gauges and tools take their equal share of the maintenance expense. It is a fallacy to believe that a low gauge account is the only criterion; the tool account, and scrapped components due to gauges with excessive wear allowance not passing them, must also be taken into consideration. It is no exaggeration to say that if gauges had to be replaced three times instead of only once, as at present, the ultimate cost of the job would, in all probability, be lower.