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When it comes to lean manufacturing and quality, Henry Ford is my hero. His first book My Life and Work is, in my opinion, the best book ever written on lean manufacturing, bar none. His second book, Today and Tomorrow (1926) addressed many of what we would today call green, or conservation issues. The third book by Henry Ford, Moving Forward, addresses a number of manufacturing issues and how Ford solved them. All three books are must reads for lean manufacturing practitioners.
I've posted below a chapter from Moving Forward which deals with quality and metrology at Ford Motor Company that I find particularly interesting. If you can't measure it, you can't control it and control is the key to assuring quality.
If you would like e-book and/or PDF copies of My Life and Work and Moving Forward, drop me a note at johnhenry@changeover.com
A millionth of an inch
Chapter XIV of Moving Forward
By
Henry Ford and Samuel Crowther
Author note: The material that follows in this chapter is based on Mr. Johansson's description of this most interesting science and has been read and revised by him.
[From footnote in chapter - JRH]
The making of things to a high measure of accuracy is not just a test of workmanship. It is a fundamental to service production. In such production there can be no fitting of parts in assemblies or in repairs. Every crankshaft must be exactly like every other crankshaft and so on through every part of the automobile or anything else that one may be making by this method.
Of course, no two parts are ever absolutely alike except by accident, for it does not pay to try for accuracy beyond a certain point. But any kind of a machine which has moving parts must be accurately made or there will be an amount of vibration through play that will shorten the life of the machine and also decrease its running efficiency. A designer always plans a machine made to absolute measurements, and then he sets the limits by which each part can be allowed to vary from the measurements. In volume production the measuring has to be done quickly, and for that reason each part is judged by two gauges of whatever designs prove most suitable for the part. These two gauges represent the allowable limits of size. If the part passes through both it is too small, and if it passes through neither it is too large; to be within the limits it should pass through one and not the other.
Some parts do not require accuracy in any high degree, while others must have limits of one ten-thousandth of an inch. If a gauge is going to check to one one-thousandth of an inch—which is not extreme accuracy—then that gauge must itself be accurate to at least one ten-thousandth of an inch. And it must be kept accurate. Thus there must be a master gauge to test it—and finally a gauge to test the master gauge. All of which presumes that somewhere on the premises one has some absolutely accurate measurement with which to test the master gauge.
In point of fact, it was not always possible to know what an inch was until we had C. E. Johansson join us. And today the functioning of our industries depends upon what goes on in a small room in the Dearborn laboratories. And so also does the functioning of many other industries, for in this room are tested the precision blocks not only for our own use but also those that we sell to other industries, for it would not be in line with our policy to keep the means of accurate measurement exclusively with us. These measuring devices are available to anyone who wants them, and they are the standards of all accurate work in this country and in most parts of the world. The obtaining of a standard of accurate measurement is not so simple as it might seem to those who measure with a foot rule and think they are measuring. The finest steel rule made is so inaccurate that, if we had to use it, we should wreck our production. That gives some idea of our necessity for accuracy.
Until recent years there has been no way of attaining a real accuracy in machine production. For we have had no standard inch for shop work. It is service production that has forced the making of standards of accuracy. The English foot of twelve inches was legalized about the year 950, but fought for more than three hundred years before it supplanted the Belgaic foot of the Tungari, which was 13.22 inches, and was brought to England at the time of the Belgian influx before the Tenth Century. The origin of the Belgaic foot is lost in the obscurity of ancient Asia and Greece.
The metric system is the result of a demand by French scientists that the unit of measure be based upon some measurable and immutable distance in the universe. The French National Assembly chose a distance equal to one ten-millionth of a quadrant of the earth through Paris. This distance was duly computed, and the resultant metre-a-traite, or measure of distance, known as the metre, was made the compulsory national standard in 1801. In 1875, the International Bureau of Weights and Measures was constituted in Paris to furnish prototypes of the metre to subscribing countries. The official French metre is represented by the distance lying between two microscopic lines on an iridio-platinum bar (now reposing in the archives of the Bureau), at zero centigrade.
The alloy and the cross-section chosen for this rod are least susceptible to change. Temperature is a vital factor in computing the metre, since, as the temperature of the rod goes above or below zero centigrade, the distance between the scribings becomes less or greater than a metre. The consideration of temperature was one of the great problems previous to the advent of the Ford-Johansson gauge.
The metre is measured at zero centigrade; the English inch at sixty-two degrees Fahrenheit, and the U. S. inch, by Act of Congress, July 28, 1886, at sixty-eight degrees Fahrenheit. Considering these temperatures it is evident that, to bring the three standards into accord, the units must be held at three different temperatures. If a standard (French) 100 mm. steel gauge, at zero centigrade, having a coefficient of expansion of .000,011,5 per one degree centigrade, is measured at the English standard temperature, it is .019 mm. longer than 1 decimetre. Measured at the U. S. standard temperature, it is .023mm. longer than 1 decimetre. Conversely, a U. S. standard decimetre gauge and an English standard gauge become correspondingly less than a decimetre when measured at the French official (metric standard) temperature.
Since these standards of measure have been adopted, another method of checking the accuracy of measures has been discovered. The instrument used is called an interferometer. Its operation is based upon the constant length of the rays of light in the spectrum. Should the official metre gauge be destroyed, it might be reproduced within one ten-millionth of its theoretical length with the aid of the interferometer. This instrument has shown that the metre is equal to 1,533,164.13 wave lengths of the red ray of the spectrum of cadmium in the air at fifteen degrees centigrade, and 760 mm. barometric pressure. But of what avail is such a method of measuring in a machine shop?
Less than forty years ago most of the difficulties experienced in commercial measuring in mediaeval times still existed. Each country, each large industry, and even each toolmaker had individual measures. While these measures all conformed to either the metric or the English standards, no two agreed unless made from the same material, in the same shop, and at the same time. Each shop had its own set of master gauges from which measurements were sometimes taken directly, but more often these gauges were used to make other individual gauges for specific work. Since gauge making was slow, expensive work, it followed that a shop doing any volume of business had an accumulation of gauges that represented an investment out of all proportion to the amount of use which they received. The matter of the investment, however, was only incidental to the gauge difficulty.
When a gauge is made of steel it must be hardened to withstand even moderate use without appreciable wear. When a piece of steel is hardened tremendous internal strains are set up within the surfaces of the gauge which cool and set first as the gauge is quenched after heat treating. These strains are gradually relieved by slow expansion of the metal—a process that may continue for a year or more after the gauge is completed. Also if a gauge is not used at the same temperature as that at which it was finished its size will vary with the difference of temperature.
Thus, what with a variation of standards, a variation of materials, and a variation of temperatures, the paths of the machine and tool manufacturers were anything but rosy. Arguments between a mechanic on the warm side of a shop with one on the cold side often became legal battles between seller and buyer which involved broken contracts, months or even years of litigation, and often not inconsiderable damage awards by courts. The court decisions could never establish precedents, for they were rendered arbitrarily for lack of any universally recognized standards of measure.
C. E. Johansson began his career as a mechanic in Eskilstuna, Sweden, in 1885, and as tool-room foreman of the government's arms factory he first faced the problems arising from an inaccurate system of measure. Although well equipped with the usual quantity of gauges, scales, and measuring devices, the arsenal did not adopt the tolerance system of measures until 1889, when they undertook the manufacturing of arms, in the present-day sense, in place of the highly individual method of arms manufacture which obtained from 1867 back to the days of the stone axe and arrow makers. During the period 1867-1889 the arms were manufactured by machines, and although the quality was good the quantity production was limited because the limit system was not used.
One of the first points which impressed him in those days was the preference of toolmakers for solid blocks of steel finished to a specific size corresponding to some specific dimension to be imparted to the manufactured part. Scales varied in their calibration and micrometers varied in the leads of their threads, but once a machinist measured a dimension to a steel block, and it was accepted, he worked speedily and confidently to the block until it was worn out. Consequently, Mr. Johansson specialized on these solid gauges. Soon there were hundreds of them in the arsenal and hundreds more being made all the time to take their places. At each change in specifications, from two to a hundred of these became obsolete—most of them long before they were worn out. Though inches, half inches, quarter inches, and the like did not change, parts could not be designed to suit the existing gauges—gauges must be made to suit the designs.
Mr. Johansson, from his experience, knew that two gauge blocks, if finished with the most painstaking care, would adhere slightly when wrung together. A close study of the finished surfaces of such blocks held a promise that several gauges might be made so accurately and so perfectly finished that they might be used separately or in combination to produce from one to a number of measure values limited only by the number of gauge blocks used. He began work along this line at once, but almost immediately another difficulty was encountered. No method for determining the size or the parallelism of the surfaces of these gauge blocks was accessible. Hours of work were necessary to measure any gauge, and even then the margin of error was great. Eventually it was solved in a manner that permitted the instant gauging not only of the flatness and the parallelism of the opposite surfaces of a gauge but the determining exactly how far these surfaces were apart. However, as this end was approached, a new difficulty became increasingly great; namely, the stabilization of the steel itself from which the gauge was made, a difficulty which seemed insurmountable.
Unless a gauge block is heat treated and hardened, its usefulness is soon greatly impaired by wear. In the case of a gauge the accuracy of which is required only within one one-thousandth of an inch—and it is now a quick and simple process to divide this accuracy by ten thousand, measuring to one ten-millionth part of an inch—this accuracy would be completely lost with very little use unless the gauge were rendered glass hard by an adequate heat treatment. It became evident, when working in such small dimensions, that a block which was heat treated and finished to within one ten-thousandth part of an inch of the measure value desired, would begin to expand, very slowly, after it was finished, until at the end of a year it might measure several ten-thousandth inches more than when finished. The larger the block and the greater volume of steel it contained, the greater the growth after finishing.
By the year 1899 gauge blocks could be made by Mr. Johansson so flat that they would adhere when wrung together. Their surfaces could be placed at a predetermined distance from each other within a margin of a few hundred thousandths of an inch. The surfaces of the blocks could be rendered so nearly parallel that, when placed in combination, the cumulative error in several blocks was no greater than the error in a single block of the same length as the combination. Selection of the steel best suited to the manufacture of gauge blocks and the discovery of the heat treatment which would render this steel sufficiently stable required experiments over a period of nine years. Then a universal gauging system passed from an ideal to an accomplishment.
The first set produced contained 102 blocks. Years of observation had shown that the temperature of the average machine shop varied between fifteen and twenty-five degrees centigrade; therefore twenty degrees centigrade (sixty-eight degrees Fahrenheit) was selected as the standard temperature at which these and subsequent blocks should be finished and used, except blocks made for use at specified temperatures other than sixty-eight degrees Fahrenheit.
Sets of these gauges were introduced into the United States in 1907, and were received with some hesitancy. Purchasing agents at first considered them too fine for their requirements. Even if they allowed themselves to be persuaded by shop superintendents to purchase sets, they frequently placed them in the office vault and permitted their use only when a difference of opinion on measures arose. While American manufacturers were accustoming themselves to the possession and use of gauges, the same gradual acceptance of this new standard was going on in all foreign countries. As the gauges became known their possibilities became increasingly evident. The limit system of manufacture, that is, the process of holding the dimensions of each part of each assembly between two limiting dimensions as determined by these new gauges, became universal. Even the limits became somewhat standardized. Thus, one standard permits a round rod to fit a round hole in four degrees. These are: running fit, push fit, driving fit, and forced fit. In the first class, the rod slides in the hole easily; in the second, it may be pushed into the hole but fits snugly; in the third, a hammer is needed to drive the rod into the hole; in the fourth, a mechanical or hydraulic press is needed to force the rod in. In the first, or running fit class, standard practice requires the rod—supposing it to be more than 1" and less than 2" in diameter—to be less than the diameter of the hole by not less than .0015", and less than the diameter of the hole by not more than .0025", or a limit of variation of .001", the difference between the two tolerances. In the case of a driving fit, the limit changes from less to greater; the diameter must not be less than .00075" greater than the diameter of the hole, nor more than .002" greater, or a limit of plus .00125"; the limit on the case of a forced fit is plus .002".
If rods similar to the one described were to be used in quantities, a gauge for measuring their diameters would be made. For the rods the gauge would consist of four pieces of steel, set in a horseshoe-shaped holder, two far enough apart to allow the rods to pass between them in their diameters were not greater than the maximum tolerance specified, and the other two close enough together to keep the rods from passing between them unless the diameters of the rods were below the low limit prescribed. Such a gauge is called a "go-and-no-go," or snap gauge, and the points may be spaced to the specified distances apart by setting them to gauge blocks, or combinations of them, of the measure value desired.
The accuracy of practically every manufacturing operation carried on to-day may be traced directly to the use of these gauges. The Ford Motor Company has one hundred and forty complete sets in daily use, besides a quantity of single blocks for purposes where complete sets are unnecessary. This number does not include many pocket sets individually owned and used daily to test the accuracy of micrometers.
The alliance of the C. E. Johansson Company with the Ford Motor Company furnishes an interesting chapter in the development of the gauges. Considering the importance to the Ford Motor Company of high accuracy at a low cost, the gravitation to it of the C. E. Johansson Company, with exactly that combination, was inevitable. Also, by means of the resources and the manufacturing talent made available the price of the gauges has been materially decreased since the Johansson Company was taken over.
Thousands of gauges and fixtures must be manufactured in our tool rooms, and by using these blocks it is possible to make them to the necessary limits. Without the Johansson blocks and tools our tool rooms would be blind. By using them the gauge inspectors also can tell at any moment the condition of the gauges and fixtures in service.
How important this is may be realized from the fact that, when one of the gauges is worn to the point it no longer checks the limits properly, one hour's run may mean several thousand pieces of scrap, tying up of the assembly line, or the holding up of some branch in a remote part of the world.
If extreme accuracy is to be obtained economically — as mass production demands — the master standard gauges must be just as near to size as it is possible to make them. Any error in the master is multiplied several times before the working gauges reach production. When production machines or the assembly lines are held up for minutes, or the fraction of a minute, the cost is very high. Every error in a gauge must be paid for in the slowing down of machinery and the loss of parts.
Take, for example, the snap gauge used in the manufacture of the push rod in the car. These cost approximately four dollars each to repair; twenty are used on each eight-hour shift, and their life is approximately four hours. On all gauges twenty per cent of the tolerance is the wear limit. That means a wear limit of one ten-thousandth of an inch or one hundred-millionths. Hence, it costs one dollar for each twenty-five millionths of an inch wear.
At least three sets of Johansson gauge blocks are needed to pass these gauges every time they are sent to the tool room for repair. One is used by the man who repairs; another by the tool-room inspector; and a third is used in checking them every four hours. If an error of many millionths exists between the three sets, more gauges would be returned to the tool room than actually wear out. Production would be retarded and costs would run high. Therefore, we have determined that it pays to hold the master gauges to the extreme accuracy of not exceeding plus or minus two millionths of an inch. Thus the inspection gauges for the tool room and for inspection have an accuracy of plus or minus four millionths, and working gauges can be supplied the production line at an exceedingly close limit, well within an accuracy of fifty millionths of an inch. All of the close-limit gauges used are checked once every day, many of them three times every eight hours. Under the guidance of Mr. Johansson himself, the workmen are able to control degrees of accuracy at a saving. He believes that it is no harder with proper means to hold to a close limit than to the old accepted accuracy.
We have searched practically the entire world for special gauging equipment. Much of this is reconditioned by the Johansson division after it is received. And it is a large undertaking to get dies, jigs, and fixtures together to build accuracy. A large proportion of the machinery we use must he rebuilt to meet the accuracy standards.
One of the chief requirements in the gauging system is a definite manufacturing—wear tolerance, established and maintained. The first thought, of course, must be accuracy; the second, speed. The third is the original cost, plus the amount of repairs and reconditioning necessary. There must be some swift, accurate way to determine when it is time to retire a gauge from use. For this, our men use the Johansson block.
The manufacturing of these blocks is carried on in the Dearborn laboratory, and they must be finished at a temperature of sixty-eight degrees Fahrenheit. This requires a special room. The constant-temperature room in which the finest gauges are finished and tested is seventeen by eleven and one-half feet. Its walls are of plaster to a height of three feet; above that they are of glass. The glass walls are double, with a space of twelve inches between, which serves as an air duct. The glass in the outer wall is of triple thickness, that of the inner of but one. Throughout, the glass is set in heavy felt and hermetically sealed in its frames. The doors, of which there are two, are fitted with weather strips. The walls and ceiling are insulated with two layers of cork, laid directly on masonry surfaces, and the floor is insulated by the same material, laid in asphalt cement. All walls have a four-inch base of cement.
A constant temperature in the room is maintained by circulating air at the proper temperature in the air duct formed by the double walls of glass. A number of electric elements supply warm air to the duct whenever it is needed. A refrigerating machine supplies cold air. Thermostats at three points control the flow of warm and cold air. The presence of workers in the room reduces the amount of oxygen and adds to the amount of moisture in the air. To make up for the first an ozonizer, having a capacity of six grams of ozone per hour, is used. With this are installed two dehydrators, capable of supplying this room with thirty litres of dry air per hour, by means of which the moisture is counteracted. Mercury tube lights are used for illumination. To prevent their heat from interfering with the temperature of the room, they are separated from it by a case of glass, and the resistance coils with which they are equipped are placed outside the room.
The making of these gauge blocks is an intricate and highly skilled process. Some of the gauges used for shop work are very ingenious, and they must be tested by special apparatus. Take the wrist pin hole in the piston of a Model A motor. It is checked by a special Johansson internal gauge, which in turn is set by a ring gauge. The wrist pin hole and the pin must fit within limits of .0003 of an inch. To test the ring gauge we have a machine that will check the inside walls of a hole. The machine is known as an "inside optometer." It looks like an elaborate microscope. The ring gauge is placed in the centre of the machine on a tiny platform, which can be swung in any direction. It is brought to position beneath two jaws, or measuring parallels, that extend downward toward the floating table. These parallels are set with Johansson blocks, and the distance between is thus known accurately. On the tips of the jaws are jewels, placed there to provide a hard, unyielding contact against the inside walls of the ring gauge. After the parallels have been set against the inside walls of the gauge the operator, by looking into an illuminated tube on the right of the machine, is able to detect whether there is any variance. If the walls are within one half of one ten-thousandth of an inch, the operator can see the difference in greatly magnified degree as through a microscope. It is enlarged so much that the operator can halve the size just mentioned—which is one hundred times finer than the size of a human hair.
This instrument may be used to detect not only a tapering in the walls of the hole but also the exact inside and outside diameters. It can also tell whether a tool or part is out of round, and by how much.
Here is another example. In the use of screw threads many dimensions must be checked to insure proper fits when the threaded parts are assembled. For example, a bolt and nut must fit together. Both the external thread on the bolt and the internal thread in the nut must be measured. That can be done with master gauges made and threaded like the parts to be fitted. With these the parts can be tested during manufacture.
But how to measure the accuracy of the master thread gauges? The answer is found in an intricate-looking machine in the gauge-inspection room. It is known as a "universal measuring microscope." With the microscope in this machine, the screw threads can be magnified to such proportions that they can be easily measured. It is the second machine of its kind to be brought to America; the first is in our Dearborn laboratory.
Checking is accomplished by use of the knife-edge method. The knife-edge is a narrow blade of steel having an angle on one end. On the knife-edge a fine line has been engraved at a given distance from, and parallel with, the edge formed by the angle at the end. In the microscope are lines that have been spaced apart the same distance as that between the engraved line on the knife-edge and the edge of the knife. Two blades of steel are used. The knife-edge of one blade is placed in contact with the flank of the screw. The other is placed on the opposite side of the screw thread so that it comes in contact with the opposite flank of the screw.
By means of the microscope one of the lines in the eyepiece is superimposed upon the engraved line on one of the knife-edges. A reading is then taken on a magnified vernier scale. The carriage of the machine is then moved until the other line in the microscope eyepiece is superimposed upon the engraved line of the other knife-edge. A reading is again taken on the magnified vernier scale, and the difference between the two readings is the correct pitch diameter.
There are three different phases of a screw, all of which will affect its accuracy, and which all must be measured. These are the pitch diameter, which technically is the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points as to make equal the widths of the threads and the width of the spaces cut by the surface of the cylinder; the lead, which is the distance a screw thread advances axially in one turn; and the angle, which is the angle included between the sides of the thread measured in an axial plane.
All of these phases can be checked on the universal measuring microscope. In fact, since the machine was installed, the accuracy of thread gauges in the plants has improved almost one hundred per cent. The machine will determine whether a surface is absolutely straight and square. It will measure either round or tapering surfaces equally well. It is especially useful in checking templates and in laying out holes. It has been said to measure as fine as one ten-thousandth of an inch.
With a dividing head, this machine can be used to determine angles or indices. The head is placed in position in a specially constructed groove. Through its magnifying lens the operator can look down on a circle, the degrees of which have been so much enlarged that each is divided into three parts and each third into twenty. The operator can thus read the minutes as well as the degrees of the angle and can check within limits of one minute.
These, of course, are not manufacturing gauges. They have to be so nearly absolute in their accuracy only because the gauges in use must be what they purport to be, else the parts turned out in production will not fit. An error at the source is multiplied many times when it reaches the part.
One ten-thousandth of an inch is a workable accuracy, but to insure that degree of accuracy one must be able to control to perhaps a millionth of an inch. That we can do.
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