Related Resources: mechanical-tolerances
A Study of tolerance limits and fits for engineering purposes, with full tables of all recognized and published tolerance systems.
H. G. Conway
Open: Engineering Tolerances
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The use of tolerances, limits, and fits in engineering design is an essential factor in modern manufacture, whether by mass, batch, or even job production. ' It is surprising, therefore, that there appears to be no. publication which explains the scope of the various published limit systems, and endeavors to instruct the designer in the proper use of such systems.
The gulf between designer and shop man—a gulf into which many platitudes have been casts not made less by the ignorance of the average designer of the meaning and significance of tolerances and fits, nor of the practical man of interchangeability requirements. This book, therefore, is an attempt at a clarification of the position, the prime object being to persuade the designer to use tolerances and limits with sincerity and genuine intention, both shop man and inspector then being in a position to treat the designer’s drawings with more respect than is current in most production shops today.
The author feels, as a result of some study and experience, that modern inspection methods have not kept pace with general technical and production development. The accuracy of absolute measurement which is possible today is, of course, very high indeed, but the inspection methods used in the average shop leave much to be desired from the point of view of accuracy in the hands of semiskilled personnel. It is probable that few designers realize the extent of the errors introduced by the comparatively crude systems of measurement (plug and gap gauges, etc.) which are being used in their production shops. Since it is the final result and its performance in the field which count, it is not suggested that these errors are necessarily serious, but a proper understanding between designer and producer can only come when the truth is known.
In the author’s experience a serious defect in the training of young designers is that they have no ready means of knowing or finding out what are the practical limitations of accuracy of the machines which are to be used to make the components they are designing. Some attempt has, therefore, been made at stating, with what authority the author’s experience may entitle him to, on the one hand, and a considerable research into published material where and if available, on the other hand, the inherent limitations of the main types of machine and production processes. It is known, for example, that a skilled tool room milling machine operator can, and does, mill to “half a thou.,’ probably less. This has no relevance, however, because the designer should not be concerned with what can be done, but with what is reasonable to expect from his production shop, making full allowance for wear of tools, a lower grade of skilled labor, the condition of the plant, etc. For a designer to expect a 4 in. lug to be milled in width to less than, say, 0*002 in. is, therefore, entirely wrong. Where tolerance qualities or grades are quoted in this book for various processes, they are certainly not the ultimate in possibility, but are quite definitely put forward as the limit to which the designer should go.
The tolerance tables which are as comprehensive as possible and which will, it is hoped, be found of value for convenience of reference, have been checked with care several times to eliminate errors ; it is a pity that the restrictions of the present day have made it impossible to set some of the more complicated tables with horizontal ruling to assist the eye in following a line of figures. As explained in the text, however, a designer should extract from the complete tables those fits which most apply to his particular products and will, therefore, prepare his own abbreviated tables.
Chapter I Fundamentals of theory of mechanical tolerances
Chapter II Limits and fits in the design office
Chapter III Quality control and their interest in tolerance specifications
Chapter IV Standard limit and tolerance systems
Chapter V Standard limit and tolerance systems
Chapter VI Tolerances in miscellaneous fabrication processes
Chapter VII Screw thread tolerances
Chapter VIII Gauge tolerances
- Micrometer reading tests
- Errors in measurement due to surface indentation during gauging
- Coefficients of linear expansion, modulus of elasticity
- Position errors expressed as angular equivalents for a radius on one inch
- Conversion table for small tolerances