fig 60

Fig. 58 An "allowance". Fig. 59 Methods if indicating tolerances. Fig. 60 Dimensional tolerances do not necessarily ensure geometric accuracy.

as the use of a fraction implies the general drawing tolerance. You will always find this on production engineering drawings in the title box in terms such as: Tolerance ±0.02in. except where stated or Tolerance on all fractional dimensions ±1/ 64" or Tolerance ±0.05mm except where stated.

To sum up this section, it is important to realize that setting tolerances on a dimension does not make the workpiece more accurate; the tolerance simply expresses the limits of error which will be acceptable in service. The application of an allowance, however, does ensure that the desired fit will, within the limits of error, be achieved. For one-off workpieces or even for multiple workpieces made by one-off methods, it is best to make the drawing to show the basic dimensions, and to leave the tolerancing to the machinist who is actually making the piece. I have one lathe „ on which I can work to 0.0002 inch, but at least one Duke of Edinburgh Trophy-winning model was made on an old flatbed Drummond which would have had problems at ten times that limit. More depends on the turner than on the machine, the drawing, or the tolerances.

Interpreting tolerances

At first sight the effect of the tolerances shown in Fig. 59 are evident; the dimension will lie between the limits shown, but there is more to it than that. Look at Fig. 60, where I show the effects of the same tolerance on various shapes; you will see at (b) and (c) that although within the limits and although the inspector's plug and gap gauges would show that all was well, the parts just 'fit where they touch'. At (d) is just one possible shape of a square which has been toleranced to 12 ±0.05mm (about 0.002"). (The distortion is, of course, exaggerated.)

Tolerances and manufacturing methods

You will recall the problem set up by possible errors in the pitch of a row of holes in Fig. 53 (d). One solution to the difficulty outlined at (i) in that sketch would appear to be to tighten the tolerance - give the pitch as (say) 1.251/1.249". Then, if the errors were randomly distributed up and down, the dimension 'x' has a chance of lying between tolerances of the same order. But, and it is a big but, how are the holes to be machined? At that level of tolerance you have assumed that they will be bored on a jig-borer or on a first-class toolroom milling machine. So, this leads to the final point in interpreting tolerances. Where these define position rather than simply lengths or diameters you will have to think very carefully indeed about how you are to cope with them. With care (and, perhaps, with some resort to abrasive polishing.) you can work to 0.0001 inch on a centre-lathe, but locating a hole is quite another matter. Now look at Fig.55 (c) again. This very brief discussion of tolerancing should not give the impression that the matter is unimportant. In all production shops it is of the greatest concern; selecting tolerances can make all the difference between a healthy profit and bankruptcy. B.S.308 has a complete volume on the subject and my copy of Machinery Handbook takes up several dozen pages of small print - as well as many tables giving the standard tolerances for everything from sheet metal to ball-bearings.

The model engineer, however, and the amateur machinist generally, will only be affected when the fit of a bought-in piece is involved. Use your judgement while doing the machining rather than putting tolerances on the drawing; no tolerance at all is far safer than one that is inappropriate.

Section 7

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