Perfectionism gets in the way of completion

Last year, I was looking at some articles by Steve Ogden and Bre Pettis who were discussing the relative merits of getting things done vs getting them perfect.

Surely the answer is to get them done right and to use Engineering parlance, to make them within tolerances.

“TOLERANCE is the allowable variation for any given size in order to achieve a proper function.”

However, how do you know what tolerance to use? Well back in the days before mass manufacture every shaft and hole had to be made as a pair otherwise you could not guarantee that they would fit. So some bright spark came up with the idea of standardising so that you could have two manufacturers make items and they would fit together.

The way this works is that you have a defined minimum and maximum size for each size of hole and the same for the shaft. Even if you had the hole at it’s smallest size and the shaft at maximum you would guarantee that they would fit ok and even if you had the other extreme the fit would not be too loose. Dependant on the type of fit you need e.g. does it need to rotate or slide vs a press or force fit then the dimensions are slightly different. This also makes the job easier for draftsmen as they don’t need to specify the detail of the dimensions on the drawings simply put the nominal size and have the fit type specified. I expect clever CAD software will put the detail of the dimension in a table automatically for you. The machinist / CNC programmer would then look up the dimensions from a chart and determine how to machine the parts. Again it’s possible that some CAM software will understand how to apply the tollerances directly.

As well as standards for holes there are equivalent standards for geometrical tolerances. For example when working with castings you could machine all of the surfaces to a high standard of dimensional accuracy. However, that rather wastes the point of casting them in the first place. Instead you need to know which surfaces need to be parallel, perpendicular, concentric, in line or at a defined angle and hence machine just those to a suitable standard.

When designing parts you need to understand how those will be made and hence don’t over engineer them with a greater tolerance than necessary. Machining parts to a defined tolerance means that you won’t need to spend hours trying to make them to fit.

References:

Tolerance, limit and fits
Tolerances and fits, terminology and definitions
Tolerancing System
ISO Tolerances For Fasteners
Fits for holes and shafts – tables ISO 286-2
Geometrical Tolerances
Idiots Guide

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