Tech Talk
Basic Hand Tools II

Tech Talk:
Hand Tools For Measuring Fit & Tolerance

by Brian Pollard

To be sure a part is able to work within another set of parts it is manufactured to a specific size. This size can be an exact size or a size with some tolerance. The tolerance is a top and bottom size that is acceptable for the part in question. Lets take an example: Now what if the manufacturing process produce a shaft the same size, or larger than the bearing? Well the bearing would not fit the shaft without heating the bearing or cooling the shaft. This might not be appropriate and could well be unworkable.

To avoid this typical problem, and keep manufacturing costs down, as all manufacturing costs are based on accuracy of finished product, a tolerance is agreed. This tolerance might well be bearing size plus or minus one thousandth of an inch. The customer would then receive shaft parts with some needing to be cooled before assembly and some needing the bearing to be heated before assembly. Both might well be acceptable if this is the agreed tolerance of the parts.

What is the 'fit' and why does it matter? The fit is the action of two parts fitting together. The 'fit' is decide before manufacture and is dependant on the tolerance of the two parts. In the previous example the fit of the bearing, on the shaft, was designed to be a snug fit with a maximum interference fit of minus one thousandths of an inch. (Note: interference is when one part is smaller than another, producing an interference, or pressed, fit when assembled).

Another type of fit might well be when a shaft is to carry a gear by friction only. The fit in this instance is that of an interference fit, probably in the region of .002" or .003". This size difference makes assembly more difficult, necessitating heating of the outer part before assembly.

There is of cause the loose fit, where parts are not touching while cold, but assume a correct size when heated up. This clearance is normal for rapidly expanding parts within an engine. The piston to cylinder bore is a prime example of a big tolerance, reducing as the engine heats up and reducing to zero, causing seizure, if the engine overheats.

So that is tolerance and fit covered, now back to measuring the sizes of shafts, bores, thickness and lengths.

The hand tools needed for accurate measuring include: Correct use of the tools...

Using a micrometer or vernier needs practice to develop the necessary skill for confident, accurate measurement. Imagine you were going to telephone a supplier and ask for a part to be made. Could you accurately quote the size of the existing part ? If you have just got the tool then probably not, if you have never bothered to learn properly the definitely not !

What does a correct measurement, with a vernier or micrometer, or feeler gauge, feel like?

It is a measurement, that if repeated will give a similar if not exact reading. It is not a loose reading, allowing the jaws of the measuring instrument to touch occasionally. It is not a reading where the measuring implement is over tightened and simulates a g clamp or a car jack, both exerting considerable force to stay in place. (Note: it is possible to strain the micrometer by over exertion so beware, and if in doubt use a test gauge to set the micrometer back to normal)

The use of the measuring implement will decide the accuracy of the finished product. Practice good practice and be confident of accurate results. No longer will you need to quote vague figures when describing the state of engines / chassis parts and asking for advice regarding refurbishment of parts.

Basic measuring tools, coupled with a manufacturers manual, give you the confidence to know when a part is ok, and when it is out of tolerance. Lastly store any hand tools with care and they will be useable when next required.

This article was produced by Brian Pollard, author of "Preparing the Gx160 for 'open' racing" which is available on CDROM, in multimedia format, as an e-book, and in paper form. All enquires should be sent to the above e-mail address.

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