Measuring roundness using V-block

You need to read "Technology of Machine Tools" It is a standard school shop manual, and pretty commonly available.

Cheers Trevor Jones (Am I the Only one that thinks these look like homework questions?)

It is possible to have a cylinder that is tri-lobed. This shape resembles a triangle. The sides of this triangle are bowed outward and each apex is rounded. If the shape is just right the distance across this shape measured through the center will be the same no matter where the measurement is taken. There is a name for this shape but I can't remember it right now. Anyway, measured with a micrometer, or with and indicator and a surface plate, the part will appear round. However, because of the three point contact using a vee block, the part will rise and fall as it's rotated. A three jaw chuck can distort a cylinder such that it becomes tri-lobed. Sometimes grinders will also do this to cylindrical parts. I have seen (and rejected) parts that measured .0001" round with a micrometer when the actual out of roundness was in excess of .008". ERS

hehehe.. u guessed it right.. it is a homework question ;) ... btw, thanks for all the info.

These are called curves of constant breadth and, as their name suggests, they measure the same with a mic wherever you measure them. They are not restricted to tri-lobes, or three 'sides'; they can be formed with any odd number of sides. The 7-sided UK 50p coin is a good example. If you use a curve of constant breadth as the cross section of a roller, when you roll a flat surface on it it is impossible to tell its not a circular roller.

The tri-lobed shape has a particular name. And this is the shape most likely to be encountered in a machine shop. Do you know the name of the tr-lobed shape? I think it sounds french. ERS

Reuleaux triangle

Regards, Marv

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That's it! Thanks Marv! ERS

Is that the same shape as a Wankel engine rotor? Those are usually called Troichoidal.

The answer is "yes." And interestingly Franz Reuleaux was German not French.