Roundness How Do You Measure It?

Roundness How Do You Measure It?

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Can You Measure roundness With a Micrometer?

We will get to Roundness but first I am going to ramble on, because I always do, but I often think of when I worked with my dad many years ago.

My dad was a brilliant old school engineer and he worked for Alfred Herberts from boy to man, apart from a brief period in the RAF. In its day Alfred Herberts was the biggest and most successful machine tool builder in the world.

New CNC Training ClassesMy dad finally lost his job and had his pension robbed when AH finally went bust. I was running an engineering company so it seemed obvious to have him working with me.

I learnt so much from my dad about the basic principles of machining (this is the tenuous link to the training courses) .

My dad’s claim to fame was that he worked the first Cri Dan machine. This was an amazing innovative machine that could hold its own, until fairly recently, against a CNC lathe for high-speed threading. There are still a few about now.

New CNC Training ClassesMy dad often joked that he could offhand grind a 1/16 of an inch drill and read a steel rule to a thousandth of an inch which I am sure was bullshit although I would never dare to say.

I remember we were trying to turn and bore some motor bodies on a Mori Seiki lathe and we could not get them round. Anyway he told me to weld up some jaws so that they would fully envelop the part.


Basically the part had nowhere to go. It was like a big collet and it worked.


They are nowadays called pie jaws which you can buy from your local tool supplier or from Gregg’s as part of a meal deal

Monster Jaws sell them

But there was another lesson to learn….

Our inexperienced inspector was trying to measure roundness with a micrometer which I am sure you all know cannot be done.

An argument ensued and profanities were exchanged. We ended up buying a Talyrond roundness checker which is the correct tool for the job.



As it happens the same thing cropped up in a customer’s only a few months back. I caught them trying to measure roundness with a micrometer. Back to basics again.

These days I have a much better way to prove my point than shouting and swearing. Please note I still make my point by shouting and swearing which us less intelligent folk always tend to do.

This method is cheap  and free of all profanity it cost 50 pence to be precise.



I threw them a 50p piece (UK Currency) or even a 20p for the real cheapskates. “Is that round” was my question. They laughed.


“Here is a micrometer now check it for roundness”.

You’d need one bigger than this.

Mmm it’s dead round when you check it with a micrometer.

The smart arses amongst you will know that a 50p piece is an “equilateral curve heptagon“. And it measures the same across any two points you care to choose. It proves that you can’t measure roundness with a micrometer. Unless of course you think a 50p is round and the earth is flat.

Oh yea and if you were wondering “why the fuck would you make it that shape?” Just try and put a regular seven sided coin in a slot machine.

The Plug

I won’t be teaching you about roundness on my training courses ,that would be silly cos you already know. If you are interested in learning basic CNC Programming for a Lathe or Milling Machine look no further.

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