Which type of V-Block is more accurate for showing out of roundness-?

I saw something interesting about a question is 60 degree more accurate than 90 degree V-Block turned between two centres.

It seems you do get a different reading rolling the round stock in the V-Block.

Now as I was thinking about this, someting just pops up in my mind and I thought I mention my thoughts regarding this:
Lets say you buy the most expensive sensitive digital micrometer available 100% calibrated at certain temp etc and you repeat measurements under same conditions.....You will be able to measure some difference if its not 100% round.

My point is that a micrometer is an acceptable measurement tool, so with a micrometer you got two paralel surfaces, not 60 degrees neither 90 degrees.The micrometer however does not have one side solid reference to a surface plate, its like "in the air floating".

How does this weigh in on this argument, or is my point irrelevant seeing you measure out of roundness in a V-Block(?) on a surface plate with a dial test indictor on a stand and thats it, the value is not important, just the fact you can se/detect it, and if correct it will be zero in all cases, it must be.

Just curious to know some extra thoughts on this.

A micrometer can not measure "roundness", unless it's a "V" anvil.

Lobing does not show up on 2 point measure.

Jim

Easy to prove Jim's point by measuring a 20p or 50p coin!

I don't know about V block angles, possibly lobes show up more at 60° because the rod doesn't have to turn so far. In which case both are equally accurate but 60° is more convenient? Apprentices were taught this stuff, I have to join the dots, often getting it wrong...

Dave

Geometrically rolling work in a V-block does not give an accurate measure of out of roundness. it measures height from two tangential support points. Professionally out of roundness measurement is done by rotating the work, or the stylus, on an ultra precision bearing.

Andrew

Can you explain “turned between two centres”?

I’m lost on the need or reason for this. Turned between centres should be round; diameter at one end might be different, but that is down to to how the machine has been, or has not been, set up. I meadure diameters with just a micrometer. I would measure straightness of a bar, by rollingbetween two matched V blocks.

As I see it one would use a depth gauge to measure differences in diameter if comparing readings by rolling in a V block? But not a particularly useful result apart from that there is a problem….

Please explain a bit more, as I am thinking you are on a different wavelength to me.

Turning between centres may not achieve roundness due to tool movement and part deflection. Best simple way to achieve near perfect roundness can be achieved by centreless grinding. Obviously few model machine shops have a centreless grinder, at least in the UK. Use of a dti positioned at the top of the bar when rotated in 60 degree vee blocks will give some indication of roundness, more so than 45.

Thanks for very interesting comments, got more to think about.

Thanks for micrometer comment, this makes me think about if the dent is the same as the opposing knop 180 degrees apart the micrometer cannot detect that(One type of reference take at 180 degrees), then makes me think further about V-Block the V giving two contact points, which brings me to a thougt that maybe thats not even accurate, say theoretically you got a rod floating in the air unefected by anything eccept its turning, and you surround it with multiple test dial indicators, what would they all indicate if recorded multiple contact points doing away with the twoo of the V concept.....Just thinking.....

Edited By Chris Mate on 29/11/2022 20:31:43

Vee anvil micrometers usually are 60 degrees, to highlight the most common type of lobing on shafts which has 3 highs and lows. There was a recent thread concerning one of those micrometers.

There is an interesting comment on this youtube video:

https://www.youtube.com/watch?v=YUeFNGslzZo

by 'Post hole digger' that suggests the 60 degree vee-blocks were adopted specifically to check parts machined by centreless grinding as they accentuate (i.e. ease detection of) the lobed shape that this machine is known to produce if not properly adjusted.

This document:

http://www.tarkkuustuonti.fi/Kampanjat/Brochure_Roundness_Booklet.pdf

makes the point that the spacing or phase of the irregularities on the piece being measured interacts with the vee-block angle.

You could reframe this as saying that to increase your chances of detecting/quantifying out of roundness using a vee-block procedure, two sets of blocks with different angles would be useful.

Finally, it is only after seeing this:

https://www.itl.nist.gov/div898/handbook/mpc/section3/mpc344.htm

that I realised that if you are checking something on a Talyrond machine, you can just drop it onto the machine platen any old how. The accuracy of the result comes from the accuracy of the machine - no false reading can be generated from poor 'workholding'.

So … as ‘encouraged’ in the last-mentioned text … I dashed-off and found this:

**LINK**

https://www.govinfo.gov/content/pkg/GOVPUB-C13-71e4a75d4f8a89b72d7567206b5f043a/pdf/GOVPUB-C13-71e4a75d4f8a89b72d7567206b5f043a.pdf

MichaelG.

I googled and found Reasearchgate has done research on V-Blocks and out of roundness, its complicated but well explained. As I understand it(My own understanding) out of roundness can be an infinity of possibilities all around all directions, which makes it complicated to fully encapture as 100% correct by your method whatever it is, so nect best methods are used, I maybe wrong. Maybe if a process is runned long enough data can be captured to paint a picture digitally for comparisons where zero out of roundness is aimed for. We can go as deep as we can measure...

Sometimes V-Blocks must be used like where the round object is too large for the other methods, like a ship driving shaft etc.