Radius error spreadsheet

[jjewell]

Guys,

I KNOW I have seen it somewhere on the web, can't find it here.

I have seen a spreadsheet that shows the amount of error in measuring rads.

It showed how once you get less than 180 DEG, the error becomes worse .

It also explained how this was calculated...

Anyone seen or have this to post ???

TIA

[Hilton Roberts]

Jim,

I will do some looking. I do not have it but would like a copy if it can be located.

[Hilton Roberts]

Jim,

Here is a link that may help.

http://www.geomet-cmm-software.com/tn-002.htm

[dtmtim]

I would be interested in this calculation also. I constantly have to explain to customers why reporting the center location of an arc when you have less than 25% of the arc length to pick up points on is a futile exercise.

I have had to go as far as showing them on cad if you vary the points defining the arc by as little as .0001" how dramatically it affects the center point location and radius of the arc depending on the size of the arc segment. It would be nice to have something on paper to show these engineers why they shouldn't have this on their drawings and why thay should have profile of a surface instead.

t.

[Hilton Roberts]

I would be interested in this calculation also. I constantly have to explain to customers why reporting the center location of an arc when you have less than 25% of the arc length to pick up points on is a futile exercise.

I have had to go as far as showing them on cad if you vary the points defining the arc by as little as .0001" how dramatically it affects the center point location and radius of the arc depending on the size of the arc segment. It would be nice to have something on paper to show these engineers why they shouldn't have this on their drawings and why thay should have profile of a surface instead.

t.

I have had the same issue. The math for calculating the center of a circle is precise. It is the precision of the gathered numbers that really causes the problem for CMMs.

It makes no difference to the formula if the points are picked up on 360 degrees of the circle or 15 degrees.

It is the normal variation one sees over 15 degrees that causes the results to go crazy. Lots of engineers do not see the disconnect.

[dtmtim]

I have had the same issue. The math for calculating the center of a circle is precise. It is the precision of the gathered numbers that really causes the problem for CMMs.

It makes no difference to the formula if the points are picked up on 360 degrees of the circle or 15 degrees.

It is the normal variation one sees over 15 degrees that causes the results to go crazy. Lots of engineers do not see the disconnect.

I suppose one way to demonstrate this would be to copy my CAD description into a word doc. I could show some arc segment and show the difference in the center of the arc by varying the defining points by .0001" There is probably a way to show this in excel if we pick a number for normal variation such as .0001" but it would be too much of a brain strain this morning.

[Crash Not]

This is not the spreadsheet, but it may help

[dtmtim]

This is not the spreadsheet, but it may help

Cool - thanks, the formulas are there too if someone wants to do a spreadsheet.

t.

[Crash Not]

Or this

[dtmtim]

Or this

Also very cool - that's exactly what I was describing before that I did in CAD.

Thanks!

t.

[Crash Not]

This may be the sreadsheet

[jjewell]

Spreadsheet Found !!!

Crash Not,

Thats EXACTLY the one I was referring to ! You are the man ! Thanks.

This is a very clear way to show ppl why a cmm can't measure a rad accurately when the chord isn't large enough. This is a tool to use that will explain it. If they can't follow this, they shouldn't be asking the question in the first place.

This should be posted in the CODE area, by the way, where did you find it ?

Thanks again

[MrComment]

Thanks from me also!

[Hilton Roberts]

Me three

[jjewell]

Hilton,

You already know it all, why would you need this sheet ? LOL

No disrespect meant, just admiration for your knowledge. If thats
what its like to get old, its not such a disappointment..