# DIY 3-point bore gauge?



## Nitrous (Apr 29, 2014)

Hi folks,

has anyone built a 3 point bore gauge?  I imagine something along the lines of a set of three round contact points, distributed equidistant with a micrometer head oriented perpendicular to the contact points. I could see a precision steel bearing ball being pressed out by an advancing micrometer spindle. The travel would be limited but I think it could be done accurately. It could also be done over a range of diameters.(using different heads into which the micrometer is fitted)

My particular application is to accurately measure shallow (ie ~.1" or less) internal bored round stock. I need high accuracy since I need to machine a companion disc to press fit into the bore hole. 

Anyone know if this project has been tackled?  

Doug


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## rowbare (Apr 30, 2014)

I have seen a DIY using 4 balls in a pyramid arrangement, where 3 balls measure the bore and one sits on top of the other three. A dial guage reads the position of the top ball and some fancy math ensues.

Of course I can't for the life of me remember what site I saw this on...

bob


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## Nitrous (Apr 30, 2014)

rowbare said:


> I have seen a DIY using 4 balls in a pyramid arrangement, where 3 balls measure the bore and one sits on top of the other three. A dial guage reads the position of the top ball and some fancy math ensues.
> 
> Of course I can't for the life of me remember what site I saw this on...
> 
> bob



I suspect that a geometric relationship can be made such that there would be a direct relationship between a micrometer head's downward spindle motion and three outwardly displaced contact points. if the rise/run of the displacing spindle are equal, I suspect that for every downward movement of "x", the three contact points would move out "x'". 

Doug


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## rowbare (May 8, 2014)

Wouldn't you know it, I tripped over the article this morning: http://rick.sparber.org/4bidg.pdf

bob


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## ScrapMetal (May 8, 2014)

Cool little article.  I'm going to have to make one of those. :biggrin:

Thanks for finding/sharing it.

-Ron


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## Nitrous (May 16, 2014)

rowbare said:


> Wouldn't you know it, I tripped over the article this morning: http://rick.sparber.org/4bidg.pdf
> 
> bob



He is a pretty prolific guy wrt self made tools. 
Thanks for the link

doug


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