swarfie said: You are right Alexander,technically,it should'nt be possible.But it is………..I am at fault also,for not stressing that to ahieve this result takes practise.If you have not used callipers before,and achieve this kind of accuracy first time,it is sheer luck, but with practice,it can be done consistently.Most home shop machinists do not have a full range of measuring equipment(myself included)and I believe that in the posting I read,members were asked if they would post any alternative methods for measuring a bore that they knew of,hence my posting.I first became interested in home shop maching( model engineering in england,where I'm originally from) in the mid 1960's,and was a subscriber to 'Model Engineer magazine',in those days,model engineers were even more cash strapped than today,and this technique was one in common use then.   Today,lathes,small mills,and other equipment that a home shop machinist needs,because of modern materials and production technology are available to many more people more cheaply than just 30 years ago,and many of the methods that were used back then to achieve a high degree of accuracy have either been forgotten,or are no longer used .                                                                                                                   

0

Alexander m said:

From "Calliper"' I assume you mean the one below and if that is the case I then question whether that can work to the said tolerance. I have one of my own and it has a 10-32 UNF screw for adjusting and the same long legs as the one in the image. The fine screw is  quite suitable but the spring and flexibility would counter its effectiveness. The distances from the fulcrum make the legs move three to four times the distance of the screws travel, the flexibility will demand greater reliance on feel; and spring will prevent the micrometers ratchet from functioning properly. What would be more ideal is the telescopic gauges, not to say they are perfect because measurement transfer devices are always poor substitute for the a direct measurement.

    

Image taken from traditionalwoodworker.com