Looking at your diagram, I disagee that 'A' must equal 'B'.
Assuming you can 'zero' the caliper when the two 'B' jaws are together then
the measurements should be as accurate as when you 'zero' the caliper when
the two 'A' jaws are together and take a measurement.
Note that this is different than saying "when the 'B' jaws are together then
the 'A' jaws must also be together", which is what I think you are after.
For the previous statements, the term 'together' means a physical means of
ensuring a distance of zero between the jaws. Very easy to do in the 'B'
case and I do not have a clue about how to do it in the 'A' case - how do
you create a hole of size zero to provide the initial reference point for
'A'. And I am not aware of inexpensive "inverted gauge blocks" or "hole
gauge blocks" that would allow you to set the 'A' distance at some other
reference - such as 1 inch or ...
Note that this "zero 'A' does not need to be the same as zero 'B'" applies
to the 'C' measurements that are taken off the end of the caliper - the
depth measurements. When the 'B' jaws are together the 'post' on the end of
the caliper is usually not flush with the end of the caliper body. So I
simply put the end of the body on a flat surface, move the readout until the
'post' is physically in contact with the flat surface, zero the readout and
go about reading any depths that I need. Note that at no time during the
process will the 'A' or the 'B' distance be the same as the 'C' - depth I am
measuring, but I don't expect them to be. But the depth measurements will be
as accurate as the overall accuracy of the tool.
So the problem as I see it is coming up with some type of reference against
which 'A' can be set, after which the measurements will be as accurate as
measurements taken with 'B' even thought the 'B' distance will not be the
same as the 'A' distance.
As a kludge to solve the "I need an accurate hole so I can create the
reference for 'A'" I grabbed one of the other HF 6 inch calipers I have
lying around, closed the 'B' jaws of caliper (2), zero'd the scale, move it
until I had 1.000 on the display, tightened the set screw on the top of the
display, then measured the distance using the 'A' jaws of caliper (1). The
distance was 1.0015, which is interesting but not relevant. I could have
zero's the display on caliper (1) at this point and been able to measure
things with the 'A' jaws reasonably accurately.
And as another side note, in the above experiment, I switched the roles
calipers (1) and (2) found setting a 1.000 'B' reading on (1) created a .996
reading on the 'A' of (2). Again interesting but not relevant to making
accurate measurements after (2) had been zero'd.
In the end, it would be nice if they were made with the precision that a
physical zero of 'A' would also be a physical zero of 'B' - your original
premise. But what the heck, at less than $10 a piece I have bought several
and can make it work for type of measurements that I am doing.
Arvid
> OK, I have a few of these Calipers and they work great, except that the
> indicators arenbt accurate and they are non-adjustable.
>
> Here is a link to a photo that I put on my website to define the problem
> Ibm having.
>
>
><http://www.megageek.com/photo/photoa~1.nsf/a7ffeab012bbfb5b85256eb1007ca7f6/1270fe034ea0feea85257660004f08aa?OpenDocument>
>
> In the picture, the OD measurement of b Ab should be the exact same as
> the
> ID measurement of b B.b But they are not. This means that as I measure
> a
> piston and the cylinder bore, Ibm not getting a good result (the bore is
> coming up SMALLER than the piston diameter.)
>
> These do work great for measurements of the same ID or ID objects however.
>
> So what I need to find is a way to make sure the calipers are b correctb
> before I buy them.
> Any suggestions?
>
> Moose
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