Subject: Re: Reading a Caliper (vernier scales)

From: [email-masked] (murr rhame)
Date: 23 Mar 1995 10:37:20 -0500

[email-masked] (Bruce Giller) writes:

>I have a 6" caliper that measures in both inches and mm.  It is
>supposed to go down to the .001 level but I haven't figured out how to
>do that; I can get to the .1 level easy enough.

>The 'moving' part of the caliper has an inch long scale attached to it
>that starts with 0 and ends with 25 and says "1/1000"on the scale.  I
>use the 0 mark to figure out the .1 level  by where it lines up with
>the 'non moving' scale.  In between the .1 marks are four marks which
>correspond to a .025 level.  If the 0 lines up with one of these four
>marks, I just add .025 to the measurement.  But when it is inbetween,
>I'm lost.   I assume that the other marks on the 'moving'  scale are
>for the .001 level but I am unable to figure out how to read them.  Has
>anyone any experience with a caliper like this?

The second scale marked 0 to 25 is called the vernier scale.  With this
you can determine where the pointer is when it's between the .025 marks on
the fixed scale.  Say you have the pointer at 1.125" plus something less
than the next .025" mark.  Move up the vernier scale until you find a mark
that is exactly aligned with a mark on the fixed scale.  Add the number
from the verier scale to the number to the main scale reading in my
example of 1.125".  This will give you the .001" resolution.  It's a bit
easier to see the alignment if you look at the scale edgewise looking down
the marks....  This is a lot easier to explain with illustrations.  Most
any machinist text book should have a section on how to read a verier
scale.  

A dial caliper is easier to read.  Dials and digitals are not necessarily
more accurate.  If you do get a dial caliper look for a unit where the
dial is .100" per revolution.  The less expensive .200" per revolution
models are nearly as hard to read accurately as a vernier. 

- murr -

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From: [email-masked] (Bruce Giller)
Date: 23 Mar 1995 13:50:59 GMT

I'm thinking of getting a dial caliper to help me with this problem;
any suggestions of a brand for intermittent use?  How are the
measurment tools from Harbor Freight and Salvage rated?

RE: Walker-Turner drill press spindle:
It turned out that the spindle itself was straight except for the
sections were the bearings sat; these were slightly out-of-round.  A
few rotations on a friends' lathe, and now the new bearings slip right
on.  Thanks for the help.

Bruce Giller

"Is dis a System????"   Mr. Natural

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From: [email-masked] (Jonathan M. Elson)
Date: 24 Mar 1995 06:25:10 GMT

Bruce Giller ([email-masked]) wrote:
: I have a 6" caliper that measures in both inches and mm.  It is
: supposed to go down to the .001 level but I haven't figured out how to
: do that; I can get to the .1 level easy enough.

This is what is called a "vernier" caliper.  I've been using one so long I
don't know how I do it, I just do it.  But, this is how it works.
The scale should have three marks between each .1"  So it directly reads out
to .025", but only every .1" is marked with a number.  So, read the tenths,
then count how many marks are past the tenth, and add .025 (25 thousandths)
for each mark on the fixed scale to the left of the "zero" mark.
Now, for the vernier.  If the jaws are not on an exact spacing of some
multiple of .025", the "zero" mark will not line up with one of the marks
on the fixed scale.  Search down the moving scale "0 5 10 15 ..." until you
find one of the lines there that lines up exactly with a line on the fixed
scale.  The line on the moving scale which is most closely aligned with
a line opposite it is the amount to add to the reading already computed.
So, you add the whole inches, tenths, 25 thousandths, and then from 1 to
24 thousandths all together to get the complete reading.  It takes a bit of
practice.  Pull a hair from your head (I recommend finding a loose one) and
measure it.  Can your eye detect that the #1 line (just right of the zero)
is better lined up with the line above than the zero is?  That's how you
read .001"  Well, the problem with a vernier is that it takes a sharp eye,
and you can make addition errors.  The problem with a dial caliper is 
that the gear can jump a tooth due to a chip in the rack.  I strongly
recommend a "shielded rack" version.  Also, a vernier measures inches and
mm on two scales.  Only the fanciest dials have two pointers and racks for
both inches and mm.  If you have the money, the digital electronic calipers
are great.  But they are quite expensive.  A fine 6" vernier goes for $39,
a good dial for $59, but I no longer see any digitals for less than $100.