I have used the digital Mitutoyo vernier for some time, not good 
enough. Tired of taking 10 measurements each time? different 
readings? What are you using these days? I have built a great guage 
and I'm willing to share.  

I don't give my secrets away, but it's important that we promote 
accuracy and the same standard, for tip openings. 

You will need a presicion ground bar of 20x20x130mm. This can also 
be 30x20x130 - but that gonna be heavy, or use alu. I have used 
brass bar and machined it.

You will need a digital micrometer, prefer Mitutoyo 293 series. 
either 0.00005" or 0.001mm, mine is 0.001mm (1 micron tolerance) 

a machined piston for the contact point (mic's tip is way too thick)

Need to saw the C arm of the micrometer in half. Need to machine a 
slot the width of the C arm, in the middle at the end of the 
20x20x130bar hold the mic. then drill a hole in the 20x20 bar, 
across the mic's contact tip, for the piston. Not drill, but bore, 
on a lathe, so the piston fits very snugly, but 100% free moving. I 
first bored the hole, 14mm, and then fitted a piston to it. 

Then align the mic to be 90 to the bar, and drill at least 2 holes 
through the arm of the mic and bar to hold it very securely. 
Countersunk allen screws.

Most digital mics have a ratchet adjuster, remove the screw and cut 
the spring in half to lessen the amount of tension. Mic apply quite 
an amount of preasure with this ratchet adjuster. Cut the spring to 
have very little tension on the tip before it starts clicking.

I used a 25 to 50mm mic. 

Now it's almost like a depth guage, but your piston need a 1.5mm tip 
of 3.5mm long where it will meet the tip rail, (ground flat, very 
accurately, if the contact tip is not 100% flat and accurate, you 
will have a bigger tolerance) and a 2mm point to touch the mic's 
hardened tip. Now you put the glass guage on the table of your 
measuring tool and adjust until the ratchet click and zero it. 

You can check the amount of concavity in the table and the tip 
opening, VERY accurately. for less than $80. I got my Mitutoyo for 
$30 locally, and it's got data output as well. just look around 
some. 

I will send a photo tomorrow. It's 12:27 here. 

JG

JG,
Thanks very much for sharing this. Can't wait for some pictures.

I am using a Mitutoyo digital depth gage that measures in both .0005" 
and .01mm. The base that comes with it is a bit too small but it is 
easy to make a base if you have a block of brass or aluminum. Of 
course, the accuracy depends on how flat the base it. The gage I use 
has been discontinued but here is a picture of a model that is 
similar:

http://www.mitutoyo.com/catalog/pdf/Q-02.pdf

The base attaches with an allen screw. A larger base just needs a 
hole through the top for the spindle and a tapped hole through the 
side for the allen screw.

Jon