Dial Bore Gauge Innards?

[sparky961]

Can anyone tell me how a dial bore gauge works?  I understand how to use it and what's on the outside but how does the motion of the plunger inside the bore get accurately rotated/translated 90 degrees to the affixed dial indicator?

[BillTodd]

Essentially, it is a wedge or cone that pushes the three arms out. The return could be handled by either springs or an internal tapered groove.

The angle of the cone and pitch of the micrometer thread are scaled to give diameter on the dial.

[sparky961]

Does this not describe an IntriMik/Holtest type of gauge?

[BillTodd]

Does this not describe an IntriMik/Holtest type of gauge?

I don't understand your question 

[sparky961]

Sorry, you can tell when I'm typing on my phone because my answers don't ramble on like usual. :)

Your description seems more like an Intrimik or Holtest gauge rather than a dial bore gauge.  The primary differences being that the former typically has 3 points of contact in the bore and a rotating thimble to bring them into contact and obtain a direct measurement.  The latter is zeroed to the desired measurement using a standard (setting stand with gauge blocks, micrometer, or similar) and has only 2 points of contact.  The latter is rocked around until the largest measurement is found.  Spring pressure keeps the points in contact with the bore.  The dial shows the difference between the set value and the measured diameter.

I'm thinking that the Dial Bore Gauge operates similarly but that's only me looking at the outside and making an educated guess.  I was looking for confirmation of the more concrete type.

These videos are ONLY for illustrative purposes.  I haven't watched them entirely or validated the content.

Dial Bore Gauge:

Holtest:

[DavidA]

The three point versions are often more accurately called Bore Comparators.

Much used in the engine re-boring industry.

Dave.

[BillTodd]

If it has a DTI mounted then it'll require a <45 degree wedge (so the legs can push the expander back) .

Since a comparator does not usually require the linearity of a measuring device , levers or rollers could also be used

The complication is usually how to keep all the bits from falling apart . The mechanism is rarely complicated.

Hopefully, this time it'll send: this picture is another wedge design for a bore gauge:


[edit] This is a diy version using balls
http://rick.sparber.org/4bidg.pdf

[sparky961]

Thanks.  That's good enough to confirm for me that there's no black magic going on inside.  I like the DIY version but geeez.... did you have to give me another project to get done?! ;)