Metrology Monday: Gauge Blocks

What Are Gauge Blocks For

Just like surface plates are a standard for flatness, gauge blocks are the machinist’s standard for length. And also like surface plates, they are less simple than they appear.

Created from well conditioned steel and polished to a very find degree, gauge blocks come in sets of different sizes that are combined to get whatever length is required. An imperial set is typically 81 blocks and covers ranges from .001-12.000″ in .001 increments and .2000-12.0000 in .0001 increments. The odd starting point for the ten thousands is because the first block is .1001 or such since nobody can make a durable block .0001 thick. Making up the required combination may count as playing with blocks. 🙂

The assembled length is used to compare or calibrate whatever you are trying to evaluate against the standard. My singular “real” usage so far has been checking the accuracy of various used instruments I have purchased. As I mentioned somewhere previously, one can mount a dial test indicator on a height gauge, zero it out on the gauge block(s) and use it to check parts for expected length.

A fun feature of gauge blocks is that you can “wring” them together. The highly polished surfaces allow a close enough fit that they will stick to each other. Very handy when you need to move a pile of four blocks.

Alternatives

If you don’t have a gauge block set, you can start with a micrometer and measure some object, take the result and compare it to the resulting measurement from other instruments to see if they agree. Some higher end micrometers come with a block usually called a “standard” which is used to check the mic. This is effectively a single gauge block.

Options

Cheap gauge block sets go for about $180CDN new and things go up from there. In my case, I opted for a used set of some personal and historical interest which I will now enthuse about:

In the 1800’s, New England was a center of industrial excellence with the precision manufacturing aspect perhaps most famously represented by the Waltham Watch Co. Where you have machinists, the tool manufacturers follow and so it was with Massachusetts. That industrial sector is a shadow of its glory days but the shadow is a long one with the L.S. Starrett Company still in business. I have more than one Starrett measuring tool but my gauge block set is a product of a less well know competitor, Van Keuren.

The Van Keuren Company still exists as a brand but no longer operates out of its building in Waltham, MA. (As an aside, I worked in the Waltham area for about six years, love those brick mills) I know they still were there in 1942 because that is when my block set was manufactured. The three digit serial number suggests either an early product or one that didn’t sell a lot.

The set sill contains the inspection certificate with the deviation from labelled size in millionths! for each block. I do wonder if they still conform to those results but am unlikely to need to care. 🙂

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