Hi Douglas,

To en engineer or a machinist, a steel ruler is not considered to be a straight edge for measurement and control purposes.  It depends what you mean by straight. In precision work, the straightness of an edge is measured as a deviation from perfectly straight over a distance. Engineering straight edges can achieve accuracies of 0.0001" per 12” or better. Steel measuring rulers you buy at the hardware store are nowhere near this. Straight edges are even certified at calibrated temperatures, such is the demand for precision. Straight edges must also be rigid so as not to bend, which affects straightness geometrically. So they are often I-beam shaped or similar. Aluminium straight edges go through a complex process of stress relief to keep them within specified tolerance. Cast iron ones are hand scraped to flatness. Straight edges never have measurement marks on them like rulers do.

The meter steel rules you find in workshops are generally about 1mm or so thick, and not only is the rule edge not straight enough for engineering, but they flex in use and this makes them unsuitable for reference work.

It’w worth having a look at some of the precision edges here:

http://machining.buschprecision.com/category/straight-edges

[Not an advertisement!]

I am just looking in their catalog at a 72” cast iron straight edge, which comes in at US$3,140. Definitely not a ruler!

One could argue that harpsichord making does not require engineering tolerances of such fine degree. But you can get relatively cheap long aluminium shop straight edges that are way better than rulers, and properly rigid. A good long one for harpsichord work may cost a couple of hundred dollars, but worth it over time.

Truly, if you get a precision straight edge and put a cheap steel measuring ruler up against it you will see the ruler is not straight. Rulers are not straight edges.

And woe betide anybody in the workshop who uses the master straight edge to cut with, or drops it! :-)

In case this is getting off topic, in relation to woodwork, I know people who use precision straight edges and reference flat surfaces to get the bottoms of planes very flat (or not exactly flat on purpose - but that’s a different topic).

Andrew


On 31 January 2016 at 23:13:30, Douglas Amrine ([log in to unmask]) wrote:

So steel rulers are NOT straight edges? 

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