Kit and Tools Tuesdays - ME-OUD-20 0-20mm External Dial Callipers archive
Tuesday 5 August 2014
Geoffrey Laycock looks at this set of external dial callipers from Allendale Electronics Ltd
No, nothing to do with the weather: metrology is the science of precision measurement. Many woodworkers have become accustomed to using callipers for measurement, recognising the greater precision possible than ruler alone. They do tend to be restricted to narrow stock and can easily be affected if held at even a very small angle to the surfaces being measured.
Looking for a means to measure wider stock across its full width, I initially looked at luthier's tools as they have a calliper gauge just for this, but at a substantial cost. Being an engineer of old led me to the outside callipers sold by Allendale Electronics having a capacity of 20mm up to 70mm from an edge. These are accurate to 0.01mm, which is roughly half of one thousandths of an inch for us older ones - accurate enough?
Now how do you check such claims? I have a precision slip set consisting of carefully machined steel blocks in graduated sizes. They can be combined by 'wringing' together and this allows increments of 0.001mm to be achieved, which for the imperialists is 0.0005in or thereabouts and complete overkill for woodworking obviously. The important result is that testing these callipers and working through the range of zero to 20mm they were consistently within 0.01mm, which is impressive and good to know as some of the poorer made examples may have slight errors at each extreme of the measuring scale.