Cutoff wavelength and undulations per revolution
The same filter, two notations. Drawings and measurement programs state either a cutoff wavelength in millimetres or a cutoff in undulations per revolution. To compare the two, you have to convert.
When probing, and especially when scanning, vibration, noise and surface texture are superimposed on the actual form signal. What the measuring machine records is the sum of part form and interference. A filter separates the two by defining from which wavelength a component still counts as form.
For circular measurements — bore, shaft, cylinder — the same limit is expressed in two ways. The cutoff wavelength states it as a length in millimetres and applies to any probing path, straight or circular. The cutoff in undulations per revolution instead counts the waves that fit on one circumference: unit UPR. It is defined only for circular measurements, because it needs the circumference as its reference.
How the two relate
The circumference of a circle is π · d. Distributing n waves over it makes each wave π · d / n long. Both directions follow from this:
This also makes clear why the UPR value alone says nothing about resolution: the same 15 UPR means a cutoff wavelength of 1.68 mm on an 8 mm bore, but 41.89 mm on a 200 mm cylinder. The value is always tied to the diameter.
Converter
The circle diameter applies to both directions. All values update immediately.
