While my machine went right to CNC, and I use the integrated program for read out, there is a healthy environment for manual or precision DRO units.
– Capacitance: lowest cost, lowest resolution, accuracy, and repeatablity
– Magnetic : middle, quadrature to achieve resolution. Can be cut and custom fit
– Optical/Glass: higher, direct reading, bulkier, cannot be shortened, can be affected by dust and coolant
Costs range from mild to extreme.
For this class of mini-mill, with the typical positional accuracy in ACME/transport grade screws, several claim the magnetics are good enough. However, to achieve the stated accuracy, they are quadrature units and interpolated. For true accuracy, Glass Scales are the way to go, but they have their own limtations; cost among them.
An excellent source of information on DRO`s in general, and the Android Open Source DRO project: TouchDRO (Wireless DRO)
Yuri`s Toys excellent primer on the differing types of scales : Selecting Scales for DRO
I`m still converting my archives to web, so more to come.