Digital read out

A digital read out (DRO) is a numeric display, usually with an integrated keyboard and some means of numeric representation. Its integral computer reads signals generated by linear encoders or (less frequently) rotary encoders installed to track machine axes, using these measures to keep track of and display to a machine operator the workpiece position (e.g., milling machines), or tool position (lathes, grinders, etc) in space.

In machine shop terminology the complete Digital Read Out system (consisting of a computer, axis position encoders, and a numeric display) is referred to by the acronym DRO. Such a system is commonly fitted to machines in today's shops, especially for metal working — lathes, cylindrical grinders, milling machines, surface grinders, boring mills and other machine tools — to allow the operator to work faster and with greater accuracy. Use of DROs is not limited to manually operated machines. CNC machines can usually be switched to manual operation, and in this case a form of DRO is simulated on its control panel.

Several 7-segment displays, or an LCD screen on more expensive models display the position of each machine axis. Three-axis systems including the X, Y, Z axes are common on milling machines; those plus U and W are used on highly sophisticated 5-axis vertical machining centers. Lathes or cylindrical grinders typically use just X and Z axes, while a surface grinder may use only a Z axis.

DROs have a lot of functionality, providing computation of common operations. The following list was taken from the user manual of a digital readout manufacturer's product:

All encoders have a scale that attaches to the moving part (the table, carriage, knee or quill) and a reader that attaches to the part that does not move. All are subject to damage from impact, so should be protected with a metallic shield.

Made from strips of high quality glass with evenly etched marks just like the marks of a ruler, but very small (the smallest are 5 μm apart). Two optical sensors (phototransistors or photodiodes) are placed very close to each other to make a linear encoder. When the machine axis moves, the dark marks move under the optical encoders triggering them in succession. If movement is from, for example, left to right, encoder A is triggered first and encoder B afterwards. So the computer can know that the scale moved 5μm to the right. And, if encoder B triggers first and A does as follow the computer knows it was in the other direction.

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