A digital read out is a small computer (display unit) usually with an integrated keyboard and some means of numeric representation. It reads the signals generated by the linear encoder

Linear encoder

A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout or motion controller.The encoder can be...

 (or less frequently by rotary encoder

Rotary encoder

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...

s) installed to several machine's axes used to keep track of workpiece position (milling and the like) or the tool's position (lathes and grinders). In the shop argot this complete system conformed by the computer and the encoders are referred to simply as DRO. Such a system is commonly fitted to most machines in today's shops: lathes, cylindrical grinders, milling machines, surface grinders, boring mills and other machine tools to allow the operator to work faster and with greater accuracy. The use of DROs are not limited to manual equipment as CNC equipment can usually be switched to manual operation and in this case a form of DRO is simulated on the Control Panel of a robotic machine.

Display unit (computer)

Several 7-segment displays

Seven-segment display

A seven-segment display , or seven-segment indicator, is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot-matrix displays...

, or an LCD screen on more expensive models display the current position of each machine axes. Such as X, Y, Z for a common milling machines and those plus U and W for highly sophisticated 5 axes vertical machining centers; x and z for a lathes or cylindrical grinders; and just z for a surface grinder.

Common Standard functions on a modern DRO

Modern DROs have a lot of functionality, giving you finger tip control over the most frequent shop operations. The following were taken from a popular brand User's manual:

1. Imperial (inch) and metric interchange.
2. The so called "1/2" function. It takes the current value of an axis and divides it by two. It is used to find the accurate center of a workpiece or an existing feature such as a hole.
3. Preset dimensions. The operator can type in the value of an axis. Used to locate a feature by using an edge finder.
4. Absolute or Incremental modes. The position of a feature given on a blue print is given by one of two methods:
   1. Absolute, which means the coordinate is relative to the part's absolute zero (usually one of its corners or its center).
   2. Relative, meaning the coordinate is referred to some other feature, usually the last one machined.
5. Bolt holes. The drilling or boring of several holes along an arch without using a rotary table
   Rotary table
   A rotary table is a precision work positioning device used in metalworking. It enables the operator to drill or cut work at exact intervals around a fixed axis...
   
   .
6. Inclines, compute a cut across an incline.
7. Memory, Stores hundreds or thousands of points.
8. Scientific calculator. A full featured calculator has been integrated to some select models.

Linear encoders

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 some sort of metallic shield.

Glass scales

Made from strips of high quality glass with evenly etched marks just like the marks of a ruler, but very small (the smallest are just 5 μm apart). Two optical sensors (phototransistors or photodiode

Photodiode

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....

s) are placed very close to each other to make a linear encoder

Encoder

An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...

 . 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.

Commercial models are enclosed in an aluminum "box" with a rubber protection on the side where the encoder slides. Used primarily where shielding from coolant and chips is necessary or where a resolution of 5μm (0.0002 in) or better is required (surface grinders).

Glass scales used to be too expensive for home use, but have recently fallen in price (late 2008) to where they are competitive with the other technologies.

Electronic scales

Instead of glass, a printed circuit on a stainless steel ruler is used to trigger at least two microelectronic hall effect sensor

Hall effect sensor

A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications....

s. Resolution is limited to 0.01mm (0.0005 in) but shielding from coolant and flying chip is not a requirement. These scales are very resistant to everyday shop contaminants and debris. Electronic scales are much cheaper than their glass counterparts.

Electronic scales are available with build in displays so they can be used independently.

Ball scales

The ball scales produced by Newall use an electromagnetic field to track ball bearings in a tube. They sell under the trade name Spherosyn and Microsyn. They work only with Newall DROs.

Rack and Gear scales

These models use a rack (toothed metal strip) that meshes with a gear that turns a rotary encoder. Claimed accuracy of .002 inches per foot, though users frequently report it Is much more accurate, with no measurable deviation over several feet of travel. Debris getting between the gear and rack is a concern.

Vertical Quill DRO

This is a speciality DRO system composed of a computer and an electronic scale in one small piece of equipment. Usually battery operated. Installed on the quill of a milling machine (whence its name). At this part of the machine coolant splash, flying chip and accidental shock are day-to-day events, therefore it is a very bad place for a glass scale used in conventional DROs. It also gives the operator a lot of comfort by being placed right in front of his eyes next to the controls that adjust the machine and it has not a single wire that can get trapped in the cluttered quill area.
A very common setup is to have a regular DRO with glass scales on the milling machine's table and a separate vertical quill DRO. This gives a resolution of 0.005mm to the table position and 0.01mm to the quill. Both exceed by far the expected "0.04mm accuracy of the milling process" (The Machinery's Handbook)

Horizontal quill DRO

It is very much like the vertical version, except in that it is designed to be installed in a horizontal position. The only difference is the orientation of the display and the buttons to be seen and operated horizontally. This device is not for standard shop equipment. It is used in research and calibration of other equipment.