Understanding Digital Read Outs (DRO’s)

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Selecting the correct digital read out for your machine can be confusing. There’s a number of options that are available based upon your specific requirements. A DRO can be very simplistic in output or much more advanced as you will see below. I’ll try to break down the basic nomenclature here so you can start your purchasing process with an informed decision about the differences between the various systems that we sell and service.
A digital read out (DRO) is a small computer display unit usually with an integrated keyboard and some means of numeric representation. It can also be just be a numerical display of a measurement. Regardless of the display out put a DRO reads the signals generated by the linear encoder, or by rotary encoders, installed to several machine’s axes used to keep track of workpiece position or the tool’s position on a milling machine, lathe or a grinder. In fact, DRO systems are commonly fitted to many kinds of equipment in a machine shop: 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 either: CNC equipment can be switched to manual operation and provide simple coordinates for workpiece or spindle location, so the term “DRO” covers quite a broad category of ‘readouts.’

Common Standard functions on a modern DRO. Once you get past the basic “readout” functions of a modern DRO they can have a lot of functionality: giving you finger tip control over the most frequent shop operations. As an example some DRO’s can: Switch between Imperial (inch) and metric display output.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.

Preset Dimensions: The operator can type in the value of an axis. Used to locate a feature by using an edge finder.

Absolute or Incremental modes: The position of a feature given on a blue print is given by one of two methods:

Absolute, which means the coordinate is relative to the part’s absolute zero (usually one of its corners or its center).

Perform “relative machining: Meaning the coordinate is referred to some other feature, usually the last one machined.

Mill Bolt hole circles: The drilling or boring of several holes along an arch without using a rotary table. Inclines, compute a cut across an incline. Memory, Stores hundreds or thousands of points.

How A Linear Encoder Works:

Two optical sensors (photo-transistors or photo-diodes) 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 (0.0002 in) 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 ¬†(0.0002 in) or better is required (surface grinders).

Glass scales used to be too expensive, but have fallen in price since 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 sensors. 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. What’s unique about these scales is that they can’t be scratched. The reader head passes over the ball bearings enclosed within the scale and determines the location based upon the diameter of the encapsulated ball bearing. \u00a0These scales never ‘wear out’

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.

Other DRO Setups:

Vertical Quill DRO:
This is a specialty 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. As coolant splash, flying metal chips and accidental shock are day-to-day events, glass scales are used mostly in conventional DRO setups. The glass¬† 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”
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.