Indispensable technology does not need to be flashy and cutting edge to be useful in today’s machine shops. So it goes with the digital readout (DRO)—a combination of position encoders, a simple computer and a numeric display that almost every shop denizen is familiar with.
One would think that with the growth of conversational CNC controls and offline CAM programming, most machines could cut parts without a human working the machine. Since the DRO provides only position information and relies on a human cranking wheels to position the cutting tool, shouldn’t the lowly DRO be on its way out?
Not so fast. They remain useful on many machines you’d find in a shop, such as knee mills, lathes, grinders, boring machines, EDMs, optical comparators and more. With the system’s ability to accurately monitor position, operators don’t worry about backlash or other mechanical issues in these machines.
DROs also make training of new or less-experienced operators much easier and less time consuming since it is relatively easy to grasp the fundamentals of machining by simply looking at the digital count of positioning on a DRO. While it seems a relic of the 20th century, today’s DRO provides much more than position readout. There are still plenty of very good reasons to purchase a new DRO or keep an old one on a manual machine.
Price is a very good reason to choose a DRO. A DRO package that includes a dedicated computer, three axis-position encoders and a numeric display that an owner could install, might run $2,000 to $3,000. A CNC control that includes three-axis control and servo-actuators for moving the cutting head might typically cost 10 times as much. “There’s still an advantage to having manual machine tools around, just from a cost perspective,” said John Parker, controls product manager for Heidenhain Corp., Schaumburg, Ill.
Manual machines are especially relevant for when a shop needs to make prototype parts or one-offs in support of maintenance and repair operations. In these cases, the required resolutions and accuracies needed are less than that required from a CNC.
“For example, on a knee mill, a five-micron resolution would be more than enough. That would allow you to count in two-tenths increments. But sometimes on a grinding application, you might need a higher resolution, so a one-micron resolution is sufficient,” said Alejandro (Alex) Escamilla, Midwest DRO sales manager for Fagor Automation Corp., Elk Grove Village, Ill. DROs typically come in either 1 µm or 5 µm resolutions.
Encoders provide that resolution. Encoders have a metal body that attaches to a moving component of the machine (table, carriage, knee or quill), while the readerhead attaches on a fixed position. The scales can be glass, electronic or magnetic. Glass scales are most popular and reliable.
“Fagor Automation uses glass scale technology for its feedback product,” said Escamilla. “[Glass scales are] more repeatable and have better accuracy. Magnetic scales can handle more contamination, but they need to interpolate between scale marks more than glass scales.” He also notes that Fagor has improved scales with, for example, updated diodes aimed at making the signal more reliable, with better sealing to keep out dust and contaminants.
It is also common for suppliers to offer a range of devices at various levels of functionality and price points. One can match a one-axis application to a one-axis DRO, up to a five-axis application including linear and rotary encoders with a sophisticated, multifunction DRO. They are available specially designed for lathes, milling and boring mills, EDMs and even optical comparators.
For example, Heidenhain’s Acu-Rite offers three ranges of product. “We have what we call an entry-level product, a mid-range and then a high-end product line,” Parker said. “Our simplest products have no advanced functions, while our most advanced can compute things like constant surface speed or bolt-hole positions. Features are also specific to the application—for example, a bolt-hole calculation wouldn’t do you any good on the lathe.”
Like anything that relies on computers and electronics, DROs are also advancing.
“Most DROs nowadays are providing you with an LCD screen,” said Escamilla. “Fagor uses a high-resolution TFT LCD screen.” Another advance is what he terms easier navigation, with onboard graphics, similar to conversational programming on a CNC control. This could include functions such as finding the exact center of a workpiece, preset dimensions, calculating a bolt-hole pattern, or a series of holes across an incline or diagonal. DROs can calculate tool offsets, as well as save operator created programs. “An interesting feature we have on our DROs is called ‘distance-to-go bars,’” said Escamilla. Using color codes ranging from green to red, distance-to-go bars alert the operator as to when their positioning is getting close to what they programmed; red means you’re almost there.
Another clever, useful display trick is on an Acu-Rite DRO. When an operator is moving an axis, the font for that axis gets bigger. This popular Dynamic Zoom feature shows which axis is in motion at that moment, according to Parker. “Our devices can also switch between what we call a daytime mode and a night-time mode—and it just reverses the color of the images on the screen. Depending on the lighting in the shop, a lighter color may be more reflective, so that darker image comes across better to the user.”
He also noted that in large machine applications with up to four axes, such as a big vertical turret lathe, the DRO can be set to view only two of those axes for even more operator flexibility. Other advanced features include finding edges of the centerline of the block material in the center of a circle easily and quickly.
It is easy to think of DROs coupled with a human as a cyborg version of a CNC machine. As DROs get smarter and computing gets cheaper, we could expect to see even more advanced features to help humans machine better.
Data is key to repeatable and reproducible production. But the data must be trustworthy; users must be confident in its veracity. If not, the reams of collected data are nothing more than numbers.
Analog gages are very precise, but that precision can be affected by operator influence. Is the operator properly trained to use a dial gage or indicator and accurately transfer the data from gage to paper?
Reading and recording data can be made simpler through the use of digital readout (DRO) technology, according to Kyle Johns, Testar division manager at Marposs Corp., Auburn Hills, Mich., a manufacturer of precision analog and digital gages and indicators.
DROs can display real-time data and make the information easy to read. Pass-fail decisions can be made at a glance. DROs such as those from Marposs offer clear indicators of problems through the use of color signals: Green=good; red=bad; yellow=caution, data may be trending out-of-specification.
The digital device can also gather and show more specific data. Each year, said Johns, tolerances get tighter and tighter, and a dial readout might not have the capability to detect those measurements. With a “dial readout, you’re not getting those sub-microns,” he said. “You physically cannot read it. Now, with DROs, you can read it from a micron to submicron to even quarter micron.”
In this Industry 4.0 world, a DRO can collect data from all over a plant or from multiple plants. The DRO can transfer data for analysis that can show trends. Data for tracking part production is an example, revealing how and when parts were tested, and by whom.
An operator can check 10 parts and save the data, then an engineer can log into the network and evaluate data to see what’s being done in the shop. “With DROs, data is key. Data tells a story,” said Johns. “Before DROs, that was not possible.”
Applications for DROs go beyond the shop and the machine tool. While applications such as body panels, bumpers and hard tops are typical, a DRO can be used in myriad, sometimes unusual cases.
In one application, a gage was sent around the world, from the U.S. to Mexico to Poland. Users switched back and forth from inches to millimeters and back to inches as they ensured the exact placement of a heads-up display on a vehicle windshield.
Another novel—and safety-driven—implementation of a DRO was to inspect the diameter of cylinders inside a nuclear power plant’s reactor. The gage was sealed to protect it against radiation, and a specially designed wrapped cable wound its way back to the DRO—and safety.
“You can’t go down in that hole to physically check a readout,” said Johns. “You want a DRO on the outside of a wall, because if you go behind the wall, you’ll melt.”
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