What to Look For When Shopping for a DRO
There are hundreds of different digital readout models on the market with prices ranging from few hundred dollars to many thousands. For someone who hasn't used a DRO before, selecting the right unit can be a daunting task.
Most sellers readily list the features of the DRO consoles they sell but you will quickly realize that at this point even the most basic digital readout consoles provide most of the common DRO functions. It's hard to find a digital readout that doesn't offer absolute and incremental coordinate systems, imperial and metric units, bolt hole pattern functions, some stored coordinate memory, etc. At the end, many of the choices come down to personal preferences and budget but it's important to keep an eye on the following important parameters:
Accuracy and Resolution
A digital readout is a precision measurement device and therefore its overall accuracy and resolution are two of the most important characteristics. Accuracy and resolution mostly depend on the scale selection, which is described in depth on the Understanding DRO Scale Parameters page. What is often overlooked is the fact that the console itself can introduce errors. This happens one of two ways. First, some low-end DROs use older (and therefore less expensive) microcontrollers that can introduce rounding errors during division, for instance when using the "Center" function. Second, and more common, problem is missed pulses from the scales, which over time lead to a buildup of error. When shopping for a DRO, especially if 1 micron scales are to be used, ensure that the unit can handle the expected traversal speeds. This is most often a problem on lathes, where longitudinal movement speed can easily exceed several inches per second.
The main reason of adding a DRO to a machine is to improve the usability of that machine, but surprisingly this is the most overlooked parameter when it comes to the DRO itself. In order to be easy to use, a digital readout unit needs to have intuitive and, more importantly, consistent user interface. Some low-end digital readouts have user interface and menus that seem to be randomly cobbled together in order to check a checkbox in the sales brochure. A good digital readout console should feel natural to use and shouldn't be confusing, or you will make mistakes and ruin parts. Before buying a DRO it's a good idea to download and study the user manual and see if the functions that you are likely to use are easily accessible.
A digital readout will be used in a relatively hostile environment of a machine shop and will be constantly subjected to dust and grime. In order to survive in such environment, a DRO needs to have some level of protection. If flood coolant is to be used routinely it's helpful to have at least IP65 level of protection, or better yet IP67 or IP68. Otherwise avoiding DROs that have cooling fans, open vents and mechanical switches is a good idea.
Side note: lower-end DRO manufacturers often include statements like "waterproof to IP54 specifications". This is a misleading sales gimmick designed to lure buyers who don't know what these numbers mean. Unless the DRO is IPxx certified, those claims are worthless. Furthermore, IP54 certification ensures that an enclosure "protects against dust, limited ingress and protects against spray from all directions", or in practical terms means that the DRO is enclosed in a plastic box without large openings.
Vibration and Shock Resistance
This is often overlooked, but an average DRO will be subjected to severe vibration during machining. Unless the circuit inside is manufactured with vibration resiliency in mind, it is guaranteed to fail rather quickly due to a cracked solder joint. Most reputable manufacturers use precisely controller processes to ensure good mechanical strength, but this is often not the case with cheap Chinese units.
Keypad Switch Durability
Most traditional DROs use membrane keypads or mechanical switches for input. Quality of switches is measured in actuations, i.e. the number of times a switch can be pressed before it fails. Top-of-the line switches can be rated at millions of actuations, while cheap non-name membrane switches can fail after only hundreds of actuations. This information is generally not provided by the DRO manufacturers, so it's important to research keypad failure rates for the specific model you are considering.
Side note: there are two variants of membrane switches, tactile and non-tactile. The former use a small cup spring to provide positive feedback when the switch is pressed. Low price units almost never come with tactile switches; more expensive name brand DROs come with both options, so it possible, you should try both to decide which one you prefer.
Linear Error Compensation
Linear error compensation is arguably the most important function of a DRO. Without it you will not be able to calibrate the scales on the machine and the readout will always be off by some amount. There are two reasons for this. First, it's virtually impossible to mount the scale perfectly parallel, so there will always be a small amount of cosine error. Second, scales have manufacturing errors that are usually linear in nature. A DRO that lacks this function should not be considered to be a precision instrument and should be avoided.
Side note: some DRO consoles offer segment (non-linear) error compensation. Generally speaking this feature can be found on more expensive units that come with purpose designed scales . It requires absolute scales (or scales with a zero reference marl) and laboratory-grade equipment for calibration. Some less expensive DROs started adding this feature but in practice it's a gimmick without proper scales.
DRO Interface Types
Digital readouts/display units have been around for several decades but have undergone surprisingly few changes. The familiar 7-segment display and physical keypad is still the most common type of user interface. The reasons for this are mostly historical: this was the best available technology for the application. Today there are much better choices, but most DROs are still using this antiquated design. Based on the display type the DROs can be grouped into the following four categories:
Basic 7-Segment DROs
These "old school" digital readout consoles use 7-segment LED displays with membrane or mechanical numeric keypads and haven't changed much since the 80s'. They offer a list of standard DRO functions, such as switching between inches and millimeters, absolute and incremental coordinate systems, zero set, centerline function, several hole pattern functions, tiny saved coordinate and predefined tool memories, etc. The biggest issue with these DROs is the user interface. Due to the limited number of buttons and inability to display more than a few characters of text, most operations require several key presses and deep menu systems.
Most of the well known name brands offer digital readout consoles in this market, but they are usually their lower-end models. That said, this particular market segment is dominated by the inexpensive DRO kits and consoles from China. While the former are generally well made, albeit expensive, the main appeal of the Chinese models is low cost. Ebay and AliExpress are full of two and three axis DRO kits from Chinese sellers for under $500; the cost from an established reseller will be 1.5-2 times as much. While the glass scales are generally pretty decent, the consoles are the place where the manufacturers cut corners by using low quality generic parts, cheap membrane keypads, suboptimal manufacturing processes and poor quality control. With proper setup and care such DROs can be pretty accurate, but since the warranty service offered by Chinese sellers is unreliable at best, you should factor in the cost of replacement console if the original one fails.
Side note: Inexpensive Chinese DRO units have three common failure modes. First, solder joints crack due to vibration, leading to intermittent resets or strange behavior. Second, low quality electrolytic capacitors dry-up in a matter of one or two years, leading to all sorts of intermittent issues or outright failures. Third, frequently used membrane switches can fail prematurely, often resulting in whole rows or columns of on the keypad becoming unusable. With luck, an oscilloscope and basic troubleshooting skills it's often possible to fix the first two issues, but the third issue is often fatal.
LCD Based DROs
Digital readout consoles that use LCDs to display the readouts are relatively newcomers to the market and instead of the 7-segment display they use graphical LCDs to display the information. This design has several benefits. First of all, it can display numbers, and especially text, in a more readable format. Second, it can display reacher information that includes graphics and other elements. Finally, the display can adapt to the task at hand, making the menu system simpler and more intuitive. LCD based DRO market segment is still mostly dominated by the high-end units from well known brands such as Heidenhain, Newall, Acu-Rite, Fagor. These are modern top-of-the line digital readouts built to last, offer many advanced features, and are a pleasure to use.They use either good quality mechanical or tactile membrane switches rated for hundreds of thousands of actuations in order to guarantee long service life, etc. The downside is that these features aren't cheap, and you should expect to pay at least $2000 for a mid-range DRO kit, and as much as $5000 for a high-end setup. Moreover, many of these manufacturers use proprietary scale protocols, especially in their higher end units, so if something fails out of warranty, the repair or replacement won't be cheap.
Over the last few years, some of the better known import manufacturers, such Easson, Ditorn, etc. have introduced LCD based DROs that look very reasonable on paper and sell for half the price of a comparable name-brand unit. As is often the case however, lower price comes at the cost of quality and reliability, and complete duds are not that uncommon. When buying an import DRO, it's important to choose a reputable reseller that has a support center in your country and will help you deal with any warranty issues.
PC Based DROs
PC-based digital readouts include systems like Caliper2PC, Yadro, and a few others. These DROs first appeared on the market in the early 2000 and have one feature in common: they separate the low-level position decoding from the display, which is replaced with a piece of software running on a laptop. The idea behind this approach is pretty sound and in theory a PC based DROs could have significant advantages over the traditional one-piece digital readouts. For example, they were able to leverage graphical user interface and superior computing power of a PC to provide better user experience. Moreover they made DROs accessible to hobby users by letting them use inexpensive capacitive scales as encoders and an old laptop for the display.
Unfortunately PC based DROs have one significant flaw: laptops are expensive, bulky, and need a keyboard and mouse to operate. For this reason PC based DROs haven't gained wide popularity, and the designs that are currently on the market are pretty old and haven't been updated for a while. Furthermore, none of the currently offered PC based DROs take full advantage of the superior processing power and flexibility: they merely emulate basic digital readout display on a PC and offer only one type of scale interface hardware. Finally, since laptops have spinning hard drives, many moving parts, cooling fans and open vents, they are easily damaged by dirt, grime and vibration. For those reasons PC based DROs aren't a good choice for a machine shop.
Tablet Based DROs
TouchDRO is currently the only digital readout system designed specifically for a tablet. Overall system design is similar to the PC based DROs, where scale decoding is done in a separate controller that communicates wirelessly with the application on the tablet and offers similar advantage without any of the drawbacks. First of all, Android tablets are similar in size to a traditional DRO console, so it's easy to mount and use. Second, TouchDRO is designed to leverage the convenience multi-touch, offering much more easy to use and convenient user interface. Finally, contrary to a common misconception, Android tablets are well suited for a shop environment. First of all, they don't have cooling fans or open vents, so out of the box they have decent dust and dirt protection. Second, most name brand tablets come with at least tempered glass, and some of the better one use Corning Gorilla glass that is virtually impossible to scratch. Finally, since there are no moving parts in a tablet, there isn't much that can wear out over time, so unless a tablet is damaged, it will last for many years of continuous use. TouchDRO's biggest drawback is the fact that it's not a "turn key" solution: you will need to buy suitable scales, Android tablet and spend time building and setting up the controller unit.
Side note: name brand tables have very low failure rates due to electrical issues. The most common failure mode for a tablet is a broken screen. This often happens when a tablet is dropped or something heavy is dropped on the screen. Since a DRO will likely be securely attached to a machine, it's unlikely to be dropped, and under normal condition things large enough to damage a screen should not be flying around the shop. That said, an inexpensive screen protector and a rugged case can be added as an added layer of protection.
Adding a DRO to a milling machine or a lathe can be a significant investment, and as is often the case will require some compromises. Your budget will likely be one of the main deciding factors, but it's important to keep in mind such things as accuracy, reliability, usability and overall interface design. While top-of-the-line name-brand digital readouts are designed with all those considerations in mind, they often cost several thousand dollars and are out of reach for most hobbyists and small shops. Less expensive models from those manufacturers are excellent in terms of quality and reliability, and cost much less. Unfortunately due to the 7-segment displays and physical keypads they sacrifice a fair bit of usability and convenience. Import DROs cost even less but can have major reliability and quality issues, since the console is the easiest place to cut corners in order to keep price low. If you decide to get a Chinese DRO kit, try to find a reputable reseller in your country who will be able to help with warranty and repair issues. The price will be 1.5 - 2 times higher than what can be found on eBay, but you will stand less of a risk of being stuck with a dud. Finally, for those on the budget who want professional features without the professional price tag a software-based DRO, such as TouchDRO can be a good alternative. By pushing some of the final assembly to the user, it can offer the benefits of high quality reliable hardware and more convenient user interface at an affordable price.