Scale Recomendations for a DIY DRO
The choice of scales for a DIY digital readout often comes down to cost. Glass and magnetic scales are more expensive than the capacitive ones but offer excellent accuracy, repeatability and reliability. These scales are pretty much immune to electric noise and are very well protected from the elements. Capacitive scales are much cheaper but require some compromises.
Glass or Magnetic Scales
Glass and magnetic scales are a natural choice for professionals and machine shop enthusiasts, where the main concern is scale performance, overall accuracy, and long-term reliability. They also make sense for a larger, rigid Bridgeport-size machine. Such machine will be able to take advantage of the higher resolution and overall accuracy offered by the scales. Finally, when using flood coolant, properly mounted glass scales or even magnetic scales are a must. A set of 5 micron scales will provide very good resolution for a milling machine or a lathe, while 1 micron scales are a better choice of a surface grinder. These scales are very reliable by design, extremely precise, accurate, and very fast. With some careful shopping [on eBay] a set of 3 5μm glass scales for a mid-size machine will cost roughly $350-$450; a set of Electronica/EMS scales will cost you time-and-a-half as much.
Outfitting a mill or a lathe with glass scales is a costly proposition and for a mini machine (SIEG X2 mini mill or the Mini Lathe) will be an overkill. These machines have so much flex that the extra resolution and accuracy offered by the glass scales will be completely negated. Unless money is not an object, much less expensive capacitive scales are more appropriate.
Shahe/iGaging Scales
Shahe/iGaging "Remote DRO" aluminum scales offer a good balance between cost and long-term durability and are a logical choice for a smaller hobby machine. Their resolution is about half of the resolution of the glass scales, but at 10 micron it's still prenty good for a typical import hobby mill or lathe. A set of scales for a medium sized mill (SIEG X3, for example) will cost a bit over $100, so you will get half of the resolution at third or even quarter of the cost.
Some people are wary of the Shahe scales due to the fact that their frame is made of aluminum and will wear out over time. In practice, once the scales are properly mounted on the machine, the machine becomes the frame.
Standard Chinese Scales and Calipers
The standard "Chinese" scales were the mainstay of DIY digital readouts about a decade ago and are still pretty popular in home shops. They offer performance similar to iGaging scales but the resolution is slightly higher. Their main drawbacks are the fact that they use 1.5V power supply and a hard-to find proprietary cable. For those reasons iGaging scales are a better choice when starting from scratch.
A number of people use inexpensive Chinese digital calipers as DRO scales. They work adequately well and offer surprisingly good resolution. They have a one big drawback: to save power the manufacturers decided to turn the calipers off completely after some period of inactivity. When used in a DRO setup this is a huge problem, since the DRO completely loses the position. Another issue is the 3Hz position refresh rate, which is too slow for a DRO, where at least 20 Hz is preferable.
Rotary Encoders
Finally for situations where a linear scales is impractical, don’t discount a rotary shaft encoder. Even though it will not account for the leadscrew backlash, you will still get some of the conveniences of a DRO. For instance, an inexpensive rotary encoder with 1000 pulses-per-revolution, installed on a lathe compound slide will offer resolution of 10,000 - 16,000 pulses-per-inch.