DRO Scale Selection Guide
Your selection of scales will have the biggest impact on performance, reliability, and accuracy of your DRO. The recommendations in this guide are based on hands-on testing of dozens of scale brands and years of feedback from TouchDRO users. It is aimed at manual mills, lathes, and surface grinders — the machines where TouchDRO is most at home. (If you're building a CMM, fitting a microscope stage, or working in a metrology lab, the requirements are different enough that most of the advice below won't apply.)
Start with the resolution you need, then figure out which scale technology fits your machine and available space. The rest — reference marks, signal type, connector pinout — is covered further down and easy to sort out once those two decisions are settled.
What Resolution Do You Need?
DRO scales are commonly available in three resolution tiers: 10µm, 5µm, and 1µm. The right choice depends on the kind of work you do and the machine you're putting them on.
5µm (0.0002"/0.005mm) is the default choice for milling machines and for the longitudinal (carriage) axis on a lathe. It is finer than what most manual machines can reliably hold, and it provides a smooth, responsive reading on the DRO display. Even in professional shops, 5µm is more than sufficient for milling work.
1µm (0.00005"/0.001mm) makes a lot of sense on a lathe cross slide, where each micron of cross-slide travel removes two microns in diameter — so 1µm scale resolution gives you an effective 2µm diametral resolution. It is also a natural fit for surface grinders and bore/jig grinders where you're routinely working to tight tolerances. For other axes, 1µm is more resolution than you need. Some older DRO controllers can struggle with the higher signal frequency that 1µm scales produce at rapid traverse speeds, but TouchDRO adapters are designed to handle the higher count rates at typical manual-machine traverse speeds — so if the price difference is small, there's no downside to choosing the finer scale.
10µm (~0.0005"/0.01mm) is typical of capacitive scales (digital calipers and similar). This is not ideal for primary axis travel, but many machinists successfully use 10µm capacitive scales on a quill or tailstock where space constraints leave few other options. At 0.0004″ per count it is coarser than 5µm, but still perfectly usable for a lot of real-world work.
To put these numbers in context:
| Task | Typical tolerance | Recommended resolution |
|---|---|---|
| General milling (slots, pockets, features) | ±0.002″ (±50µm) | 5µm |
| Careful manual milling / hole patterns | ±0.001″ (±25µm) | 5µm |
| Turning to length (shoulder location) | ±0.002″ (±50µm) | 5µm (carriage) |
| Turning to diameter (general) | ±0.001″–0.002″ (±25–50µm) | 1–5µm (cross slide) |
| Transition / light press fits | Tens of µm (size dependent) | 1µm (cross slide) |
| Press fits | Tens of µm (size dependent) | 1µm (cross slide) |
| Surface grinding | ±0.0002″ (±5µm) | 1µm |
| Quill depth / drilling | ±0.004″ (±100µm) | 10µm |
Nominal vs. Effective Resolution
An important caveat: the resolution number on the label is not always the resolution you actually get. With optical scales, the actual resolution is close to the nominal resolution, with very small or non-existent non-linearity error — a 5µm glass scale genuinely resolves to 5µm. Magnetic and capacitive scales rely on heavy interpolation, and their real-life resolution is almost always lower than the label suggests. See the Magnetic Scales section below for details.
Glass vs. Magnetic vs. Capacitive
Glass (optical) scales are the de facto choice for most DRO installations. For applications where glass scales don't fit or need to be cut to length, magnetic scales are a good alternative. Capacitive scales are a practical choice for a quill on a milling machine or a tailstock quill on a lathe.
| Glass (Optical) | Magnetic | Capacitive | |
|---|---|---|---|
| Typical resolution | 5µm or 1µm | 5µm or 1µm (nominal) | 10µm |
| Effective accuracy | Close to nominal | ~10µm for most models | ~10µm, noise-dependent |
| Form factor | Enclosed bar (slim options available) | Compact two-part (tape + head) | Compact bar with display or remote |
| Can be cut to length | Difficult | Easy | Relatively easy |
| Environmental protection | Good (sealed housing) | Very good (no optics to foul) | Poor (no sealing) |
| Signal type | Quadrature (TTL or RS-422) | Quadrature (TTL or RS-422) | Proprietary digital |
| Price range | $80–$1,000+ | $150–$1,000+ | $50–$400+ |
| Best for | Primary axes (most machines) | Tight spaces where glass won't fit | Quill and tailstock |
Glass Scales
Glass DRO scales use a precision glass encoder strip with laser-etched marks and an array of optical sensors to measure travel distance. The glass has a low coefficient of thermal expansion, so the encoder strips can be very precise and accurate. Dollar for dollar, glass scales deliver better accuracy than any other scale technology. Even inexpensive imports are generally very decent, since the encoder glass is made by only a handful of manufacturers who have their process well dialed in. Where cheap scales cut corners is in the electronics, weather sealing, quality control, and after-sales support — not in the glass itself.
The traditional full-size enclosed glass scales (the Ditron-style form factor most people picture) can't be easily cut to size and need to be mounted with the opening facing away from chips and coolant, which can make installation tricky when space is tight. That said, compact and slim glass scales are increasingly available — from high-end models like the Acu-Rite SENC 50 to affordable Chinese slim scales that take up less room than many magnetic alternatives.
When ordering glass scales, keep in mind that the travel length (measuring range) is shorter than the overall housing — typically by 120–140mm for full-size enclosed scales. Make sure the total housing length fits your available mounting space, not just the travel distance.
Glass scales range in price from around $80 for no-name imports to several hundred for established brands, and well over $1,000 for precision scales from Acu-Rite or Heidenhain.
Magnetic Scales
Magnetic scales use magnetic tape and a reading head that measures a relatively coarse magnetic field using Hall effect or magnetoresistive sensors, then applies sine/cosine interpolation to produce a fine position readout. The technology was originally developed by Sony (now Magnescale) in the mid-1960s for applications in extremely dirty or high-vibration environments where optical scales wouldn't survive. For most machine shop use, the main appeal is the compact two-part construction — a thin tape strip and a small reading head — which provides a lot of mounting flexibility and can be cut to length for tight installations where glass scales won't fit.
The main disadvantage is accuracy. Magnetic scales interpolate a coarse sine/cosine wave (2–5mm pole spacing) down to 1–10µm steps, and that heavy interpolation is inherently less precise than reading a fine optical grating. High-end brands like Newall and RLS use proprietary processes and expensive tape to achieve accuracy that approaches a 5µm glass scale, but those scales are priced accordingly. Dollar for dollar, magnetic scales will be significantly less accurate than glass, and the quality and repeatability of budget offerings is very inconsistent.
Magnetic scales are rarely the best choice for an entire machine, but there are axes where they may be the only viable option. TouchDRO makes it easy to mix scale types on the same setup, so a common approach is to use glass scales where they fit and a quality magnetic scale on the axis where a glass scale won't.
Capacitive Scales
Capacitive scales are digital calipers — the same technology, packaged as a linear scale. They are commonly used as a quill scale on milling machines or on a lathe tailstock, where the compact form factor fits spaces that glass and magnetic scales can't. Resolution is typically 10µm, and prices range from around $50 for an iGaging scale to over $400 for a dedicated Mitutoyo quill scale.
These scales use proprietary digital protocols and are generally not interchangeable between brands. Traditional DROs don't support them. TouchDRO supports the most common types: iGaging EZ-View DRO, iGaging Absolute DRO, and Mitutoyo SPC (Digimatic). Some capacitive scales have a built-in display, while others use a remote display. The remote-display versions are more convenient with TouchDRO: they don't need a battery, have a faster data transfer rate, and won't time out and shut off mid-cut — something the display versions are prone to.
The main drawbacks are noise sensitivity and limited protection from the environment. Capacitive scales have poor shielding and are susceptible to ground loops and electromagnetic interference, which can make readings unreliable. The frames and reading heads have no weather sealing, so they are easily contaminated by swarf and cutting fluids. That said, for quill and tailstock duty they are a natural fit, and TouchDRO adapters do a good job of mitigating most noise and stability issues.
Reference Track
All the scales discussed on this page are incremental encoders: they measure movement from a starting point, but they don't know their absolute position. When you power off the DRO, the position is lost. When you power it back on, you need to re-establish where each axis is. You can do this manually by touching off a known surface, but many scales offer a reference track that automates this process.
A reference track is an additional signal channel (usually labeled Z, R, Reg, or Fiducial) that provides one or more precisely located markers along the length of the scale. After a power cycle, you move the axis past a reference mark, and the DRO uses it to restore the saved coordinate system. There are two common arrangements:
Periodic reference marks are spaced at regular intervals along the scale (typically every 50mm). After power-up, you only need to move the axis until it crosses the nearest mark — usually a short distance. This is the most common arrangement on Chinese glass scales from brands like Ditron, Aikron, and Sino. Some manufacturers also offer magnetic tape with periodic reference marks.
A single reference mark is placed at one specific location on the scale, usually near the middle or at one end. After power-up, you need to traverse that specific point, which may require moving the axis across a significant portion of its travel. High-end scales from Acu-Rite (SENC 150) typically come with a single reference mark, though other configurations are available on request.
Not all scales include a reference track. Many entry-level scales omit it to keep costs down. If automatic position recovery after power loss matters to you, make sure the scales you're considering have reference marks. The TouchDRO TDA-410 and TDA-420 adapters support both periodic and single reference marks. For scales without a reference track, these adapters also support home switches that can achieve the same result.
TTL vs. RS-422: Does It Matter?
Quadrature scales come in two electrical flavors: TTL (single-ended) and RS-422 (differential). RS-422 uses a pair of complementary signals per channel, which makes it more resistant to electrical noise and allows longer cable runs. TTL uses a single 5V signal per channel — simpler, but more susceptible to interference over distance.
In practice, TTL is more than adequate for most manual machines. The cable runs are typically well under 5 meters, and the signal is a robust 5V square wave inside a double-shielded cable. If RS-422 scales are the same price or close to it, they're a cheap insurance policy — but it's generally not worth paying a significant premium for differential signaling on a typical mill or lathe installation. That said, if your machine has a VFD or BLDC spindle motor, RS-422 is worth considering — these drives can generate enough electrical noise to occasionally upset a single-ended signal.
Which Scales Should You Get?
The right combination depends on your machine. Here are some common setups to use as a starting point.
Milling Machines
Full-size mill (Bridgeport-style): 5µm glass scales on the table (X and Y) and the knee (Z), with a capacitive scale on the quill. TouchDRO can sum the knee and quill axes, giving you a single Z reading that accounts for both coarse positioning and fine quill travel.
Mini mill: Space is the constraint. A common approach is 5µm slim glass scales on the Y and Z axes, and a quality magnetic scale on X, mounted on the back of the table. This preserves the full Y travel — a full-size glass scale on X would eat into it — and keeps the scale out of harm's way against the column.
Lathes
Full-size modern lathe (square-way castings): 5µm glass scale on the carriage, mounted behind the bed with the opening facing down. 1µm slim glass scale on the cross slide, mounted on the chuck side.
Vintage or hobby-size lathe where even a slim glass scale won't fit on the cross slide: glass scale on the bed for carriage travel, and a quality miniature magnetic scale on the cross slide. Mount the magnetic scale on the tailstock side and add a wiper or cover to keep metal chips off the tape.
Mini lathe: Slim glass scale on the carriage (fits on the back side) and a miniature magnetic scale on the cross slide.
Tailstock and Quill Scales
A capacitive scale on the tailstock quill is useful for controlling drilling depth and precision boring. Some TouchDRO users have a neat setup where they install two reading heads into a single glass scale frame on the bed: one head attached to the carriage and the other to the tailstock. The readout from the tailstock head can be summed with the quill scale, giving you precise depth control referenced to the carriage position.
Frequently Asked Questions
[Expand]What resolution DRO scale do I need for a milling machine or lathe?
5µm is the default choice for milling machines and lathe carriage axes — it's finer than what most manual machines can reliably hold. 1µm makes sense on a lathe cross slide, where each micron of travel removes two microns in diameter, and on surface grinders and bore/jig grinders. 10µm capacitive scales are commonly used on quills and tailstocks where space is limited.
What resolution scale should I use on a lathe cross slide?
A 1µm scale is recommended for a lathe cross slide. Because every micron of slide travel changes the workpiece diameter by two microns, a 1µm scale gives you an effective 2µm diametral resolution — useful for precision turning and facing work.
Are 10µm capacitive scales good enough for a DRO?
10µm capacitive scales are not ideal for primary axis travel, but they are a practical choice for quills and tailstocks where space constraints leave few other options. At 0.0004" per count they are coarser than glass or magnetic scales, but many machinists find them perfectly usable for drilling depth, boring, and general positioning.
What is the difference between glass, magnetic, and capacitive DRO scales?
Glass (optical) scales use a precision encoder strip and offer the best accuracy for the price — they're the default choice for most DRO installations. Magnetic scales use magnetic tape and a reading head in a compact two-part form factor that can be cut to length, making them ideal for tight spaces where glass won't fit. Capacitive scales are essentially digital calipers packaged as linear scales; they're commonly used on quills and tailstocks.
Are magnetic DRO scales as accurate as glass scales?
No. Magnetic scales interpolate a coarse sine/cosine wave (2–5mm pole spacing) into fine steps, and their real-life resolution is almost always lower than the nominal resolution. Even high-end magnetic scales from Newall and RLS have a published accuracy of ±10µm. Dollar for dollar, glass scales deliver significantly better accuracy.
Are glass DRO scales too fragile for a machine shop?
No. Despite marketing claims from some magnetic scale vendors, modern enclosed glass scales have robust weather sealing and are designed for machine shop use. When mounted with the opening facing away from chips and coolant, they can last for decades. If a scale does get contaminated, it can usually be disassembled and cleaned with isopropyl alcohol.
Should I choose TTL or RS-422 DRO scales?
For most manual machines, TTL (single-ended) scales are more than adequate. Cable runs are typically short and the 5V signal is robust inside a double-shielded cable. RS-422 (differential) is worth considering if your machine has a VFD or BLDC spindle motor, since these drives can generate enough electrical noise to occasionally upset a single-ended signal. If RS-422 scales are priced close to TTL, they're a cheap insurance policy.
Can I mix different DRO scale types on the same machine?
Yes. TouchDRO makes it easy to mix glass, magnetic, and capacitive scales on the same setup. A common approach is to use glass scales on the primary axes where they fit and a magnetic or capacitive scale on axes where space is limited, such as a mini-mill X axis or a lathe tailstock.
Do I need a reference track on my DRO scales?
A reference track allows the DRO to automatically restore your coordinate system after a power cycle, instead of requiring you to touch off a known surface manually. It's a convenience feature, not a requirement. Many entry-level scales omit it to keep costs down. If you want it, make sure your scales have reference marks and that your DRO adapter supports them. TouchDRO TDA-410 and TDA-420 adapters also support home switches as an alternative for scales without a reference track.
Do I need to replace my existing DRO scales to use TouchDRO?
Often no. Many existing glass and magnetic scales can be reused with TouchDRO. If your scales output a quadrature signal (TTL or RS-422), it's usually just a matter of matching the connector pinout. Check the TouchDRO supported scales page to confirm compatibility, or get in touch if you're not sure.
Next Steps
Ready to pick your scales? Head over to our Recommended DRO Scales page for specific brands, models, and where to buy them. Once you've settled on scales, the TouchDRO Adapter Comparison page will help you choose the right adapter to connect everything.