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Digital Readout (DRO) Basics for Machinists
Guide · Updated October 28, 2026
A digital readout (DRO) reads table position from glass or magnetic scales, not the leadscrew, so backlash in the screw does not change the reading. It makes hole patterns, zeroing and reading positions quicker and less error-prone.
Check the scales
Check DRO accuracy over a gage block stack with an indicator. The free Gage Block Calculator builds the stack.
Step by step
- Pick units: Inch or metric.
- Find the edge: Touch off with an edge finder and enter half the tip diameter.
- Zero absolute: Set the part datum as absolute zero.
- Use incremental: Switch to incremental for features relative to the last one.
- Set lathe mode: On a lathe, set the X axis to diameter mode.
- Use functions: Bolt circle, line holes and calculator functions save math.
- Check accuracy: Check against gage blocks periodically.
Worked example: 6-hole bolt circle by hand
A 3.000 in bolt circle diameter (radius 1.500 in), 6 holes, first at 0°. Each hole: X = 1.5 cos θ, Y = 1.5 sin θ. 0°: (1.5000, 0). 60°: (0.7500, 1.2990). 120°: (−0.7500, 1.2990). 180°: (−1.5000, 0). 240°: (−0.7500, −1.2990). 300°: (0.7500, −1.2990). The DRO bolt circle function gives the same values; check one by hand.
Accuracy
Scales typically resolve 0.0002 or 0.0005 in, but accuracy depends on mounting and scale grade. Check the scale over a known stack with an indicator.
Edge finding
With a 0.200 in edge finder, enter ±0.100 at the edge, or touch off and set 0.100 so zero is the edge.
Worked example: zeroing with an edge finder
Edge finder tip diameter 0.200 in. Touch the left edge of the part; the tip center is 0.100 in left of the edge. Set X to −0.100 in absolute, so X0 is the edge. Do the same on the front edge for Y. Then move to X1.2500 Y0.7500 for the first hole. Check by touching the opposite edge of a 3.000 in wide part: the DRO should read 3.100 in.
Lathe diameter mode
In diameter mode, moving the cross slide in 0.010 in reads 0.020 in, because the part shrinks by twice the depth. Turn a test diameter, mic it, and set X to the measured diameter so the DRO reads true size after that.
Checking scale accuracy
Clamp an indicator to the spindle, zero it against a gage block stack end, move along the stack by its length, and compare the DRO with the stack size.
Common problems
Count drift: dirty or damaged scale. Wrong distances: scale not parallel to travel (cosine error). Bad diameters on a lathe: radius mode on.
Tips
- Edge: edge finder.
- Cosine: cosine error.
- Backlash: leadscrew backlash.
FAQ
What is a DRO?
A digital readout of axis position from scales.
Absolute vs incremental?
Absolute is from a datum, incremental from the last zero.
Does a DRO remove backlash?
The reading ignores it, but the table still has it.
Diameter mode?
X reads diameter, not radius, on a lathe.
How accurate is a DRO?
Resolution is 0.0002–0.0005 in; check accuracy with gage blocks.
Related guides and calculators
- Edge Finder Basics
- Cosine Error with Indicators
- Leadscrew Backlash Basics
- How to Use a Rotary Table
- How to Use a Dial Indicator
- How to Use a Slotting Attachment on a Mill
- How to Cut Spur Gears with a Dividing Head
- How to Transfer Hole Locations
- How to Lay Out a Bolt Circle by Hand
- All free machinist calculators
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