How to Indicate a Hole Milling CNC Machine: Step-by-Step

• 1
  Key takeaways
• 2
  What hole indication actually does
• 3
  Tools and preparation before you touch the spindle
• 4
  Method 1: dial test indicator sweep
• 5
  Method 2: coaxial indicator
• 6
  Method 3: spindle probe and macro
• 7
  Indication targets and when to stop sweeping
• 8
  Common errors and how to correct them
• 9
  Step by step: indicate a bore and set the offset
• 10
  Which indication method fits which job

Indicating a hole means aligning the spindle axis with the true center of a bore that already exists. The bore can be a machined bore you need to ream or thread, a cast hole that needs enlarging, or a hole in a fixture plate that sets the origin for the whole job. The goal is always the same: spindle center and hole center on the same line, then zero the work offset there.

This is not the same as edge finding. An edge finder locates a face. Hole indication locates an axis. If you edge-find a round bore, you get two tangent points and a lot of trigonometry. If the bore is out of round or has a chamfer, the tangent point moves and your origin drifts.

The tolerance you need drives the tool you pick. A ±0.1 mm fixture hole does not need a 0.001 mm test indicator and twenty minutes of sweeping. A dowel bore in a mold insert at ±0.005 mm does. Match the method to the print, not to the habit.

• 1
  True position vs. runoutIndication sets position and also reveals how round and how straight the bore is.
• 2
  One bore, two readingsSweep near the top and near the bottom to catch a tilted or tapered hole.
• 3
  Zero follows the toolMove the axis, re-zero, then re-sweep. Never assume the offset is still good.

You need a dial test indicator with 0.01 mm or 0.002 mm graduations, a magnetic or clamp-style holder with fine adjustment, a coaxial indicator, or a spindle probe with a known stylus. Add a soft brush, an air gun, and a clean lint-free wipe. Debris inside the bore is the single biggest source of error in this job.

Check the indicator first. Sweep the stylus against a known ring gauge or a ground dowel pin. If the needle jumps or the reading does not repeat, the indicator or the holder has a problem. A sticky stylus will show a clean sweep and still lie to you.

Set the indicator so the stylus tip contacts the bore wall at roughly 15–20° off perpendicular. Too flat and the contact slips. Too steep and the cosine error grows. For a 20 mm bore, a stylus with a 20–30 mm arm and a small ball is easier to control than a long, heavy arm.

• 1
  Indicator with 0.01 mm divisionsFine enough for most bores. Use 0.002 mm for gage work.
• 2
  Rigid holder with fine feedA holder that shifts when you touch the adjuster wastes the whole sweep.
• 3
  Clean bore and clean stylusWipe the ball tip and blow the bore before the first contact.

This is the method most people mean by hole indication. Lower the spindle until the indicator stylus sits inside the bore, with the ball touching the wall at about half the bore depth. Rotate the spindle by hand, slowly, and watch the needle. Note the highest and lowest readings. The difference is the total indicated runout, or TIR.

To correct, move the table or the spindle in X and Y by half the TIR, in the direction that splits the error. This is the part most people rush. Move half, not the full reading. Then sweep again. Two or three passes usually bring the needle inside 0.01 mm on a 20–30 mm bore.

Common errors: sweeping too fast so the needle lags, touching the adjuster with the indicator in contact, and forgetting that a tilted bore reads differently at the top and bottom. On a bore deeper than 30 mm, sweep at two heights. If the TIR changes between heights, the hole is not parallel to the spindle axis.

• 1
  Rotate by hand, not by spindleSpindle rotation at speed is a safety risk and makes the needle unreadable.
• 2
  Move half the TIRFull-reading moves overshoot and add passes.
• 3
  Record the final TIRWrite it on the setup sheet so the next operator knows the starting condition.

A coaxial indicator holds a stylus on an eccentric tip and reads on a dial that does not spin with the spindle. You rotate the body by hand and watch a stationary needle. The advantage is speed: you can read the error and adjust without stopping to interpret a moving dial.

It is less sensitive than a good test indicator. Typical resolution is 0.01 mm, and the mechanism adds a small amount of play. For bores held at ±0.05 mm or looser, that is fine. For a dowel bore at ±0.005 mm, use the test indicator and accept the slower cycle.

Keep the stylus short. A long stylus amplifies every bit of holder play and every chip you failed to blow out. Check the coaxial indicator against a ring gauge once a month. If it drifts more than 0.01 mm, it needs service.

• 1
  Good for ±0.05 mm workFast setup on fixture plates and second-operation bores.
• 2
  Not for gage-level workMechanism play limits repeatability.
• 3
  Short stylus, clean boreLong arms and debris dominate the reading.

A spindle probe finds the bore wall at several points and calculates the center in the control. On a 3-point or 4-point bore cycle, the probe touches the wall at a known feed, records the trigger position, and writes the center into the work offset. No hand sweeping, no needle reading.

Probing is the only method that repeats the same way at 7 a.m. and at 11 p.m. That matters on a production run. It is also the only method that logs a number you can compare part to part. Set the probing feed slow, typically 50–100 mm/min for the touch, and use a stylus ball small enough to reach the bore without rubbing the chamfer.

The limits are real. A probe cannot measure what it cannot reach. A deep, small bore needs a long stylus, and long stylus equals more bending. A probe also needs a clean bore. A chip on the wall shifts the center by half the chip thickness, and the control will not warn you.

• 1
  Best repeatabilitySame cycle, same feed, same result across shifts.
• 2
  Needs a clean boreDebris shifts the calculated center silently.
• 3
  Stylus length mattersLong stylus adds bending error on deep bores.

Match the TIR target to the feature tolerance. A hole with a ±0.05 mm position tolerance does not need a 0.005 mm sweep. A dowel bore at ±0.005 mm does. Over-indicating burns spindle time and adds nothing the print asks for.

A practical starting rule: aim for TIR at one-third of the position tolerance or better. On a ±0.03 mm position callout, target 0.01 mm TIR. On a ±0.1 mm callout, 0.03 mm is plenty. Write the target on the setup sheet so the next operator does not guess.

Heat matters on long cycles. A spindle that has been running for two hours is not the same length as a cold spindle. On tight work, indicate, cut a test feature, and re-check. If the offset moved, re-zero and run. This is normal, not a mistake.

• 1
  Target one-third of the toleranceLeaves margin for tool wear and thermal growth.
• 2
  Re-check after warm-upCold spindle and hot spindle do not indicate the same.
• 3
  Document the TIRA written number turns a guess into a repeatable setup.

The needle will not settle. Check the holder first. A loose clamp or a fine-adjust screw that backs off under vibration keeps the reading moving. Tighten the holder, then re-sweep.

The reading repeats but the part is still off. The bore may be out of round or tilted. Sweep at two depths. If the TIR changes, the bore axis is not parallel to the spindle. That is a setup or a part problem, not an indicator problem.

The probe cycle finds a different center each run. Look for chips in the bore or coolant film on the wall. Blow out and re-run. If it still jumps, check the stylus for a bent shaft. A bent stylus reads a different center at every touch.

• 1
  Unstable needleTighten holder and fine adjust, then re-sweep.
• 2
  Repeatable but wrongCheck bore roundness and tilt at two depths.
• 3
  Probe center driftsClean the bore, then check the stylus for bend.