Appearance
A manufacturer producing 50 mm × 1,200 mm bores in 4140 steel using BTA drilling finds that 15% of parts are rejected for out-of-round conditions exceeding 0.05 mm. Traditional CMM inspection can only measure the first 100 mm of the bore due to probe reach limitations. Implementing an on-machine laser measurement system with a multi-sensor probe that traverses the full bore length reveals that roundness error follows a three-lobed pattern caused by resonant vibration of the drill tube at 120 Hz. Adjusting spindle speed from 1,800 rpm to 1,500 rpm changes the forcing frequency and reduces roundness error from 0.06 mm to 0.02 mm — a 67% improvement.
Roundness and Cylindricity Parameters
Roundness and cylindricity are defined by ISO 12181 and ISO 12180 respectively. Roundness describes the deviation of a circular cross-section from a perfect circle. Cylindricity describes the deviation of the entire bore surface from a perfect cylinder, combining roundness and straightness.
Roundness Parameters
| Parameter | Name | Definition |
|---|---|---|
| RONt | Total roundness deviation | Radial distance between two concentric circles that just contain the actual profile |
| RONq | RMS roundness deviation | Root mean square of radial deviations from the least-squares circle |
| LSC | Least-squares circle | Circle minimising sum of squared radial deviations |
| MZC | Minimum zone circle | Two concentric circles with minimum radial separation containing the profile |
| MIC | Maximum inscribed circle | Largest circle that fits inside the profile |
| MCC | Minimum circumscribed circle | Smallest circle that contains the profile |
Cylindricity Parameters
| Parameter | Definition |
|---|---|
| CYLt | Total cylindricity deviation — radial distance between two coaxial cylinders containing the entire surface |
| Unroundness | Maximum deviation from the mean cylinder at any cross-section |
| Taper | Rate of diameter change along the bore axis |
| Barrel/ hourglass shape | Convex or concave longitudinal profile |
| Straightness deviation | Deviation of the bore axis from a straight line |
Typical Deep Hole Drilling Capabilities
| Process | Roundness (mm) | Cylindricity (mm/m) | Straightness (mm/m) |
|---|---|---|---|
| Gun drilling (solid carbide) | 0.005–0.020 | 0.010–0.030 | 0.02–0.08 |
| Gun drilling (carbide-tipped) | 0.010–0.030 | 0.020–0.050 | 0.05–0.15 |
| BTA drilling (brazed head) | 0.010–0.025 | 0.015–0.040 | 0.05–0.15 |
| BTA drilling (indexable head) | 0.015–0.035 | 0.020–0.050 | 0.05–0.15 |
| BTA reaming | 0.005–0.015 | 0.008–0.025 | 0.03–0.10 |
| Gun reaming | 0.003–0.010 | 0.005–0.015 | 0.02–0.08 |
Roundness Error Patterns in Deep Hole Drilling
Roundness errors in deep hole drilling follow characteristic patterns determined by the dynamics of the long, slender drill tube.
Lobing Patterns
| Lobe Count | Typical Cause | Characteristic | Corrective Action |
|---|---|---|---|
| 2 lobes (oval) | Spindle bearing wear; workpiece clamping distortion | Oval cross-section | Check spindle bearings; reduce clamping force |
| 3 lobes (tri-lobe) | Resonant vibration at 3× rotational frequency | Triangular shape | Adjust spindle speed to avoid resonance |
| 4 lobes (quad-lobe) | Guide pad harmonics; 4-start thread runout | Square-like shape | Check guide pad clearance; verify thread concentricity |
| 5+ lobes | High-frequency vibration; chatter | Polygonal shape | Increase damping; adjust cutting parameters |
| Random lobes | Guide pad wear; chip interference | Irregular shape | Replace guide pads; improve chip evacuation |
Roundness Variation Along Bore Length
Roundness is not uniform along the bore. Research using multi-sensor measurement systems shows three characteristic zones:
| Zone | Location | Roundness Trend | Dominant Error |
|---|---|---|---|
| Entry zone | 0–20×D | Best roundness | Entry lobing from bushing clearance |
| Mid zone | 20–80×D | Increasing roundness error | Drill tube vibration; torsional oscillation |
| Deep zone | > 80×D | Maximum roundness error | Accumulated vibration; wear effects |
Measurement Methods
| Method | Diameter Range | Depth Capability | Roundness Accuracy | Cylindricity | Speed | Cost |
|---|---|---|---|---|---|---|
| CMM with long-reach probe | > 10 mm | < 200 mm (standard) | 0.001–0.005 mm | Yes | Slow | High |
| CMM with custom extension | > 20 mm | < 500 mm | 0.005–0.010 mm | Yes | Slow | Very high |
| Rotary roundness tester | > 3 mm | < 300 mm | 0.0005–0.002 mm | Limited | Moderate | High |
| Air gauge (2-point) | > 5 mm | < 3 m | 0.005–0.020 mm (diameter only) | No | Fast | Moderate |
| Air gauge (3-point) | > 5 mm | < 3 m | 0.003–0.015 mm (lobing detection) | No | Fast | Moderate |
| Laser on-machine (OMMS) | > 32 mm | Full bore length | 0.002–0.007 mm (vs CMM) | Yes | Fast | High |
| Multi-sensor probe system | > 20 mm | Full bore length | 0.002–0.010 mm | Yes | Moderate | High |
| Bore scope with optical sensor | > 6 mm | < 3 m | 0.010–0.030 mm (qualitative) | Limited | Fast | Low |
| Ultrasonic thickness gauge | > 10 mm | < 2 m | Indirect (wall thickness) | Indirect | Moderate | Moderate |
CMM Measurement Limitation
Standard CMM probes can only reach approximately 100 mm into a bore before the stylus shank contacts the bore wall. Custom extension styli extend reach to 500 mm but introduce measurement uncertainty from stylus bending. For bores deeper than 500 mm, CMM measurement is impractical, and alternative methods must be used.
Air Gauging
Air gauging is the most common production method for deep hole diameter measurement. Pressurised air flows through a small nozzle at the gauge head; the back pressure varies with the gap between the nozzle and the bore surface.
| Air Gauge Type | Measurement | Lobing Detection | Production Use |
|---|---|---|---|
| 2-point (opposing jets) | Diameter | Cannot detect odd lobes | 90% of all roundness gauging |
| 3-point (120° jets) | Diameter + initial lobing | Detects 3-lobe patterns | Supplement to 2-point |
| Multi-orifice (4+ jets) | Average diameter | Limited | Large bores |
| Slot-type | Continuous profile | Good | Research; calibration |
Critical limitation: A 2-point air gauge cannot detect odd-numbered lobing (3, 5, 7 lobes) because the opposing jets both see the same radius simultaneously. A 3-lobe error of 0.05 mm produces zero reading on a 2-point gauge. This is why 15% of parts may be rejected for roundness even though 100% pass air gauging — the gauge cannot measure the error pattern.
Laser On-Machine Measurement System (OMMS)
The OMMS uses laser displacement sensors mounted on a probe that traverses the bore. The system measures roundness at multiple cross-sections and calculates axis straightness simultaneously.
| Parameter | Specification |
|---|---|
| Sensor type | Laser triangulation displacement |
| Probe diameter | 28–30 mm (for > 32 mm bores) |
| Measurement points per cross-section | 200–2,000 |
| Cross-section spacing | 5–50 mm (adjustable) |
| Roundness accuracy | ±0.003 mm (compared to CMM) |
| Straightness accuracy | ±0.005 mm/m |
| Measurement time | 3–8 min per bore (full length) |
| Signal processing | Wavelet packet decomposition; 3σ noise rejection |
Straightness Measurement
Straightness deviation of the bore axis is a component of cylindricity and a critical quality parameter in its own right.
| Method | Principle | Accuracy | Depth |
|---|---|---|---|
| Laser alignment | Laser beam aligned to spindle axis; detector traverses bore | ±0.01 mm/m | Unlimited |
| Mandrel and dial indicator | Precision rod inserted in bore; indicator measures deflection | ±0.02 mm/m | < 2 m |
| Multi-sensor probe (OMMS) | Three-point circle fitting at multiple cross-sections | ±0.005 mm/m | Full bore |
| Gravitational (piano wire) | Tensioned wire along bore axis; microscope measurement | ±0.05 mm/m | < 5 m |
| Ultrasonic array | Multiple wall thickness measurements | ±0.10 mm/m | < 3 m |
In-Process Straightness Compensation
Recent research (Gerken et al., 2022) has demonstrated a mechatronic system for in-process monitoring and active compensation of straightness deviation in BTA drilling. The system uses:
- Real-time measurement of cutting forces and tool position
- A model-based algorithm predicting straightness deviation
- Active compensation through a piezo-actuated tool guide
The system reduced straightness deviation by 30–50% in controlled trials, demonstrating that closed-loop control of deep hole drilling form errors is achievable.
Factors Affecting Roundness and Cylindricity
| Factor | Effect on Roundness | Effect on Cylindricity | Mechanism |
|---|---|---|---|
| Spindle speed | Changes lobing pattern | Affects uniform or non-uniform wear | Alteration of resonant vibration frequency |
| Feed rate | Moderate effect | Moderate effect | Chip load variation alters cutting force distribution |
| Guide pad clearance | Significant effect | Significant effect | Pad chatter creates lobing and taper |
| Coolant pressure | Minor direct effect | Moderate | Chip evacuation affects pad contact |
| Tool wear progression | Gradual degradation | Gradued taper development | Worn cutting edge increases forces; pad wear increases clearance |
| Drill tube length | Major effect | Major effect | Longer tube = lower natural frequency = more vibration |
| Workpiece clamping | Major effect at clamping zone | Moderate | Distortion from clamping force creates ovality |
| Bushing condition | Major at entry zone | Minor beyond entry | Worn bushing allows entry lobing |
| Material hardness variation | Localised effect | Gradual taper | Hard spots deflect drill |
| Coolant temperature | Minor | Moderate | Thermal expansion of tube changes clearance |
Troubleshooting Roundness and Cylindricity Problems
| Problem | Measurement Signature | Likely Cause | Corrective Action |
|---|---|---|---|
| Oval hole (2-lobe) | Lobe peaks 180° apart | Spindle bearing wear; excessive clamping | Check spindle runout; reduce clamping force |
| Triangular hole (3-lobe) | Lobe peaks 120° apart | Resonant vibration at 3× RPM | Adjust spindle speed by 15–20%; add damping |
| Square hole (4-lobe) | Lobe peaks 90° apart | Guide pad harmonic; thread runout | Check pad clearance; verify tube straightness |
| Tapered bore | Diameter changes linearly along length | Non-uniform guide pad wear; cutting edge wear | Replace pads and inserts; check coolant flow |
| Barrel shape | Bore larger at ends, smaller in middle | Drill tube deflection; coolant heating | Increase tube stiffness; control coolant temperature |
| Hourglass shape | Bore smaller at ends, larger in middle | Guide pad pressure too high at entry/exit | Adjust pad clearance; reduce feed at entry and exit |
| Spiral mark on bore | Helical pattern on surface | Drill rotation combined with axial feed mark | Reduce feed; check chip breaker function |
| Random roughness peaks | Non-periodic spikes in roundness trace | Chip debris embedded between pad and bore | Improve coolant filtration; flush before removal |
| Increasing roundness with depth | Progressive degradation | Guide pad wear; increasing vibration | Replace pads; add tube damping; reduce speed |
| Sudden roundness change at depth | Step change in trace | Chip blockage; momentary tool stoppage | Improve chip evacuation; ensure continuous feed |
FAQ
What roundness can BTA drilling achieve?
BTA drilling typically achieves 0.010–0.035 mm roundness depending on diameter, depth, and parameters. Gun drilling with solid carbide tools achieves 0.005–0.020 mm. BTA reaming improves roundness to 0.005–0.015 mm. The best roundness is achieved at stable resonant-free speeds with sharp cutting edges and properly adjusted guide pads.
How do you measure roundness in a deep hole?
Roundness in deep holes is measured using air gauges (2-point for diameter, 3-point for lobing detection), laser on-machine measurement systems (OMMS) that traverse the full bore length, multi-sensor probe systems, or coordinate measuring machines (CMM) with long-reach probes (limited to approximately 500 mm depth). For bores exceeding 1 m depth, OMMS or air gauging are the only practical production methods.
What causes three-lobed roundness error in deep hole drilling?
Three-lobed (tri-lobe) roundness error is caused by resonant vibration of the drill tube at a frequency three times the rotational speed. The long, slender drill tube acts as a vibrational system with natural frequencies that can be excited by the cutting forces. When the 3× rotational frequency coincides with a natural frequency of the tube, the vibration amplitude increases, creating a characteristic triangular bore shape.
How does spindle speed affect roundness?
Spindle speed directly affects roundness by controlling the forcing frequency applied to the drill tube. If the spindle speed places a harmonic (typically 3× RPM) near a natural frequency of the tube, roundness error increases dramatically. Changing spindle speed by 15–20% shifts the forcing frequency away from resonance and can reduce roundness error from 0.06 mm to 0.02 mm.
Can a 2-point air gauge detect all roundness errors?
No. A 2-point air gauge cannot detect odd-numbered lobing patterns (3, 5, 7 lobes) because the opposing measuring jets both see the same radius simultaneously. A hole with a 0.05 mm tri-lobe error reads as perfectly round on a 2-point gauge. For complete roundness measurement, a 3-point gauge or full-profile measurement system is required.
What is the difference between roundness and cylindricity?
Roundness measures the deviation of a single circular cross-section from a perfect circle. Cylindricity combines roundness, straightness, and taper into a single parameter describing the entire bore surface. A bore can be perfectly round at every cross-section but still have poor cylindricity if the axis is bent or the diameter tapers.
How is straightness measured in deep holes?
Straightness is measured by inserting a laser alignment system (laser emitter aligned to spindle axis with a position detector traversing the bore), a multi-sensor probe that measures axis position at multiple cross-sections, a precision mandrel with dial indicator, or a tensioned wire with microscope measurement. On-machine measurement systems can measure straightness simultaneously with roundness.
What causes barrel-shaped bores in deep hole drilling?
Barrel-shaped bores (larger diameter at entry and exit, smaller in the middle) are caused by drill tube deflection under cutting loads. The tube deflects at mid-depth, reducing the effective cutting diameter. Other contributing factors include coolant heating (thermal expansion of the tube reduces clearance at depth) and guide pad wear (worn pads at depth increase pressure on the remaining pad area).
How does guide pad condition affect bore geometry?
Guide pad condition is the most influential factor in bore geometry. Worn pads create excessive clearance, allowing the drill to vibrate and produce lobing. Uneven pad wear creates taper. New pads with improper clearance can create oversize or undersize bores. Pad pressure directly affects the burnishing action that determines surface finish and can distort the bore shape under excessive pressure.
What is the ISO standard for roundness measurement?
ISO 12181 defines roundness specification and measurement. ISO 12180 defines cylindricity. ASME B89.3.1 covers measurement of out-of-roundness. For deep hole roundness measurement, the evaluation should follow the minimum zone circle (MZC) method per ISO 12181-1, with a minimum of 200 measurement points per cross-section and cross-sections spaced no more than 50 mm apart for full cylindricity evaluation.
Summary
Roundness and cylindricity measurement in deep hole drilling requires methods that can access the full bore depth — air gauging (2-point and 3-point), laser on-machine measurement systems, and multi-sensor probes. Typical BTA drilling achieves 0.010–0.035 mm roundness and 0.015–0.050 mm cylindricity. Roundness errors follow characteristic lobing patterns (oval, tri-lobe, quad-lobe) determined by drill tube vibration dynamics, with tri-lobe error most common from 3× rotational frequency resonance. Adjusting spindle speed by 15–20% detunes the resonance and reduces roundness error. CMM measurement is limited to bore depths below 500 mm, making on-machine or air gauge methods essential for deep hole quality control.