CNC turning runout control: maintaining concentricity on multi-diameter components
Content Menu
● Understanding Runout and Concentricity
● Sources of Runout in Multi-Diameter Turning
● Conclusion: Building Repeatable Precision
Understanding Runout and Concentricity
Runout is the total variation in position as a workpiece rotates 360 degrees around its datum axis. It is measured with a dial indicator and reported as total indicator reading (TIR). Radial runout affects the cylindrical surface; axial runout affects faces perpendicular to the axis. For a multi-diameter part, combined runout captures both and shows how errors compound from one section to the next.
Concentricity is a geometric tolerance that controls the location of the median points of a diameter relative to a datum axis. In practice, it means all turned diameters share the same centerline. A typical multi-diameter shaft might have a 60 mm section, a 40 mm section, and a 25 mm section. If the 40 mm section is offset by 0.015 mm from the 60 mm datum, the 25 mm section will inherit that error and add its own.
In one production run of hydraulic piston rods, the large end was gripped in a three-jaw chuck and the small end was supported by a live center. Initial TIR on the small diameter was 0.045 mm. After re-centering the tailstock and using soft jaws contoured to the large diameter, the reading dropped to 0.009 mm. This example shows how fixture alignment directly controls axis alignment across multiple diameters.
Types of Runout in Stepped Parts
Radial runout appears as lobing or ovality on a single diameter. Axial runout shows up when a face is not square to the axis. In multi-diameter turning, the most common issue is cumulative runout—each new diameter is turned relative to the previous one, so any offset stacks. A study on mild steel bars found that feed rate above 0.25 mm/rev increased radial runout by 18 % on the second diameter due to higher cutting forces.
Another case involved a gearbox input shaft with four stepped diameters. The largest diameter was turned first and used as the datum. The smallest diameter, 200 mm away, showed 0.038 mm TIR. The root cause was spindle thermal growth after 45 minutes of continuous cutting. A 10-minute coolant break and lower spindle speed reduced the error to 0.011 mm.
Why Concentricity Matters in Assembly
Parts that rotate at high speed—transmission shafts, pump impellers, motor rotors—require concentricity to avoid imbalance. Even static parts like valve bodies need it for sealing. A 0.020 mm offset on a 30 mm diameter creates a 0.040 mm total runout, which can cause seal leakage or bearing overload. In medical bone drills, concentricity below 0.008 mm is mandatory to prevent wobble during surgery.
Sources of Runout in Multi-Diameter Turning
Runout starts at the machine and propagates through the process. The main contributors are fixture errors, cutting forces, thermal effects, and material behavior.
Machine and Fixture Errors
Spindle runout is the baseline. A worn spindle can add 0.010 mm TIR before the part even touches the tool. Test it with a ground test bar and indicator at the spindle nose. If the reading exceeds 0.005 mm, bearings need attention.
Chuck grip is critical. Hard jaws on a three-jaw chuck apply uneven pressure to round bar, pushing the axis off-center. Soft jaws machined to the exact diameter distribute force evenly. In a run of 150 mm long stepped pins, switching from hard jaws to bored soft jaws cut setup runout from 0.065 mm to 0.012 mm.
Tailstock misalignment tilts the axis. A 0.015 mm height error over 300 mm length creates a 0.030 mm runout at the far end. Use a dial indicator between centers to align within 0.003 mm.
Cutting Parameters and Forces
Cutting force deflects the workpiece and the tool. Higher depth of cut increases deflection. A factorial experiment on 4140 steel showed that depth of cut contributed 42 % to concentricity variation, feed rate 31 %, and spindle speed 19 %.
Feed rate affects surface generation. At 0.30 mm/rev, the tool leaves distinct feed marks that appear as lobing when measured. Dropping to 0.15 mm/rev smoothed the surface and reduced runout by 22 % on a 35 mm diameter section.
Spindle speed influences chip load and heat. Too low, and built-up edge forms; too high, and vibration grows. Optimal range for medium carbon steel is 900–1200 RPM for roughing and 1400–1800 RPM for finishing.
Thermal and Material Effects
Heat from cutting expands the workpiece. A 500 mm long shaft can grow 0.012 mm in diameter after 20 minutes of turning. Flood coolant keeps growth under 0.005 mm.
Material hardness varies. In a batch of 4340 steel, hardness ranged from 28 HRC to 34 HRC. The harder sections required lower feed to avoid deflection, keeping runout consistent across diameters.
Practical Control Methods
Control starts with rigid fixturing and continues through parameter selection and in-process checks.
Fixture Design for Stepped Parts
Use a collet chuck for the largest diameter and a live center for the small end. For parts longer than 4× diameter, add a steady rest at the midpoint. In a 450 mm long compressor shaft, a hydraulic collet on the large end and a steady rest 180 mm from the chuck held runout to 0.007 mm across three diameters.
Custom soft jaws are bored in-place to match the gripped diameter. This eliminates bell-mouthing and keeps the axis true.
Parameter Selection Using DOE
Design of experiments (DOE) identifies optimal settings. A Taguchi L9 array tests three levels of speed, feed, and depth. Results from one study showed:
- Speed: 1000 RPM
- Feed: 0.12 mm/rev
- Depth: 0.4 mm These settings gave 0.004 mm coaxiality on a 50 mm to 30 mm stepped part.
ANOVA quantifies contribution. In the same experiment, depth of cut had p-value < 0.01 and explained 48 % of variation.
In-Process Measurement
Touch probes mounted on the turret measure diameter and position after roughing. The CNC adjusts finish allowances automatically. In a run of 800 aerospace tie rods, probing reduced rework from 6 % to 0.8 %.
Laser scanners check runout without stopping rotation. They flag errors above 0.010 mm and trigger compensation.
Real-World Examples
Example 1: Automotive Camshaft Diameters: 48 mm, 38 mm, 28 mm. Material: chilled cast iron. Initial runout on small end: 0.052 mm. Fix: hydraulic chuck, two steady rests, feed reduced to 0.10 mm/rev. Final TIR: 0.009 mm.
Example 2: Pump Impeller Shaft Length: 620 mm, four steps from 75 mm to 32 mm. Stainless 17-4PH. Thermal growth caused 0.028 mm drift. Solution: intermittent cutting with 5-minute coolant pauses, speed 850 RPM. Result: 0.006 mm runout.
Example 3: Robotic Arm Pivot Titanium 6Al-4V, 45 mm to 22 mm steps. Vibration at transition. Damped boring bar and 0.08 mm/rev feed. Concentricity: 0.005 mm across all sections.
Troubleshooting Guide
- High runout after chucking: Re-bore soft jaws, check bar roundness.
- Runout increases with length: Add steady rest, reduce overhang.
- Runout jumps after tool change: Check insert seat, balance holder.
- Thermal drift: Use flood coolant, allow warm-up cycle.
Emerging Approaches
Adaptive control systems adjust feed in real time based on spindle load. Sensor-equipped lathes predict runout and compensate before the error grows. Hybrid machining—turning after additive pre-forms—reduces material removal and deflection.
Conclusion: Building Repeatable Precision
Runout control in multi-diameter turning requires attention to fixture rigidity, cutting force management, and thermal stability. Studies using factorial designs and Taguchi methods show that depth of cut and feed rate dominate variation. Shops that implement soft jaws, steady rests, and in-process probing routinely achieve concentricity below 0.010 mm. The process is iterative: measure, adjust, verify. Start with one improvement—better chucking or lower feed—and track the gain. Over time, these small changes compound into reliable, high-precision output.
Frequently Asked Questions
Q1: What is the fastest way to reduce setup runout on a stepped shaft?
A: Bore soft jaws to the largest diameter and use a live center on the small end.
Q2: How much does feed rate affect runout on a 40 mm diameter?
A: Reducing feed from 0.25 to 0.12 mm/rev typically cuts runout by 15–25 %.
Q3: Can coolant type change concentricity?
A: Yes—flood coolant limits thermal growth better than mist, often by 8–12 microns.
Q4: When should I add a steady rest?
A: For length-to-diameter ratio above 4:1 on the unsupported section.
Q5: How do I verify concentricity without a CMM?
A: Mount the part between centers, use a dial indicator in a V-block, rotate and read TIR.