How to Guarantee 0.005mm Coaxiality? Full Process Breakdown of CNC Milling Machine Spindle from Forging to Fine Grinding

Hansheng Automation, as a professional CNC precision parts manufacturer, would like to share with you a machining case study of spindle components - CNC milling machine spindle. The content covers drawing analysis, process analysis, the entire machining process, and more. If you are interested in our machining capabilities, or if you have drawings and are looking for a suitable partner, please feel free to contact us to learn about our machining capabilities.

1.In the CNC milling machine spindle shown in Figure, the coaxiality tolerance of the φ90js5 outer diameter, the φ88g7 outer diameter, and the 7:24 taper bore to the common axis A‑B is φ0.005 mm.

2.The left end face and the left and right end faces of the gear teeth shall have an axial circular runout tolerance of 0.005 mm with respect to the common axis A‑B.

3.The part material is 15CrMn steel.

4.The carburized case depth shall be 0.6 to 1.0 mm, and after quenching and tempering, the hardness shall be 59 to 64 HRC.

A process flow chart:

Overview of Process Challenges and Solutions (Concise Edition)

Overall Process Characteristics

• Adopt an alternating sequence of multiple heat treatments and finishing operations to effectively balance material properties and final precision.
• Maintain datum unification throughout the entire process to minimise the number of clamping setups and accumulation of errors.
• Through reasonable allowance allocation (0.3–0.5 mm for finish turning before grinding, and approximately 0.1 mm for semi-finish grinding), ensure that final dimensions and geometric tolerances are consistently achieved.

1. This part is a spindle used in the accessory milling head of a CNC milling machine. The coaxiality, roundness, and cylindricity tolerances of all outer diameters are exceptionally tight. In particular, the coaxiality and surface roughness of the two support journals – the φ90js5 outer diameter and the 7:24 taper bore – are critical and must be ensured.

2. In the process sequence, the forging is normalized after forming. After finish turning, it undergoes carburizing and quenching, followed by finishing operations.

3. The φ90js5 outer diameter, the φ88g7 outer diameter, and the 7:24 taper bore have high precision requirements. Grinding allowance is left during finish turning, and the final sizing is performed on an cylindrical grinder.

4. Achieving the required 0.005 mm coaxiality between the φ90js5 and φ88g7 journals and the 7:24 taper bore is the biggest challenge. To meet this tolerance, we first finish-grind the journals to final dimensions and then use them as the primary datum for grinding the taper bore. Gear finishing is scheduled as the last operation.

5. After semi-finish grinding, an additional heat treatment aging step is introduced to improve and ensure the final precision and stability of the spindle. This aging effectively eliminates residual stresses generated during the machining process.

CNC Milling Machine Spindle – Mechanical Machining Process Sheet (Unit: mm)

Phase 1: Blank Preparation and Rough Machining

Phase 2: Semi-Finishing and Gear Tooth Machining

Phase 3: Heat Treatment and Surface Treatment

Phase 4: Preliminary Precision Grinding (Rough Grinding)

Phase 5: Aging Treatment and Final Precision Grinding (Finish Grinding)

Phase 6: Final Inspection and Warehousing

7:24 Taper Bore Machining: How to Ensure Coaxiality with the Journals?

he coaxiality requirement between the 7:24 taper bore and the φ90js5 and φ88g7 outer diameters is φ0.005 mm – the most challenging geometric tolerance to achieve on the entire spindle. Our core approach follows a clear principle: grind the journals first, then the bore, while maintaining a single unified datum throughout. By completing the critical outer journals to final size before machining the taper bore, we significantly reduce datum conversion errors.

The specific procedure is as follows: Once the two support journals (φ90js5 and φ88g7) have been ground to final dimensions, the workpiece is mounted on a CNC internal grinder using a custom locating sleeve on the φ90js5 journal. We then carefully indicate the journal with a dial gauge, ensuring radial runout stays within 0.002 mm. Only then do we proceed with grinding the 7:24 taper bore. The two V‑blocks on the special fixture are tipped with carbide, and their center height must be precisely aligned with the center height of the grinding wheel spindle; otherwise, the generatrix of the taper bore will be hyperbolic, compromising contact accuracy.

Allowance allocation: Grinding allowance is left during the finish turning operation. Rough grinding leaves 0.03–0.08 mm, while finish grinding leaves approximately 0.02 mm. Finish grinding must be performed after rough grinding has removed geometric errors – finish grinding cannot correct errors left from rough grinding. During grinding, the wheel extension arbor should be as short and rigid as possible, and the wheel width should be appropriately reduced to minimise vibration.

High-Hardness Outer Diameter Grinding (59–64 HRC)

After carburizing and quenching, the workpiece hardness reaches 59–64 HRC, which is a typical case of grinding hardened high-hardness steel.

Wheel selection: Priority is given to CBN (cubic boron nitride) grinding wheels with a ceramic bond. CBN wheels offer extremely high hardness and thermal stability, making them suitable for grinding hardened steels, tool steels, and other high-hardness materials. Their service life is 10 to 50 times that of alumina-type wheels. For finish grinding, a fine grit size (120–240 mesh) is selected to meet the surface roughness requirement of Ra0.8 μm. The wheel hardness should be on the soft side (grade D–F) to promote grain fracture and self-sharpening, preventing grinding burns caused by wheel dulling.

Grinding parameters: Wheel peripheral speed is controlled at 30–35 m/s. The depth of cut is 0.02 mm for rough grinding and 0.01 mm for finish grinding. The longitudinal feed rate of the workpiece is kept small during finish grinding.

Grinding strategy: A multi-pass approach is adopted – rough grinding → semi-finish grinding → finish grinding. Approximately 0.1 mm of allowance is left after semi-finish grinding, with final sizing achieved during finish grinding. Adequate coolant is applied between each grinding pass to prevent surface burns or microstructural changes caused by grinding heat.

Carburizing and Quenching Deformation Control

Carburizing and quenching inevitably introduce some bending distortion in 15CrMn steel spindles due to thermal and phase transformation stresses. Controlling this deformation is essential for achieving final precision. The control measures consist of two steps: straightening and aging.

Straightening: Straightening should be performed in the hot state immediately after quenching, before the workpiece has fully cooled to room temperature. At this stage, the retained austenite content is higher and the material exhibits better plasticity, making straightening easier and reducing the risk of cracking compared to straightening after tempering. During straightening, the following points should be observed:

• (1) Stress concentration occurs at the sharp corners of stepped sections with varying diameters. Excessive pressure at these locations should be avoided; instead, apply multiple corrective points over adjacent areas.
• (2) Oil holes, relief grooves, and other features are stress raisers after carburizing and quenching. If subjected to high tensile stress during straightening, they are highly prone to cracking.
• (3) Immediately after straightening, a low-temperature tempering treatment shall be carried out to eliminate the additional stresses induced by the straightening process.

Aging: After semi-finish grinding, an artificial aging treatment is introduced (in an oil furnace, at low temperature), with the temperature controlled at 160–180°C. This temperature range is below the tempering temperature of 15CrMn steel (typically 180–220°C), with the aim of relieving grinding stresses without reducing hardness. If the temperature is too high (>200°C), surface hardness may drop by 1–2 HRC; if it is too low, stress relief will be insufficient, and deformation may still occur after finish grinding. For high-precision parts, arranging an aging treatment after semi-finish machining is a critical measure to ensure final accuracy.

Gear Tooth Finishing Strategy

Hobbing allowance: During hobbing, a grinding allowance must be left on the tooth thickness. In general, the grinding allowance per side should not exceed 0.2 mm. When determining the allowance, the following factors must be considered:

• (1) The thermal distortion of the tooth profile after carburizing and quenching must be accommodated within the allowance.
• (2) If hobbing and gear grinding do not share the same machining datum, the tooth profile runout from hobbing must be controlled. This prevents inconsistent grinding allowances that could result in non‑uniform case depth on the tooth flanks, thereby reducing wear resistance.

Gear grinding datum: Gear grinding is scheduled after finish‑grinding the outer diameters and before finish‑grinding the taper bore. The locating datum is the φ90js5 or φ88g7 outer diameter and its adjacent end face, both already ground to drawing dimensions. This approach achieves "datum unification" – rough turning, hobbing, cylindrical grinding, and gear grinding all use the same outer diameter datum for locating, minimising errors from datum changes. Appropriate cutting parameters must be selected during gear grinding to avoid grinding burns and cracks.

This complete process - from forging and normalizing through carburizing, quenching, stress-relief aging, and final precision grinding - has proven highly effective for our high-precision spindles. More importantly, the same principles can be successfully applied to other shaft-type components that demand both high hardness and tight tolerances.

Rather than relying on a single "high‑end machine" or an experienced operator's "touch" to achieve results, this approach ensures consistency and stability through the rational design of the process chain, with each tolerance broken down into specific operations.

If you are currently facing similar precision challenges with spindle or shaft components, we would be happy to review your drawings and discuss the most suitable process solution based on our extensive production experience.