CNC Machined Bushings: A 45 Steel Precision Machining Case Study

This aritcle covers the full manufacturing process for a 45 steel CNC machined bushing, from raw material preparation to final inspection. The part requires a Ø30H6 bore, Ø0.005 mm concentricity, 45–50 HRC hardness, and Ra 0.8 μm surface finish.

We explain how we structure the machining sequence, transfer the machining datum, and use precision grinding to meet these requirements.

The drawing defines a 45 steel bushing with the following critical requirements:

The Ø45 OD axis is the datum. The Ø30H6 bore axis must remain concentric to this datum within Ø0.005 mm. The left end face also has an axial circular runout requirement of 0.005 mm relative to the same datum.

The engineering challenge is maintaining the geometric relationship between the bore and OD after heat treatment and precision grinding, not simply machining each feature to size.

Without this level of process control, a bushing can meet the individual bore and OD dimensions while still failing the concentricity requirement. Heat treatment can also shift the relationship between machined surfaces, making it risky to rely on the pre-treatment datum for final finishing.

For this reason, the process establishes the Ø30H6 bore after heat treatment and uses the finished bore as the reference for the final OD grinding operation.

Our machining strategy follows a defined datum sequence:

OD → ID → ID as the new datum → OD

We first establish the OD as the initial machining reference, then finish the H6 bore before transferring the datum to the finished ID for the final OD grinding operation.

The part is manufactured from 45 steel. For this 45 steel bushing machining case, the material selection is part of the overall process plan because the final part must undergo heat treatment before precision finishing.

Quenching and Tempering to 45–50 HRC

The specified heat treatment is quenching and tempering, with a final hardness of 45–50 HRC.

We place heat treatment after rough machining rather than after final finishing. The rough machining establishes the basic geometry first, while the precision dimensions and surface finish remain for the operations after heat treatment.

This sequence prevents the final H6 bore machining and grinding operations from being completed before the heat-treatment stage.

Why Precision Finishing Is Scheduled After Heat Treatment

For this CNC machined steel bushing, the process is divided into three clear stages:

Rough Machining → Heat Treatment → Precision Grinding

Rough machining establishes the basic OD, bore, and part geometry. We then perform quenching and tempering to reach the specified 45–50 HRC hardness. After heat treatment, precision grinding completes the critical bore, end face, and OD requirements.

This separation allows the final grinding operations to establish the datum relationship needed to control bushing concentricity.

Step 1 - Blank Preparation and Rough Turning

We start with a Ø50 × 300 mm round steel bar. We use a sawing machine to prepare the stock and a horizontal lathe for rough turning.

Because the finished bushing is relatively short, we machine it from long stock and cut it off after completing the main turning operations. The rough-turning sequence includes:

• Face turning
• Drill Ø20 mm
• Bore to Ø29.70 mm
• Turn the OD to Ø45.30 mm
• Machine C1.3 chamfers
• Cut off at 22.6 mm
• Reverse the part
• Face to the final 22.1 mm length
• Finish the chamfers

The purpose at this stage is to establish the basic geometry and leave the required material for subsequent finishing. We intentionally complete the rough machining before heat treatment, so the critical dimensional and surface requirements can be addressed during the post-heat-treatment grinding operations.

Step 2 - Heat Treatment: Quenching and Tempering

After rough turning, the part undergoes quenching and tempering using a salt bath furnace and tempering furnace. The specified final hardness is 45–50 HRC.

The process sequence is:

Rough Machining → Heat Treatment → Precision Grinding

After heat treatment, the part goes to precision grinding rather than additional turning. The heat-treated part moves directly into the precision finishing stage, where grinding establishes the critical bore, face, and OD requirements.

Step 3 - Precision Internal Grinding and Face Grinding

After heat treatment, we use a CNC internal cylindrical grinding machine to finish the Ø30H6 bore and the left end face. The specified surface finish is Ra 0.8 μm for both surfaces. The finished bore will serve as the new precision datum for external grinding.

Step 4 - External Grinding with a Taper Mandrel

The finished Ø30H6 bore now becomes the precision datum for external grinding. We locate the part through the finished bore using a taper mandrel, then grind the OD on a CNC external cylindrical grinding machine.

The target OD is Ø45 (+0.089/+0.050) mm, with a final surface roughness of Ra 0.8 μm.

To control the Ø0.005 mm concentricity between the bore and OD, we use the finished bore as the positioning reference during external grinding. A taper mandrel locates the part through the bore, allowing the OD to be ground relative to the finished bore axis. This transfers the final bore datum into the external grinding operation and provides the geometric basis for achieving the required concentricity.

We position the part from the finished bore, not the original rough OD. The taper mandrel transfers the bore axis into the external grinding operation, allowing the OD to be finished relative to the actual bore axis.

This allows the final OD to be machined relative to the finished bore axis, which is essential for meeting the Ø0.005 mm concentricity requirement.

Step 5 - Final Inspection, Oiling and Packaging

After grinding, the finished CNC machined bushing moves to the inspection station. We verify the drawing-critical requirements:

• Ø30H6 bore
• Ø45 (+0.089/+0.050) mm OD
• Ø0.005 mm bore-to-OD concentricity
• 0.005 mm end-face axial circular runout
• Ra 0.8 μm surface roughness

The process then follows:

Inspection → Oiling → Packaging → Storage

The inspection stage confirms that the dimensional, geometric, and surface-finish requirements have been maintained through the complete manufacturing process.

Achieving 0.005 mm concentricity in a precision bushing requires more than accurate individual dimensions. The process must control the relationship between the bore and OD from rough machining through final grinding.

1. Build the Process Around a Controlled Datum

We complete rough machining before heat treatment and precision-grind the Ø30H6 bore after heat treatment.

Once finished, the bore becomes the new precision datum. A taper mandrel uses this bore to position the part during final OD grinding.

This creates a direct machining relationship between the finished bore axis and the OD rather than relying on the original rough OD.

2. Control Grinding Allowance and Grinding Heat

Grinding requires enough stock for the operation while avoiding unnecessary removal at the finishing stage.

We leave material during rough machining for post-heat-treatment grinding and complete the final dimensions through controlled grinding operations. Grinding also requires control of heat generation to avoid compromising the finished geometry and surface quality.

The available process data specifies the final Ra 0.8 μm surface finish but does not define a specific grinding allowance or thermal-control parameter. These values must be determined from the actual production setup rather than treated as fixed values.

3. Use Grinding for Final Geometric Accuracy

The final OD is produced by external cylindrical grinding, not by turning. This operation establishes the Ø45 (+0.089/+0.050) mm OD and Ra 0.8 μm surface finish while maintaining the datum relationship established through the finished bore.

That sequence defines the manufacturing strategy for meeting the Ø0.005 mm concentricity requirement.

Engineering Takeaway

For precision CNC machined bushings, the final result depends on how the machining sequence aligns with the drawing requirements. In this case, the finished Ø30H6 bore becomes the reference for the final OD grinding operation, allowing the bore and OD relationship to be controlled through the actual manufacturing process rather than through separate dimensional checks alone.

At Hansheng Automation, we start with the drawing, identify the geometric relationships that matter, and build the machining route around them.