Machining Process Analysis and Special Fixture Design

for Thin‑Walled Stainless Steel Sleeve‑Type Parts

 

1．Part Structure Analysis

As shown in Figure 1, this component is a typical small‑sized, thin‑walled rotational part. The material is Y1Cr17 (GB/T 1220) stainless steel, supplied as bar stock, and it is intended for mass production.

 

Main structural features:

• Minimum wall thickness: 1 mm
• Inner diameter: Φ10–Φ12 mm, with tolerance 0.01–0.02 mm
• Outer diameter: Φ14–Φ13 mm, with tolerance 0.02 mm
• Symmetrical inner bores at both ends, each with an internal groove of 0.5 mm × 0.1 mm
• Several fillets with very small radii, ranging from 0.15 to 0.25 mm

This part presents four major difficulties: thin walls prone to deformation, high precision requirements, compact structure, and poor machinability of the material. These demand stringent process planning and tooling design.

 

2．Process Difficulties and Corresponding Control Measures

2.1 Thermal Deformation Caused by Cutting Heat

Thin‑walled workpieces have low thermal capacity and slow heat conduction. Cutting heat can cause rapid temperature rise, making dimensional control extremely difficult.

Countermeasures:

• Reduce cutting deformation and cutting forces to minimize heat generation at the source.
• Increase the heat dissipation area of the cutting edge and ensure an ample supply of cooling and lubricating fluid.
• Use emulsions containing sulphur‑ or chlorine‑based extreme‑pressure additives to provide both anti‑adhesion and cooling performance.
• Maintain sufficient flow rate and pressure of the cutting fluid.

 

2.2 Vibration and Deflection Under Cutting Forces

Radial cutting forces tend to bend the workpiece. In addition, fluctuating chip‑breaking forces can excite vibration in the tool‑workpiece system, adversely affecting dimensional accuracy, form and position tolerances, and surface finish.

Countermeasures:

• Select machine tools with high rigidity and adequate power.
• Increase the stiffness of the tool and workpiece system – for example, by enlarging the tool shank cross‑section and minimizing tool overhang.
• Optimize tool geometry (e.g., increasing rake angle) to reduce cutting forces.

 

2.3 Clamping Deformation

When a three‑jaw self‑centering chuck is used, radial clamping forces can easily deform the thin wall into a triangular shape, causing roundness errors. If clamping force is reduced too much, the workpiece may become loose during machining.

Limitations of conventional methods:

• Using open transition rings or increasing the jaw contact area – suitable only for relatively thicker walls and simpler geometries.
• Axial clamping (via a nut) – despite minimal deformation, the lengthy auxiliary setup time makes it unsuitable for high‑volume production.

Therefore, a dedicated fixture must be designed to simultaneously ensure accuracy and productivity.

 

3．Special Fixture Design

3.1 Locating Datums and Clamping Method

• Radial datum: the pre-machined outer diameter Φ(14.3 ± 0.009) mm. Axial datum: the finished left end face.
• Axial datum: the finished left end face.

Because the wall thickness is only 1 mm, the cutting allowances, cutting forces, and clamping forces must all be kept moderate. Hence, a pneumatic elastic rubber fixture is adopted, as shown in Figure 2.

3.2 Fixture Structure and Working Principle (Figure 2)

The fixture consists of a connecting plate, sealing gasket, large‑thread plug, M6 studs, threaded plug, rubber plug, etc.
The connecting plate is mounted on the machine spindle. Its central hole (Φ20 mm) is connected to the compressed‑air line of a rotary pneumatic device. Compressed air passes through three symmetrically arranged air passages and acts on the elastic rubber sleeve, which expands uniformly to clamp the workpiece.
The locating holes for the gripper jaws are machined after the fixture has been properly installed on the machine, ensuring concentricity with the spindle.

 

3.3 Gripper Jaw (Figure 3)

The gripper jaw is the main actuating element. Its structural design must ensure uniform elastic deformation and high repeated positioning accuracy. Detailed dimensions are given in Figure 3 (drawing omitted here).

 

4．Selection of Tool Materials and Cutting Parameters

4.1 Tool Materials

Y1Cr17 is a ferritic free‑cutting stainless steel. Its machinability is comparable to that of free‑cutting structural steels, while its mechanical properties are similar to those of high‑quality structural steels.

• Drills, taps, and other standard tools: use advanced high‑speed steels such as W12Cr4V4Mo, W12Cr4V5Co5 (T15), and W9Cr4V5Co3.
• Turning tools (external and internal): select cemented carbide grades YG, YW1, YW2, YH1, YH2, or PVD‑coated ultra‑fine grain cemented carbide and PVD‑coated cermet.

Tool geometry (a larger rake angle reduces cutting forces, while a moderate clearance angle maintains edge strength):

Operation	Rake angle γ₀	Clearance angle α₀
Rough turning	12°–20°	6°–10°
Finish turning	20°–30°	10°–20°

4.2 Cutting Parameters

Operation	Depth of cut (mm)	Feed rate (mm/r)	Cutting speed (m/min) or spindle speed (r/min)
Rough turning (OD)	0.3–0.5	0.02–0.05	approx. 3000 r/min
Finish turning (OD)	0.1–0.15	0.02–0.05	approx. 3000 r/min
Rough boring (ID)	0.2–0.3	0.02–0.05	approx. 3000 r/min
Finish boring (ID)	0.1–0.15	0.02–0.05	approx. 3000 r/min
Grooving / Parting off	—	0.02	approx. 2000 r/min
Drilling (Φ9.5 mm)	—	0.07	approx. 1200 r/min

Note: When using ultra‑fine grain cemented carbide or PVD‑coated tools, cutting speeds can exceed 295 m/min. Within the machine’s speed capability, the highest possible spindle speed should be selected to improve efficiency.

 

4.3 Cutting Fluid Selection

For thin‑walled workpieces, cutting fluids that provide both cooling and lubrication are preferred. Recommended types include sulphurized oil, sulphurized soybean oil, kerosene mixed with oleic acid or vegetable oil, carbon tetrachloride mixed with mineral oil, and high‑concentration emulsions.

 

5．Recommended Process Sequence

Operation I (bar stock clamping):

1. Drill a Φ9.5 mm pilot hole.
2. Rough bore the inner hole.
3. Rough and finish turn both end faces to size.
4. Rough turn the outer diameter.
5. Semi‑finish turn the external profile.
6. Finish turn the external profile to drawing requirements.
7. Part off.

Operation II (special fixture clamping):

1. Semi‑finish and finish turn the inner bore and end face of one end to size.
2. Cut the internal groove.
3. Reverse the workpiece; semi‑finish and finish turn the inner bore and end face of the other end.
4. Cut the internal groove on that side.

 

6．Conclusions

By employing the pneumatic elastic rubber fixture in conjunction with appropriate tool materials and cutting parameters, the deformation problems caused by clamping forces, cutting forces, and thermal effects have been effectively mitigated, while simultaneously ensuring high production efficiency suitable for mass production. In practice, the following points should also be observed:

• Regularly inspect the elasticity and sealing performance of the rubber sleeve.
• Strictly control the cutting fluid supply to avoid sudden temperature rises.
• Leave sufficient semi‑finishing allowance before finish machining to compensate for deformation caused by stress release.