High-strength, wear-resistant polyetheretherketone (PEEK) composite material S-0203 represents a significant advancement for gear components operating in demanding aqueous environments. This material excels under conditions ranging from room temperature and high-temperature steam to dry friction scenarios. Its application in aerospace electrolytic circulation pump gear assemblies, utilizing a composite/metal pairing instead of traditional metal/metal, effectively mitigates common failures like high friction, excessive wear, seizure, and galling. These fixed-clearance gear pumps function by using an electric motor to drive meshing gears, pressurizing fluid at the inlet and delivering it to the outlet. Achieving the required high precision on gear end faces and tooth flanks during gear machining presents unique challenges. This paper details the investigation into resolving critical processing defects – end face chipping and tooth flank scoring – encountered during the gear machining of S-0203 composite gears.
Gear assemblies manufactured from S-0203 consistently exhibited two primary defects post-machining. Firstly, after the press-fit assembly of the composite gear onto its metal shaft, visible chipping occurred at the end face interface. Examination under a stereomicroscope (15x magnification) revealed distinct fracture morphology at the chipped locations. Secondly, following the tooth grinding stage, visually apparent scoring marks running along the tooth profile direction were observed on the flank surfaces.
The root causes were systematically investigated through material property analysis and process review. S-0203 is a carbon fiber short-fiber reinforced PEEK composite. PEEK, a semi-crystalline aromatic linear thermoplastic engineering plastic, offers an exceptional combination of properties: low density (1.44 g/cm³), excellent chemical stability, high-temperature resistance, acid/alkali resistance, hydrolysis resistance, wear resistance, and self-lubrication, coupled with good impact toughness. However, its relatively low Shore D hardness (86-88 HD) classifies it as a hard plastic. While initially machinable with aggressive parameters using sharp tools, its machining performance degrades rapidly with tool dulling or wheel loading, leading to spalling and chipping defects.
Analysis pinpointed the chipping defect to two sequential operations: press-fitting and end face grinding. The interference fit between the gear bore and shaft (0.03–0.05 mm) meant that conventional room temperature press-fitting exerted significant radial and axial compressive forces on the composite material. This force concentration at the bore/shaft end face junction initiated cracking and chipping. Furthermore, if the press-fit resulted in the shaft end being recessed below the gear end face, that unsupported composite region became highly susceptible to spalling under the shear forces generated during the subsequent end face grinding operation.
The tooth flank scoring defect was traced to the gear machining process, specifically the form grinding operation. The gear machining setup involved holding a long, slender workpiece (total length 43.5 mm, shaft diameter Ø4 mm, length-to-diameter ratio ≈ 11) between centers on a CNC form grinding machine. The initial gear machining strategy involved direct form grinding from the blank state (Ø16.5 mm tip diameter, Ø9.5 mm root diameter, total stock removal of 3.5 mm) using a 5SG100-FG12VS3P aluminum oxide grinding wheel. The inherent tendency of the composite to shed debris during gear machining caused rapid wheel loading if dressing was inadequate. Combined with the workpiece’s low rigidity (prone to bending) and the relatively soft nature of the original wheel (leading to premature grit dislodgement before becoming fully dull), the conditions were conducive to grit particles dragging across the tooth flank, causing scoring.
Refinement of Press-Fit Assembly Process
To eliminate the mechanical stresses causing chipping, the press-fit methodology was fundamentally redesigned. Leveraging experience with thermal expansion techniques for other material pairs (e.g., graphite sleeves in aluminum housings), a differential thermal expansion approach was adopted for the composite gear and metal shaft assembly. The composite gear requires heating to expand its bore, while the metal shaft requires cooling to contract its diameter. Theoretical thermal expansion calculations guided initial temperature parameters:
Composite Gear Expansion: $$\Delta D_{gear} = \alpha_{gear} \cdot D_0 \cdot \Delta T$$ Where:
- $\alpha_{gear}$ = Coefficient of thermal expansion (CTE) ≈ 5 × 10⁻⁵ /°C
- $D_0$ = Nominal bore diameter ≈ 4.15 mm
- $\Delta T$ = Temperature increase (e.g., 170°C from 20°C → 190°C ≈ ΔT=170°C)
$$\Delta D_{gear} = (5 \times 10^{-5}) \times 4.15 \times 170 \approx 0.035 \text{ mm}$$
Shaft Contraction: $$\Delta D_{shaft} = \alpha_{shaft} \cdot D_0 \cdot \Delta T$$ Where:
- $\alpha_{shaft}$ = CTE (Steel) ≈ 14 × 10⁻⁶ /°C
- $D_0$ = Nominal shaft diameter ≈ 4.15 mm
- $\Delta T$ = Temperature decrease (e.g., 20°C to -40°C → ΔT=-60°C)
$$\Delta D_{shaft} = (14 \times 10^{-6}) \times 4.15 \times (-60) \approx -0.0035 \text{ mm}$$
The net diametral clearance becomes: $$Clearance \approx \Delta D_{gear} – |\Delta D_{shaft}| – \text{Original Interference}$$ $$Clearance \approx 0.035 – 0.0035 – 0.04 = -0.0085 \text{ mm}$$ This calculation indicates a slight residual interference. Refining parameters, heating the gear to 180°C (ΔT=160°C) and cooling the shaft to -50°C (ΔT=-70°C) yields:
- $\Delta D_{gear} = (5 \times 10^{-5}) \times 4.15 \times 160 \approx 0.0332 \text{ mm}$
- $\Delta D_{shaft} = (14 \times 10^{-6}) \times 4.15 \times (-70) \approx -0.0041 \text{ mm}$
- $Net \ Clearance \approx 0.0332 – 0.0041 – 0.04 = -0.0109 \text{ mm}$ (Still interference)
Further adjustment, targeting a gear temperature of 190°C (ΔT=170°C) and shaft at -55°C (ΔT=-75°C):
- $\Delta D_{gear} = (5 \times 10^{-5}) \times 4.15 \times 170 \approx 0.0353 \text{ mm}$
- $\Delta D_{shaft} = (14 \times 10^{-6}) \times 4.15 \times (-75) \approx -0.0044 \text{ mm}$
- $Net \ Clearance \approx 0.0353 – 0.0044 – 0.04 = -0.0091 \text{ mm}$
Practical trials confirmed that heating the gear to 170-180°C (held for 10 minutes in a forced-air oven) and cooling the shaft to -50°C to -40°C (held for 10 minutes in a low-temperature freezer) created sufficient differential expansion for a near-zero force assembly. Due to the small thermal mass of the parts, rapid transfer and assembly are critical to prevent significant temperature equalization before completion.
Complementing the thermal method, a precision press-fit fixture was designed. This fixture positively locates the shaft on its Ø4.15⁺⁰·⁰² mm journal (see Figure 7 schematic). The gear is pressed onto the shaft until its end face contacts the precisely machined datum face of the fixture. The critical feature is the minimal land width (0.220–0.235 mm) on the shaft and a tight clearance fit (min. 0.01 mm) between the shaft locating diameter and the fixture bore. This ensures shaft perpendicularity and guarantees that both the shaft and the composite gear end flush with the fixture face after assembly, eliminating protrusions susceptible to grinding shear forces.
These modifications addressed both root causes of end face chipping: eliminating crushing forces during assembly and ensuring a flush end face to provide full support during grinding. Post-implementation inspection under 15x magnification confirmed the absence of chipping defects.
Optimization of Gear Machining (Grinding) Process
Grinding Wheel Selection
Selecting the optimal grinding wheel was paramount for eliminating tooth flank scoring. Given the novelty of machining PEEK composites within our operations, selection was based on material properties, handbook guidelines, and empirical testing. Key wheel characteristics were evaluated:
| Wheel Characteristic | Selection Criteria | Options Considered | Rationale & Test Results | Final Selection |
|---|---|---|---|---|
| Abrasive Type | Material Compatibility, Form Grinding Capability | Aluminum Oxide (A/WA), Silicon Carbide (C/GC) | Silicon Carbide is preferred for non-metallic materials and form grinding applications. Effective for roughing and finishing in one setup. | Silicon Carbide (GC) |
| Hardness Grade | Workpiece Material Hardness, Grinding Mode | Soft (H, I, J, K), Medium (L, M, N), Hard (O, P, Q) | PEEK composite is “soft” relative to steel but tough/fibrous. Soft wheels (J, K) allow dull grits to shed before excessive force builds up, reducing scoring risk. Form grinding also benefits from softer grades. | Soft (J/K) |
| Bond Type | Wheel Porosity, Cutting Sharpness Retention | Vitrified (V), Resinoid (B) | Carbon fiber debris readily clogs pores. Vitrified bonds offer superior porosity for chip clearance and are stable at standard grinding speeds (≤35 m/s). Resin bonds, while stronger/faster, have lower porosity. | Vitrified (V) |
| Grit Size | Surface Finish Requirement, Gear Module Size | Coarse (46#, 60#), Medium (80#), Fine (100#, 120#) | Module 1.5 gear requires fine surface finish (Ra 0.8 μm) and tight tolerance (0.015 mm total composite variation). Coarse grits (46#, 60#) compromised finish. 80# produced acceptable but inconsistent results. 100# delivered stable, superior surface quality meeting Ra 0.8 μm. | 100# |
Based on this analysis and testing, a GC100JVK silicon carbide wheel was selected to replace the original 5SG100-FG12VS3P aluminum oxide wheel. Trials confirmed the GC wheel significantly reduced loading, maintained sharpness better, and eliminated tooth flank scoring defects.
Grinding Parameter Optimization
Effective gear machining of PEEK composites requires managing wheel loading and minimizing forces on the slender workpiece. The grinding cycle was segmented into distinct roughing and finishing stages. Crucially, due to the material’s propensity to shed debris and load the wheel, a dressing pass was implemented after *every* grinding pass, regardless of depth, to maintain wheel sharpness and profile accuracy. Wheel speed was maintained at 30 m/s. The optimized parameters are detailed below:
| Stage | Pass # | Depth of Cut (mm) | Feed Rate (mm/min) | # of Strokes | Infeed per Stroke (mm) |
|---|---|---|---|---|---|
| Roughing | 1 | 0.58 | 3000 | 16 | 0.05 (first 5), 0.03 (last 11) |
| 2 | 0.36 | 3000 | 12 | 0.03 | |
| 3 | 0.30 | 3000 | 10 | 0.03 | |
| 4 | 0.30 | 3000 | 10 | 0.03 | |
| 5 | 0.24 | 3000 | 8 | 0.03 | |
| 6 | 0.16 | 3000 | 8 | 0.02 | |
| 7 | 0.16 | 3000 | 8 | 0.02 | |
| Finishing | 8 | 0.035 | 2000 | 3 | 0.015 (first), 0.01 (last 2) |
| 9 | 0.010 | 1500 | 2 | 0.005 |
This parameter strategy aggressively removes stock in the roughing phase while maintaining control, progressively reduces forces and stock removal as the finish dimension is approached, and employs slower speeds and minimal infeeds during finishing to achieve the required surface integrity and dimensional accuracy. Frequent dressing is the key enabler for stable gear machining throughout the cycle.
Process Validation and Conclusions
Implementation of the thermally assisted press-fit assembly with precision fixturing and the optimized grinding parameters using the GC100JVK wheel resulted in the consistent production of defect-free S-0203 composite gears. Stereomicroscope examination (15x) of end faces confirmed the elimination of chipping. Visual and tactile inspection of tooth flanks showed no evidence of scoring. Dimensional checks and surface roughness measurements (Ra ≤ 0.8 μm) consistently met specifications.
Key learnings for successful gear machining of high-strength, wear-resistant PEEK composites like S-0203 are summarized:
- End Face Chipping: Primarily caused by mechanical crushing forces during conventional press-fitting and unsupported grinding shear forces due to non-flush assembly. Solved via differential thermal expansion assembly and precision fixturing.
- Tooth Flank Scoring: Resulted from wheel loading (clogged pores) and premature grit dislodgement from an unsuitable (too soft) wheel. Addressed by selecting a silicon carbide abrasive (GC), a slightly softer hardness grade (J/K) to promote controlled grit shedding, a vitrified bond (V) for high porosity, and a fine grit size (100#) for required finish. Crucially, this must be coupled with an appropriate dressing regimen.
- Grinding Strategy: PEEK composites demand careful wheel selection and proactive wheel maintenance. Frequent dressing is non-negotiable to combat loading, maintain cutting sharpness, and preserve profile accuracy throughout the gear machining process. Parameters must balance stock removal efficiency with minimizing forces, especially for slender components, transitioning smoothly from roughing to finishing.
The successful resolution of these defects through targeted process modifications demonstrates that robust and reliable gear machining of advanced PEEK composites is achievable, enabling their full potential in demanding tribological applications like aerospace fluid pumps.