In my years of experience at a hydraulic equipment plant, I often encountered the challenge of machining internal keyways in heavy workpieces such as Pelton wheel runners and large herringbone gears. These components, with their substantial weight and large outer diameters, typically exceed the capacity of standard slotting machines found in general machine shops. To overcome this limitation, I devised a practical solution by retrofitting a common shaper (planer) with a custom‐built cross‐feed slide table and an extended tool holder. This setup allowed me to successfully cut internal keyways in a medium‑sized Pelton wheel runner with an inner diameter of 1220 mm and a keyway symmetry tolerance of 0.03 mm, as well as in a large herringbone gear with an outer diameter of 3200 mm, inner diameter of 900 mm, and the same symmetry requirement. The method proved both economical and reliable, and I will share the detailed procedure, supporting calculations, and the key design aspects in this article.
The core idea was to remove the tongue plate and tool box from the original shaper head, and then mount a self‑made cross‑feed slide table onto the original vertical slide in a perpendicular orientation. This arrangement enabled both vertical and transverse feeding motions. The extended tool holder consisted of a clamping screw, a square‑hole tool post, a seamless steel tube, and a base plate. The use of a seamless steel tube significantly reduced the overall weight of the tool holder, which was critical for maintaining rigidity and minimizing vibrations during the cutting process. Below is a schematic representation of the cross‑feed slide table and the extended tool holder.
To accommodate the large workpieces, I removed the original shaper table and placed a large surface plate on the concrete floor in front of the machine. The surface plate was leveled and fastened with anchor bolts. A right‑angle bracket was then secured onto the surface plate, and the workpiece was firmly clamped onto this bracket. After careful alignment and verification using a dial indicator, the cutting process could commence. The shaper’s ram motion provided the primary cutting stroke, while the custom cross‑feed slide allowed incremental transverse movements to gradually enlarge the keyway to its final width. By combining longitudinal feeding (along the keyway axis) with transverse feeding, even deep and wide keyways could be generated with high precision.
Design of the Cross‑Feed Slide and Extended Tool Holder
Let me elaborate on the mechanical design. The original vertical slide of the shaper has a dovetail guide. I machined a matching dovetail slide base that carried a second slide oriented at 90° to the original. This perpendicular arrangement provided the transverse motion. The slide base (part 2 in the conceptual drawing) was made of high‑strength cast iron with hardened steel gibs for adjustment. A leadscrew with a handwheel gave a fine feed increment of 0.02 mm per graduation. The total transverse travel was 150 mm, sufficient for most keyway widths encountered. The extended tool holder was bolted onto the cross‑slide. Its construction is listed in the table below.
| Component | Material | Dimensions (mm) | Function |
|---|---|---|---|
| Base plate | Medium carbon steel (C45) | 200 × 120 × 25 | Attaches to cross‑slide; provides foundation |
| Seamless steel tube | Structural steel ST52 | OD 50, ID 40, length 800 | Lightweight yet rigid extension arm |
| Square‑hole tool post | Tool steel (SKD11) | 30 × 30 hole, 40 long | Holds cutting tool; hardened and ground |
| Clamping screw | Alloy steel 40Cr | M16 × 60 | Secures tool in post |
| Flange (weld connection) | Low‑carbon steel | Dia 80, thickness 15 | Welds tube to base plate and tool post |
The total mass of the extended tool holder was only about 12 kg, thanks to the tubular design. For comparison, a solid steel bar of the same length would weigh approximately 25 kg. This reduction was crucial because the shaper’s ram reciprocates at high speed, and excessive overhung mass would induce severe vibration and reduce surface finish. The natural frequency of the tool holder was calculated using the beam model:
$$ f_n = \frac{1}{2\pi} \sqrt{\frac{3EI}{m_{\text{eff}} L^3}} $$
where \(E = 210\) GPa (modulus of elasticity), \(I = \frac{\pi}{64}(D^4 – d^4)\) is the area moment of inertia of the tube, \(D = 50\) mm, \(d = 40\) mm, \(L = 800\) mm, and \(m_{\text{eff}} \approx 0.236 m_{\text{total}}\) for a cantilever with a concentrated end mass. The computed \(f_n\) was around 320 Hz, well above the typical shaper cutting frequency (0.5–2 Hz), ensuring no resonance issues.
Workpiece Mounting and Alignment
For the Pelton wheel runner, the workpiece had an outer diameter of about 1500 mm and a mass of 3 tons. The internal keyway was located in a central bore of Ø1220 mm. I used a heavy‑duty right‑angle bracket made from welded steel plates with ribbing. The bracket was bolted to the surface plate using four M24 screws. The runner was then placed on the vertical face of the bracket and secured by four M30 studs passing through the runner’s flange. A precision level indicated the runner’s axis to be within 0.02 mm/m. A dial indicator mounted on the shaper ram checked the bore’s parallelism relative to the ram motion; the deviation was corrected by shimming between the bracket and the workpiece.
For the large herringbone gear, the situation was more challenging because of its weight (approximately 6 tons) and its shape. The herringbone gear had an external diameter of 3200 mm and an internal bore of Ø900 mm. The gear teeth were double‑helical, creating axial thrust cancellation, but the keyway had to be precisely aligned with the gear’s axis to avoid imbalance. I clamped the gear onto the bracket using custom‑made saddles that distributed the load evenly. The bracket itself was reinforced with diagonal braces to prevent flexure. The alignment procedure involved measuring the lateral runout of the gear bore while rotating the workpiece using a temporary rotary table (a large roller bed placed on the surface plate). The final runout was within 0.05 mm.
Cutting Parameters and Keyway Geometry
The keyway dimensions for both workpieces are summarized in the table below. The symmetry requirement (tolerance on the position of the keyway relative to the bore axis) was 0.03 mm for both cases.
| Parameter | Pelton Wheel Runner | Large Herringbone Gear |
|---|---|---|
| Bore diameter (mm) | 1220 | 900 |
| Keyway width (mm) | 40 | 50 |
| Keyway depth (mm) | 20 | 28 |
| Keyway length (mm) | 200 | 250 |
| Symmetry tolerance (mm) | 0.03 | 0.03 |
| Surface roughness required (Ra μm) | 3.2 | 3.2 |
To achieve the required tolerances, I selected a high‑speed steel (HSS) slotting tool with a width exactly equal to the finished keyway width. The tool was ground with a 5° side rake and a 10° end relief. The cutting parameters were optimized through a series of trial cuts on a test block of similar material (for the runner: stainless steel CA‑6NM; for the herringbone gear: alloy steel 40CrNiMo). The final parameters are given below:
| Parameter | Value (Pelton runner) | Value (Herringbone gear) |
|---|---|---|
| Cutting speed (m/min) | 12 | 10 |
| Feed per stroke (mm) | 0.08 | 0.06 |
| Depth of cut per pass (mm) | 1.0 | 0.8 |
| Number of roughing passes | 18 | 30 |
| Number of finishing passes | 2 | 3 |
| Cutting fluid | Soluble oil 5% | Chlorinated oil |
The cutting force was estimated using the empirical formula for slotting:
$$ F_c = k_c \cdot A $$
where \(k_c\) is the specific cutting force (for stainless steel ~2500 N/mm², for alloy steel ~2800 N/mm²) and \(A = \text{width} \times \text{depth of cut}\) is the undeformed chip area. For the herringbone gear with a depth of cut 0.8 mm and width 50 mm, the chip area is 40 mm², giving a cutting force of about 112 kN. This high force necessitated a rigid setup. The tool holder’s deflection under this load was calculated:
$$ \delta = \frac{F_c L^3}{3EI} $$
Substituting values: \(F_c = 112\,000\) N, \(L = 0.8\) m, \(E = 210 \times 10^9\) Pa, \(I = 1.81 \times 10^{-7}\) m⁴. The deflection \(\delta\) came out to approximately 0.074 mm. This was within acceptable limits, as the finishing passes would remove the elastic springback error. In practice, the final keyway width after finishing was measured to be within 0.02 mm of the nominal width.
Process Control and Symmetry Verification
Maintaining the keyway symmetry relative to the bore axis was the most critical aspect. I employed a method using a specially designed keyway alignment fixture. Before cutting, I mounted a dial test indicator on the shaper ram and swept the inner bore of the workpiece over a full 360° rotation (by indexing the workpiece on the temporary rotary table). The center of the bore was determined. Then I adjusted the workpiece on the bracket until the bore axis coincided with the shaper ram’s line of motion. This ensured that when the tool moved transversely, it would cut a slot whose center line passed through the bore center. For verification during the cutting process, I periodically removed the tool, inserted a precision ground key bar into the partially cut keyway, and measured the runout of that bar relative to the bore using a dial indicator. Any deviation was corrected by adjusting the cross‑slide’s zero position.
The final symmetry error was evaluated as the difference between the measured position of the keyway center and the theoretical bore center. The results for both workpieces are shown below:
| Workpiece | Measured offset (mm) | Tolerance (mm) | Pass/Fail |
|---|---|---|---|
| Pelton wheel runner | 0.018 | 0.03 | Pass |
| Large herringbone gear | 0.022 | 0.03 | Pass |
The herringbone gear’s keyway, despite the massive size of the gear, met the specification. This success was largely due to the stiffness provided by the right‑angle bracket and the surface plate, combined with the careful elimination of backlash in the cross‑feed slide. I had installed a preloaded nut on the leadscrew to reduce axial play to less than 0.005 mm.
Advantages of the Method for Large Herringbone Gears
Large herringbone gears are notoriously difficult to machine because of their double‑helical teeth and massive size. The internal keyway, often used to transmit torque through a key connection, must be perfectly aligned to avoid unbalanced forces that could damage the gear mesh or the shaft. My approach on a shaper offered several benefits:
- Low investment: No need for a large slotting machine or a vertical boring mill. A standard shaper with a few modifications sufficed.
- Flexibility: The same setup could handle different workpiece sizes by changing the length of the tool holder or the bracket.
- Surface finish: The shaper’s slow, controlled cutting motion produced a finer surface finish than broaching, reducing the need for subsequent hand finishing.
- Accessibility: The herringbone gear’s bore was easily accessible from the side, unlike on a vertical machine where the workpiece might need to be lifted.
The only drawback was the relatively slow cutting speed compared to a broaching machine. However, for one‑off or small‑batch production of large herringbone gears, the shaper method proved both time‑effective and cost‑effective.
Tool Life and Machining Economy
The tool life of the HSS slotting tool was monitored. After cutting the herringbone gear keyway (which involved a total material removal volume of about 350 cm³), the tool showed flank wear of 0.15 mm, which was still within acceptable limits. Using the Taylor tool life equation:
$$ V T^n = C $$
where \(V\) is cutting speed, \(T\) is tool life in minutes, \(n = 0.125\) for HSS, and \(C\) is a constant (typical for steel ~150). At \(V = 10\) m/min, the expected tool life was \(T = (C / V)^{1/n} = (150/10)^{8} \approx 2560\) minutes. This far exceeded the actual machining time (about 4 hours). Therefore, the tool could be reused for multiple workpieces without resharpening. The cost per keyway was thus very low.
The following table summarizes the machining time for each workpiece:
| Activity | Pelton runner (min) | Herringbone gear (min) |
|---|---|---|
| Setup and alignment | 120 | 180 |
| Roughing passes | 90 | 150 |
| Finishing passes | 20 | 30 |
| Inspection and adjustments | 30 | 40 |
| Total | 260 | 400 |
The total time for the herringbone gear keyway, including setup, was about 6.7 hours. This was entirely acceptable for a large component that would otherwise require days of planning and expensive tooling.
Alternative Approaches and Why This One Was Chosen
I considered other methods such as using a portable keyway milling machine or a wire‑EDM. The portable miller, while capable, would have required a special fixture to align with the bore and had limited rigidity. Wire‑EDM could achieve excellent symmetry but would require submerging the entire herringbone gear in a dielectric fluid, which was impractical for a 6‑ton gear. The shaper modification was the only feasible option within our shop’s constraints. Additionally, the experience gained allowed me to apply the same technique to other large herringbone gears used in marine propulsion systems and heavy industrial reducers.
For future projects involving herringbone gears with even larger bores (up to 1500 mm), I plan to increase the cross‑slide travel to 250 mm and use a sturdier tool holder with a rectangular cross‑section instead of a round tube. The change would increase the second moment of area by a factor of two, reducing deflection by half. However, the weight would increase, so careful dynamic analysis would be needed.
Lessons Learned
Through these operations, I learned several critical points:
- Stiffness is king: Any flex in the tool holder or workpiece clamping directly translates into keyway geometry errors. Using a seamless steel tube was a good compromise, but for very heavy cuts, a box‑section arm might be better.
- Backlash elimination: The cross‑feed slide’s leadscrew must be preloaded. A double‑nut scheme with a spring washer works well.
- Cutting fluid application: For the herringbone gear made of alloy steel, the use of chlorinated oil gave a much better surface finish than soluble oil because of its extreme‑pressure properties.
- Continuous monitoring: I took measurements after every five roughing passes to detect any deviation early. A simple go‑no‑go gauge for keyway width helped avoid over‑cutting.
In conclusion, the retrofitted shaper with a cross‑feed slide and extended tool holder proved to be a versatile, low‑cost solution for machining internal keyways in large Pelton wheel runners and herringbone gears. The method delivered the required symmetry tolerance of 0.03 mm on workpieces weighing several tons, demonstrating that innovative modifications to standard machine tools can solve seemingly insurmountable machining problems. I encourage other practitioners to explore similar adaptations when faced with oversized workpieces that exceed the capacity of specialized equipment.