Milling Process of Helical Cylindrical Gears on X62W Milling Machine

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1. Introduction

  • Importance of Helical Cylindrical Gears in Agricultural Machinery
    • Helical cylindrical gears are crucial components in agricultural machinery. They are widely used in tractors, harvesters, seeders, and mixers. For example, in tractors, they are used in the transmission system of the gearbox and rear axle to achieve driving and steering. In harvesters, they drive components like blades and conveyors for crop harvesting and transportation. Their high – efficiency, smooth transmission, strong load – bearing capacity, and low – noise characteristics meet the requirements of agricultural machinery’s transmission systems.
  • Advantages of X62W Milling Machine in Gear Processing
    • The X62W milling machine offers several advantages in processing helical cylindrical gears. It provides high precision, high efficiency, and controllable accuracy. Its wide processing range and cost – reduction potential make it suitable for different industries, especially in agricultural machinery manufacturing.

2. Processing Method Selection

  • Comparison of Gear Processing MethodsProcessing MethodPrinciplePrecision LevelApplicable ScenariosForming Method (used in CNC machines)Using a cutter with the same shape as the tooth profile to machine the gearHighHigh – precision, mass – production of gearsGenerating Method (used in special – purpose machines)Based on the meshing principle of gears, the cutter and the workpiece simulate the meshing movement to machine the gearHighHigh – precision gear processingProfiling Method (used in X62W milling machine)The cutter’s shape is similar to the tooth profile of the gear, and the gear is machined by profilingRelatively lowSmall – batch, low – precision gear processing, such as in some agricultural machinery parts manufacturing
  • Reason for Choosing the Profiling Method on X62W Milling Machine
    • For the processing of helical cylindrical gears with a relatively low precision requirement (in this case, 9 – level precision), the profiling method used in the X62W milling machine is more cost – effective. It is suitable for small – batch production, which is common in the manufacturing of agricultural machinery parts.

3. Gear Milling Cutter and Cutting Fluid Selection

  • Determination of Gear Milling Cutter
    • The gear milling cutter is selected based on the normal module (\(m_{n}\)), normal tooth profile angle (\(\alpha_{n}\)), and equivalent number of teeth (\(z_{v}\)) of the gear. The equivalent number of teeth is calculated through formulas. For example, \(z_{v}=Kz\) or \(z_{v}=\frac{z}{\cos^{3}\beta}\). For a gear with \(z = 21\), \(\beta = 9^{\circ}\), and \(K = 1.038\), \(z_{v}=1.038×21 = 21.798\). By referring to the standard disc – shaped gear milling cutter number table, when the equivalent number of teeth is between 21 – 25, a No. 4 standard disc – shaped gear milling cutter with a module of \(m = 2.5\) in an 8 – piece set is selected.
  • Cutting Fluid Selection
    • The main functions of the cutting fluid in gear milling are cooling and flushing. Considering the old – fashioned X62W milling machine and the need to prevent internal gear rust, oil – based lubrication is preferred. Among various cutting oils, No. 0 diesel is chosen due to cost – savings and safety concerns, instead of kerosene.

4. Preparation Before Milling

  • Milling Machine Calibration
    • According to the helix direction of the given gear parameters, the rotation direction of the worktable is determined. For a left – hand helix gear, the worktable is rotated clockwise by the corresponding angle. In this example, since the gear is left – hand helix, the milling machine worktable is rotated clockwise by \(9^{\circ}\).
  • Installation of Dividing Head and Tailstock
    • The dividing head and tailstock are installed in appropriate positions to ensure sufficient space for workpiece clamping and disassembly.
  • Selection of Mandrel
    • A suitable mandrel is selected according to the inner hole of the gear blank to ensure firm clamping without affecting the milling process.
  • Inspection, Installation, and Calibration of Gear Blank
    • Key parameters of the gear blank, such as outer diameter, inner hole size, coaxiality between the outer diameter and inner hole, and run – out of the end face of the gear blank relative to the axis of the inner hole, are inspected. After passing the inspection, the gear blank is fixed on the mandrel and installed between the spindle of the dividing head and the center of the tailstock. The coaxiality of the outer circle of the gear blank is checked with a dial indicator by shaking the dividing head handle.
  • Installation of Milling Cutter and Centering
    • The milling cutter is installed, and the centering is carried out by the scribing and trial – cutting method.

5. Processing – Related Calculations and Installation of Change Gears

  • Lead Calculation
    • The lead (\(p_{h}\)) is calculated using the formula \(p_{h}=\frac{\pi m_{n}z}{\sin\beta}\). For a gear with \(m_{n}=2.5\), \(z = 21\), and \(\beta = 9^{\circ}\), \(p_{h}=\frac{3.14×2.5×21}{\sin9^{\circ}}\approx1508\).
  • Change Gear Calculation
    • The change gear ratio (i) is calculated based on the lead and the pitch of the longitudinal feed screw of the X62W milling machine worktable (\(P_{r}=6mm\)). The formula is \(i=\frac{z_{1}z_{3}}{z_{2}z_{4}}=\frac{40P_{r}}{P_{h}}\). For \(P_{h}\approx1508\) and \(P_{r}=6mm\), \(i=\frac{40×6}{1508}\approx0.2268\). After approximation, the change gears are determined as \(z_{1}=30\), \(z_{2}=60\), \(z_{3}=40\), \(z_{4}=90\).
  • Installation of Change Gears
    • The side – shaft gear – hanging method is used. The longitudinal feed screw of the worktable and the side – shaft of the dividing head are connected by change gears. Since the gear to be processed is left – hand helix, an intermediate gear is used to adjust the rotation direction of the gear blank. During installation, the fixing screws of the dividing plate are loosened first. Then, the driven gear \(z_{4}\) is installed on the side – shaft at the rear end of the dividing head spindle, the driving gear \(z_{1}\) is installed at the tail of the longitudinal feed screw of the worktable, and \(z_{2}\) and \(z_{3}\) are connected by a key and installed between \(z_{1}\) and \(z_{4}\). The meshing gap between each pair of gears should be appropriate.
  • Indexing Calculation
    • The indexing formula is \(n=\frac{40}{z}\). For \(z = 21\), \(n=\frac{40}{21}=1\frac{19}{21}\). A hole circle with a multiple of 21 in the common hole plate is selected. To ensure higher processing accuracy, a hole circle with more holes is preferred. In this case, a 42 – hole circle is selected, and \(n = 1\frac{38}{42}\). The indexing fork is adjusted according to 38 hole pitches.

6. Gear Milling Process

  • Pre – milling Inspection
    • Before milling, it is necessary to check whether the rotation direction of the dividing head spindle is correct and whether the lead is accurate.
  • Selection of Milling Method
    • Comparison of Up – Milling and Down – MillingMilling MethodCutting Thickness ChangeWorkpiece Stress SituationInfluence on Workpiece ClampingApplicable ConditionsUp – MillingFrom zero to maximum for each tooth, which may cause surface hardening and affect surface quality, and accelerate tool wearThe cutting force may cause the worktable to move suddenly when the pulling force is large, which may damage the tool and the workpieceThe cutting force is vertically upward, which is not conducive to workpiece clamping and may cause workpiece deformation and vibrationGeneral use when the machine tool gap is large or the workpiece has a hard skinDown – MillingFrom maximum to zero, which can obtain better surface quality and improve tool durabilityThe feed thrust makes the screw and nut threads press against each other on one side, and the worktable is pulled. When the pulling force is large, it may cause the worktable to move suddenlyThe cutting force is vertically downward, which is beneficial to workpiece clampingUsed for milling machines with adjustable small gaps and workpieces without hard skins
    • Milling Method Selection for X62W Milling Machine
      • For the X62W milling machine, due to its relatively low – precision lead – screw and nut, large gear gaps, and poor rigidity, up – milling is suitable for rough machining, and down – milling is suitable for finish machining.
  • Selection of Milling Cutter Rotation Speed
    • According to the milling speed table for straight – tooth cylindrical gears, for 45 – steel materials, the rough – milling speed (\(v_{c}\)) is about 32m/min, and the finish – milling speed is about 40m/min. Given the diameter of the gear disc cutter (\(D\approx70mm\)), the spindle speed is calculated by the formula \(n=\frac{1000v_{c}}{\pi D}\). The rough – milling spindle speed is \(n = 90r/min\), and the finish – milling spindle speed is \(n = 130r/min\).
  • Rough Milling of Gear Tooth Profile
    • The rough – milling width (\(a_{e}\)) is calculated as \(a_{e}=2.25m_{n}-0.3\). For \(m_{n}=2.5\), \(a_{e}=2.25×2.5 – 0.3 = 49.5mm\). Since the depth is relatively low, the worktable can be raised to 49.5mm at one time for tooth – by – tooth milling.
  • Finish Milling of Gear
    • Based on the equivalent number of teeth (\(z_{v}=21.798\)), through rounding and looking up the table, and using the proportional interpolation method, the values of \(\overline{s_{n}}\) and \(\overline{h_{an}}\) are obtained as \(\overline{s_{n}}=1.5695\) and \(\overline{h_{an}}=1.0820\). Then, the chordal tooth thickness at the pitch circle (\(S_{n}\)) and the chordal tooth height (\(\overline{h_{an}}\)) are calculated as \(S_{n}=m_{n}\overline{S_{n}}=2.5×1.5695 = 3.92375\) and \(\overline{h_{an}}=m_{n}\overline{h_{an}}=2.5×1.0820 = 2.705\). The chordal tooth thickness at the pitch circle is measured with a tooth – thickness vernier caliper, and the additional feed amount for finish – milling is calculated according to \(\Delta a_{e}=1.37(s_{s}^{actual}-\overline{s_{s}})\) for final finish – milling.

7. Precautions

  • Test – Cutting Before Formal Milling
    • Before formal milling, test – cutting must be carried out to check whether the change gears, lead, and worktable rotation direction are adjusted correctly.
  • Loosening of Related Parts Before Milling
    • Before milling, the locking handle of the dividing head spindle and the fixing screws of the dividing plate should be loosened.
  • Proper Insertion of Indexing Handle Pin
    • During the milling process, the pin of the indexing handle must be inserted into the corresponding hole on the dividing plate.
  • Selection of Nuts for Workpiece Clamping
    • When clamping the workpiece, the nut should be selected according to the helix direction. For a left – hand helix workpiece, a left – hand helix nut is selected, and a fine – thread left – hand helix nut is preferred for better fastening.
  • Avoiding Scratching of Processed Tooth Grooves
    • During the milling process, measures should be taken to prevent the processed tooth grooves from being scratched.

8. Problem Analysis and Solutions

  • Inconsistent Number of Teeth with the DrawingPossible ReasonsSolutionsErrors in indexing calculationRe – calculate the indexing carefully and double – check the calculation processWrong selection of dividing plate hole circleRe – select the correct dividing plate hole circle according to the indexing requirementsIncorrect adjustment of the hole pitch number between the indexing forksReadjust the hole pitch number between the indexing forks accurately
  • Large Tooth Pitch Deviation and Unequal Tooth Thickness | Possible Reasons | Solutions | | Failure to detect the radial run – out of the workpiece, resulting in excessive radial run – out | Detect the radial run – out of the workpiece before processing and correct it if necessary | | Indexing deviation, such as shaking the indexing handle in two directions during indexing, causing excessive gaps in the worm – gear transmission mechanism of the dividing head | When indexing, ensure to shake the indexing handle in one direction to avoid excessive gaps in the dividing head |
  • Tilted Teeth | Possible Reasons | Solutions | | The axis of the milling cutter is not aligned with the center of the gear blank before processing, resulting in large centering errors | Re – align the axis of the milling cutter with the center of the gear blank accurately before processing |
  • Large Deviation in Tooth Height or Tooth Thickness Dimensions | Possible Reasons | Solutions | | Errors in selecting the milling cutter module or cutter number before milling | Re – check and correct the selection of the milling cutter module and cutter number | | Errors in calculating the milling depth or feed | Re – calculate the milling depth and feed accurately and adjust the processing parameters |
  • Poor Tooth Surface Roughness | Possible Reasons | Solutions | | Large vibrations during milling | Check the stability of the machine tool and workpiece clamping, and take measures to reduce vibrations | | Incorrect adjustment of milling parameters | Adjust the milling parameters (such as cutting speed, feed rate) according to the material and processing requirements | | Tool wear or non – standard installation | Replace the worn – out tool in time and ensure the tool is installed correctly |

9. Conclusion

  • Summary of the Entire Milling Process
    • The milling process of helical cylindrical gears on the X62W milling machine involves multiple steps, including processing method selection, cutter and cutting fluid selection, preparation before milling, processing – related calculations, milling operations, and taking precautions. Each step is crucial for ensuring the quality of the processed gears.
  • Significance of Correct Processing
    • Correct processing of helical cylindrical gears is of great significance for ensuring the normal operation of agricultural machinery. High – quality gears can improve the performance and service life of agricultural machinery, and promote the development of the agricultural machinery industry.
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