Introduction to Multi-Start Worm Hob

Structural Features

• Multi-Helical Tooth Design The teeth are distributed in multiple helical lines along the cylindrical surface of the hob, with the number of starts equal to the number of starts (z₁) of the worm being machined (usually 2-6 starts, with special cases reaching more than 10 starts). The multi-helical design allows for a larger feed per revolution of the hob, significantly improving efficiency compared to single-start hobs.
• Chip Flute Adaptability Depending on the processing precision requirements, straight flutes or helical flutes can be used:
  • Multi-start worm hob with straight flutes: The chip flutes are parallel to the axis, making manufacturing simple and suitable for rough or semi-finishing of multi-start worms with small to medium modules (m≤5mm) and moderate precision requirements.
  • Multi-start worm hob with helical flutes: The chip flutes are helically distributed, resulting in a more reasonable cutting angle and smooth chip removal. Suitable for medium to large modules (m≥3mm), high-precision (grade 6 and above) multi-start worm machining, especially suitable for worms with large helix angles.
• Tooth Profile Conjugate Matching The tooth profile of the hob must be precisely conjugate with the worm tooth groove. This is achieved through involute or Archimedes spiral design to ensure uniform meshing clearance between the hob and the worm blank during machining, avoiding tooth surface interference or overcutting.
• Reinforced Rigidity Structure Multi-start hobs have a large lead and long helical line span. The hob diameter and length are usually larger than single-start hobs of the same module. The hob material needs to be strengthened through forging, with optimized fillet radii at the bottom of the flutes and at the root of the teeth to avoid chipping due to stress concentration during cutting.
• Positioning and Clamping Design The two ends of the hob usually have a center hole or shoulder positioning structure. Some high-precision hobs use a tapered shank or flange clamping to ensure coaxiality and rotational stability during machining, reducing runout error.

Core Parameters and Design Points

1. Number of Starts (z₀) ：
   The number of starts of the hob must strictly match the number of starts (z₁) of the worm being machined. This directly determines the transmission ratio and lead of the worm. The more starts, the larger the worm lead and the faster the transmission speed, but the higher the requirements for the speed ratio synchronization between the hob and the blank during machining (speed ratio error must be ≤0.01%).
2. Lead (P₀) and Pitch (p₀) ：
   The lead is the distance the hob moves along the axis per revolution, calculated as: P₀ = z₀ × p₀ (p₀ is the pitch), and must be exactly equal to the lead (P₁) of the worm being machined (P₀ = P₁). The lead of a multi-start hob is usually z₀ times that of a single-start hob. During design, the helical lead error control (≤0.02mm/100mm) must ensure uniform tooth spacing.
3. Helix Angle (β₀) ：
   Calculated from the hob lead and outer diameter: tanβ₀ = P₀ / (πD₀) (D₀ is the hob outer diameter). It needs to match the worm helix angle (β₁), ensuring that the cutting edge is consistent with the worm tooth direction during machining to reduce tooth surface scratches. For intersecting axis machining, the axial angle correction also needs to be considered.
4. Rake Angle and Relief Angle ：
   The rake angle of a straight-fluted hob is close to 0°, and an auxiliary rake angle (usually 2°-5°) needs to be formed by grinding. The helical-fluted hob naturally forms a working rake angle (5°-12°) through the helix angle, reducing cutting resistance. The relief angle is ensured through grinding (3°-8°) to ensure that the tooth profile accuracy is maintained even after tool wear.
5. Module and Pressure Angle ：
   The hob module (m) must be consistent with the worm module. The pressure angle (α₀) is usually 20° (standard value), and 15° or 25° can be used in special conditions. It needs to be calibrated using a tooth profile template or gear measuring center.

Manufacturing Process

1. Material Selection ：
   • Multi-start hobs with small to medium modules (m≤8mm) commonly use high-speed steel (W6Mo5Cr4V2) or powder metallurgy high-speed steel (ASP-30), with a hardness of 63-66HRC and good toughness.
   • Hobs for large modules or high-speed machining (m≥10mm) use cemented carbide (such as WC-Co alloy) or coated high-speed steel (TiAlN coating), increasing wear resistance by 30%-50%, suitable for machining quenched steel or high-strength cast iron.
2. Helical Flute Machining ：
   Multi-helical flutes are machined using a CNC helical milling machine or a five-axis machining center. The helix angle, lead, and groove symmetry need to be precisely controlled. The line division accuracy of the multi-start hob is crucial; the tooth spacing error between adjacent helical lines must be ≤0.005mm, otherwise it will cause uneven worm tooth thickness.
3. Heat Treatment and Grinding ：
   • High-speed steel hobs undergo overall quenching (860-880℃ heating) + three tempering (560℃) to eliminate internal stress and stabilize hardness; cemented carbide hobs use vacuum sintering to ensure density ≥99.5%.
   • A dedicated hob grinder is used to grind the tooth profile, front rake face, and back relief face. An online measurement system is used to correct grinding parameters in real time to ensure tooth profile error ≤0.003mm and helical line error ≤0.008mm/300mm.
4. Precision Inspection ：
   The lead accumulation error, tooth pitch deviation, tooth profile error, etc., of the multi-start helix are measured through the gear center detection, conforming to Grade A (precision grade) or Grade AA (ultra-precision grade) standards in GB/T 6084, with multi-head wire deviation being a key inspection item.

Working Principle

Application Scenarios

• Large lead transmission worm processing Such as lifting mechanisms lifting worms, conveying equipment drive worms (lead usually 50-200mm), requiring multi-head design to achieve fast transmission.
• High-load precision worm processing Such as machine tool worktable feed worms, servo motor gearbox worms (module 3-15mm, precision grade 6-7), multi-head design can increase load capacity by increasing tooth surface contact area.
• Special tooth profile worm processing Adaptable to Archimedes worms, involute worms, normal straight-sided worms, etc., realizing mass production of non-standard worms through custom hob tooth profiles.
• High-strength material processing Can process 45 steel, 40Cr quenched and tempered steel (HRC 25-35), ductile iron (QT600), etc. When equipped with a cooling system, it can also be used for lightly quenched steel (HRC ≤40) processing.

Advantages and Disadvantages Analysis

Core Differences from Single-Head Worm Hobs

Key Usage Precautions

1. Equipment matching During processing, ensure the synchronization accuracy of the machine tool spindle and feed system, wire division error ≤0.01mm, to avoid uneven worm tooth pitch.
2. Cooling and lubrication Use high-pressure cutting fluid (pressure ≥5MPa) for forced cooling to reduce the impact of cutting heat on tool wear and worm accuracy.
3. Blade grinding and maintenance When sharpening, a special branching device must be used to ensure the consistency of the multi-head spiral lines. After each sharpening, the tooth shape and lead error must be checked.
4. Material Matching When processing high-strength materials, it is preferable to choose cemented carbide multi-head hobs and reduce the cutting speed (linear velocity ≤30m/min).