Worm turning
The worm is the active component in worm transmission, and cooperates with the worm wheel to realize motion and power transmission between staggered axes. It has the characteristics of large transmission ratio, compact structure, and smooth operation. It is widely used in machine tools, lifting machinery, automation equipment and other fields. The structure of the worm is cylindrical, and its outer surface is machined with spiral teeth. The tooth shape is usually an Archimedean spiral. According to the number of heads, it can be divided into single-head, double-head and multi-head worms. The turning process of the worm is a key link to ensure its transmission accuracy. The tooth shape accuracy, pitch error and surface quality of the spiral teeth need to be controlled. Due to the large lead angle and complex tooth shape of the worm, the turning process faces problems such as difficult tool selection and difficult control of spiral processing accuracy, and a specialized process solution needs to be adopted.
Before worm turning, process preparation requires clear technical requirements and CNC machining benchmarks. Key worm technical parameters include module ( m ), number of starts ( z ), lead angle (γ), tooth profile angle (α), pitch circle diameter ( d₁ ), and tooth height ( ha ). These parameters must be calculated and determined based on the transmission requirements. For example, for a worm with a module m = 3mm and number of starts z = 2 , its lead Ph = πmz ≈ 18.85mm and lead angle γ = arctan (Ph/πd₁) . The CNC machining benchmark is typically the center hole at each end of the worm. The roundness and coaxiality of the center hole must be guaranteed to achieve high-precision CNC machining using a double-thrust clamp. For worms without a center hole, a single-clamp, single-thrust clamping method can be used, with the outer diameter and end face of one end used for positioning. The surface roughness of the positioning surface must be ≤ Ra3.2μm , and radial runout ≤0.02mm. The blank is generally made of 45 steel or 40Cr forgings. After rough turning, it needs to be tempered and the hardness is controlled at 220-280HBW to improve cutting performance and enhance comprehensive mechanical properties.
The tool selection for worm turning must match the tooth profile parameters and CNC machining stage. For rough turning, a carbide worm cutter (such as YT15) should be used. The tool’s profile angle should be equal to the worm’s (standard: 20°). The rake angle should be 5°-10°, and the relief angle 6°-8°. The lead angle should be adjusted according to the lead angle to avoid tool interference during cutting. For finish turning, a high-speed steel worm cutter (such as W18Cr4V) should be used. The cutting edge should be finely ground to a surface roughness of Ra ≤ 0.025μm to ensure tooth surface quality. For multi-start worms, an indexable or multi-edge tool should be used. An indexing device should be used to sequentially machine each tooth. Indexing accuracy must be maintained within ±5°, otherwise uneven load distribution will occur across the teeth. When installing the tool, ensure that the tool tip is at the same height as the worm axis and the tool shank centerline is perpendicular to the worm axis. Excessive deviation will result in tooth profile angle error. For example, if the tool tip is 0.1mm above the axis, the tooth profile angle error will increase by approximately 1°.
The CNC machining method for worm turning depends on the number of turns and required precision. Single-start worms can be machined using either the straight-in method or the left-right method. The straight-in method is suitable for worms with small modules (m ≤ 3mm). The tool feeds directly in the radial direction, resulting in high efficiency but high cutting forces. The left-right method is suitable for worms with large modules. While the tool feeds radially, it simultaneously moves left and right along the axis, reducing cutting forces and vibration. For turning multi-start worms, one helical tooth is first machined. The workpiece is then rotated 360°/z using an indexing head or the C-axis function of the CNC lathe before the remaining teeth are machined. For example, a double-start worm requires 180° rotation, and a triple-start worm requires 120° rotation. During rough turning, a 0.2-0.5mm allowance should be allowed for each tooth, and during finish turning, the tooth profile should be calibrated using a template to ensure a profile angle error of ≤±10° and a tooth thickness deviation within the h11 range. For worms with a lead angle greater than 10°, the effect of the helix angle on the tool’s actual rake and clearance angles must be considered and compensated for by tilting the tool shank. For example, for a worm with a lead angle of 15°, the tool clearance angle must be increased by 15° to ensure that the actual clearance angle remains between 6° and 8°.
The cutting parameters for worm turning need to be optimized based on the material and CNC machining stage. For rough turning, carbide tools should be used at a cutting speed of 80-120 m/min, a feed of 0.15-0.25 mm/r, and a depth of cut of 1-2 mm to quickly remove excess stock. For finish turning, high-speed steel tools should be used at a cutting speed of 30-50 m/min, a feed of 0.05-0.1 mm/r, and a depth of cut of 0.1-0.3 mm to ensure surface quality. The choice of cutting fluid should balance cooling and lubrication. For rough turning, use an extreme pressure emulsion (5%-8% concentration), while for finish turning, use a sulfurized cutting oil. High-pressure fluid should be delivered to the cutting zone via a high-pressure jet to reduce cutting temperatures and friction. A steady rest or steady rest should be used to support the worm during CNC machining, especially for slender worms with an aspect ratio greater than 10. The steady rest support block should be made of wear-resistant material (such as cast iron), and the support force should be adjusted to prevent workpiece bending without causing noticeable indentations.
Precision inspections for worm turning must cover dimensional accuracy, form and position accuracy, and surface quality. Dimensional accuracy inspections include the tip diameter (using a micrometer) and tooth thickness (using a tooth thickness vernier caliper or normal micrometer). The tooth thickness limit deviation is determined according to design requirements (usually h11 or g11). Form and position accuracy inspections include cumulative pitch error (using a pitchmeter), tooth profile error (using an involute tester), and lead angle error (using a sine rule). The cumulative pitch error must be controlled within 0.02-0.05mm/100mm, and the tooth profile error must be ≤0.01mm. Surface roughness is measured using a roughness tester. The tooth surface Ra value should be ≤1.6μm, and for high-precision worms, ≤0.8μm. Common quality issues and solutions: If the tooth profile is asymmetrical, it may be caused by tool installation tilt or indexing error, requiring recalibration of the tool or indexing device. If the pitch error is excessive, check the lathe screw accuracy and make corrections. If chatter marks appear on the surface, reduce the cutting speed, increase tool rigidity, or adjust the steady rest support. Through rigorous inspection and process optimization, high-precision turning of the worm can be achieved, ensuring good meshing performance with the worm wheel.
