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What is a worm shaft?

CNC machining of worm shaft
What is a worm shaft?
CNC machining of worm shaft

Worm shaft refers to a gear that has one or several helical teeth and meshes with a worm gear to form a cross-shaft gear pair. The curved surface of the index can be a cylindrical surface, a conical surface or a torus surface. Worm shafts have four categories: Archimedes worm shafts, involute worm shafts, normal straight profile worm shafts, and cone enveloped cylindrical worm shafts.

CNC Machining Method of Cylindrical Worm
The tooth surfaces of ordinary cylindrical worms (except ZK worms) are generally machined on a lathe with a turning tool with a straight edge. According to the different installation positions of the turning tool, the tooth profile curves of the machined worm tooth surface in different sections are also different.

Transmission structure
The worm drive is composed of a worm and a worm gear, which is used to transmit the movement and power between the interleaved shafts. The interleaved angle of the two shafts is usually 90°. In general worm shaft transmission, the worm is used as the driving part. From the appearance point of view, the worm is similar to a bolt, and the worm wheel is very similar to a helical cylindrical gear. When working, the worm gear teeth slide and roll along the spiral surface of the worm. In order to improve the contact of the gear teeth, the worm wheel is made into a circular arc shape along the tooth width direction to enclose the worm part. In this way, the meshing of the worm and worm gear is in line contact instead of point contact.

rchimedes worm shaft
Classification of worm shafts
According to different tooth profile curves, ordinary cylindrical worm shafts can be divided into four types: Archimedes worm shafts (ZA worm shafts), normal straight worm shafts (ZN worm shafts) and cone-enveloped cylindrical worm shafts. GB10085-88 recommends using ZI worm shaft and ZK worm shaft. Now the worms and matching worm gear tooth profiles used in the above-mentioned four kinds of ordinary cylindrical worm shaft drives are respectively introduced in the following:
The processing method of cylindrical worm: Rotary cutting worm

Archimedes worm shaft
The Archimedes worm shaft (ZA worm shaft) is on a plane (that is, the end face) perpendicular to the axis of the worm. The tooth profile is an Archimedes spiral, and the tooth profile on the plane containing the axis (that is, the axial tooth profile) is a straight line, and its tooth profile angle α0=20°. It can be turned on the lathe with a single turning tool with a straight edge (when the lead angle γ≤3°) or a double turning tool (when γ>3°). When installing the tool, the top surface of the cutting edge must pass through the axis of the worm, as shown in Figure 1. The grinding of this kind of worm shaft is difficult, and the machining is inconvenient when the lead angle is large.

Normal straight profile worm shaft
Normal straight profile worm shaft (ZN worm shaft) "The end tooth profile of this worm shaft is an extended involute (Figure 2), and the normal surface (N-N) tooth profile is a straight line. The ZN worm shaft is also turned on a lathe with a single turning tool or a double turning tool with a straight edge. The installation form of the tool is shown in Figure 2. Grinding of this kind of worm shaft is also difficult.

Normal straight profile worm shaft
Involute worm shaft
Involute worm shaft (ZI worm shaft) "The end tooth profile of this worm shaft is involute, so it is equivalent to an involute cylindrical helical gear with a small number of teeth (the number of teeth is equal to the number of worm heads) and a large helix angle. The ZI worm can be turned on a lathe with two turning tools with linear blades. The top surface of the blade should be tangent to the base cylinder, one of which is higher than the axis of the worm and the other is lower than the axis of the worm. The tooth profile angle of the tool should be equal to the helix angle of the base cylinder of the worm. This worm shaft can be ground on a dedicated machine tool.

Cone Enveloping Cylindrical Worm Shaft
Cone Enveloping Cylindrical Worm Shaft (ZK Worm Shaft)   This is a non-linear spiral curved worm shaft. It cannot be machined on a lathe, it can only be milled on a milling machine and ground on a grinder. During processing, in addition to the spiral movement of the workpiece, the tool simultaneously makes a rotary movement around its own axis. At this time, the envelope surface of the rotary surface of the milling cutter (or grinding wheel) is the helical tooth surface of the worm, and the tooth profile on the I-I and N-N sections are all curves. This kind of worm is easy to grind, the precision of the worm is higher, and the application is becoming more and more extensive. As for the tooth profile of the worm gear paired with the above types of worms, it is completely different with the tooth profile of the worm. Worm gears are generally processed on hobbing machines with hobs or flying knives. In order to ensure the correct meshing of the worm and the worm wheel, the tooth profile of the hob of the cutting worm wheel should be consistent with the tooth profile of the worm shaft; The center distance during deep cutting should also be the same as the center distance of the worm drive.
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