The main areas of application are the production of:
- Internal gears for smaller quantities
- Internal gears in blind holes (= non-through holes)
- External gears when there is no run-out, i.e. when hobbing is not possible
Examples of workpieces that are often geared by gear shaping:
- Internal ring gears for planetary gears
- Coupling hubs
- Drive shafts
When shaping internal gears, the workpiece and the shaping tool form an internal gear. The cutting tool rolls like a planet in the ring gear. Workpiece and shaping tool rotate continuously around their axes.
In addition to the rotary movement, the shaping tool makes a lifting movement and removes chips from the workpiece during the downward movement. At the lower end of the toothing, the tool lifts off the material (see animation) so that the cutting edges do not touch the material during the upward movement.
In order to achieve the required tooth thickness on the workpiece, the shaping tool moves radially into the material of the workpiece as a third movement.
The shaping tool is basically a pinion with cutting edges on the lower face.
Slotting tools with a shank and Morse taper mount (shank cutting wheels) as well as slotting tools in disk or bell shape are common.
Tool steels are used for production, which are often coated with titanium nitride (TiN) or other layers to extend tool life.
When choosing the cutting tool for producing internal gears, a few points must be observed, as otherwise interference between the workpiece and the cutting tool can occur:
- The shaping tool must always have a few teeth less than the workpiece. Otherwise the teeth of the shaping tool could wear away parts of the workpiece teeth during rolling that have to remain. The required difference in the number of teeth depends very much on the geometry of the toothing (pressure angle, tooth height, profile shift of workpiece and cutting tool).
- The length of the cutting tool must be sufficient for the respective workpiece.
By changing the infeed depth of the shaping tool, the tooth thickness of the workpiece teeth can be freely selected within certain limits.
Since the tooth thickness cannot be measured directly, the tooth thickness is usually determined indirectly by measuring the distance between two rollers that are placed opposite in the tooth gaps.
If the workpiece has an uneven number of teeth, there is a tooth opposite each gap. The second roll is therefore placed in one of the tooth gaps next to the opposite tooth. This "oblique" measurement is taken into account when calculating the roll dimension .
The measurement with rollers or balls can be used for internal and external gears.
These measurements can be made by the manufacturer on the gear cutting machine. In addition, all gears can also be measured on gear measuring machines or coordinate measuring machines.
Tips for practice
The following points are important for designers:
- The cutting tool must emerge from the material at the (lower) end of the toothing so that the chip breaks off.
For this purpose, a recess must be provided behind the toothing, which must not be less than a width of 5 to 10 mm. This width is required because the pusher tool overflows slightly after exiting the material for lifting. In addition, the cutting surface pushes the chip in front of it, which must also have space in the recess. Radii and chamfers in the recess do not count as run-out.
- The teeth must not be too long.
Conventional gear shaping machines have stroke lengths of 80 to 200 mm, rarely up to 400 mm. Therefore, the lower end of the toothing to be butted should not be deeper than 200 mm in the hole. If shank shaping tools have to be used, the tooth length must be adapted to the usually available shaping tools.
- Thin, long internal gears are difficult to cut.
The shaping tool must be correspondingly thin and long for such gears, which favors the tool to evade under the cutting forces. As a result, such internal gears cannot be manufactured so precisely. The harder the workpiece material, the more problematic the processing. In such cases, broaching or, in the case of particularly hard workpieces, spark erosion of the internal teeth is recommended .
- Toothings lying deep in the hole should be avoided.
If the beginning of the toothing is deep in the hole, it is very difficult or sometimes impossible to measure the toothing. In addition, machining on conventional machines is less favorable because the shaping tool hits the material at a higher speed. Normally, the impact speed is almost zero, as the top reversal point of the stroke movement is just above the workpiece surface. At the reversal points, the speed is zero.