Power skiving is a continuous gear cutting process in which a gear-like tool and the workpiece roll against each other on crossed axes, like a crossed-axis helical gear pair. It cuts internal and external gears, and it is especially useful for internal gears and for teeth close to a shoulder, where a hob has no room to run out of the cut. A patent for the idea was filed in 1910, but it only became practical with fast direct drives, wear-resistant cutting materials and coatings. This page explains how it works, the tool, how it compares with hobbing, shaping and broaching, which machines can do it and where it stops working well. It ends with a short note for owners of such machines.
What is power skiving?
EMAG describes power skiving as a combination of gear hobbing and shaping: continuous milling with axial feed, suited to internal and external gears and an alternative to broaching and gear shaping. According to Klingelnberg, the idea comes from Wilhelm von Pittler, who filed a patent on 1 March 1910 for cutting gears with a gear-like tool with cutting edges on the tooth end faces. For a long time it failed: Klingelnberg names too short tool life, machine vibrations and difficult chip removal as the reasons. Liebherr speaks of a renaissance of a process more than 100 years old, made possible by fast direct drives, wear-resistant cutting materials and new coatings.
Kinematics: crossed axes instead of a stroke
It helps to compare it with gear shaping. In shaping, the cutting motion is the stroke of the gear-shaped tool in the axial direction, and because nothing is cut on the return stroke, time is always lost, as Klingelnberg puts it. In skiving, the crossed arrangement of tool and workpiece axes creates an axial relative speed that provides the cutting motion. During one revolution each cutting edge passes through different tooth heights on the workpiece.
Besides the axis-crossing angle (the angle between tool axis and workpiece axis), Klingelnberg says the process needs a tilt angle that sets the kinematic clearance angle between tool and workpiece flank, two linear movements (centre distance and feed along the workpiece axis) and a superimposed differential rotation. Tool and workpiece spindles therefore have to run electronically coupled with high precision. Cutting speed follows from the rotational speed, the ratio of workpiece to tool diameter and the axis-crossing angle.
How large the axis-crossing angle is depends on the source and the task:
| Source | Process | Axis-crossing angle |
|---|---|---|
| EMAG (blog) | Hobbing | 45° to 135° |
| EMAG (blog) | Power skiving | 10° to 25° |
| EMAG (technology page) | Power skiving, usual values | 15° to 25° |
| FVA 661 II | Power skiving, conventional | 20° to 30° |
| FVA 661 II | Power skiving, research for teeth at shaft shoulders | 5° to 15° |
Klingelnberg works out the difference in cutting engagements. At 2,000 rpm, a cutting speed of 200 m/min and a tool with 25 teeth, there are 50,000 engagements per minute. A hob with eight gashes and two starts at the same cutting speed turns at 1,200 rpm and reaches only 19,200. For dry milling with carbide, Klingelnberg considers cutting speeds of 150 to 250 m/min sensible.
The tool: the skiving cutter
The tool is a skiving cutter. Klingelnberg distinguishes three designs:
- Cylindrical skiving cutter: easy to re-sharpen, but with unfavourable wear behaviour because the clearance angle is small.
- Conical skiving cutter: better wear behaviour, but it can be reground only a few times, and regrinding is complex.
- Blade-type skiving cutter: carbide blades with a ground cutting profile in a base body. Rake and clearance angles can be chosen freely, and re-sharpening is a controlled process.
Gleason builds a re-sharpening function into the 300PS so that fewer tools need to be held.
Power skiving, hobbing, shaping and broaching compared
| Criterion | Hobbing | Gear shaping | Power skiving | Broaching |
|---|---|---|---|---|
| Cutting motion | continuous, axes 45° to 135° | stroke with idle return stroke | continuous, axes 10° to 25° | linear pull, multi-tooth tool |
| Internal gears | large diameters only, small ring gears not | yes, also in a blind bore (relief groove 5 to 10 mm needed) | yes, also against a shoulder, without relief groove | yes, usually in one stroke |
| Batch size | large series | single parts, small series | one piece to medium series | large series |
| Tool | hob | cutter wheel | re-sharpenable skiving cutter | profile-specific broach, expensive |
| Special feature | first choice for external gears in the soft state | thin, long internal teeth are difficult | turning and gear cutting in one clamping possible | no helix correction such as crowning |
Sources for the table are EMAG, Wikipedia (gear shaping, hobbing, broaching), a trade article on produktion.de and Liebherr. There is no single factor for the time saved against shaping: EMAG gives about 50 percent faster and 2 to 3 times the productivity and tool life, a gearbox manufacturer four to five times shorter cycle times at modules of 0.5 to 2. Against broaching, the FVA 661 II research project found skiving times 2 to 3 times shorter than the reference process, broaching, even at small axis-crossing angles; that result applies to the parts and conditions studied there. Our page on how gears are made shows how these processes fit together, including for internal gears.
What power skiving is used for
- Ring gears for planetary gearboxes: a gearbox manufacturer skives internal gears of module 0.5 to 2 (the module is the tooth size in millimetres); EMAG names ring gears for planetary gearboxes in automotive and internal gears for electric drives as a growing field.
- Teeth against a shoulder or interfering contour: for external gears with adjoining shoulders, where hobbing needs too much tool run-out, skiving can cut the teeth.
- Combined with turning: the EMAG VSC 400 PS works with up to four skiving tools and six turning tools on two spindles, machines parts up to 340 mm diameter and 420 mm height in one clamping, and so keeps the teeth concentric with the bearing seat.
Which machines can do power skiving?
Power skiving is a process, not a machine class, and it runs on three concepts:
| Machine concept | Example (manufacturer data) | Working range |
|---|---|---|
| Skiving machine with turning function | EMAG VSC 400 PS | up to 340 mm diameter, 420 mm height, up to 4 skiving and 6 turning tools |
| Compact skiving machine | Liebherr LK 180-280 | module 3, 180 to 280 mm diameter, integrated chamfering |
| Medium to large skiving machine | Liebherr LK 300-500/700 M | module 8, 300 to 700 mm diameter |
| Skiving machine family | Gleason 100PS, 300PS, 400PS to 600/800PS | from small shafts and gears to medium and large gears |
| Hobbing machine with skiving function | EMAG K 160 Longbed XL | hobbing machine with skiving capability |
| Machining centre with turning function | Heller HF series (option) | driven rotary table with spindle coupling, Siemens control with input screens |
| Universal turn-mill centre | machine cycle with process design software (Fraunhofer IWU) | retrofit of existing turning and milling centres |
For operators of older equipment: skiving does not replace every other process. According to Wikipedia, gear shaping remains important for internal gears in blind bores and small quantities, and broaching for large series of fixed geometry. The machines for those processes are covered on our pages for gear hobbing machines, gear shaping machines and broaching machines.
Hard skiving: skiving hardened gears
Skiving is not limited to the soft state. Liebherr describes hard skiving as an alternative to grinding and honing for internal gears and spline profiles; it allows hardening distortion to be compensated deliberately and offers, according to Liebherr, flexibility, tool life and cost efficiency. Gleason goes further and writes that hard skiving can replace other hard finishing processes at comparable quality for a fraction of the cost; that is a manufacturer’s statement. For orientation: skive hobbing, according to EMAG, is hobbing of hardened external gears in which only the tooth flanks are removed. It reaches ISO 1328 quality 6 to 7 at about Ra 0.5, while hobbing in the soft state reaches class 6 to 8. Machining hardened gears with carbide became possible only from the mid-1980s, according to EMAG. For grinding of hardened gears, see selling gear grinding machines.
Process design, simulation and closed loop
Because kinematics and chip formation are complex, skiving needs process design. Klingelnberg describes a closed loop: first the gear is designed, then tool and kinematics are calculated, and a manufacturing simulation shows rake and clearance angles that the user can optimise. After skiving, the part is measured with a topographic grid, and the deviations flow back into the machine control as correction values for the kinematics. For the software SkiveAll, developed by researchers at Fraunhofer IWU, Maschinenmarkt reports that the basic version designs and optimises technology variants and the 3D version calculates tool profiles and simulates machining. On the EMAG VSC 400 PS, the gear and tool data are enough: the user interface calculates the whole machining program automatically. Gleason names a process-guidance dialogue and a process simulation.
Limits and requirements
In project FVA 661 II (IGF no. 19053 N), the Forschungsvereinigung Antriebstechnik investigated the economic and technological limits of skiving at small axis-crossing angles. The aim was to cut teeth close to shaft shoulders and keep tool run-out small. Results:
- The conventional angles of 20° to 30° were reduced to 5° to 15°; below 10° the process window becomes narrow.
- Machining times were 2 to 6 times longer than at conventional axis-crossing angles, but 2 to 3 times shorter than the reference process, broaching.
- The limit depends strongly on the material: wear patterns were stable in 16MnCr5, while pre-hardened 30CD12 ran unstably with much shorter tool life.
- The deciding design criterion at small angles is the minimum effective rake angle. Tool head rake angle and multi-tooth distribution counteract negative effective rake angles.
Chip formation adds to this. Klingelnberg gives rake angles of at best 0° and down to minus 50° in the process, which raises cutting forces and demands contour accuracy, dynamics and stiffness from the machine. DVS asks for extremely rigid machine structures and optimised control loops; EMAG names stable clamping, high tool quality and high speeds as conditions. Liebherr puts it this way: skiving does not forgive mistakes, and deviations within manufacturing tolerances can decide whether machining works at all.
If you own a machine used for skiving
We buy your machine. Sale through our buyer network: commission stated upfront; you are paid before pickup. For a skiving-capable machine, we need the nameplate data, year, workpiece diameter and module range, control version and the skiving cycles installed, spindle coupling and tooling, plus photos and any inspection report.
The offer usually follows 24–48 hours on working days after we have all the details once we have all the information. On a direct purchase, payment is made by bank transfer before pickup, and we organise dismantling and transport, with the question of whether costs are included stated in the offer upfront. The whole process usually takes 7 to 15 working days on a direct purchase.
See selling gear cutting machines for the overview, what your gear machine is worth for the value factors and how it works for the process.