Maintaining Synchronization

Mazak Multi-Tasking Machines Synch Up for Gear Cutting

Basic gear tooth milling is possible on any standard full simultaneous 5-axis multi-tasking machine. However, gear power skiving and hobbing on those machines requires two additional key elements – spindle speed synchronization and advanced software specifically for programming those particular gear machining operations.

For both power skiving and hobbing, a multi-tasking machine must precisely maintain synchronization between the rpm of its B-axis milling spindle and that of the workholding C-axis turning spindle throughout the cutting process. This is critical because of the minute amount of allowable margin for error with this rotational synchronization during these gear cutting operations.

It should also be noted that power skiving, as compared to gear hobbing, requires an even higher level of synchronization precision because spindle speeds for that operation are much greater than those used in hobbing. For example, hobbing a 20DP 60 tooth spur gear with a single start 2.0” diameter hob at 328 sfm will require the tool to rotate 627 rpm and the part to rotate at 10.45 rpm; whereas, power skiving the same gear with a 1.45” diameter, 29 tooth cutter tilted at 20 degrees cross angle will require the tool to rotate 2,526 rpm and the part to rotate at 1,220.9 rpm to produce the same cutting velocity of 328 sfm.

This is why multi-tasking machines for power skiving must also have rotary scale feedback capabilities for those spindles to ensure such synchronization. Scale feedback allows the machine to keep track not only of the rotational velocity of the spindles, but also their locations respective to one another

To illustrate, consider the simple operation of drilling a hole. If the spindle’s/drill’s speed varied by 5% during actual drilling, it would not affect the outcome of the process or the quality of the hole and would not damage the drill. In power skiving, such a fluctuation in rotational speed would generate vibration that – once started – is difficult to eliminate and would inhibit gear tooth surface quality.

Although gear cutting on a multi-tasking machine can require high spindle speeds (regarding milling processes), it does not necessarily involve excessive amounts of machine power/torque. Depths of cut are light to medium, much like those of a conventional end milling operation on a die or mold surface.

In addition to spindle synchronization, multi-tasking machine controls must be equipped with advanced software specifically for gear cutting program development. Such software allows users to quickly and easily develop gear cutting programs at the machine and eliminates any potential for human error resulting from manual programming.

In the past, power skiving and hobbing programs were written by hand, line by line, code by code. Without extensive code writing experience, the process could be laborious and challenging. A single minor typographical error wouldn’t surface until actual machining and potentially result in a scrapped part.

Advanced gear cutting software such as Mazak’s SMOOTH Gear Cutting, for example, makes programming gears a lot less intimidating for operators. Working in tandem with a machine’s rotary axis scale feedback, the software eliminates the guesswork often associated with gear cutting, particularly in terms of programming and now the synchronization of turning/milling spindles at high rpm. The results are a far more refined cutting action and smoother surface finishes on gear teeth.

SMOOTH Gear Cutting software eliminates the need to program the part offline or on a CAM system and ensures precise synchronization. It provides a graphical user interface/modules on the machine’s control, and operators simply fill in various fields with information such as number of gear teeth, pressure angles and the like. The control then generates the toolpath to machine the gear teeth.

To ensure accurate gear cutting results, it is also recommended that machines have some type of on-machine metrology capabilities/systems to measure cut gear teeth. For instance, after power skiving an O.D. gear, Mazak’s Smooth Gear Check along with a standard probe is used to locate or measure a tooth space and to determine work offsets. When the operator calls up those offsets, the machine can rotate the part to the proper position.

For a more advanced solution, shops could opt for SMOOTH Gear Check Plus, which uses a scanning probe to actually scan over a gear tooth surface so, shops can measure tooth lead and profile. The results of the lead and profile scan are displayed in a chart on the machine’s control.

Over the years, what started with 5-axis cutting of larger-diameter ring-gear sets on multi-tasking machines has now advanced to the ability to power skive and hob gears as well. These machines continue to prove themselves as viable processing alternatives for low to medium-volume production.

Unlike high-volume dedicated systems, multi-tasking machines provide the flexibility to perform other part operations. Shops can, for instance, turn a part’s I.D. and O.D. as well as cut its other mating features – all on the same machine that will also power skive the part’s gear tooth pattern. Such capability significantly improves overall part accuracy because all the machined features run true to the gear pitch diameter. Additionally, the same multi-tasking machine that power skives a gear one day can machine completely different, non-geared components the next.

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