Hobbing VS Profile Milling, Invomilling technology 4

[mfg11eng11guy]

Doing some research on manufacturing gears. Has anybody done any research on the pros and cons of profile milling a gear vs. hobbing? Hob cutters and hobbbing machines are very expensive and I'm thinking of alternatives like profile milling and Sandvik has Invomill now. Obviously the hobbing machines are extremely rigid and can produce a very high quality gear. Can the multi-axis mills and technology match what a gear hobber can do?

Please Advise, thanks in advance.

 

[geesamand]

They are slow. That's the difference.

 

[tbuelna]

The tooling and machining approach proposed by Sandvik for milling external gears is pretty impressive. But it requires specialized tooling, software and good quality CNC machines. After it's all said and done, you might be better off buying a good quality used gear machine. Depending on the type and quality of gears you will be making, you might also need to have additional machines for shaving, grinding, honing, etc.

 

[verknde]

Firstly it is important to know that 5-axis milling of gears is an extension for a flexible production of gear geometries with pros and cons. This technology does not replace the traditional methods like hobbing and the use of gear shapers, grinders and gear shavers. This technology is not interesting for a serial production especially of small gear types and modules.
The advantage is the usage of a high precise 5-axis simultaneous capable machining centre for different types of gears (cylindrical gears, external and internal toothing, each type of bevel gears straight/skew/spiral like Gleason, Klingelnberg, Konvoid, Kurvex etc.) in soft and hard machining with non-profiled solid carbide tools and inserts on the same machine ("low-budget tools" compared to special hobs, shaper cutters, bevel gear cutters etc.) and the possibility to use the machine for non-gear parts, too.
The 5x gear milling technology is based on the single indexing system - compared to hobbing it is much slower depending on the size of the tooth gap. 5x milling is more efficient for bigger modules = larger tooth gaps. Otherwise you work with very small tool diameters and it looks like a work with a needle
For cylindrical gears the 5x milling makes sense
a) for single pieces and small lots
b) when the hob is not available and a short delivery time is required
c) prototyping with special tooth profile (unusual PA, basic rack profile, protuberance etc.)
d) double helicals with a small gap (which cannot be manufactures by hobbing)
e) Herringbone gears
f) very large modules

Spur gears can be manufactured by 4 axis, but helical gears and spiral tooth forms requires a 5x simultaneous capable machining centre equipped with a high precise machine rotary table for the indexing from tooth gap to tooth gap. This is an important point. Indexing by the head/spindle is not accurate enough. The rotary table must achieve the necessary tolerances for the pitch measurement. Furthermore the tool wear influences the pitch quality.

A professional software for the generation of 3D tooth shapes (instead of a 2D drawing) is necessary as well as a CAM software in order to prepare the milling programs. A professional gear calculation software is important, because there are software tools on the market with does not calculate the complete tooth profile (without tooth root for example). Due to the load capacity calculation and other points a professional solution is absolutely a must. Simplified tooth profile calculations cannot be a basis for a qualified gear production. A 5x milled gear must have the identical tooth form like a traditional manufactured gear.

 

[mfg11eng11guy]

Thanks Guys, very good conversation. And very good observations.