Re: Climb vs conventional hobbing


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Posted by BillS on April 24, 2008 at 12:19:51:

In Reply to: Re: Climb vs conventional hobbing posted by RAGHAVENDRA on March 10, 2008 at 06:23:58:

Yes, I agree with what everyone has said so far. If you are cutting a ductile iron blank, it can "ring like a bell" and Raghavendra is right about how important it is to fixture out close to the cutting action.

But I think lbmakem hit the nail on the head with choice of cutter hand for high gear angles. I believe this was the primary reason for the tool breakage. So, I hope he doesn't mind me going into some detail to try and explain why.

Choosing a LH hob forces you to reverse the index rotation direction which , for a RH gear, is not loading the index wormgear and geartrain properly. (maybe you had the LH cutter to cut its mating member?) Any backlash or slack in the index worm/wormgear and thrust bearings is not being removed by the cutting forces, and the gear is allowed to "jerk" rather than rotating smoothly. This could explain why the hob broke.

In effect, with a high RH angle, the machine is climb cutting, i.e. the cutter is trying to pull itself into the workpiece.

Look at it this way: In any cutting operation there is a feed force and the opposing force on the cutter due to cutting action. You always want the opposing force from the workpiece to be opposite to the tool's feed direction.

Your situation (LH cutter and high-angle RH gear) produce a cutting force from the tool that is in the same direction as the direction of workpiece feed due to rotation - if you were thinking about any milling operation, this would be called climb cutting, and the feed mechanism has to be virtually 0 backlash to avoid tool breakage. Unlike a milling machine, hobbing action produces two feed directions - one due to table feed the other due to workpiece rotation.

When "Conventionally" hobbing, the cutting force is primarily against table feed for small table angles. In Climb hobbing the table feed is reversed and the cutter tries to pull itself into the workpiece - need a tight machine for this! This covers forces involved with table feed.

With high gear angles, a larger portion of cutting force is either opposed (or aided) by workpiece rotation. This is where direction of workpiece rotation must oppose cutter force to avoid climb cutting. Choosing the appropriate cutter hand will determine the correct direction of workpiece rotation.

As long as table angle setting is small, very little of the cutting force is in the direction of rotation, so normal friction and inertia help prevent "jerkiness" that would occur because of climb cutting.

If possible you always want to setup the machine for opposition between cutting force and rotation. If not, you are depending on friction or a very tight machine.

The only sure cure, as Lblmakem said, is to use a RH cutter.


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