Can a lead crown be applied with a hob? 1

[ExRanger]

On a simple involute spur gear - I was under the impression that in order to apply a crown in the lead direction it is necessary to use a grinding wheel and either plunge the wheel into the part as it traverses the face width or etc. But is it possible to get a lead crown using a hob cutter?

Thanks.

 

[gearcutter]

I'm assuming you are talking about a helical gear wich has been crowned. Crowning helical gears can easily be done on a hobbing machine with either a (for older machines) crowning attachment or with any CNC hobber. The hob is virtually plunged in and out, as you suggest, to create the profile.

 

[ExRanger]

Thank you gearcutter. I'm talking about hobbing just spur gears, not helicals, but I assume the same thing applies vis-a-vis the crowning.

Would you happen to know the best surface finish that is achieveable by hobbing using the best equipment available today?

 

[gearman1234]

The surface finish (in lead direction) depends on the depth of the feed scallop marks generated by the hob, which in turn depends on the hob dia, feed rate, no. of hob gashes etc. For a freshly resharpened and TiN coated hob with 15 gashes , 3.5" dia. 0.06 in per rev feed rate, the finish could be about 20-25 mu Ra.

 

[gearcutter]

Unfortunately it's like the old "How long is a piece of string" question. Quality of finish depends on several factors. Apart from the obvious things like condition of the machine, type of work and tool holding, fixtures etc, and along with the valid points gearman1234 makes, other considerations are; the type of material (generally the harder the material the less pronounced is feed scalloping), number of teeth in blank (generally the less number of teeth the more pronounced are the generating flats produced by the hob, design of the hob (is it spiral or straight gash), type of cutting oil. I'm sure there are many more considerations I can't think of right now. You really need to answer a few of these points before working out what sort of finish you might end up with. As always, it's always best to try to set up a trial to best answer these questions. Sorry I can't give you a straight answer.

 

[israelkk]

Isn't the surface finish is embeded in the AGMA class number? If for example one specify AGMA Q10C and do a rolling check against amaster gear, isn't the total composite tolerance or more accurately the tooth to tooth tolerance actually defines the surface finish?