hobbing helical gear

I have no idea what kind of gears these are. Information is a bit scant, really.
I just follow my own curiosity here.

Hm, crossed helical gears, there is always something more to look up

@CORBETT
I don't really know a lot about gears. I thought maybe I could glean some information by following this project. Alas I have the distinct feeling this is not going to be the busiest of threads. Feels more like milking stones.
Maybe just a language thing though. And he probably gets more stuff done than me.
Anyway, you seem to have one of those gear hobbers sitting around, maybe you could convert that with all the bells and whistles now?

After all there is no time like the present, is there?

Ah, another one.
So are you also refitting an older manual machine?

Aciera wrote:

I think I'll let Andy figure that out overnight and wait for him to present his solution (including a GUI).

Unfortunately I was not really reading the forum when this was discussed.

I keep meaning to make a helical gear, but have not done so yet.

There are two things to do:
1) Angle the hob to the gear by the normal amount (hob lead angle) and by the helix angle
2) Add in an extra movement of the hob relative to the blank proportional to the distance travelled along the gear shaft axis.

Exactly how the maths works out will depend on the machine. On my mill the X feed direction swings with the table (which sets the hob angle), whereas on a conventional hobber I don't think that it does.

This second part could be done in two ways:
a) By rotating the blank by an extra amount. The problem here is that the extra amount depends on the radius of the cut
b) By shifting the hob along the hob axis. This should work at any radius (but I haven't analysed it, this is just visualising it). It has the advantage of evening the wear on the hob. It has the disadvantage that the hob might not actually be long enough.



b) Would be easy on my my milling-machine hobber, but might not even be an option on a real hobbing machine.

Both are equally easy in HAL. Just another scale and sum. You don't need to worry about the absolute position of the linear axis even, as the starting point will be in the numbers from the start.

If I was to make helical gears there would be a lot of nylon or wax practice parts made first, whilst I checked the maths.