hobbing helical gear

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.