Hobbing helical gear on Barber Colman 16-16

[cuttergrinder]

I have hobbed helical gears on my barber colman #12 but we just got a Barber colman 16-16 and I have to hob some helical gears on it. It came with manuals but It doesnt have actual operators manual. It does have tags on the machine on calculating the change gears for the differential. It says 7.5/desired lead=driver/driven. I understand this but The only hob I have is a double start hob. Will this affect my lead angle? I know it needs to be accounted for when gearing for the number of teeth but im not sure about when gearing the differential. My helix angle will not be real critical because we have to make matching gears but I would like to get them close.

 

[Dan from Oakland]

The diff ratio will remain the same, but you will have to adjust your index ratio to account for the dbl start hob. Gear your index for ½ the number of teeth- same as when cutting a spur gear on your 12.

 

[cuttergrinder]

Thanks for the reply. I have hobbed helical gears on my #12 but they were all the same and a guy figured the index and fead gears for me.

This gear I need to copy doesnt have a drawing only a sample gear. We are making a left and right so they dont need to be perfect but I would like to get them close. I mounted my sample in a super spacer and mounted that to a mill and put a pin in the spindle. I started with the super spacer on zero degrees and I touched the gear tooth with the pin in the spindle at about the pitch diameter and zeroed the x axis and then moved the x axis until the super spacer turned 20 degrees and noted my travel in x. Then multiplied this number by 18 to get my lead of the helical tooth. Does this sound reasonable.

 

[10 fingers]

RE: Does this sound reasonable?

If the job was in my shop, we would insist on an accurate center distance dimension. Then, reverse engineer gears such that the helix angle yields pitch diameters that agree with the CD. Finally, confirm lead on a 12H instrument. Need to be mindful of long & short addendum designs, transverse pitch, circular pitch, and crazy specials. Such work is a minefield of complications.