error hobbing gears?

[dansawyer]

I am trying to hob 10 tip acme threads in an aluminum disks. The first test with a 90 count gear worked. The aluminum disk was turned to 2.861 inches diameter. The disk was then placed in a jig replacing the compound feed in a SB 9a. An Acme 10 tpi tap was held in a three jaw chuck and the end centered with a live center on the tailstock. The jig is a spacer, a thrust bearing, the gear blank a second thrust bearing, and a top spacer and a nut. The parts are assembled on a 1/2 inch threaded rod. The whole stack is held by a machined tapered adapter that replaces the compound feed in the cross feed.
The first blank is 9.011 inches in circumference (yes it is a bit large.) The hobbing process produced 90 teeth.
The second blank is 5.737 inches in diameter and 18.023 inches in circumference. (yes also a bit large). The hobbing process produced 182 teeth.
Does anyone have a clue as to what happened? My math predicts it should be pretty close to 180 teeth.

 

[benmychree]

Generally, when gears are hobbed, there is a geared connection between the cutter spindle and the gear blank spindle, this keeps the cutter and gear blank in time together for the correct number of teeth. Worm gears can be hobbed by first gashing the blank with a suitable cutter, then using the hob to finish the teeth, letting the hob revolve the blank and feeding in to the finish depth. It sounds like the hob created multiple starts on the blank resulting in the overabundance of teeth.
You do not give enough information about the setup and cutting tools used to say much more, perhaps some pictures? On the second disc, an error in diameter of the blank could have caused the multiple tracking.

 

[dansawyer]

Thanks for the reply. You raise one possibility. The tap which is being used as the hob is being held in the chuck. This creates a narrow space between the cutting teeth of the tab and the chuck. With the small gear I am able to move the apron close enough to the head to use the deeper teeth on the tap. I started or gashed the teeth in the gear on the first pass.
On the larger gear this was not the case. The cutting / gashing area was over half way toward the entrance end of the tap. The cutting teeth on the lead end may be narrow to fully rotate the gear. I will proceed to test that theory.