How does a non-differential hobbing machine cut helical gears?
The concept is simple. Let’s say the gear to cut is 100 teeth. If it were to be a spur gear (helix angle = 0) then the index gear ratio would be Index Constant divided by teeth. For an index constant = 30, the gear ratio should be 30/100 as shown in the prior topic. Using 30/100 would synchronize the hob to the workpiece so that when the workpiece completes each revolution, the hob would be meshed precisely in the same tooth. Moving the hob table slowly (feeding) through the workpiece parallel to the workpiece axis will add to the length of the milled tooth spaces, and the tooth space will be parallel to the gear axis. What has this to do with a helical gear, you ask? The gear teeth in our example are parallel to the gear axis only because the index gear ratio is EXACTLY equal to 30/100, or EXACTLY 0.30000. So, we are going to select index gears so that the ratio is not matched exactly. From the example above, let’s change one of the idler gears: 30 x 60 ———– 61 x 100 Now the ge
