Posted by BillS on December 14, 2007 at 14:48:54:
In Reply to: Re: Hobbing Machine posted by John Cerra on December 12, 2007 at 14:17:51:
Hi all,
John has hit the nail on the head with his concise explanation.
It's always good to know your hobbing equipment intimately and to see what is required to set these machines up.
As John states, the difference between having and not having a differential tends to encourage obtaining a machine with differential. I think tangential feed is a good option, too. But hey, we do what we can...
Cutting a helical gear with a differential machine is a better way to go since it is easy to take roughing and finishing passes, especially important when cutting large pitches. It is easier to setup, (again for helical) since the index sets up for the spur number of teeth, and the helical angle is generated by the differential. As John said, you solve the differential formula for the pitch, teeth, helix angle, etc. and you find a differential gearset to closely approximate that decimal ratio.
Calculating index and feed gears for helical gears on a NON-differential machine is very doable, but I wouldn't want to do it over and over unless I had a complete inventory of index and feed gears. And you want to always double-check your formula calculations, gearset resizing, etc. to avoid making scrap.
Getting the decimal ratios for index and feed on a NON-differential hobber can be trial and error using the "Pickup Constant" method, but sometimes it works out on the first attempt! By the way, this method is fully described in the Machinist Handbook.
Some folks have used the "14000 Gear Ratio" book by Page (and there are surely others) to find gearsets.
Also, there are free software packages that will accept the decimal ratio that comes out of the formulas, and give
you gear sets, whether the formula is for index or feed gears on the hobber.
People that have made these utilities freely available are doing the gear community a great service.
But...
Okay, here's another plug for the "shameless commerce division" -
We designed CPC-RATIO to complete the gear set process in one wack without going to the machine or doing any trial and error. You can see for yourself if CPC-RATIO has the features you need to determine change gear sets under less than ideal conditions.
One important thing to consider - Large numbers of teeth, large prime teeth, missing change gear sizes can be impossible for some circumstances - yes, the devil is in the details. You might appreciate knowing if a change gear set is impossible given your shop environment, or that you really need to cut a new change gear size.
CPC-HOB is another offering that is designed to handle some specialized requirements, such as cutting large prime gears on differential machines. Primarily it provides a convenient (no mistake) way to get hobbing machine setups. CPC-HOB has a number of other significant advantages over CPC-RATIO, but you can test it for yourself. Feature lists don't mean much if you never need them.
Well, I deserve a "flame" for mentioning the software available here. But we're proud to serve the gear community with quality affordable PC based solutions to gear cutting. Admittedly it's not free, but I've always tried to give value for the money.