making small involute spur gears with a CNC mill/lathe + gear hob?

hi,
i've been researching how to make small involute spur gears (96 or 120 DP, 60-120 or so teeth) using my equipment: a taig CNC with 4th axis rotary table + taig lathe (not CNC).
i have a book on making gears on order ("Gears and Gear Cutting" by Ivan Law) as recommended by people who responded to my previous post...but i was researching a bit on the net and came across several manufacturers of hobs. i was wondering if i can purchase a hob (rather than making one) and use it with my CNC and/or lathe in order to cut gears.
here's a link to one company that sells hobs (not meant to be promotional, just an example):
http://www.ashgear.com/hobfulldia.htm
i'm not averse to purchasing a bit of equipment...so i was wondering if i could use something like this to cut gears with my equipment. the homebrew techniques seems to focus on building the hob...i thought perhaps i could just save some time and purchase one.
any help would be appreciated.
thanks, tom

On Mon, 29 Jan 2007 14:35:41 -0800, tj wrote:
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Hobs are faster, by far, and don't require cnc. While I have never used an actual hob, I have cut worm wheels to ANS threads using a tap. The first time I did it, I was in a shop class and the instructor wanted to see it cut. I touched off the cutter just to get a good start before calling him over, and I had cut to depth before I realized it! I had to set up another blank and show him and the rest of the class. It was a very rewarding experience.
Ron Thompson On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA
http://www.plansandprojects.com My hobby pages are here: http://www.plansandprojects.com/My%20Machines /
Visit the castinghobby FAQ: http://castinghobbywiki.plansandprojects.com /
The member map is here: http://www.frappr.com/castinghobby

tj wrote:
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Be sitting when you see the price !
On the other hand, you could also use some of that CNC stuff to make a hob.
A CNC co-ordinated 4th axis would be pretty sweet setup for hobbing gears with a commercial (or shopmade ) hob.
IIRC there was a fair bit in print on this subject, in Model Engineer, or Model Engineer's Workshop magazines a while back.
Cheers Trevor Jones

‘

Thanx for the great link, Wes. I have it bookmarked for extra reference when using Ivan Law's book.
Bob Swinney
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Tom,
You're on the right track, but not quite there yet.
I am contemplating the same action you are, hobbing gears using cnc equipment.
Pre-enjoyed hobs are quite reasonable in price for the small sizes; I seem to recall $US 25 to 75 each, depending on the remaining life expectancy. Considering that one hob will cut all gear sizes from minimum teeth pinion to rack, of a given pitch and pressure angle, the cost is reasonable.
As for cnc equipment, neither the cnc lathe nor a 4 axis mill will do the job UNLESS the mill has an encoder on the cutter spindle so that you can specify the "gear ratio" between the cutter spindle and the rotary / indexing spindle.
Consider what it is you are trying to achieve:
The hob must be driven at an rpm that is related to the number of teeth you wish to hob; with a single start thread hob, cutting say 57 teeth, the hob needs to make 57 revolutions for one revolution of the work (gear blank) spindle.
In a mechanical hobbing machine this is achieved through a gear train, a different gear train is required for each number of teeth to be hobbed. This is where cnc shines because you don't have to muck about with changing gear trains.
Consequently using cnc software designed for a screw-cutting cnc lathe will have the attributes required for hobbing.
Add an encoder to the milling machine spindle, have a stepper motor driven work spindle, connect everything with the cnc lathe software, and hardware, and you are ready to hob. You will need to figure out the parameters to achieve the correct "gear ratio" between the two spindles.
Just keep in mind that to hob parallel tooth spur gears requires the hob spindle to be inclined with respect to the work spindle to an angle called the helix or lead angle. The hob will have this angle engraved on it. With small hobs the vertical axis milling machine is handy because this angle is easily adjusted by swivelling the vertical head. To hob parallel tooth spur gears on a horizontal mill (required for big hobs) a universal milling machine is required because it permits swivelling of the table to set it's axis to the helix angle with respect to the work spindle.
Anyhow, I encourage you to give it a try and keep us informed on the method you choose, and how you make out.
Good luck!
Wolfgang
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There's a great NG on Yahoo just for this interest: http://groups.yahoo.com/group/mwmills2 /
You want to read through all the old posts.

Perhaps - wire edm could work -the cost maybe higher than you thought - but we have been successfull in short gear type jobs for keyways, teeth, ect. Just a thought--visit our web site for more info.....
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