Cutting gears with a simple hob

T

Tom Griffin

Forum Guest
Register Today
I came across a site the other day about how to make a hob on the lathe for cutting involute gears. The hob was basically just five tooth rack of the same pitch with V grooves cut at the same pressure angle of the gears. It is used as a form cutter and the gear blank is indexed by it tooth by tooth to generate the teeth. It seems too good to be true, but it looks like a very simple way to cut gears. Check it out: A simplified Gear Hob

Tom
 
Should work just fine. I remember an obscure method of gear profile generation using a shaper and a single, flat sided tool that was advanced across the top of the gear as the blank was rotated. It was not a factory method, for sure, but a clever setup by some thinking individual. I think the same principle would be at work on this.
 
This is a method called "approximate hobbing" There is a guy on youtube named "hobbynut" who has a series of videos (5 or 7 as I recall) where he shows how to machine, harden, sharpen, and use these things. Good stuff. I have tried them out. Work pretty well in fact. They tend to work better on larger diameter (read, more teeth) gears.

http://www.youtube.com/user/Hobbynut
 
That's good to hear. I have a clock project coming up that I will try it out on.

Tom
 
I make an use home made hobs quite frequently. They do work quite well except as stated for gears with smaller tooth counts. As the blank is indexed from one tooth to the next the hob shaves of a little bit of stock giving somewhat of an involute curve to the tooth. One way of making the lower count teeth have a better profile is to index by one half and shift the cutter the appropriate amount, then do the proper indexing from that point. It sure saves on buying a set of involute cutters.
gbritnell
 
I'm missing something here. (no surprise) What material is being used for the hob? Is it hardened and ground afterwards. Are the cutters made or purchased. Remember guys, the only stupid question is the one I didn't ask.
dickr
 
The cutters are made from drill rod (tool steel). Made as in "shop made". The profile is cut on a lathe with a tool that looks like a threading tool, then you cut teeth with a mill and rotary table or indexer. Harden, then sharpen the faces with a stone. Take a look at hobby-nut's videos I linked above, they walk you through everything.
 
dickr,

If you go to the first link I provided and look on the right side of the page under site map, you'll see the whole process. Basically, you just make a single point tool like you would for cutting a thread, except instead of using a 60º point, you make it double the pressure angle of the gear you want to cut (40º for a 20º PA gear). Then you cut a thread with it in a piece of O1 drill rod, turn it into a cutter by milling some flutes into it and harden the cutter. Then you mount the cutter in a mill, the gear blank in a dividing head and take successive cuts, indexing the blank for each tooth. As the teeth index by the rotating cutter, the involute shape is generated and you end up with a gear.

I would highly recommend the Workshop Practices Series book "Gears and Gear Cutting" for more information if you are considering trying this out.

Tom
 
There is also a DVD by Jose Rodriguez covering hobbing that they sell on LMS. I bought it and it is interesting. Have not tried yet.

You can make two main types, one that has a spiral thread like a tap and one that just has spaced teeth (like a spline with v grooves in it). Some crude hobbing can be done with a tap as well. Some are available commercially. I picked up two a while back from China, but again, they are on my to-do list.
 
I also bought the Jose Rodriguez DVD from LMS and the information in it is pretty good. Nothing against Jose, but he does have a bit of a monotone voice, so if you sit down to watch it at the end of a long hard day, you might find yourself missing most of it. There are some charts that come along with the DVD also, and it certainly looks like a good method to try and will be far less expensive than buying commercially made gear cutters. I do not yet have a method for indexing, so have not tried it yet, but I'm working on it.

I also viewed Hobbynut's videos and they are good too!
 
Last edited by a moderator:
You must log in or register to reply here.
Back
Top