Subject: Hobbing Gears II

From: [email-masked] (Daryl Bender)
Date: 5 Jun 1995 21:02:18 GMT

I have a feeling (the more I think about it) that "freewheeling" could be
"ify". 

1) How do you gash the blank without driving it?
2) If you fudgingly reduce the pitch diameter slightly ie a 60 tooth gear
cut on a 59 tooth pitch diameter blank, (this is something I'll have to do)
do you just cut less deep as when using milling cutters? 

It certainly makes me want to see a real hobbing machine, the gear box
required to do every reasonable number of teeth would be MASSIVE. Therfore,
I thought I'd feel out the crowd on plan B.
I have a shaper, soooo if I ground some tool bit to make a rack tooth (or
rather space) form then used the shaper to generate a rack. Then use the
rack as a cutter for a blank mounted in an index head. For large numbers of
teeth I think it would work, you'd also get some generation since you cut
various parts of many teeth on each pass. The problem comes with low
numbers of teeth. The large curvature on the tooth profile would be only
roughly approximated. It seems the solution would be to rotate (or rock)
the rack cutter back and forth somewhat. My shaper can do this.

3) My question here is what is the center of rotation, the tooth space root
or the space pitch line? My gut tells me its the pitch line.

4) How much do you rotate, the angle subtended by two teeth?

5) Lastly, on an somewhat unrelated topic, how are elliptical gears cut?

Thanks
Daryl 
-- 
Working but not speaking for BNR

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From: fairfield <[email-masked]>
Date: Wed, 7 Jun 1995 21:23:02 GMT

> 5) Lastly, on an somewhat unrelated topic, how are elliptical gears cut?
Yeah, well.. you've just re-invented the Sunderland Gear Planer, one of
the early devices for generating decent gears. I've used one in the dim 
past, and was most impressed by the awful mathematics of setting it up.
Heath Robinson would have really been proud of it. If you use a rack cutter,
you rotate the gear blank on its axis & traverse the rack to stay in mesh.
But you need to gear rotation & rack traverse together. Good results, but slow 
& noisy. Can't think of any way to approximate it with an ordinary shaper.
Pass on the ellipticals, they've gotta be hobbed, don't know how.

Ray Fairfield
Univ. of Sth Queensland

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From: [email-masked] (gary preckshot)
Date: 8 Jun 1995 19:45:54 GMT

> [email-masked] (Daryl Bender) wrote:
Ray,

I'm not really sure what you're trying to do, but you sure seem to be
choosing the hardest ways to do it.  Hobbs are used for production
machining of gears, like when you want 2000 gears all the same.  They're
also almost necessary for doing helical gears.  However, if you're doing
spur gears, why don't you do what everybody else does for onesies and
twosies.  Get, or make (from a screw slotting saw), a cycloidal-form gear
tooth gullet cutter.  Set the blank up on a rotary table or dividing head,
and cut the gullets one at a time, rotating the blank by one tooth angle
each time.

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From: [email-masked] (gary preckshot)
Date: 9 Jun 1995 16:56:07 GMT

In article <[email-masked]>, [email-masked] (Daryl
Bender) wrote:
> 
> Alright I'll bite.... How do you cut accurate cycloidal or involute shapes
> into slotting saw teeth?

You take a diamond nib set in one of those fancy grinding wheel truing
fixtures and shape a grinding wheel to make the involute form.  Then you
grind the cutter to form, with the wheel at a slight angle to relieve the
cutting edge.  I'm not going to go into the details, but I'll admit that
working out cutter sharpening angles and grinding wheel forms is not
simple.  Grind each side of the slotting cutter with a wheel trued to form
for that side.  It would obviously be better to start with a proper blank,
since you may end up with poorly supported edges depending on the cutter
you started with.  However, my suggestion was for a quick & dirty way to do
it for onsies and twosies, using a cheap import cutter.  It's clearly
better to get the right cutter from a professional tool maker.

---

From: [email-masked] (Daryl Bender)
Date: 8 Jun 1995 21:00:43 GMT

Alright I'll bite.... How do you cut accurate cycloidal or involute shapes
into slotting saw teeth? The Idea of the hob and rack method is that they
are straight sided rack teeth (easy to be accurate). Both can be used to
generate the curvature of 12 tooth gear say. How do I get an accurate 12
tooth profile by other means? For the record I have been cutting gears with
purchased involute cutters its just that recently I've hit a wall.
64DP-128DP cutters are as far as I can see unavailiable. I'll redescribe
what I'm thinking with ASCII art.

                    PIVOT HERE                        Shaper is in behind
page                            
                     |                      A                             
                     |                      |                            
     RACK CUTTER     V         RACK CUTTER  |                              
         
 ************************************************                     
 *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *                         
              
  |               *    *                                                   

  |              *  O   *                                                
  V              *      * BLANK           Cutting direction                
                     
                  *    *  (index arbor    is in and out of                 
               
                    **    is in center)    page                            

Using the shaper you push the rack through the blank (similar to cutter)
but you cut parts of many teeth on a pass (picture a comb going through
your hair). For a 12 tooth gear you wouldn't get complete generation since
there are so few teeth. Therefore you rock the rack (crude arrows above
show one direction) back and forth to fully generate your curved 12 tooth
profile. Then index to the next position and repeat. My shaper can do 200
strokes per minute so this shouldn't take too long and the benefit would be
fully generated teeth, theoretically more accurate than the cutters. For 60
or more teeth you probably wouldn't even have to "rock the rack". Your
blank remains fixed to the index arbor same as cutters. As far as I've
seen, shapers have pivoting heads so no special jigs to build there (maybe
just an adjustable pivot stop).                                            
                          
As I think about this now you could make your rack cutter out of some
angle; one angle has a wide rack (for resharpening) the other is used for
mounting.
One last thing... If I could buy it (the cutters) believe me I would. I'm
just desperate and exploring possibilities.                                

Cheers
Daryl