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Miniature coil winder
philf:
Doc,
Are you just making cuts to full depth with the cutter centred on the gear? (If that makes sense!)
To generate a true involute you'd have to do many cuts rotating the gear slightly each time and raising or lowering the cutter accordingly.
I found a pretty good explanation here http://david.rysdam.org/machining/gears.html.
He got acceptable results just rotating the gear 1/2 tooth between cuts.
With just one pass he could feel the inaccuracy and the gears wouldn't run when fixed at the theoretical centre distance. With two passes the gears ran together smoothly.
That process would be very easy to automate on a cnc mill with a 4th axis. (Maybe a cnc 4th axis should be another project for you!)
Phil.
Doc:
Using the hob method each pass actually cuts 3 teeth on the gear at different angles. I've used this method before and works like a charm. Yes you probably won't get an absolute perfect gear but you will get a gear dam close and usable.
awemawson:
If the cross sectional form of your cutter is that of a rack of the correct dimensions, and it is long enough to encompass any teeth in mesh then surely by definition it must cut the correct profile.
Obviously the teeth in mesh will vary dependent on the tooth count
philf:
--- Quote from: awemawson on October 07, 2018, 02:13:41 AM ---If the cross sectional form of your cutter is that of a rack of the correct dimensions, and it is long enough to encompass any teeth in mesh then surely by definition it must cut the correct profile.
Obviously the teeth in mesh will vary dependent on the tooth count
--- End quote ---
Not so Andrew!
In a gear planer the cutting rack moves tangentially between strokes and the gear blank rotates a small amount generating the involute form.
With just one pass of a rack form cutter at centre height you get:
Rotating the blank a tooth (in this case 30 degrees for 12 teeth) you get:
That's as good as it gets - the involute is approximated with 2 flats.
Add a pass where you rotate the blank 1/2 tooth and raise the cutter the equivalent of 1/2 tooth you get:
The next pass would be another 1/2 tooth rotation dropping the cutter down to it's original height producing:
The involute is now approximated with three flats but some horrible looking artefacts at the root of the teeth.
So to generate a perfect involute you'd have to do an infinite number of passes rotating the blank and adjusting the cutter height accordingly. A compromise somewhere in between would give a very acceptable gear.
I chose a 12 tooth gear because this would accentuate the problem.
Hope this makes sense.
Cheers.
Phil.
awemawson:
Makes sense, I'll stick to normal hobs :thumbup:
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