Gallery, Projects and General > How to's

Cutting Spur Gears -- A neat technique

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philf:
Paul,

Your cutter profile looks all wrong to me and would explain why the resulting gear teeth look thin.

The cutter teeth should resemble a rack profile and the tooth/space should be equal at the pitch circle line and not at the tip of the tooth which yours looks.

Phil.

BillTodd:

--- Quote ---Then a circular array of this cut is done 'Tooth count' times to form the gear.
--- End quote ---
That's not how a gear hob works.

your cutter needs to be a symmetrical rack form and (in your reference frame) it needs to move along the tangent as it is copied around the blank (as it it were rolling around the circumference).

If you need a gear model (reasonable accuracy, good enough for printing) I have a couple, mod & dp,  based on a modified button method (uses elliptical buttons for a better involute match). mod version attached

Bill

RussellT:
Hi Paul

I've just been looking at your first diagram.  The figures you have used for pitch and depth of cut are exactly as described on Helicron's web page linked at the beginning of this thread but what isn't particularly clear on his page is that these figures are for a 20 degree pressure angle.  If you change the angle in your solidworks model I think you will find it works.

I have tried this method of gear cutting - my first attempt at gear cutting to test my home made dividing head - and the teeth when examined closely have three distinct facets on each side rather than the two seen in your model.

Russell

BillTodd:
Using the straight hob  method described you'll have a different number of facets depending on the numbercof teeth engaging with the hob, larger gears (more teeth)  will have more facets as they'll be flatter against the hob

Have a look on you tube for a method using a straight hob on a small cnc mill that indexes both the gear blank and hob to get many more facets for a closer involute form.

bill

RussellT:
The original post describes using a 5 tooth cutter central to the gear blank.  Under those circumstances it should only give three facets using this method.  If the gear were small enough there might only be two facets but there were three on the 16 tooth gear I tried.

Russell

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