Gear Cutter Question

Brown and Sharp type gear cutters each cover a limited range of teeth numbers. E.g. a number 6 cutter covers 17-20 teeth gears, and a
number 5 cutter covers 21-25 teeth. As I understand it, the cutters are exact for the lowest number of teeth, becoming less and less accurate for higher numbers in the range.
So is it better (more accurate) to use, say, a number 6 cutter to cut a 20 tooth gear (3 steps away from the most accurate) or a number 5 cutter (1 step away from the most accurate). Logic tells me that the number 5 cutter is the better choice, but I wonder if I'm missing something.
Mike
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I found a reference that said the same though it sounds illogical. One might expect the most accurate to be one in the centre of the range. Must be something to do with the approximations made.

With a quoted useful range it makes sense to use the one quoted for 20 teeth. It will be interesting to see what the experts have to say and any explanations offered.
Henry
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In article

I'm no expert - never cut a gear in my life, though I've read up on it in preparation to trying it soon. I wonder if the issue is one of interference. The small gears (or more precisely, those with least teeth) have gaps between the teeth which are greater in angle than those with more teeth, to allow the opposing teeth to mesh, but not interfere, over a larger angle of rotation. For a gear with very many teeth, its angular rotation from start to finish of contact is small, and for a rack of course it is zero. By "greater in angle" I guess I mean "has more cut away from the full profile of the rack form with straight-edged, sharp-cornered trapezoidal teeth at the nominal pressure angle".
A cutter which is perfect for an n-toothed gear will allow non-interference for n+1 teeth or more, but for n-1 teeth it could allow interference between teeth.
...Unless of course someone with some actual useful experience or knowledge points out the error of my theory.
David
--
David Littlewood

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On Tue, 21 Jul 2009 16:13:08 +0100, David Littlewood

That's exactly what happens. If you use a cutter for a gear with a smaller number of teeth than it's rated for, you will get interference. It there is no alternative, a slightly larger between centres distance or greater depth of cut will be required to restore workable meshing. DAMHIKT ;-)
Mark Rand RTFM
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David & Mark
Thanks - that makes perfect sense.
Mike
Mike
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