Gear Cutter Question

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mikecb1 - 21 Jul 2009 12:07 GMT

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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Dragon - 21 Jul 2009 15:28 GMT

> 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.

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.

> 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.

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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David Littlewood - 21 Jul 2009 16:13 GMT

In article
<7a4e541b-7772-4913-8230-c59a2e1d57cc@j32g2000yqh.googlegroups.com>,
mikecb1 <mike.crossfield@virgin.net> writes

>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
[quoted text clipped - 7 lines]
>number 5 cutter is the better choice, but I wonder if I'm missing
>something.

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

Signature

David Littlewood

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Mark Rand - 21 Jul 2009 17:57 GMT

>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
[quoted text clipped - 4 lines]
>
>David

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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mikecb1 - 21 Jul 2009 18:56 GMT

> On Tue, 21 Jul 2009 16:13:08 +0100, David Littlewood
>
[quoted text clipped - 16 lines]
> Mark Rand
> RTFM

David & Mark

Thanks - that makes perfect sense.

Mike

Mike

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Model Forum / General / Models / July 2009

Gear Cutter Question