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Posted by Robert Swinney on May 2, 2008, 3:07 pm
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I have ivan Law's book, "Gears and Gear Cutting" and a copy of John Stevenson's
paper, "Cutting
involute Gears with Form Tools". In each are detailed instructions for making
"button type" form
tools for the fabrication of gear cutters. Choose any specific gear and
pressure angle and the 2
sets of instructions show different sizes of buttons, pin centers, and etc. Can
anyone elaborate on
why this is so? I would think the specifications for making precision gear
cutters would be
consistent among "authorities".
Bob Swinney
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Posted by Karl Townsend on May 2, 2008, 8:56 pm
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> why this is so? I would think the specifications for making precision
> gear cutters would be
> consistent among "authorities".
I guess you gotta choose your authority. I've made gears with Ivan Law's
instruction and it worked perfectly, the second time. (The first time was
for seed)
Karl
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Posted by Bruce in Bangkok on May 3, 2008, 1:32 am
Please log in for more thread options On Fri, 2 May 2008 19:56:21 -0500, "Karl Townsend"
>
>> why this is so? I would think the specifications for making precision
>> gear cutters would be
>> consistent among "authorities".
>
>I guess you gotta choose your authority. I've made gears with Ivan Law's
>instruction and it worked perfectly, the second time. (The first time was
>for seed)
>
>Karl
>
If I remember correctly it is impossible to cut perfectly accurate
involute gears except by generating them. Thus any calculation to make
a "gear cutter" would be, at best, an approximation so a difference in
calculations might well be the norm.
Bruce-in-Bangkok
(correct Address is bpaige125atgmaildotcom)
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Posted by Ed Huntress on May 3, 2008, 7:50 am
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> On Fri, 2 May 2008 19:56:21 -0500, "Karl Townsend"
>
>>
>>> why this is so? I would think the specifications for making precision
>>> gear cutters would be
>>> consistent among "authorities".
>>
>>I guess you gotta choose your authority. I've made gears with Ivan Law's
>>instruction and it worked perfectly, the second time. (The first time was
>>for seed)
>>
>>Karl
>>
>
> If I remember correctly it is impossible to cut perfectly accurate
> involute gears except by generating them. Thus any calculation to make
> a "gear cutter" would be, at best, an approximation so a difference in
> calculations might well be the norm.
I don't think that's the case, Bruce. It depends on how much undercutting is
required at the base of the teeth. And that's a function of the pressure
angle and the number of teeth per gear, plus the geometry of the tips of the
opposing teeth.
That is, for straight-cut gears. The geometry of most such gears is
perfectly achievable with a milling (or straight shaping) cutter. But there
are some extreme ones that require generating.
--
Ed Huntress
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Posted by Bruce in Bangkok on May 3, 2008, 8:44 am
Please log in for more thread options On Sat, 3 May 2008 07:50:32 -0400, "Ed Huntress"
>
>> On Fri, 2 May 2008 19:56:21 -0500, "Karl Townsend"
>>
>>>
>>>> why this is so? I would think the specifications for making precision
>>>> gear cutters would be
>>>> consistent among "authorities".
>>>
>>>I guess you gotta choose your authority. I've made gears with Ivan Law's
>>>instruction and it worked perfectly, the second time. (The first time was
>>>for seed)
>>>
>>>Karl
>>>
>>
>> If I remember correctly it is impossible to cut perfectly accurate
>> involute gears except by generating them. Thus any calculation to make
>> a "gear cutter" would be, at best, an approximation so a difference in
>> calculations might well be the norm.
>
>I don't think that's the case, Bruce. It depends on how much undercutting is
>required at the base of the teeth. And that's a function of the pressure
>angle and the number of teeth per gear, plus the geometry of the tips of the
>opposing teeth.
>
>That is, for straight-cut gears. The geometry of most such gears is
>perfectly achievable with a milling (or straight shaping) cutter. But there
>are some extreme ones that require generating.
I can't quote an authority as it is something I remember (I think)
from my apprentice days - that the only method of producing a
perfectly accurate involute gear was to generate it. Now there is a
possibility I "learned" this little fact when visiting Fellows Gear
Shapers, who would be somewhat prejudiced, being in that business, but
it is stuck in my memory from somewhere.
Having said that we habitually cut perfectly acceptable gears using a
set of gear cutters and In fact I have ground single point tools using
the gear itself as a template and then used a fly cutter type device
to cut the gear with the single point cutter.
It was a one off for a large computer installation in the A.F. with no
part numbers, drawings or data available about the "special gear". As
far as I know the thing was still running some years later when I left
the Base.
I throw in the "war story" to illustrate the fact that sometimes
"close enough" works and you don't need perfect parts. Thus my remarks
about two different calculations for "close enough" being possible.
Bruce-in-Bangkok
(correct Address is bpaige125atgmaildotcom)
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> gear cutters would be
> consistent among "authorities".