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Posted by Ed Huntress on May 3, 2008, 9:11 am
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> 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.
I think that's true, and you can make a small compromise in the tooth tip
shape to avoid undercuts, which, IIRC, is done with some fairly
high-performance gears that are production-cut with hollow broaches.
But gear geometry is something I've long forgotten. I do remember reading a
paper on broach-cut gears that showed how to adjust simple gear shapes to
avoid undercuts, with no loss of performance.
On the other hand, the most sophisticated paper on gears I ever read was for
some work I did for Hoescht Celenese a few years back, about plastic gears,
which showed how even hobbed gears were really compromised from ideal
shapes -- and which you could achieve with relative ease in plastic. The key
example was an injection-molded plastic planetary set that would transmit
something like 150 ft-lb of torque at high speeds. Pretty impressive.
--
Ed Huntress
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Posted by on May 4, 2008, 10:59 am
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On Sat, 03 May 2008 19:44:14 +0700, Bruce in Bangkok
>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)
Involute gears have the tooth flank form required to produce
constant velocity transmission i.e. constant ratio. Any deviation
from this results in slight variations in instantaneous ratio
(causing noise and roughness) as individual teeth move in and out
or mesh.
Generation methods are necessary if the"perfect" constant
velocity tooth form is needed. However the flank form of involute
gears with large numbers of teeth is very close to a single
radius curve. Acceptable gears can be cut by a cutter formed with
a simple cylindrical cutting edge.
The best centre position and the curve radius changes
with the number of teeth in the gear. This change is small for
gears with a large number of teeth but rapidly increases for
gears with smaller numbers of teeth.
Purpose made gear cutters for milling teeth come as set
of 8. No 2 covers the range 55 to 134 teeth but No 8 only covers
the range 12 to 13 teeth.
Jim
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Posted by Trevor Jones on May 3, 2008, 9:56 am
Please log in for more thread options Robert Swinney wrote:
> 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
>
ISTR seeing a mention of the discrepancy in Model Engineer a while back.
The answer had something to do with differences in the reference
points that the measurements were based upon. I was interested enough to
read the stuff, but was not taking notes, if you know what I mean.
Same John Stevensen that hangs out on uk.rec.model.engineering ? Ask
there, if you can access it.
IIRC, George H Thomas and the crew he conspired with, came up with a
form releiving tool called the "Eureka", that was to be used with such
cutters. It was in relation to one of these form releiving tools that I
recall reading about the different sets of formulae for coming up with
the buttons.
Cheers
Trevor Jones
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Posted by Robert Swinney on May 3, 2008, 11:26 pm
Please log in for more thread options Thanks Trevor,
The Eureka is mentioned in Ivan Laws book.
Bob Swinney
Robert Swinney wrote:
> 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
>
ISTR seeing a mention of the discrepancy in Model Engineer a while back.
The answer had something to do with differences in the reference
points that the measurements were based upon. I was interested enough to
read the stuff, but was not taking notes, if you know what I mean.
Same John Stevensen that hangs out on uk.rec.model.engineering ? Ask
there, if you can access it.
IIRC, George H Thomas and the crew he conspired with, came up with a
form releiving tool called the "Eureka", that was to be used with such
cutters. It was in relation to one of these form releiving tools that I
recall reading about the different sets of formulae for coming up with
the buttons.
Cheers
Trevor Jones
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Posted by Paul K. Dickman on May 3, 2008, 2:46 pm
Please log in for more thread options
>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
Years back, as a mental exercise I worked out the math to duplicate
Stevenson's chart for 14 1/2 deg pa.
When I cross checked my math, I found some discrepancies with his 20 deg pa
chart. It wasn't much, but I lost interest and never followed it up. It
might have been a clearance issue or simply which tooth count is really the
average of the cutter range.
Now I'll have to get a copy of Law's book and see if my formulas match his
numbers.
Paul K. Dickman
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