gear cutting help needed

Our machining class needs information on generating single point (fly) cutters with involute geometry to cut spur change gears.
Eyeball grinding of the tool ala Gingery produces entirely useable gears but we are looking for something more exact.
Ivan Law in his book " Gears and Gear Cutting " [see http://www.transatlanticpub.com/cat/workshop/gears.html for details] [another US source for UK workshop practice series of books is http://www.powells.com/psection/Engineering.html ] shows a technique to generate very close circular approximations to the involute curve for both single point and circular form tools by using two circular cutters of specific diameters spaced a given distance apart. The tool blank is then fed to a specific depth between the circular cutters to generate the correct involute form.
Data take from Gears and Gear Cutting by Ivan Law ISBN 0-85242-911-8 Data to generate circular approximation of involute curve For 20 degree pressure angle gears                              Base table for 1" diametrical pitch gears         Inch                     Cutter #    Teeth    Diameter    C/C        In Feed    Width 1        135-R    51.300    49.600    17.790    4.000 2        55-134    32.150    31.600    11.470    4.000 3        35-54    15.070    15.510    5.870    4.000 4        26-34    10.260    11.030    4.270    4.000 5        21-25    8.550    9.400    3.710    4.000 6        17-20    7.800    8.700    3.440    4.000                          dimensions in inches         Table for DP gears    25.400    <== input DP     Metric Module 1.00 Cutter #    Teeth    Diameter    C/C        In Feed    Width 1        135-R    2.020    1.953    0.700    0.157 2        55-134    1.266    1.244    0.452    0.157 3        35-54    0.593    0.611    0.231    0.157 4        26-34    0.404    0.434    0.168    0.157 5        21-25    0.337    0.370    0.146    0.157 6        17-20    0.307    0.343    0.135    0.157
The same technique is shown at http://www.metalwebnews.com/howto/gear/gear1.html , but with slightly different dimensions for the cutters, spacing and in-feed. ( I have an excel spread sheet to do the calculations if anyone wants a copy - send me an email or I can post to the dropbox)
==> The problem is that both charts show C/C spacing distances LESS than the specified diameter of the two circular cutters for the form tools for larger numbers of teeth.
Has any one used this technique? If so how are the #1 and #2 cutters formed?
We will be making 1.0 m/m module change gears out of 6061 T6 and/or phenolic / micarta, and possibly a 127 tooth metric conversion gear.
Given that it takes some time to hand grind a form cutter and this is a introductory machining class, we would like to minimize tool breakage. Has anyone tried using a slotting saw to remove most of the material and then cleaning up the slot using the form cutter?
Using the advise from these NGs, our first attempts were done using a spin indexer to make a 40T gear with entirely satisfactory results. We just purchased a rotary table to produce the gears the spin indexer can't divide. This is a WT 1990-0015 (see at http://www.wttool.com/p/1990-0015 )
Any advise would be appreciated.
GmcD
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On Fri, 14 Jan 2005 16:14:54 -0600, F. George McDuffee

Very common practice, even when using form milling cutters. Saves some wear and tear on the cutters.
Greybeard
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This subject is exhaustively covered in the British metalworking mags.
I know a guy who wrote a program to plot out an involute curve at 20X actual size, and then he plotted that on a transparency and put it on the screen of his optical comparator. He made a single-point tool and used a round lap to remove metal, comparing with the transparency often. He said it went really well and he made a whole set of gears for a lathe. He did all of this at home, but then he's a really good machinist compared to me. Seems like something you could do at a school where they have programming guys, plotters, and optical comparators around.
GWE
F. George McDuffee wrote:

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We have a version of the gear generation program written in lisp for auto/intelli-cad called truegear by Eugene Kalney [see http://www.cad.dp.ua/english/program-e.html to download as truegear.zip w/ support files and coumentation] this seems to work very well.
One problem is we don't have an optical comparator and we have a zero budget. This is not all bad in that it forces us to do machining projects that stress ingenuity over attachments.
One of my major concerns, given the current economic trends, is a situation where industry and manufacturing is forced into survival mode where replacement parts are no longer available, either because the equipment is so old, a world wide economic implosion has occurred or because the parts are only made overseas and the suppliers will no longer accept worthless American dollars. [Consider the situation of an Argentinean manufacturer who has a failure of a critical part on an old American made machine…]
It appears possible we can adopt the plans for a radius/ball turning lathe attachment to use a pencil type die grinder to grind the required radii in HSS lathe bits if the forming information in Law's table is correct (or can be corrected).
Thanks for the suggestions - these newsgroups are the best resource I have ever found. =================On Fri, 14 Jan 2005 15:31:44 -0800, Grant Erwin

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A cylindrical cutter with multiple rows of teeth(about 5) in the shape of a rack of the desired pitch (easily made on the lathe) will generate involute gear teeth of good quality. There is info available on this gear cutting method if you are interested.
Randy
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Would greatly appreciate the information -- URLs? Books?
How do you get the clearance for the teeth? Simple eccentric offset with only one "gash"?
Thanks
GmcD
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wrote:

=======================Thanks for the tip. I checked and some of the older machining books [Lindsay reporints] show these cutters.
As usual, the details of use are sketchy at best.
When I make a 25 X CAD layout it appears with the smaller gears the extra teeth on the cutter do not contact and thus the spaces in the gear are too narrow. Do you have to make several passes with the cutter with the gear slightly rotated and cutter slightly displaced to get a involute curve (approximated by a series of straight lines) and wider spaces? If so how many passes?
GmcD
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I think there might be a small market for 127 tooth 1 module gears among the owners of mini-lathes.
Like me.
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jtaylor wrote:

among the

Did you try compound gearing for a 80/63 ratio? That is maybe close enough for hobbiest work anyhow. Since 80 = 20 * 4 and 63 = 21 * 3 you can build a compound gear train from a 20 tooth, 21 tooth, and another pair of gears with a 3:4 ratio. Finding 20 tooth, 21 tooth, and either 15 tooth or 25 tooth gears should be easier than finding 127 tooth gears. 80/63 = 1.2698 Basically this gets you within .0015 percent.
Maybe someone will have enough ambition to develop electronic gearing for a mini lathe. Even if it doesn't have the ball screws for CNC threading, it could work with electronic gearing and a stepper driving the lead screw... But I digress.
Pete
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On 14 Jan 2005 20:27:53 -0800, snipped-for-privacy@centurytel.net wrote:
Several sources have suggested using a 37/47 gear combination which is susposed to produce "2/100 of 1% accuracy" [see http://www.lathe.com/metric_threading ]
We can't find a source for those gears either.
GmcD =====================================>> I think there might be a small market for 127 tooth 1 module gears

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My numbers are more accurate with the 80/63 than the 47/37. And most sets of gears should have a 20 and 21 tooth gears in them. I posted that because I am sure that 20, and 21 tooth gears should be cheaper, more accuarate and easier to find than the 47 and 37 tooth gears, If someone wants to go that route.
Pete
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I am not sure but I think Scott Logan at Logan Actuator has the 37/47 gears for metric threading on Logan Lathes. Don't have a link right now but there is a web site. Don Young
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snipped-for-privacy@fpc.cc.tx.us says...

Also available from Ametric..
http://www.ametric.com/catalogs.htm
Ned Simmons
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On Sat, 15 Jan 2005 12:49:01 -0600, F. George McDuffee

We carry those gears. Our online store is temporarily down for maintenance, but you can call our office [(815)943-9500] for details.
We have two different sets available, both 16DP, 14-1/2 NPA.
One set is 7/16" wide with a 5/8" bore and 5/32" keyway. These are designed for Logan 9", 10", 11" and some 12" Lathes.
The other set is 5/8" wide with a 15/16" bore and 5/32" keyway. These are designed for Logan 14" and some 12" Lathes.
We also have available 100T and 127T gears with the same specs for mathematically correct transposition. These gears are, of course, substantially larger and more expensive.
The 37T gears are $60.50 each, the 47T gears are $74.50 each.
The 100T gears are $162.25 each, and the 127T gears are $213.25 each.
--
+--------------------------------------------+
| Scott Logan - ssl "at" lathe.com |
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snipped-for-privacy@deletethis.hfx.andara.com says...

They're available from Ametric in 15mm face width...
http://www.ametric.com/catalogs.htm
Ned Simmons
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r
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Correct. You have to grind flats on the two cutters so they won't interfere with each other. The radius given by the formula is needed only where the two "circular cutters" actually cut. -Dave
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email: snipped-for-privacy@mindspring.com
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"Has anyone tried using a slotting saw to remove

This is a very old approach in watchmaking and it is still the preferred approach for cutting pinions. Pinion cutters for clock/watches are very dear (can't be had from MSC) and every effort is made to spare them.
In the very early days (1700) the wheel was divided with a slitting saw and then a special file was used to shape the teeth. Later, this evolved into the "rounding up" tool; a self indexing tool that is used to modify an already divided wheel. When I have a wheel that does not conform to current cutters (tooth is shorter or longer) then I divide the wheel and use the rounding up tool to shape the tooth. Only when the wheel is too large for the tool will I make a fly cutter.
BTW, in watchmaking/clockmaking we make fly cutters in the lathe out of round stock. Since we use hand gravers (ala wood turning), we turn the end of the rod to a profile that is an exact fit between two good teeth of the wheel we are replacing. File the profile to the center line, harden and temper. Grind relief on the tip and polish. Takes about 30 minutes.
--
Regards,
Dewey Clark
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and
current
end
Nice website Dewey. You appear to be a true craftsman. Did you serve an apprenticeship? I can appreciate working with small stuff. I used to run a factory that made dental handpiece motors. Spindles, chucks, impellers, all with seemingly zero tolerance. At a half million rpm .001 run out will waste a bearing in a short period of time.
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<snip>

====================Sounds like a good idea. Cutter will automatically be centered and both sides will be symmetrical. Does the tool have enough strength for anything beyond sheet brass? What is the working radius of the tool? Do you just have a flat on the tool to orient in the holder or is there some special technique?
Do you use soft drill rod / silver steel and harden or grind round hard tool steel? We are limited to a propane torch and a bucket of water for hardening (but if the old timers could do it … )
GmcD
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