Emergency hobbing on a CNC lathe?

Emergency gear generation on CNC machining centers?
One segment of increasing machine shop activity in my area is machine repair that frequently requires the scratch production or
remanufacturing of non-stocked parts. A recent thread in this newsgroup indicates this is becoming an increasingly common activity in other areas.
One area of concern is the production of non-standard gears, especially those with odd diametrical pitches/modules and/or pressure angles.
I have come across two articles for the home shop or hobby machinist that shows how to generate the involute curve, using only a single point tool ground to the shape of the rack [straight sides] with a slight additional length to provide clearance at the root. Standard M2 lathe tool blanks appear adequate, and most any shop with access to a surface grinder should be able to grind such a tool very accurately, even with odd pressure angles. Where 14-1/2 degree pressure angles are used, an ACME thread gage and a carbide style grinder [with a little care] should prove adequate.
Basically this requires a shaper with the means to rotate the gear as it is traversed past the reciprocating tool as if it were rolling on the rack, along with a means to index the gear for the required number of teeth. In the article from the 1950s UK, a hand-powered shaper was used.
To review the articles [these have some suggested tool geometries in addition to the how-to] click on http://www.eurospares.com/graphics/Tools/shapers/shaper%20gear%20cut.pdf and http://www.eurospares.com/graphics/Tools/shapers/second%20article.pdf
It occurs to me that if you have a CNC lathe with a fine indexing spindle, you can simulate the shaper reciprocating action by using the Z motion, and can traverse the tool in sync with the rotation of the gear blank to generate the required involute teeth. Of course the tool would have to be adjusted to the correct height and clamped. For ease of setting the tool vertical and the holder parallel to the Z-axis, something fabricated out of square section CRS should work well. Assuming that the top of the bar was parallel, I can see setting with gage or space blocks after touching off on the gear OD. This would allow accurate roughing and finishing cuts.
While not producing AGMA class 8 gears, this could be a useful dodge when a non-standard gear is required ASAP and where the involute B&S style disk cutters are not immediately available.
Basically, if you can turn the blank, you should be able to make the gear. No clapper box should be necessary as there is no feed until the tool is clear of the part [with proper programming] or the part can be back-rotated "just a tad"
Indeed, if the tool-holder/bar was set at the correct helix angle, it appears this dodge could be used to generate helical gears as well as well as straight cut gears if simultaneous/synchronized X, Z and controlled rotation are allowed, which it appears to be possible on at lest some of the lathes that produce the "Higby" style thread starts.
Has anyone used this dodge? Does it seem like it should work? Comments? Suggestions?
It may be helpful along with that roll of racer tape and coil of bailing wire in the bottom of your toolbox to get things running again quickly.
Unka' George [George McDuffee] ===========Merchants have no country. The mere spot they stand on does not constitute so strong an attachment as that from which they draw their gains.
Thomas Jefferson (1743-1826), U.S. president. Letter, 17 March 1814.
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F. George McDuffee wrote:

At one time I machined gear electrodes on a 4 axis milling machine. These were all non-standard gears. The diametral pitch was modified for casting shrinkage and the backlash, od, and id modified for edm overburn. I used a single lip cutter ground in a deckel single lip cutter grinder and a high speed spindle. The 4th axis was set up parallel to the x axis at the left side of the table. Starting at a negative Y, I ran a pass at the blank X- then X+. Then roll the blank a little and move Y+ a little. Make a X- and X+ pass. Repeat until you cut air. This gives you 1 space. Reset, index A axis, repeat.
If the gear was big enough you could do the same thing with a straight sided end mill. Instead of incrementing Y, you would increment +Y+Z for 1 side and +Y-Z for the other side. Tan Y/Z would be the pressure angle.
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On Mon, 11 Jun 2007 22:54:16 -0400, Steve Austin

=======Sounds like a plan. Was the single lip cutter ground to the rack profile?
Unka' George [George McDuffee] ===========Merchants have no country. The mere spot they stand on does not constitute so strong an attachment as that from which they draw their gains.
Thomas Jefferson (1743-1826), U.S. president. Letter, 17 March 1814.
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F. George McDuffee wrote:

Yes. A little under the space thickness. Instead of the -X+X cut that I described, the cut was actually +Y -X -Y(x2) +X +Y. I'd cut two opposite spaces first and adjust the Y to bring the wire measurement in.
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F. George McDuffee wrote:

That's how they were made in the beginning. On a shaper.
I started (or halfway finished) to make something like this for my shaper. A banjo, some gears and a rack on the traversal axis. It wasn't accurate enough, so I have put it back.
Some weeks ago, I thought that a digital scale on the traversal axis and a rotary table driven with a stepper and some CPU in between will do it much easier. No problem with selecting gears, different pitches etc. I'll come back to this idea when I have time ...
Nick
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On Mon, 11 Jun 2007 20:35:45 -0500, F. George McDuffee

As it happens, I'm in the process of form grinding a rack tool to produce a 14t 22DP pinion on the shaper at the moment. This one is for the HLV cross slide leadscrew and can't easily be produced with a hob or an involute cutter. There is a needle roller bearing journal right next to the pinion that is larger than the base diameter of the pinion, so the involute cutter makes a mess of it DAMHIKT.
I am using a template on a Diaform wheel dresser to make a 3 tooth rack on a bit of 1/2" HSS. It probably would have been easier to grind a single point tool, but what the heck, I'm not doing this for money.
I guess with a modern CNC T&C grinder, it might be possible to grind a reasonable sized gear directly without too much trouble. I doubt if I'd get a friendly response if I suggested borrowing the use of our new one at work for that purpose though. And that still wouldn't help me with this particular pinion.
Mark Rand RTFM
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On Wed, 13 Jun 2007 13:23:27 +0100, Mark Rand

=========>As it happens, I'm in the process of form grinding a rack tool to produce a

============I can see how a shaft with an integral gear or gear cluster would be a problem.
The objective of this was to provide the small shop with a cnc lathe with an indexing spindle an emergency way to make a involute gear. This won't be cost effective for production, but could be a "life saver" for an emergency or rush job, particularly when a non-standard gear PD/module and/or PA and/or "stub" profile is involved.
A single tooth rack profile cutter should be easy(er) to grind, and you could machine and "hob" in one chucking.
My only major concern would be the amount of force required to use the Z motion to simulate a shaper, but very light cuts (0.001 inch or less) are taken because of the small cross slide and rotational increments to generate the involute tooth profile should keep this to a minimum.
I have sketched out a holder for this, sized to fit a 3/8 and 1/4 square lathe tool bit [AutoCAD 2000 format, but "free" viewers are available for download]. I no longer have access to any cnc equipment [retired] but would be happy to share. 1 piece of 1X2X8, 1 piece of 1X1X2 CRS and two 3/8X1_3/4 SHCS required.
Anyone know if any of the cnc lathes have the X, Z and spindle rotation synchronized enough to allow generation of a helical gear?
I can see this might be very helpful for the repair/maintainance shop.
Any interest in the group for a program to run under windows to generate the g-code for straight cut gears? You would input the PD, number of teeth, thickness of gear, and desired X increment. Unka' George [George McDuffee] ===========Merchants have no country. The mere spot they stand on does not constitute so strong an attachment as that from which they draw their gains.
Thomas Jefferson (1743-1826), U.S. president. Letter, 17 March 1814.
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F. George McDuffee wrote:

Yes. They can even cut polygons with an extra driven spindle. That's the best prove that they can keep axes in sync to within fractions of a degree. <
http://www.youtube.com/watch?v=oGq-9NNmr3o

Nick
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Thanks to all the regulars on AMC and RCM for their observations, suggestions, etc. that got me this far. Please let me know if you find this of interest and/or help.
==> Supplied as freeware with no warranty for fitness of use, not even as a bad example.<= click on http://mcduffee-associates.us/PE/HOB03C.ZIP to download hob03c.zip which contains hob03c.bas [powerbasic source] and hob03c.exe which should run under most versions of windows. This does not require a DOS box, although it is text based. Feel free to browse the rest of the site.
The generated programs have not been test run and this program generator is provided only for your comments and suggestions, of which I hope to receive quite a few. Use the email in the header to contact me.
Based on user input information such as pitch diameter, number of teeth, spindle resolution, desired X step increment, gear face thickness, etc. three programs are generated:
O02.cnc which cuts two opposing spaces for checking over pins for adjustment.
O099.cnc which cuts all the spaces.
O0314.cnc which strokes the Z axis in and out, rotates the spindle and increments X.
O0314 is a sub program called by both O02 and O099.
This is an alpha(?) test level (v0.3c) of a program generator to allow you to use a CNC lathe with indexing spindle as a light duty low volume gear hobber/shaper.
One big advantage is that it will generate the correct involute (curved) profile using only a rack profile [straight sided] single point tool that can even be hand ground for emergency use, but certainly on any surface grinder. Only the side angles and the flat at the tip are critical.
It is assumed that a knee type tool holder will hold the tool above the part at a distance of 1/2 the pitch diameter to the Z axis and the tool is at right angles to the Z axis. In use the tool is positioned (and part zero is) slightly behind the rear edge of the gear blank and in line with the front face. Z axis travel is used as the ram of a shaper, and the tool is fed to the front of the lathe in synchronization with the blank rotation to generate the involute profile.
You should be able to cut involute splines on a shaft although a relieving grove at the end of the stroke will be required.
This will be slow but eliminates a second operation and possible outside processing. It also eliminates the need for special gear cutters, and the long lead times for "specials."
If I have just reinvented fire, please let me know.
Enjoy
Unka' George [George McDuffee] ===========Merchants have no country. The mere spot they stand on does not constitute so strong an attachment as that from which they draw their gains.
Thomas Jefferson (1743-1826), U.S. president. Letter, 17 March 1814.
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