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
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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.
Reply to
F. George McDuffee
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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.
Reply to
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.
Reply to
F. George McDuffee
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 ...
Reply to
Nick Mueller
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.
Reply to
Steve Austin
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
Reply to
Mark Rand
============= 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.
Reply to
F. George McDuffee
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.
Reply to
Nick Mueller
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.
Reply to
F. George McDuffee

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