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Aligning the Intersection of a Herringbone Gear

This article deals with aligning the intersection of the teeth of a herringbone gear to the midpoint between
gear sections.
Ideally, the teeth should meet exactly at the midpoint. This midpoint is usually marked as a small groove
halfway between herringbone halves. This groove is axially located in reference to bearing mounting surfaces
to assure proper location of gear member in the gearbox.
In practice, it is seldom necessary to achieve perfect alignment at the hobbing machine since ideal tooth
contact at assembly usually requires shimming bearings to shift pinion and/or bull gear. However, if 'best'
herringbone alignment is achieved when hobbed, then maximum teeth in contact can be expected. The following
procedure will accomplish 'best' herringbone alignment and therefore maximum service.




This picture shows a herringbone still on the hobber with the Right Hand gear completed. The bottom gear blank
must be cut Left Hand so that RH and LH teeth will intersect at the midpoint. The groove shown references the
desired intersection.



Picture above shows the center groove where the teeth should intersect. Obviously, the location of the center
groove must be correct for the intersection to be correct as well.
After finishing the first half of the herringbone, a set of shallow cuts are made. Depending on the diameter of
the center section, you may need to move the table slightly toward the cutter to make these marks. Be careful not to
go into the uncut blank if possible.



The shallow cuts can cross the center groove or just come up to it, as in picture above. These shallow cuts and
the center groove provide the reference for aligning the other half of the herringbone.

Now you want to turn the pinion over to position the uncut portion as the upper half. Reverse the hob table angle,
correcting for hob cutter angle (a RH cutter cutting a LH gear : Table angle = Helical angle + Cutter angle). Reverse
the differential rotation for LH cutting.
Next scribe two vertical lines in the exact center of two adjacent teeth. Then, scribe a third line exactly between
the two lines. The center line at face of gear blank is the ideal location for center of tooth space.



When you slightly touch the surface with the cutter, there is only one hob tooth doing that "marking". This tooth,
closest to the gear surface, is the only tooth that will operate at full depth. Thus the "marking" tooth locates
the root of the finished tooth space. If we identify this "marking" tooth we can align it with the center scribe.

Here is a procedure that, when done properly, will not leave a "mark" on the finished gear. Start with cutter at
a slight distance from the OD and near the top of scribe mark. (see picture above). You want to be sure your cutter's
"marking" tooth is over the OD away from an edge.

Find the cutter's "marking" tooth by LOOSELY holding a piece of paper between the cutter and work OD while the
machine is running. Use a full sheet of paper - KEEP HAND AT SAFE DISTANCE! Slowly move table toward cutter until
paper is taken gently from your hand. Now you know that the "marking" tooth is approximately 0.001-.003 from work.

This pinch-point is likely between the "marking" tooth and maximum runout of gear. If runout of gear is greater
than .003, you should double (or triple) the number of sheets of paper to avoid marking the gear surface.

Start machine and bring work around to where outer scribe line will be near or just starting to roll under
cutter. Turn machine off and pull machine by hand (hopefully you can access the main drive belts) until "marking"
tooth is closest to work surface (use sheet(s) of paper again). This should require less that one cutter revolution.
Avoid "jogging" machine (quick power on/off) since you can't control backlash or tooth position. At this time
you are not concerned about location of scribe line. The idea is to position the closest cutter tooth over the OD
of workpiece.

With power still off, unlock index and crank worktable around until center scribe line is aligned under "marking"
tooth. Lock index as close to this position as possible.

Disengage and lock the differential before this next step. You want to move the cutter table down to the gear
face where the start of LH tooth would be, but you don't want the table to rotate at all. When cutter is in position
over start of imaginary tooth space, re-engage the differential before starting or moving anything.

Now you can run the machine and barely touch the workpiece to verify that cutter is aligned with center scribe.
Remember, this new mark should be at the face of the gear and exactly across the gap from a RH tooth space. If
gear needs CW or CCW adjustment, stop machine, unlock the index and crank table to compensate. Relock index and
"mark" again under power.

Any cutter marks made at this time should be well within the future LH tooth space. When satisfied, back cutter
away from gear, move cutter to "safe" starting position, adjust cutter to proper depth, and cut gear.
This alignment should be adequate for most herringbone work.


As an alternative, you can use "marks" to get visual intersection of LH and RH teeth without using scribe lines.
This may be the most accurate method, and probably just as fast, and maybe less prone to making a mistake -
you decide.

Bring the hob into position just above the center groove. Make a set of shallow cuts on the opposite side of
the center groove.



You will note the teeth do not intersect at exactly the center goove - very close but not exactly. This is the
degree of accuracy you can expect from the scribed line procedure above and again, this is sufficiently accurate
for almost any application.

To make the intersection exactly on the center groove, you need to rotate the work table relative to the index
position of the hob. (The next article details how to shift the table with respect to the cutter - Ed)
From the picture, you can see the work table (gear) needs to be rotated counter clockwise. With the index release
and hand crank, rotate the work table a very small increment, say one turn. Now, run the hobber. The upper teeth
marks will become wider. Continue this procedure until you achieve "best" alignment. Optical magnification can
be used for even greater accuracy.

Notes:
If for cosmetic purposes, you do not want the shallow teeth marks at the center line, you can use tape and make
the shallow cuts in the tape rather than in the workpiece. Depending on the space between the two herringbone
halves, you may have to infeed a stationary roughing cut to allow the hob to reach the center groove. The scribed
line procedure is more than accurate for this. You may need just enough clearance for the hob to put test marks
near the center groove. Once satisfied with location, the table is withdrawn, cutter is returned to start of cut,
depth is set, and 2nd gear is cut.




Information and photos contained in this article have been generously provided by:

August Lehman,
Managing Director
Lehman Associates
Manilla, Phillipines

Website and contact information



Posted 2/05/20012

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