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Changing Hobs [Re-Zeroing and Synchronizing]

If you have a tool and cutter grinder that holds the arbor from your gear hobber, you can simply remove the
arbor with hob, sharpen the hob, restore the arbor and continue hobbing (more-or-less). However, in many cases
you will need to change hobs in the course of making a single gear. This article deals with the situation
when you change hobs while making a single gear. In particular, this article deals with re-zeroing and
synchronizing a hob for the final finishing cut.
Let us say you have just removed the rough-cut hob and changed it with a new, freshly sharpened hob. The
problem is how to get the newly installed hob back to zero and synchronized with the gear you are making.
Re-Zeroing:
First, you cannot assume the rough-cut hob and the finishing hob are the same diameter. Their diameters may
differ by as much as 1/8 of an inch. So you must re-zero the outside diameter of the hob to the outside diameter
of the gear. To do this you must align the tips of the hob teeth to the tips of the gear teeth.

To align the hob teeth tips with the tips of the gear teeth, you must be able to move the work table (the gear)
independently of the index position of the hob. In other words, you must be able to rotate the work table
without moving the hob. You will rotate the work table (the gear) until the hob teeth are aligned with the tips
of the gear teeth, see picture below.



To align the hob teeth and the gear, your hobber must have a means of disengaging the index and rotating the
work table independently of the hob.



(If your machine does not have the index lock/unlock and crank, this procedure is still very doable. You
can unmesh the index gear train to unlock, and manually rotate the final driver gear. This will functionally
unlock the index so you can rotate the work table.- Ed)

With the Index Release in the unlocked position, you are free to rotate the work table, clockwise or counter
clockwise. You now rotate the work table to achieve the desired alignment.

The gear ratio of the crank to the work table will be very large. It may take several thousand turns of the crank
to rotate the work table 1 turn (360-degrees) - not that you would ever want to but be aware the work table may
not seem to move (rotate) when you turn the crank. Just keep cranking and the movement will become apparent.

When you think you have the desired alignment, lock the index release and run the hobber. Backlash in the gear
train may shift the alignment you thought you had; just repeat the procedure until you are satisfied the hob
teeth tips are aligned with the tips of the gear teeth, (see first picture).

Next very slowly bring the work and the hob together until the hob teeth just mark the surface of the gear
teeth tips. This will be your new zero point for setting the depth of cut.

SPECIAL NOTE:
In the first picture you will see the depth of the zeroing-cut is more than necessary. This was intentionally
done for the article BUT ALSO; this provides an EXCELLENT (and fast) check on the concentricity of the work.
If the work (the gear) has shifted during hobbing, the length of the cuts will not be the same. On one side of
the gear, the cuts will be short and on the opposite side, the cuts will be long. The visual nature (difference
in length) of the shallow cuts allows you to instantly determine if the gear is concentric with the axis of
rotation (to the work table).

Synchronizing:
Now that the hob has been re-zeroed, the hob must be synchronized with the teeth of the gear. This is not a
simple process. Let us assume the depth of your final (finishing) cut is 0.005-inch and the included angle of
the hob tooth is 20-degrees. The tips of the hob teeth will cut 0.005-inch deeper but the flanks (side) of the
hob teeth will only advance into the face of the gear teeth by the sin of 20-degrees = 0.342; meaning the depth
of cut on the face of the gear teeth is only about 0.002-inch. It is very easy to cut down just one side of the
gear teeth on the finishing cut unless you are careful or have a deep finishing cut.
First, use your index release and crank to visually align the hob teeth to the gear. Relock the index release,
run the hobber and check the alignment while the hobber is running. Repeat the procedure you are satisfied with
the first pass synchronization.

Now bring the hob and work together some safe distance where you are sure the hob is not going to cut into the
gear. Now work with your index release and crank to achieve a better synchronization. Keep bring the hob and
work closer until you are close as possible, without cutting the gear.
Now refer to next picture.



Place tape on the adjacent flanks of two adjacent teeth. This tape will give you
a standoff (margin of safety) for bringing the hob and work closer without cutting the gear. You will probably
want to tape the gear every 90-degrees so you do not have to wait so long to see the results (of bringing the
hob and work closer).

Very slowly bring the work (gear) and the hob closer, say 0.001-inch at a time. Check your tape and keep
advancing 0.001-inch at a time. At some point, the hob will cut into the tape.



At this point you are close, so use very small increments. You will see the cut is on the right side, so the
work has to be rotated counter clockwise. With your index release and crank, try one turn of the crank and relock.
Run the hobber and look for cuts in the left side. Keep repeating this procedure until you get a cut on the left
side. Remember to count the number of turns. When you get the first sign of a cut on the left side, bring the
work table back clockwise 1/2 of the number of turns. You can color the tape with felt-tip pen to ‘heal’ the cuts.



and very slowly, bring the hob and work together no more than 0.001-inch at a time; look for cut
marks. If your tape is say 0.005-inch thick, then a total of 0.003-inch should be safe. If, after 0.003-inch, you
have cut marks on only side, back up and repeat this step. When you have cuts on both sides of the tape - you
are almost done.

Now, spray the contact zone (360-degrees) for the hob teeth with fast drying paint.



You can use die blue but it is expensive and paint is thicker. Now bring the hob and work together no more than
0.001-inch at a time. Look for the hob to scrap the paint off of one side, as in the picture above, the left
side. This means the work table needs to be rotated clockwise.  Again, with the index release and crank, turn the
table clockwise no more than 1/2-turn of the crank. Keep at this 1/2-turn at a time until you have scrap marks
on the right side. Remember to count the number of turns. Now back up (counter clockwise) the work table 1/2 of
the number of turns. Your hob is now synchronized. Back the hob out (away) from the work and move it to the
starting position. Move the hob and work together for the final depth of cut based on the re-zeroed position.

A few notes:
The index release will not relock at just any position. Depending on the design, it may relock every 360-degrees,
every 180-degrees or even every 90-degrees. DO NOT FORGET TO RELOCK THE INDEX RELEASE. If you do, the hob will
try to cut a band through all of the teeth - and break something along the way.
Be careful about using heavily sharpened hobs just before the finishing cut.  Hob teeth get shorter as they are
sharpened. Very short hob teeth may cut so much off the sides of the gear teeth that nothing is left (on the
sides of the gear teeth) for the final cut. The tip of the hob on the final cut will cut the new (additional)
depth but cut nothing on the sides of the teeth.



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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