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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


(August Lehman does it again! His next article (which was his first article) delves into the secrets of an unknown hobber. Ultimately he reveals all, even his hobber's differential constant. He uses his universal milling machine and geared index head. Even if you know all about your hobber, you can still appreciate Mr. Lehman's next three articles where he uses ordinary tooling and unordinary insight to make good things happen. - Ed.)

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