Machinist

[Lawrence London <lfljvenaura@gmail.com>]

http://www.practicalmachinist.com/vb/general/sharpening-involute-gear-cutter-274710/

1. sharpening an involute gear cutter

   I usually hang out in the antique machine sections since that is all I own. I recently made two gears for my lathe. I used steel for the gear material. The gear cutter is a 10 DP 20 PA #4 cutter. I noticed that it took longer to cut the last three teeth of the last gear, and figured out that the tips (or points) on the cutter were beginning to lose their edge.
   
   How are these cutters sharpened? Can they be 'touched up' at home or do I have to send the cutter somewhere to be sharpened?

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2. 11-05-2013, 10:56 PM #2

   L Vanice

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   The flat face of each tooth has to have an equal amount of material ground off, keeping the angle the same as the original grind. You have to remove material until all of the wear on the sides of the teeth is gone. It requires a rather simple fixture and some sort of grinding machine like a tool and cutter grinder or a surface grinder. You could adapt other machines and other types of grinders. It is up to you to get creative, or decide to pay an expert.
   
   Larry

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3. 11-05-2013, 11:38 PM #3

   blake in spokane

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   Many moons ago at a shop I was told to index off a good area of the front side of a profile/gear cutter - lightly grind the back side till even - then index off the back & grind the face evenly. Just my 2 cents worth!

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4. 11-05-2013, 11:43 PM #4

   stephen thomas

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   This has been done a few times in the past year or so.
   
   The important note on any form cutter but especially gear cutters, is to first go around (on the T & C grinder or set up on the surface grinder) and kiss the back of each tooth, using a finger rest on the face. Keep going around and be just as particular as if it was being sharpened. When there is an equal flat on the back of every tooth, the cutter can be put back on the fixture, or between centers, now with the finger on the back. Set the correct angle (probably neutral, but maybe not. be sure to verify) If the face angle is not correct, the form will not be correct on the work.
   
   There are a lot of simple ways that could be rigged to do the face, but the method needs to be precise for angle, and it needs to repeat every tooth the same.
   
   Edit: Looks like Blake types faster than me! but we both learned the same method.
   
   smt

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5. 11-05-2013, 11:48 PM #5

   sfriedberg

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   The key point in Larry's post is that form cutters, like involute gear cutters, are not sharpened the same way as most milling cutters. If the cutting faces were radial (perpendicular to the bore), they need to stay radial, so you are grinding off a narrow wedge, wide at the circumference tapering to zero at the bore axis and you need to grind the same amount off each tooth. That second condition is normally met by indexing off the back of the teeth.
   
   If the cutting faces were not radial, because the OEM wanted some rake on the teeth, then you need to figure out the base circle the faces are tangent to, and resharpen the faces so they stay tangent to that circle. Unfortunately, the OEM's usually stamp the original rake angle on the cutters, when what the resharpener needs is the base circle diameter!
   
   Cincinnati had a fairly simple fixture for grinding form cutters which could be approximated in your shop without too much trouble. I'm going to assume you will use a thin disk wheel on a grinder to reach between the cutter teeth. Arrange a short vertical arbor on your grinder table for your cutter to rotate on. Put a shoulder or spacer on it tall enough that the disk wheel can reach the entire tooth while clearing the grinder table top. Cross feed the grinder table and rotate the cutter so a tooth face aligns with the business side of the disk wheel. That initial alignment step is important to preserve the shape of the cutter teeth! Rotate (not cross feed!) the cutter slightly further, just the amount you need to sharpen the teeth. Now arrange for an indexing finger to hold each cutter tooth in that position successively. Hold each tooth in the index position and advance the tooth into the disk wheel. Retract and index to the next tooth. Cincinnati's fixture had the additional features of letting you "dial in" the additional rotation in case you had to take a 2nd pass, and the arbor could be tilted right or left to accomodate cutters with side rake (mostly not on form cutters, though.) Neither feature is essential for a shop lash-up.