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We did it.

We did it.

We did it.

ref http://www.eng-tips.com/viewthread.cfm?qid=383133
Our sister company purchased an involute shaped end mill and has cut their first test herringbone pinion on their machine center. The measurements we can check look good but now we need to check the involute curve to see if it is correct. Any ideas about how/where this can be accomplished will be greatly appreciated. Thanks, K.

RE: We did it.

Most reputable gear shops will have CMMs that can test your gear.
Some will even have portable equipment.

Unless the tool designer has allowed for the fact that it's a helical gear; I doubt that the profile will be correct. You won't mill the correct profile if the cutter has been designed for a spur gear. The reason is; as soon as the helical movement is interpolated past the cutter, you are now generating an involute helicoid. The higher the helix angle, the more pronounced the effect becomes.
Theoretically - you'd need a different cutter for different helix angles.

A gear cutting company that I know of found this out the hard way.
They converted their hobbing machine into a gasher, by replacing the hob tool with a form milling cutter. Instead of a continual process, the machine now cut one tooth gap at a time.
The milling cutter utilized replaceable carbide inserts. The form on the inserts was created in-house on a special profile grinder which used a template as the reference for the form to be ground.
One of the mech engineers was given the task of coming up with the form for the template. He used a program called GearCad to create the theoretical profile for a spur gear, not the helical gear they needed to produce.
We're talking about a gear which was about 10 meters in diameter. Fortunately they had portable profile checking CMMs and were able to detect the error while the blank was still on the hobber.
They were scratching their heads for quite some time trying to figure out what it was that was producing the error.

RE: We did it.


Ouch! I hope your company did not make a costly mistake. As gearcutter correctly points out, machining/grinding gear teeth with 3D surface geometry (like your helical gear) usually requires tools with cutting profiles designed to "generate" the proper flank surface, based on the combined motions of the cutter and blank. A form cutter based on a 2D spur gear profile would not produce a correct 3D helical gear tooth profile.

RE: We did it.

Thank You gearcutter and tbuelna for considering and responding to my post. Yes, the helical angle was supposedly taken into consideration by the manufacturer that created the end mill. But even with that manufacturers assurance that the profile they created is correct, everything about this project is new to us so confirming the involute curve is a priority. Since I have never worked with a gear lab anywhere, I was hoping someone had good experience with a gear lab and be willing to pass that info along. Again, Thanks All, appreciate your input. K.

RE: We did it.

kthree -

I hope that your tooling supplier has taken the helix angle into account.
It's a somewhat complicated process, coming up with the correct form of the cutter.
Litvin shows how to go about coming up with the correct form in his book. He calls the type of cutter you've been using a Finger-Shaped Tool.


RE: We did it.


I don't recall seeing any mention of the size, number of teeth, quality requirements, etc. for this DH pinion. When you look for a vendor to perform an FAI on your pinion, you'll need to take these considerations into account.

Once you confirm that your end mill form cutter has the correct profile needed for machining the tooth flanks of this DH pinion, here are a couple other things to watch out for. Most importantly, with the type of cutter and machining technique you are using, tool wear can be a concern. An end mill type form cutter (with fewer cutting edges) will wear faster than other types of cutters. This can result in variation of profile between the teeth on a particular gear if a single cutter is used to perform all of the machining. So you might consider making both roughing and finishing cutters. You might also look at the possibility of adding some cutter wear compensation to your CNC program.

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