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- Dec 23, 2019
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Helicals are notoriously difficulty to reverse engineer. Even measuring the helix angle is not trivial, and this has a bearing on many other dimensions. The formula for the pitch diameter D, for example is: D = N/(Pn cos@)
Where @ is the helix angle, Pn is the normal diametral pitch (defined as being perpendicular to the tooth face) and N is the number of teeth. Note too that everything is calculated from the pitch diameter, meaning that you need to measure the helix angle at that diameter. It's really, really not easy to do accurately.
Try running some numbers through a calc or two, or consult the Machinerys Handbook to get a feel for it. Your chances of successfully making a high speed, highly stressed gear of that type based on measurement are not high, I'm afraid. Making a pair based on the centre distance you're tied to... that's another matter and probably more achievable, though still a fair task in a home shop. When you know and control every aspect of both gears in mesh, you're in a much better position.
A extract relevant overall from the excellent KHK primer here:
"The helical gears made by KHK can be classified into two groups by the reference section of the gears being in the rotating plane (transverse module) and normal plane (normal module). If the reference section is in the rotating plane, the center distance is identical to spur gears as long as they are the same module and number of teeth. This allows for easy swapping with spur gears. However, in this case, they require special hobbing cutters and grinding stones, leading to higher production cost. On the other hand, if the reference section is in the normal plane, it is possible to use spur gear hobbing tools and grinding stones. However, the same module and number of teeth in spur gears no longer match the center distance of helical gears, and swapping becomes very difficult. In addition, the center distance is usually not an integer."
I don't mean to blunt your enthusiasm, more to hopefully guide your decision on whether it's an achievable job.
Where @ is the helix angle, Pn is the normal diametral pitch (defined as being perpendicular to the tooth face) and N is the number of teeth. Note too that everything is calculated from the pitch diameter, meaning that you need to measure the helix angle at that diameter. It's really, really not easy to do accurately.
Try running some numbers through a calc or two, or consult the Machinerys Handbook to get a feel for it. Your chances of successfully making a high speed, highly stressed gear of that type based on measurement are not high, I'm afraid. Making a pair based on the centre distance you're tied to... that's another matter and probably more achievable, though still a fair task in a home shop. When you know and control every aspect of both gears in mesh, you're in a much better position.
A extract relevant overall from the excellent KHK primer here:
Helical Gears | KHK Gear Manufacturer
Helical Gears have teeth with a helix angle for higher torques and noise reduction. KHK produces various standard/special helical gears.
khkgears.net
"The helical gears made by KHK can be classified into two groups by the reference section of the gears being in the rotating plane (transverse module) and normal plane (normal module). If the reference section is in the rotating plane, the center distance is identical to spur gears as long as they are the same module and number of teeth. This allows for easy swapping with spur gears. However, in this case, they require special hobbing cutters and grinding stones, leading to higher production cost. On the other hand, if the reference section is in the normal plane, it is possible to use spur gear hobbing tools and grinding stones. However, the same module and number of teeth in spur gears no longer match the center distance of helical gears, and swapping becomes very difficult. In addition, the center distance is usually not an integer."
I don't mean to blunt your enthusiasm, more to hopefully guide your decision on whether it's an achievable job.