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Helical gear cutting-tools

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gbritnell:
BACKGROUND:
When I taught apprenticeship classes for the machine trades one of the books we used was titled 'Technical Drawing'. There is a section in the book devoted to gears, mainly slanted toward drawing gears when making a drawing but with other technical information. One of the charts is titled 'Wellman's Involute Odontograph. When I got into making my own gears I used the calculations in the chart for creating the profiles for my gears. My earliest engines that used gears were the hit and miss type and usually the gears were included with the castings. It wasn't until a few years later that I started making my own.
I purchase the book 'Gears and Gear Cutting' by Ivan Law. The book along with online information built my knowledge of gear cutting. One of the recommended ways of making the cutters is by the button method whereby a holder and hardened steel buttons are made to form involute curve on the gear tooth. I only used this method twice because of having to make buttons for every range of teeth, much like a set of involute cutters. Instead I made my own cutters, which are a hob type used only for making spur gears.

TECHNICAL:
We'll stick with the tooling for cutting only helical gears. Some of the following information you probably already know but it's worthwhile so I'll repeat it. Let's start with what is a helical gear. A helical gear is a spur gear with the teeth arranged in a helix or spiral around a hub. Helical gears can transmit motion at varying axis angles depending on the angle of the mating helixes. Two 45 degree helixes will produce an exact right angle but combinations of 80/10, 70/20 and so on will also create a right angle. The two gears need to be of the same hand, meaning 2 rights or 2 lefts. Opposite hands would be used for parallel shafts. Pictures #1-2.
Helical gear teeth are the  same as spur gear teeth in their form and construction. A 20 D.P. gear tooth form would be the same for both. The difference is the spacing of the teeth along the cylindrical shaft.  Spur gears teeth are measured from tooth to tooth around the pitch diameter. Helical gear tooth spacing is measured the same way except the teeth aren't at right angles to the pitch diameter. A 20 D.P. spur gear of 20 teeth will have a pitch diameter of 1.00 inch and the circumference (3.1416) divided by 40 (teeth and spaces) will yield a width of .075,  but the same D.P. and tooth count on a 45 degree helical gear will have a pitch diameter of (teeth and spaces) .111 x 40 = 4.443/pi = 1.413. When cutting a spur gear with an involute cutter the cutter selected will be for the proper tooth count of that gear but when cutting helical gear compensation needs to be made as to which cutter to use. The reason being that although the helix might be at 45 degrees there is no straight line formed that is 45 degrees. The helix at the centerline of the gear is a developed curve bending toward and away from the theoretical 45 helix so with a standard involute cutter you would be cutting more from both sides of the standard tooth form.
When doing all my research on cutting helical gears I came across 2 books online that are very complete in explaining the cutting of helical gears and the compensations that are needed.

CUTTER:
First off I didn't want to invest in buying a set of involute cutters for the occasional gear cutting that I do. Along with that I didn't feel comfortable with a 2.00 cutter cutting a small gear in the Chuck Fellows fixture so I make my own. I use .375 diameter drill rod/silver steel. I use O-1 which I find machines a little easier than W-1. Going back to the background section I use the Odontgraph and my CAD to develop the involute curve. Rather than making the button cutters I just grind a lathe tool blank with the appropriate curve on it. I calculate my measurements, infeed and spacing and cut the drill rod. A rigid setup required cutting close to the chuck or collet because there is a lot of load on formed with the cutter. After the teeth are formed I transfer the shaft to the mill and gash it to form the individual teeth. Then using my headband magnifier and hand grinder I grind away some of the material behind the cutting edge for clearance. The cutter is then hardened. I generally don't normalize the cutters and they have held up quite well. I have used these cutters to cut drill rod and they work fine for a small number of pieces but don't have the proper integrity that a proper tool would have.

crueby:
This is going to be good!   :popcorn: :popcorn: :popcorn:

gbritnell:
As a follow up to my last post by using the small diameter cutters I don't make compensation for the shape of the cutter because it's so small that it doesn't skim the teeth like a 2.00 cutter would.
I tried to post a link to the books but the files are too large. One is a Treatise on milling by The Cincinnati Milling machine company and the other is The Navy Machinery Repairmans book.

crueby:
This page has the milling treatise book:
http://vintagemachinery.org/pubs/detail.aspx?id=3873
The other book is available here:
https://www.maritime.org/doc/pdf/machinery-repairman.pdf

gbritnell:
Thanks Chris

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