Cutting Spur Gears -- A neat technique

Several years ago, I did a series on cutting spur gears using the methods in the “Workshop Practice Series” book #17, “Gears and Gear Cutting” by Ivan Law:



Mr Law's method works great. You can follow his techniques without deviation. However, recently I found another method to cut gears that approximates involute tooth using only one cutter to form gears of any size within the diametral pitch. The method is described by "Helicron" at his website.

http://www.helicron.net/workshop/gearcutting/

I followed Helicron's technique with good results.

First step is to make the cutter tool. My lathe is not rigid enough to make the cutter as Helicron shows. Instead mine is a single point cutter with most of the blank removed before forming the cutting teeth.



The steel for the cutter is alloy “O1”. O1 is the most widely used tool steel and is available from McMaster Carr and other suppliers. O1 is very easy to work with and harden. Shape and form the tool to the desired size. Heat with a torch to 1400 degrees F, which is cherry red heat in room light. Plunge in oil to quench. I used some leftover salad oil from the kitchen. Leave in the oil quench about 30 seconds, it cools down slowly. Quenched tool steel is too brittle at this point and needs tempering. To temper, suspend the tool in a small pot of salad oil on the kitchen stove. Heat to 350 degrees F using a candy thermometer to check the temperature. Leave it in about 10 minutes, pull out and let air dry. The O1 steel comes out very hard and will cut just about anything.

For an upcoming project I want to cut some 48 diametral pitch gears with 52, and 16 teeth. First, the 52-tooth gear. These are from 1/8” thick flat brass bar. First, make a shank from length of 1/2" diameter steel in the lathe cut and turn a 5/16” diameter boss. Thread the end to accept a nut. Gang up four pieces of brass stock on the shank.



Turn the ganged stock to the final diameter for the gear. In this case 1.125"



To cut the gears I used my little, homemade Harold Hall dividing head. It is a “direct indexing” head that uses the change gears from the Atlas 6” lathe for indexing. The spindle hole is tapered with #2 Morse Taper (2MT). A 2MT collet holds cinches stock tightly in the dividing head be means of a draw bar.



Secure the 52-tooth lathe change gear to the spindle of the dividing head. Mount the dividing head to the mill table. Use a 1/2" 2MT collet to chuck the gear blanks. Chuck the gear cutter in the mill spindle and use a height measure device to set the gear cutter height.



Adjust the depth of cut and start cutting. depth of cut on 48DP gears is 0.045". Advance the dividing head spindle one notch on the indexing gear.



Finish the 52 tooth gears by soft soldering an appropriate hub. Drill and tap for a set screw.



Next time we will cut the 16 tooth gear and do something with the results.

I've used (and recommended) the same site and techniques in the past and I agree - very useful, simple and great results, all without spending money on a set of cutters.

Thanks Al.

The 16-tooth pinion cuts in the same general manner as the 52’s. The notable difference is that the hub is integral, rather then hub less.

Change out the 52 tooth indexing gear on the dividing head and replace it with a 32 tooth. During machining, skip every every other tooth to get 16 divisions.

A 16 tooth, 48DP gear has a rough stock diameter of exactly 3/8”which is convenient. Use a 3/8” collet to chuck some 3/8” stock. Cut as before.



With the dividing head set up I did some 32 tooth gears too.



Chuck the 16 tooth gear in the lathe and turn a hub. Tap for some set screws and it’s done.



These gears are going into a scaled up Ernest Glaser “Cracker”



They mesh perfectly. The chassis rolls smooth as silk.



Observations:

- Helicron's method is a great way for cutting gears. Simple to follow, simple to do. A single cutter rather then a set of 7 or 8 is so much better.

- My lathe is not rigid enough to make the cutter blank as he shows. So I went with the single tooth cutter. Perhaps feeding the lathe bit in at an angle would be better then plunging straight in as I did.

- Overall grade: A-

A silly question maybe .
The helicron website says to make the cutter using a V shaped lathe tool
with an included angle of twice the pressure angle .

What determines the pressure angle of a gear or a set of gears ?

I know about , and have made both Module and DP gears in the past
using std involute gear cutter sets and a dividing head .

I also know what the pressure angle is , in short : the line tangent to the contact point of 2 meshing teeth , and the gears center - centre line .

But I've never used or needed the pressure angle when calculating gear blanks
and or machining them .
For a given nr of teeth and mod ( or DP ) it's just a few formula to calculate od and depth of cut .
Depth of cut can usually also be found on the cutter itself .
So where does that pressure angle come in ?



Pat

Pressure angle is arbitrary, but standardised. Your standard gear cutters could be 14.5° or 20° degree cutters. They should be marked accordingly. They should not be mixed because the two standards produce different tooth profiles which will not mesh properly. Anyone making their own cutter needs to know. 20° is the modern standard.

So if I understand this correctly it is quite possible to make a
Z40 MOD1 gear with a pressure angle of 14.5° ,
and it is also possible to make a
Z40 MOD1 gear with a pressure angle of 20° .

And altough they'll both have a correct involute profile they won't mesh , or won't mesh as they should .


Didn't know that , I've always been told that any gear of thesame Mod or DP can work together .

Pat

I've been using this method of cutting gears for the past month or so and it works great. A couple thoughts come to mind. If you look closely at the picture in post #3 you'll note that while you have a bunch of cutting faces only 3 contact your gear blank. Thus you only need 3 cutting faces, one of which must be accurately centered on the gear blank to create the proper tooth shape.
The second thought is that you have only one cutter to engage the gear blank per revolution. I'm a newbie so take what I say with several grains of salt but when I tried a single cutter per revolution I get a lot of vibration. When I made a cutter with the 3 protrusions I used a Dremel with an abrasive wheel to make many cuts which forms many cutting teeth, much like a slitting saw might have. When making gears from Delron this make a very smooth milling as each tooth only needs to take a very small bite.

The pressure angle is also the angle of the face of the tooth on a straight rack which has straight sided teeth.

Steeper pressure angles make for stronger teeth,as the forces are directed more to the center of the gear and less shear, but result in higher forces pushing the gears apart, trying to bend the shafts. I have seen some using 30 degree pressure angles.

If you have a gear with the proper pitch/pressure angle that is hardened you can inprove the tooth form on your cut gears by running them together to burnish your new gears.

RM-MN said:

If you look closely at the picture in post #3 you'll note that while you have a bunch of cutting faces only 3 contact your gear blank. Thus you only need 3 cutting faces, one of which must be accurately centered on the gear blank to create the proper tooth shape.

Click to expand...

This is likely a function of the size of the gear being made. I've used a 5 tooth cutter of the same design and had all 5 teeth engage, although the outer 2 teeth weren't cutting a whole lot. They did provide an extra facet on each tooth though, so they contributed something to the overall shape.

Sorry to bring this up but a single tooth cutter wont work unless the involute form is cut into the cutter.You need a min of 3 teeth if they are cut as a rack form,the 2 adjacent teeth also take a nibble as the central tooth cuts to approxmate the involute form

bazmak said:

Sorry to bring this up but a single tooth cutter wont work unless the involute form is cut into the cutter.You need a min of 3 teeth if they are cut as a rack form,the 2 adjacent teeth also take a nibble as the central tooth cuts to approxmate the involute form

Click to expand...

You are right about the involute form needed for a single tooth cutter. Fortunately that form is easily made. http://mikesworkshop.weebly.com/making-gear-cutters.html

If all the gears in your train are made with the same cutter then the pressure angle of the tool won't matter and the gears will mesh.

I did not mention it, but my forming cutter is 30 degree total for a 15 degree pressure angle. The forming tool was made using the "V" notch in a screw cutting gauge as a guide. I guess that's close enough to 14.5 degrees.

I had no idea how many faces to put in the cutter. It has 8 (?). You can't see in the picture, but 5 faces engage the 52 tooth gear, the 2 outer very slightly. 3 fully engage the 16 tooth gear.

I have to experiment more making the cutters.

These are the best close up pics I could get. Comparing these to a commercial 48 DP, they are very close.



On the pinion, you can see the flat spots from the approximate involute. Even with these flat spots, they run well.