Author Topic: Chucks Helical Gear Cutting fixture  (Read 12741 times)

Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #30 on: May 15, 2020, 12:23:04 AM »
Hi Chris,
Helical gears are basically spur gears with the teeth at an angle. That's an over simplification but here's why I said it.  A spur gear and a helical gear can have the same diameter pitch (DP) The Addendum and Dedendum are the same. This is based on the spur gear calculations.
For most pairs of helical gears the helix angle is 45 degrees. If both gears are the same hand (twist goes the same direction) then the gears will operate at 90 degrees to each other. If one is left and and one is right then they can be paired like spur gears. (parallel axis)
If one looks at Chucks information they will see how the helix applies to different diameters. A gear having a 45 degree helical angle and 1.00 pitch diameter will have a lead calculated on how far a point moves along the axis from -0- degrees twisting around the PD until that point comes back to the same -0- degrees, or 360 degrees.
Helical gears can be cut with involute gear cutters. The difference between spur and helical is that for a spur gear with a given number of teeth you would use a number X involute cutter (8 cutters to cut the complete range of teeth from 12 to rack) When cutting the same amount of teeth on a helical gear you wouldn't use the same cutter because with the cutter at an angle to the axis it cuts a wider swath through the gear blank so there needs to be compensation so that the gear tooth spacing and profile would match a spur gear.
When you want to change the ratio between 2 spur gears you just change the tooth count on each gear. With a helical gear just by changing the helix angle will change the tooth count for a given diametral pitch.
This is a simple explanation of the two types of gears but once you have gone through the calculations it all makes sense.
gbritnell
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Offline crueby

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Re: Chucks Helical Gear Cutting fixture
« Reply #31 on: May 15, 2020, 12:49:32 AM »
Very interesting, thanks. Makes me want to cut some to get first hand interaction to really get an understanding. That fixture of Chuck's is makeable and usable without CNC, right?

Offline Craig DeShong

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Re: Chucks Helical Gear Cutting fixture
« Reply #32 on: May 15, 2020, 01:28:45 AM »
Also, you need to wrap the ‘angle piece’ clockwise or counter clockwise around the fixture depending on whether you want a right hand or left hand gear.

I use mine on my vertical mill, not CNC,  you need to experiment with some throw-always to get the procedure down.  It’s easier than you might think to use, but it’s very repetitive in use and let your mind wander for a second or two and you will probably end up with a ruined gear.
« Last Edit: May 15, 2020, 01:32:27 AM by Craig DeShong »
Craig
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Offline Don1966

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Re: Chucks Helical Gear Cutting fixture
« Reply #33 on: May 15, 2020, 01:44:20 AM »
Love the explanation George and to all who uses the spreadsheet it calculates the cutter to use for you. The helix gear calculation sheet can be found here... http://www.modelenginemaker.com/index.php/topic,4871.0.html with it is Chuck Fellows helix fixture PDF. THE SHEET ALSO CALCULATES THE TEMPLATE FOR CHUCKS FIXTURE.

REGARDS Don

Offline jadge

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Re: Chucks Helical Gear Cutting fixture
« Reply #34 on: May 15, 2020, 09:10:22 AM »
Cutting a helical gear with an involute cutter is inherently inaccurate as the helix angle of the gear varies across the tooth depth. A cutter running at a fixed angle will cut at that angle over the whole tooth depth. Of course, in practice it often doesn't matter and the resultant gears are fine. One of the Brown and Sharpe books on gear cutting gives some advice on tweaks to improve the accuracy, usually by setting the cutter at slightly less than the helix angle.

None of the above is anything to do with the cutter number to be used. The cutter number changes from that used for a spur gear (helical gear with a helix angle of zero) because the teeth on the normal diametric pitch are on an ellipse, not a circle. The local radius of the ellipse is larger than the radius of the blank. So the equivalent number of teeth is larger than the number of teeth on the actual gear. The cutter number is selected according to the equivalent number of teeth, not the actual number. The equivlant number of teeth is the actual number of teeth divided by the cosine of the helix angle cubed:

N' = N/cos3(A)

Cutting a helical gear the old school way using an involute cutter with a universal mill and dividing head:



Andrew

Offline Jo

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Re: Chucks Helical Gear Cutting fixture
« Reply #35 on: May 15, 2020, 09:14:17 AM »
None of the above is anything to do with the cutter number to be used. The cutter number changes from that used for a spur gear (helical gear with a helix angle of zero) because the teeth on the normal diametric pitch are on an ellipse, not a circle. The local radius of the ellipse is larger than the radius of the blank. So the equivalent number of teeth is larger than the number of teeth on the actual gear. The cutter number is selected according to the equivalent number of teeth, not the actual number. The equivlant number of teeth is the actual number of teeth divided by the cosine of the helix angle cubed:

N' = N/cos3(A)

The cheats amongst us look it up on a chart  :-X

Jo
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Offline Zephyrin

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Re: Chucks Helical Gear Cutting fixture
« Reply #36 on: May 15, 2020, 09:28:38 AM »
I did the Chuck Fellows setup to cut tiny helical gears,(0.5 Module), and it worked a treat...
I simply used the tool post for setting the height of the blank, but the useful height range is limited under the lathe mandrel as compared to the mill setup of George Britnell.

<a href="https://www.youtube.com/watch?v=wr6H7kfVFp8" target="_blank">http://www.youtube.com/watch?v=wr6H7kfVFp8</a>
<a href="https://www.youtube.com/watch?v=WKbYn27H9HM" target="_blank">http://www.youtube.com/watch?v=WKbYn27H9HM</a>

Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #37 on: May 15, 2020, 02:07:45 PM »
I have always been fascinated with making gears. As silly as that might sound I just find that to make them and have them work is very satisfying. I bought the book, Gears and Gear Cutting by Ivan Law, I downloaded scans of several books that are public domain on the internet. One is the Cincinnati-Milicron 100 years book, another is the Navy Machine Repairmans book and a third is The Treatise on Milling.
All of them have extensive coverage of gear making from spur to worms.
I cut 90% of my spur gears and generally with home made cutters.
My first association with making helical gears came way back in the late 80's when I was building my 302 V-8 engine. While designing the cam, block and distributor I knew that the distributor and cam had to run at the same speed but couldn't get my head around why the gears were 2 different sizes. They had the same tooth count but the P.D. was different. It was then that I started studying what helical gears were.
When Chuck Fellows came up with his home shop fixture I knew I had to make one and try it out. The first application was when I was building my Ford 300 inline 6 cylinder engine. I wanted to replicate the engine as much as possible which meant placing the distributor mid block. This negated the use of miter gears but as with the full sized engine would require a pair of helicals. Fortunately the gears are at right angles to each other so only one radial template was needed.
<a href="https://www.youtube.com/watch?v=B6AFWrWV7vU" target="_blank">http://www.youtube.com/watch?v=B6AFWrWV7vU</a>
The next time I made helicals was for my flathead engine. I tried to replicate the flathead as much as possible which meant making all the internal gearing helical. There are 3 gears which drive the oil pump, the 2 timing gears and then the distributor drive gears.
This meant making several different helical templates. Whenever the P.D. changed it needed a template. Whenever the gears needed to run parallel or at 90 degrees this required separate templates. This also necessitated making a couple of different index plates.
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Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #38 on: May 15, 2020, 02:11:33 PM »
Not wanting to modify or make fixturing for my lathe I used my helical fixture in my mill vise.
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Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #39 on: May 15, 2020, 02:20:35 PM »
This is the gear set for the oil pump drive on the flathead engine. On the right is the gear that goes on the camshaft. In the center (brass gear) is the intermediate gear which provides the spacing to get over to the oil pump centerline. The gear on the left is the right angle helical which mounts to the oil pump drive shaft.
The next picture is one of the home-made cutters that I used for cutting the gears. The profile of the cutter was created using one of Don's worksheets.

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Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #40 on: May 15, 2020, 02:28:08 PM »
The last set of pictures is the helical gearing that I used for the Maudslay marine oscillating steam engine. This was another of Chuck's many projects. When the drawings were published I scaled them down by half and when compete I thought that to add a little more interest to the engine I would make a tiny governor, not a working one but just the spinning mechanism.
Another set of helical templates were needed because these gears have 2 different helical angles.
Thanks Chuck for all you contributed to the model engineering community. You are missed!
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Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #41 on: May 15, 2020, 02:31:12 PM »
When you get down this small with a governor there's just not enough weight and mass to make it fully functional, at least not at slow speeds.
<a href="https://www.youtube.com/watch?v=1EKaYGyOmEU" target="_blank">http://www.youtube.com/watch?v=1EKaYGyOmEU</a>
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Offline crueby

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Re: Chucks Helical Gear Cutting fixture
« Reply #42 on: May 15, 2020, 02:55:08 PM »
With Chuck's fixture, it looks like you need to keep the moving center portion pushed up against the guide wheel as you turn it to make the cut, correct? Or is there some other bit that keeps it in place? The feed rate must take some 'feel' to get right.

Offline gbritnell

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Re: Chucks Helical Gear Cutting fixture
« Reply #43 on: May 15, 2020, 03:25:58 PM »
Chuck had a spring on his fixture to keep constant load against the roller bearing. I decided to try it without it and just make sure that I always has pressure against the wheel. It worked fine. The only thing you have to remember is to conventional cut otherwise the cutter will pull the blank into it.
gbritnell
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Offline stefan-k

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Re: Chucks Helical Gear Cutting fixture
« Reply #44 on: May 16, 2020, 08:03:59 PM »
So, with the help of all the nice guys here i managed to finish my fixture and cut the very first Helical gear with it. Attached you find some pics of my Fixture and the Helical gear i just cut. Next gear will be the matching one and then we`ll find out if they run together. This gear has a 34 DP or Modul 0,75 , 12 teeth and 60° Helix angle. The matching one will have 34 DP, 24 teeth and 30° angle. At the moment i have only 34 DP involute gear cutters at Hand. Means I´m a little limited in making gears.
How can i make my own gear cutters? specially for small gears with less than 12 teeth.? the involute ones are a little pricey......

Stefan
« Last Edit: May 16, 2020, 08:08:07 PM by stefan-k »

 

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