Constructing a gear (improved version)

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Constructing a gear (improved version)

Postby FixitMike » Sat Nov 15, 2014 9:13 pm

My original instructions for constructing a gear required offsetting the tooth profile to make the root profile. This ended up with a Bezier curve with a corner that gave my controller problems. After a few nights sleeping on the problem, I came up with an improved method which includes a direct construction of the root profile, eliminating the Bezier curves. The zipped file has the complete instructions in a Word document.
Gears (Small).JPG

Drawing a gear in Aspire or VCarve Pro (2).zip
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Re: Constructing a gear (improved version)

Postby PaulRowntree » Sun Nov 16, 2014 12:04 am

Thanks Mike!
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Re: Constructing a gear (improved version)

Postby velvetpig » Sun Nov 16, 2014 9:04 am

Thanks M8, This will come in handy :D
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Re: Constructing a gear (improved version)

Postby FixitMike » Mon Nov 17, 2014 11:26 pm

Note that the parameters I used for constructing gears are per the American Gear Manufacturers Association (AGMA) standards. Metric gears will use different factors and calculations, but the method presented will still work.

The full radius root used is for gears that are to be cut with a CNC end mill. Metal gears are designed around being machined with a hob, and as a result the root contour is different. (Unless a special full radius hob is used.)
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Re: Constructing a gear (improved version)

Postby mattd » Tue Jan 06, 2015 4:37 pm

These are excellent instructions! I've been wanting to machine some gears for a clock project, but couldn't find great instructions. I did some searching for how to draft involute gears, but the instructions were incomprehensible. These instructions are great and they will be very helpful.
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Re: Constructing a gear (improved version)

Postby FixitMike » Tue Jan 06, 2015 5:08 pm

With the way wood can move as the result of humidity changes, I would like to add the suggestion that a large BLA (Backlash allowance) is best to get smooth operation. I made a a planetary set, and had to sand the tooth surfaces to make it run smoothly.

Planetary gears.JPG

The planet carrier is not necessary, but I added it anyway.
Use the same system to draw the internal gear, but add the BLA to the circle radii when you draw the tooth surface, rather than subtracting it.
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Re: Constructing a gear (improved version)

Postby NewAgent45 » Thu Mar 05, 2015 5:03 pm

Help! I’m attempting to make a matched rack and pinion gear for a 4th Axis accessory on a CNC Shark HD. We are carving these in ¾” thick HDPE. The design is much like “Bob’s simple 4th axis” posted on the CNC Shark Forumn. Some dimension changes were necessary to fit my machine and a larger pinion gear was necessary. My first attempt to make a perfectly matched rack and pinion set failed. Close by not a winner and the pinion wants to climb and jump teeth. Not a good thing! <smile> This is my first time attempting making gears. I’m back to the drawing board and looking for help. I have read FixitMike’s drawing instructions and I believe I can follow them for drawing the vectors for the pinion gear in VCarve Pro. Nice instructions Mike!

I will need a pinion gear that has a PD ~= 3.5 inch and I have a little adjustment in the center height over the rack.

Without a full understanding. I’m looking for help. How do I draw the rack to match the pinion?

Have a great day!

……….. Rod
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Re: Constructing a gear (improved version)

Postby FixitMike » Thu Mar 05, 2015 5:18 pm

For a rack drawing, see my post at viewtopic.php?f=48&t=20899&p=148038&hilit=rack#p148041

For your application, I would suggest an 8 DP (28 tooth) gear.

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Re: Constructing a gear (improved version)

Postby NewAgent45 » Thu Mar 05, 2015 9:52 pm

Mike:

Thanks! I'll look at the new information first. I did some trial and error using the Gear Gadget to draw a new pinion gear. Funny! I came up with a 27T 8DP 20 deg PA with approximately a 3.375 inch PD. I'm not sure about using the gadget. Do you have any experience with it? It don't look like it creates a gear with "full radius root". But it may be simpler to use for a novice.

Thanks again!

........ Rod
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Re: Constructing a gear (improved version)

Postby FixitMike » Fri Mar 06, 2015 12:58 am

The gadget produces a gear with a root with sharp corners that you cannot cut with a round cutter. Also, you cannot add backlash allowance or change the dedendum. I suspect that since it will not cut a full radius root, there may be interference between the tip of the mating gear and the root of the gear as cut.

That is why I developed my method. It takes a lot more steps, but it will give you a more satisfactory gear. Once you use it a few times, the methodology will become obvious.
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Re: Constructing a gear (improved version)

Postby armbrusterco » Fri Mar 06, 2015 4:37 am

Thanks, Mike, for sharing this. I will have to try some gears.

You may enjoy what Michael Zalewski discusses on his "Guerrilla Guide to CNC machining, mold making and resin casting" about making gears of many different shapes. http://lcamtuf.coredump.cx/gcnc/ch6/

There is so much for me to learn!

Bob
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Re: Constructing a gear (improved version)

Postby gordread » Fri May 29, 2015 9:44 pm

I so much want to try making gears.... first I need to retire.. /sigh.
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Re: Constructing a gear (improved version)

Postby Ms Wolffie » Sat May 30, 2015 10:59 am

OK so I googled it and could not find an answer:
What is a full radius root?
Cheers
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Re: Constructing a gear (improved version)

Postby RichardF » Sun May 31, 2015 9:19 am

Found your search term at books.google :

https://books.google.de/books?id=NOqBnp ... ar&f=false

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Re: Constructing a gear (improved version)

Postby FixitMike » Sun May 31, 2015 3:35 pm

A full radius root is where the bottom of the space between the teeth is a full radius, approximately a half circle. This contour permits the use of the largest bit possible to cut the teeth.
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