PDA

View Full Version : layout of gears



tronica
05-19-2006, 03:01 PM
I'm wanting to dive into gear making. But i'm not sure about laying out the gear. Not sure how to go about spacing the teeth apart. Or how many teeth for the diameter of the gear?? i'm lost. can someone point me in the right direction. Thanks.

Evan
05-19-2006, 03:09 PM
Try this thread

http://homeshopmachinist.net/bbs/showthread.php?t=19195

SGW
05-19-2006, 03:12 PM
Get yourself a copy of "Gears and Gear Making" by Ivan Law. Or maybe it's "Gears and Gear Cutting." It's part of the Argus Workshop Practice series, and it will explain everything. There's enough detail involved so I hesitate to try to give you a summary. None of it is difficult though.

The book has been discussed several times in the past -- try a search of the archives for more information on it.

John Stevenson
05-19-2006, 03:37 PM
I'm wanting to dive into gear making. But i'm not sure about laying out the gear. Not sure how to go about spacing the teeth apart. Or how many teeth for the diameter of the gear?? i'm lost. can someone point me in the right direction. Thanks.

http://homepage.ntlworld.com/stevenson.engineers/lsteve/files/arrow.jpg
http://www.rbookshop.com/law/t/Technology_Law/Gears_and_Gear_Cutting_Workshop_Practice_Series__0 852429118.htm
http://www.rbookshop.com/law/t/Technology_Law/Gears_and_Gear_Cutting_Workshop_Practice_Series__0 852429118.htm
.

John Foster
05-19-2006, 04:28 PM
By all means get one of the books recommended.
From 35 years of teaching machine shop I found that there are two terms that you need to understand before gearing makes sense. They are Pitch Diameter and Diametral Pitch.

Pitch Diameter is the diameter of a an imaginary circle (for all practical purposes) half way down the tooth.

Diametral Pitch is the number of teeth per inch of Pitch Diameter. Say that over and over. Now pick up a gear, count the number of teeth on a gear and divide that number by Pitch Diameter. You can estimate the Pitch Diameter by guessing where that imaginary circle that I mentioned, half way down the teeth. You should come out with a number very close to a whole number as it is very difficult to cut say 10 and 1/2 teeth. (no that is not true, as I have cut half a tooth more than once) Diametral pitch will be a whole number.

An example. Your gear has 20 teeth and the PD is 2 inches (outside diameter will be greater). 20/2 = 10. You take the mating gear which has 40 teeth and the PD is 4. 40/4 = 10. You can not run a gear with a Diameteral Pitch of 8 with a gear of 10. It must always come out the same for gears to mesh and the teeth to be the same size.

Basically all calculations are based on Diametral Pitch. Once you understand PD and DP you are well on your way to cutting gears.
I know this is brief but again, from experience, this is where to start. John

J Tiers
05-19-2006, 05:54 PM
And, amplifying.....

"laying out the teeth" is easy enough, since you will use a standard form cutter (probably) to cut them. There is a series of 8 form cutters that cover practical gears from 12tooth or so to a rack.

The series of 8 is for EACH diametral pitch, so to be able cut ANY arbitrary gear right now you need access to a lot of cutters. But usually you have a range of teeth, DP etc that you work with most often.

As far as spacing them, two factors.

First, is the right number of teeth ..... solved by a dividing head, super spacer, spin index, etc, depending.

Second is the right spacing, solved by cutting to the correct "whole depth of tooth" for the gear you are cutting. That is how far depth wise you feed the cutter into the blank.

Another good book is the Colvin and Stanley book "Gear Cutting Practice".. With either, and a good copy of "Machinery's Handbook" you should be good to go as far as info.

You will need a mill, preferably a horizontal mill, but a vertical or mill-drill will work. I have even cut gears on a shaper. With the shaper you grind your own cutters, and they are cheap.... milling form cutters are $25 or more per each, new.

mklotz
05-19-2006, 07:22 PM
I recently had to make a worm and gear set for a model I'm making. Lacking any
proper cutters, I simply ground a lathe tool to a 40 deg included angle with a
tiny flat on the end. Said lathe tool was mounted in a fly cutter to form what
amounts to a 90 deg fly cutter. After computing the size of the gear blank
with a program I wrote (see GEARSPUR on my page),

-------------------------------------
Enter whatever data you know. Enter zero (0) for unknowns.
You must enter two data items to obtain an answer.

OD of gear [2.35 in] ? 0
Number of teeth [45] ? 36
Diametral Pitch [20] ? 64

Diametral Pitch = 64.0000
Module = 0.3969
Number of teeth = 36
Outside Diameter = 0.5938 in = 15.0813 mm
Addendum = 0.0156 in = 0.3969 mm
Dedendum = 0.0188 in = 0.4763 mm
Whole Depth = 0.0344 in = 0.8731 mm
Circular Pitch = 0.0491 in = 1.2468 mm
Tooth Thickness = 0.0236 in = 0.5985 mm
Pitch Diameter = 0.5625 in = 14.2875 mm
-------------------------------------

I mounted the blank in the Spindex and cut 36 teeth to a depth of 0.034" (the
indicated whole depth above). The resulting cobbled gear meshed nicely with
some commercial 64 DP gears I had.

The Circular Pitch (0.0491" above) is very close to a pitch of 20 tpi so, for
the worm, I threaded a 3/16" piece of brass using the same tool used to make
the gear.

The resulting worm and gear are now incorporated into the model and working
very nicely.

While I certainly wouldn't call this professional gear making, I described the
process so that amateurs who have never tried making a gear could give it a try
without investing in expensive cutters. If you're new to gear making, such an
exercise can help you understand the process and mathematics before going on to
doing it the right way. Chances are you'll mess up your first few tries (I
sure did) so this is a cheap way to learn the procedure.

Wirecutter
05-19-2006, 08:19 PM
Get yourself a copy of "Gears and Gear Making" by Ivan Law. Or maybe it's "Gears and Gear Cutting." It's part of the Argus Workshop Practice series, and it will explain everything. There's enough detail involved so I hesitate to try to give you a summary. None of it is difficult though.

The book has been discussed several times in the past -- try a search of the archives for more information on it.


The book is "Gears and Gear Cutting", by Ivan Law. I got it on "the Earl's" (thanks, John) recommendation and I'm delighted. I've had to read certain parts over and over, but it's there. If what you need to know is not there, you'll at least know what questions to ask. Law's book even goes into how one of us normal folk can make gear cutters. (A "whole set" of gear cutters is probably thousands of pieces and a whole pile of money - you only want to buy/make what you need) We're not talking here about production gear hobs or anything like that, but enough to cut a few of most types you might need.

What might be an interesting thing to study is the making of helical gears, like those in a car's rear end. I hear it's a pretty daunting thing to do, but it's probably part of the "Advanced Course" in gear cutting. I'm told there are some pretty mechanically fascinating machines that did the job before the days of CNC.

torker
05-20-2006, 01:59 AM
I'll second (or third or fourth) Ivan Laws book.
And yes...Thanks Sir John!
Russ

LES A W HARRIS
05-20-2006, 06:41 AM
Comercial gear form tools,
http://i37.photobucket.com/albums/e97/CURVIC9/GEARS06/FORMCUTTERS01.jpg

Homemade single tooth cutter, use with boring head in verticle mill.
http://i37.photobucket.com/albums/e97/CURVIC9/GEARS06/22helgearcutter01.jpg


cheers,