View Full Version : Gear cutting for dummies.

JTToner

01-09-2010, 10:02 AM

For starters, I'm neither machinist nor engineer. I've no experience with gears other than regularly breaking them in my 1949 Ford's tranny when I was in high school. So, I'm looking for a book on cutting gears. I tried to fathom Machinery's Handbook, but it was well over my head. Is there some reference that distills it down to depth of cut for a given number of teeth on a blank of a given major(?) diameter and a given PA? That's what would serve me well. For example, say I want to make a pair of gears for 2:1 reduction. One of my gear blanks might be from 2" dia. stock, and I want a 20* PA. Where do I go from there? I looked to some books at Amazon, but I don't even know enough about gears to decide which book would be appropriate. I need "Gear Cutting For Dummies".

johnny

Glenn Wegman

01-09-2010, 10:07 AM

Scroll down to the link for gears. It will give you a little basic info as far as size calculations.

http://shopswarf.orconhosting.net.nz/sindex.html

JCHannum

01-09-2010, 10:50 AM

The Ivan Law book, Gears and Gear Cutting is the best introduction to theory and cutting of gears bar none.

http://www.amazon.com/Gears-Gear-Cutting-Ivan-Law/dp/0852429118

precisionmetal

01-09-2010, 11:08 AM

Johnny,

There are some general rules that apply to most all gears.

The OD of a blank to make a gear is (N+2)/DP

In other words, if you were cutting say... a 21 tooth gear that is 12DP 20°PA, you would simply do: (21+2)/12

The OD of the blank should be 1.917".

The depth to cut (aka: "whole depth") should be 2.25/DP (2.25/12 in the above example) which would be .1875".

This of course assumes everything standard, no form modifications, etc. If you're cutting this gear on your milling machine using a form cutter, you would buy the cutter that is correct for the DP, PA, and the range of teeth you will be using it in (form milling cutters have a usable range of teeth they will cut, so it's important to use the correct cutter).

To calculate center distance, you simply add together the pitch diameters of both gears and divide by 2.

e.g. One gear is a 30 tooth, 12DP 20° gear, and the other is a 15 tooth, 12DP 20° gear.

The pitch diameter is N/DP, so the 30 tooth gear has a PD of 2.500", and the 15 tooth gear has a PD of 1.250" (note, the ODs of the gears would be 2.667" and 1.416" respectively)

(2.500" + 1.250") / 2 = 1.875" (that is the correct center distance for that gear pair).

Much more can be added to this.... but those are some fundamentals.

PM

Your Old Dog

01-09-2010, 11:36 AM

"Gear cutting for dummies." Is that kind of like an Oxymoron? :D

Gear cutting is way over my abiity but then I freely admit to not being the brightest bulb on the machinist Christmas tree. If you are really stupid, you should buy your gears like I do :D Lifes too short for me to tackle gear cutting.

John Stevenson

01-09-2010, 11:57 AM

"Gear cutting for dummies." Is that kind of like an Oxymoron? :D

No, an Oxymoron is an idiot gas welder.

.

Glenn Wegman

01-09-2010, 11:59 AM

(2.500" + 1.250") / 2 = 1.875" (that is the correct center distance for that gear pair).

PM

With zero backlash figured into the PD of the gears...

Center distance is correct.

JTToner

01-09-2010, 01:30 PM

Glenn, thanks - in addition to the gear formulas, a ton of other good information. I've got it bookmarked.

Johnny

JTToner

01-09-2010, 01:50 PM

A lot to learn and practice. Probably more sensible to just buy gears as needed. I'm not sure why, but I really want to learn these things. Up to this point, gear cutting has been, for me, among the "black arts". Best I might do at this time would be to grind an HSS bit to match the cut on an existing gear. When I got my lathe, it came with a lot of tooling about which I was clueless. One such item was large dia. chuck-like object, shaped like an "UFO", and bearing the name Jacobs. Seems it uses some sort of collets that were not included. The other item that left me scratching my head is a gear cutter. Must be for a large gear, and is marked C.M.M. CO H S No.2 55 To 134 T. 4DP-D+F=.539". For me, it might as well been Sanskrit. Like I said, got a lot to learn. Thanks to all,

Johnny

John Stevenson

01-09-2010, 02:21 PM

The UFO is a Jacobs rubberflex collet chuck, unfortunately the collets are mega expensive.

The gear cutter can be read using the Serbo Croat on the side.

C.M.M. - Probably a makers name

CO H S - Type of material, Cobalt High speed Steel. ?

No.2 - Range of cutter, there are 8 in a set.

55 To 134 T. - Numer of teeth the No. 2 cutter can cut

4DP - Size of gear tooth, there will be 4 teeth per inch measured on the pitch circle

D+F=.539" - Depth to wind cutter into the blank to get depth[D] and clearance [ f ]

.

Paul Alciatore

01-09-2010, 02:32 PM

...

4DP - Size of gear tooth, there will be 4 teeth per inch measured on the pitch circle

....

John, you must be thinking metric. 4DP means that there are four teeth PER INCH of DIAMETER of the pitch circle. Or four teeth in 3.141" (pie inches) along the pitch circle. That's about 1.27 teeth per inch on the pitch circle.

Yep, pretty big gear.

texas_po_boy

01-09-2010, 02:34 PM

"Gear cutting for dummies." Is that kind of like an Oxymoron? :D

Gear cutting is way over my abiity but then I freely admit to not being the brightest bulb on the machinist Christmas tree. If you are really stupid, you should buy your gears like I do :D Lifes too short for me to tackle gear cutting.

Where is the best place to purchase ready made gears.

John Stevenson

01-09-2010, 02:46 PM

John, you must be thinking metric. 4DP means that there are four teeth PER INCH of DIAMETER of the pitch circle. Or four teeth in 3.141" (pie inches) along the pitch circle. That's about 1.27 teeth per inch on the pitch circle.

Yep, pretty big gear.

Correct, trying to answer 3 emails all at once and ........forget it, I just have ...........:D

.

Paul Alciatore

01-09-2010, 02:51 PM

To use your 4DP gear cutter, here is all you need to do:

1. Decide on a gear with a tooth count between 55 and 134. That's the range of the cutter. Call this number T.

2. Make a blank. The OD will be (T+2)/4. Tooth count plus two and the result divided by the DP.

3. Mount the blank in your mill with an indexing head or rotary table.

4. Optional but nice step. Gash the tooth spaces with a rectangular profile cutter to remove most of the metal between teeth. Index the blank by one tooth (360 deg/T) for each cut. Some math is required here or a CAD drawing with accurate tooth profile to determine the width of the cutter and the depth of the cut.

5. Using the 4DP cutter, touch off on the original OD and then feed in by the amount indicated on the cutter (D+F=.539").

6. Cut the teeth, indexing the blank by one tooth (360 deg/T) for each cut.

Simple, isn't it.

But for a 4DP gear with at least 55 teeth, you are going to need a pretty big blank (57/4 = 14.25" diameter) and a pretty big mill. And that's just the smallest gear you can cut with it. But it will make a nice paper weight. Or perhaps a shop clock. Or a lamp.

But the procedure above will work with smaller gears as well.

JTToner

01-10-2010, 02:05 AM

I think I'm beginning to get it. The number of the cutter relates to the range of teeth it cuts, in the case of mine, a # 2 ,cuts from 55 to 134 teeth while the value of "D" will relate to the diameter of the gear blank. Depending on "D", all #2 cutters will do 55 to 134 teeth but the gears will be different sizes. Is that close or am I way off base?

Johnny

Don't know if you've seen this site, but its a pretty informative gear resource.

http://homepage.ntlworld.com/peter_harrison/workshop/gearcutting/index.htm

precisionmetal

01-10-2010, 02:27 AM

Slick method which can easily be done on a mill.

The only thing that caught my eye is that on his "involute gears" page at the bottom, it should be pi/DP or pi*module, not 3.146 (pi rounding off to 3.1416, not 3.146).

PM

Paul Alciatore

01-10-2010, 03:07 AM

I think I'm beginning to get it. The number of the cutter relates to the range of teeth it cuts, in the case of mine, a # 2 ,cuts from 55 to 134 teeth while the value of "D" will relate to the diameter of the gear blank. Depending on "D", all #2 cutters will do 55 to 134 teeth but the gears will be different sizes. Is that close or am I way off base?

Johnny

Close, but not exactly.

The "D" on the cutter (D+F=0.539) is not the diameter of the gear. It is the Depth of the tooth, not counting "F" which is the clearance at the root of the tooth being cut. D+F is the amount you must advance the depth of the cut after touching off on the OD. This does not depend on the diameter of the gear: it is the same for all gears in the range that the cutter will cut.

JTToner

01-10-2010, 12:33 PM

I'm learning, little by little, but its coming. I'm beginning to see that a machinist needs also to be an engineer, but the converse is not necessarily true.

Arrrggggh!

Johnny

Tony Ennis

01-10-2010, 12:43 PM

Not too long ago there wasn't a difference.

lazlo

01-10-2010, 01:21 PM

Not too long ago there wasn't a difference.

They've never been the same. Machining is a manual skill: craft and art. Engineering is design and analysis, which is why young engineers spend the first year of college taking calculus, differential equations, and vector math classes.

Tony Ennis

01-10-2010, 02:13 PM

Heh. Go back a little further.