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mdred68
04-23-2007, 11:43 PM
i have to draw a gear in my cad class. i figure if i draw one tooth i can aray the rest. how do i draw that one tooth? Is there a website that explains gear geometry out there. thanks.

Elninio
04-23-2007, 11:52 PM
I have a great book title "Elements de Machines - edition de l'ecole polytechnique de montreal". It covers alot of different topics among which are gears, maybe you can find an english version of it. I will try to scan some pages...

dp
04-23-2007, 11:59 PM
http://en.wikipedia.org/wiki/Involute

slipup
04-24-2007, 12:09 AM
http://www.ul.ie/~nolk/gears.htm#GEARS

Elninio
04-24-2007, 12:34 AM
http://i111.photobucket.com/albums/n122/elninio123/gear1.jpg

http://i111.photobucket.com/albums/n122/elninio123/gear2.jpg

i can scan some more (tomorrow) if you want.

Nick Carter
04-24-2007, 12:50 AM
http://www.cartertools.com/involute.html

gbritnell
04-24-2007, 09:26 AM
mdred68, generally for drawing a gear it's not necessary to draw the complete involute curve for representation. I have taught drawing for years and use several CAD systems for my work. There is a simplified method of construction using what is called the Wellman's Involute Odontograph. It will give you a very close approximation of a gear tooth and then it can be arrayed for the amount of teeth you need in the gear.
http://img.photobucket.com/albums/v43/gbritnell/Modelengines/ASSOCPART4055.jpg

Evan
04-24-2007, 10:52 AM
I have a great book title "Elements de Machines - edition de l'ecole polytechnique de montreal".

Sounds like the French translation of the book I have, Elements of Mechanism. It has an entire chapter devoted to gears and drawing gear teeth for different types of gears.

The publisher is John Wiley and Sons. No idea if it is still in print, I have the sixth edition by Schwamb, Merrill and James, Revised by Doughtie from 1952. Excellent reference.

LES A W HARRIS
04-24-2007, 04:53 PM
i have to draw a gear in my cad class. i figure if i draw one tooth i can aray the rest. how do i draw that one tooth? Is there a website that explains gear geometry out there. thanks.


I draw the tooth space on centerline!

Drawing a Gear in 2D Cadd To produce a SPACE on Vertical
Centerline (90.00° Absolute).

This is one method used to establish the templates used in Gear Grinding, Except for low numbers of teeth, 28 or less, the Involute form will be within Class 12 tolerance, for low numbers use three arc method or spline thru 10 points, then connect the last point at the Base Diameter; Either! Radially to the Origin, or extend the arc to the Root Diameter, then fillet the resulting intersection at the Root Diameter. This form does not undercut the tooth like Hobbing does.


First: Determine the Gear Data.

E.g.: 40 Teeth, 10 Diametral pitch, 20° Pressure Angle,
0.2200” Whole Depth, Fillet Radius 0.3/DP = 0.0300”Rad,
Circular Tooth Thickness 0.155080”, ((Pi/2*DP) - 0.0020”)

Second: Calculate to six decimal places, points along the Involute Curve,
From the Outside Diameter to the Base circle Diameter, (I use ten).



Third: In Cadd, set layer, line type, color, etc.
Draw a construction line from Origin, (0, 0) to centerline of root space,
(0, 1.8800”). Draw a second construction line from Origin (0, 0) to
Centerline of the adjacent tooth, (180/N) = 4.5° before vertical, (85.50°).

Forth: From the calculations, add five points on the involute curve at the Outside diameter, (1), at a point between the OD & Pitch Diameter, (2), at the Pitch diameter, (3), at a point midway between pitch Diameter & either the Root Diameter or the Base Diameter, (4). (Number of teeth will determine this) As the number of teeth decrease, the Root Diameter will be below the Base Diameter. (This is where Hobbing starts to undercut the Flanks of the teeth). And finally a point at the Root Diameter or Base Diameter, (5).

Fifth: Draw a two point arc from centerline of tooth to the point (1), on the OD of involute.

Sixth: Draw a three point arc from, (1) to (2) to (3).


http://i37.photobucket.com/albums/e97/CURVIC9/07%20GEARS/collage.jpg

Seventh: Draw a second three point arc from, (3), to (4), to (5).
In a case of Low number of teeth, extend this arc to the Root Diameter, or add a radial line from point (5) to Origin.

Eighth: Draw a two point arc, from the Root Diameter Radial line, to a point inside of the tooth. (The Root Diameter Arc).


Ninth: Fillet the second three point arc, (or the radial line), to the Root Diameter Arc. You now have half a tooth space!

http://i37.photobucket.com/albums/e97/CURVIC9/07%20GEARS/collage1.jpg

Tenth: Mirror these arcs about vertical centerline. ( You now have a tooth space).

http://i37.photobucket.com/albums/e97/CURVIC9/07%20GEARS/collage2.jpg


Finally: Radial Copy,(Or whatever your Cadd Command may be), 40 at 360°.
You have a 40 Tooth Gear!

http://i37.photobucket.com/albums/e97/CURVIC9/07%20GEARS/collage3.jpg

In this example add a 1.000" bore.

Rich Carlstedt
04-24-2007, 05:05 PM
My...My
Either this post has a hidden agenda, or you fellows are not supporting Village Press, the supporter of this very web site. ?

Mdred68..you need to get Volume 1 No. 2 issue of Village Presses new magazine :
Digital Machinist
It was published this winter and has a detailed ,step by step procedure, on drawing a involute curve gear tooth form on a CAD program.
Its a 5 page article by Nick Carter nad very well done

I believe this is the most recent issue as I do not have the Spring magazine yet.
You can see the Cover of Issue NO 1 on the Home page of this Board
Rich

Rich Carlstedt
04-24-2007, 05:08 PM
Les...Well Done !
You posted the answer while I was composing my last message

rich

Nick Carter
04-24-2007, 05:17 PM
My...My
Mdred68..you need to get Volume 1 No. 2 issue of Village Presses new magazine :
Digital Machinist
It was published this winter and has a detailed ,step by step procedure, on drawing a involute curve gear tooth form on a CAD program.
Its a 5 page article by Nick Carter nad very well done
Rich

See the link I posted below...

Spin Doctor
04-24-2007, 07:22 PM
Good explanation Les, but I think I would draw a Spline from the end point of the root tooth thickness arc through the PD tooth thickness arc to the crown tooth thickness arc. This should give you a true involute. Now do your root fillets and crown fillet if required. Construct an arc for the root diameter and Trim it to the center point of the Gullet. Mirror the tooth profile and Array the tooth profile around the center point.The old rule applies. More than way to skin the cat.*

*Note all commands AutoCad

Elninio
04-25-2007, 07:31 PM
Sounds like the French translation of the book I have, Elements of Mechanism. It has an entire chapter devoted to gears and drawing gear teeth for different types of gears.

The publisher is John Wiley and Sons. No idea if it is still in print, I have the sixth edition by Schwamb, Merrill and James, Revised by Doughtie from 1952. Excellent reference.

Its the book with half a gear and half a bearing drawn toghether on the cover page , the background is purple?

LES A W HARRIS
04-26-2007, 02:43 AM
i have to draw a gear in my cad class. i figure if i draw one tooth i can aray the rest. how do i draw that one tooth? Is there a website that explains gear geometry out there. thanks.

Drawing a gear in 3D Cadd.

First: Determine Your Gear Data:

E.g.: 20Teeth, (N), 16 Diametral Pitch, (DP), 14.50° Pressure Angle, (PA).

From Standard Gear Equations is obtained the following additional Data:
Calculations are normally done to six places and then rounded to four for the drawing dimensioning.

Outside Diameter (Do) = [1.3750”] (Basic)
Pitch Diameter (D) = [1.2500”] (Basic)
Base Diameter (Db) = [1.210185”] (Basic)
Root Diameter (Dr) = 1.1054”
Root fillet radius, Given on a print, or not larger than
(.3/DP) = 0.0188” (E.g. will use .015”)
Circular tooth thickness, (t) = (Pi/2) / DP = .0982”
In addition an amount for backlash should be subtracted for E.g. 0.0020” giving t = 0.096175”.

For this E.g. three pairs of x – y co-ordinates are given from the above data.
At Do = 0.081839”, 0.682612”
At D = 0.050034”, 0.622994”
At Db = 0.045095”, 0.603410”

Second: Open 3DCadd, Select Plane, select sketch, Draw circle “Do” from origin, For this E.g. add a bore in the gear blank, say 0.500”, draw second circle, fix circles, end sketch.
Select features, extrude to say 0.375”.

Third: Select face of gear, select sketch, add three points at calculated co-ordinates, 3 point arc thru points, fix arc to points, as in 2D extend arc to (Dr), fillet Dr and arc, Mirror to obtain tooth space, add a horizontal line above Do, extend tooth arcs to this line, trim as necessary and fix tooth space sketch. Exit sketch, select features, extrude/cut sketch 2 thru gear blank. Select this last feature, then Circular pattern, 20 places about centerline of gear.



http://i37.photobucket.com/albums/e97/CURVIC9/07%20GEARS/collage5.jpg

http://i37.photobucket.com/albums/e97/CURVIC9/07%20GEARS/collage4.jpg

Done!

dicks42000
04-26-2007, 12:46 PM
Hi all;
Although my machine design & drafting experience pre-dates wide spread use of auto-cad, I would still ask why would you have to draw a complete gear anyway. My former bosses wouldn't allow the time to be wasted in manual drafting....
Iirc, when disigning a gear train you need to know the reduction ratio desired, centre distances & dimensions you have to work within, pitch circle dia. & tooth form desired. The cutter, (available or used) is a form tool and will dictate the root dia. etc. The profiles are standard and available from the literature. You don't have to draw each gear tooth, just represent the major dia. pcd. & root dia. as outlines.
No doubt CAD makes it easier if you have a gear shape in a library and can just call up the dimensions & the machine draws the picture.....
As a machinist using the drawing & cutting the gears, I don't really care about a pretty picture of a gear (tool Co. calendars are better... :) ), as I said the cutter or hob and a good operator) dictates the gear.
Thanks.
Rick

cynic
04-26-2007, 01:00 PM
Sounds like the French translation of the book I have, Elements of Mechanism. It has an entire chapter devoted to gears and drawing gear teeth for different types of gears.

The publisher is John Wiley and Sons. No idea if it is still in print, I have the sixth edition by Schwamb, Merrill and James, Revised by Doughtie from 1952. Excellent reference.


You can read it here.


http://www.archive.org/details/elementsofmechan030118mbp

Nick Carter
04-26-2007, 02:11 PM
Hi all;
Although my machine design & drafting experience pre-dates wide spread use of auto-cad, I would still ask why would you have to draw a complete gear anyway. My former bosses wouldn't allow the time to be wasted in manual drafting....
Rick

Well you can do things like CNC mill/plasma cut/waterabrasive jet/laser cut a gear from flat stock, you can make a template for routing large wooden gears or hand sawing/filing gears in the absence of any gear cutting equipment.
Plus you can learn something about the hows and whys of gear geometry, for your own edification!

LES A W HARRIS
04-26-2007, 07:00 PM
As a machinist using the drawing & cutting the gears, I don't really care about a pretty picture of a gear (tool Co. calendars are better... ), as I said the cutter or hob and a good operator) dictates the gear.
Thanks.
Rick

Yes but then there is the Inspector of a company that knows (diddle squat) about gears,and splines, whose company had an order for the gears, and farmed them out, using a misdrafted 1961 drawing on micro fitch, hob runoff stated as (.63” MAX RAD) I guess the gear machinist did not notice that and ran 150 parts with a 2.50” dia spline hob. And the gear company Inspector was not paying attention either, and supplied a super pretty "Certificate of conformance". The machined off tooth was drawn with about 0.060”rad on a scrap section, which was dimensioned 1.00”MIN Rad, the parts had been end milled leaving a slot, the journal 0.0003” tolerance, well it was some what chewed up. Two hours of 3D Cadd, at least one could see what was dimensioned on the original drawing, and what it was supposed to look like.

http://i37.photobucket.com/albums/e97/CURVIC9/DRAFTING/105d3473f01a2.jpg

http://i37.photobucket.com/albums/e97/CURVIC9/DRAFTING/105d3473f01c01.jpg



My former bosses wouldn't allow the time to be wasted in manual drafting....

Mine neither, have a load of old gear tooth templates, commercial and home made, for blueprints and planning sheets, I know, in the good old days, all the gear machinist got was:

(“Cut spiral bevel gear teeth oversize to allow 0.005” stock for grinding and adjust pattern to allow for heat treat distortion”).

By the 90’s Customers had to approve the planning, need speeds & feeds, sketches of the tooth patterns, splines required lead & involute charts, and insisted that calibrated gages be used even for 10 pieces, couldn’t hire an operator who could figure MOW sizes, had none of their own tools.

Anyway, the trainee drafter now has quite a few ways of completing his assignment.