topic Re: Spur gear drawing using arraypolar in AutoCAD Forum

Spur gear drawing using arraypolar

Anonymous — Tue, 09 Apr 2019 06:57:42 GMT

Hi,
I'm new to AutoCAD

I'm trying to drawing a spur gear.

I've used the article here:
https://www.rcgroups.com/forums/showatt.php?attachmentid=263847

The author has described how to calculate these offsets for the shape of one half of a single spur tooth:

Angle(Deg)	Arc Length(mm)
0	0.00
5	3.56
10	7.12
15	10.68
20	14.24
25	17.81
30	21.37
35	24.93
40	28.49
45	32.05
50	35.61

I've done this:

1. Draw a circle using circle command

2. Draw a line from circle center to circle circumference - mark circle center with dimcenter then use line command

3. Draw a tangent to the line in 2. using line command

Then comes the part I do not understand:

"Create polar array of this tangent line about the center of the circle at increments that you used to calculate the involute radius (5° in my case) and about 10 of them should be enough..."

I've create a polar array:

a. arraypolar

b. Selected the tangent drawn

Question: How do I input the offsets in the table shown in the remainder of the command?

Re: Spur gear drawing using arraypolar

GrantsPirate — Tue, 09 Apr 2019 14:40:50 GMT

Use the LENGTHEN command as directed in the instructions. First erase the horizontal line. Start the Lengthen command, Total option, enter the 3.56, enter, now select the first line at a point furthest from the circle so the correct side gets adjusted to the length desired, repeat for the other lines moving down in your list for each line length.

Re: Spur gear drawing using arraypolar

hencoop — Tue, 09 Apr 2019 15:04:11 GMT

I'm not sure where they were going with those instructions. As you can see, the array polar command creates multiple copies of the selected geometry rotated about the center point. To create your spur gear, first you should draw a single tooth of your gear. It could help to layout a few radial lines that show where the key points of a single tooth are. I.E. draw a line from the center to the toe of the Dedendum/Edge of the Bottom land (without its Fillet radius); draw a line from the center to the other edge of the same Bottom land (without its Fillet radius); draw a line from the center to each end of the Tooth thickness (intersecting the Pitch circle); draw lines from the center to the left and the right of the Top land. Construct the tooth geometry (arcs or other required curves). Note that your geometry should include only one complete tooth beginning at any point you select as the start point and continuing to that same point of the next tooth.

After your tooth geometry is created, use the ARRAY/POLAR command with your tooth geometry. The angle to fill is 360°. The number of items is the Circular pitch divided by 360°.

BTW I am not a machinist or a mechanical designer. I found this spur gear nomenclature using Google. If any of you mechanical professionals see that it is not correct, I apologize. If so, would you please correct it?

HTH

Re: Spur gear drawing using arraypolar

Anonymous — Tue, 09 Apr 2019 22:55:04 GMT

Thanks for your detailed response.
The offset table is a set of points (degree, length) that shows one half of a single gear tooth.
I think these are polar co-ordinates.
Once the table points have been mapped out then I think I'm supposed to:
- join the endpoints of each of these lines via a polyline(?)
- remove the draft lines
- mirror the shape of the polyline so giving the tooth shape

Re: Spur gear drawing using arraypolar

Anonymous — Tue, 09 Apr 2019 22:56:04 GMT

Thanks.
I assume I'm to draw each of the polar coordinates by "hand".

Re: Spur gear drawing using arraypolar

GrantsPirate — Wed, 10 Apr 2019 14:40:47 GMT

The table you show in your post doesn't match the one the author gives you. If you follow the instructions on the pdf you will end up with the gear expected. I followed it all the way through and it works. Your calculations were off (assuming you are trying to draw the exact gear shown).

Once you have the arrayed lines to the lengths specified (the arc length they represent) you connect the end points with a spline as the instructions tell you. Continue with the instructions on how to find the center line of the first tooth so you can mirror about that line, and continue from there.

See attached image.

Re: Spur gear drawing using arraypolar

Anonymous — Wed, 17 Apr 2019 01:44:18 GMT

Thanks very much for going through the tutorial.

I did change the dimensions to tailor for a gear I wanted to make - hence the dimension change.

In AutoCad I thought there maybe a command like "arraypolar" that took a table of offsets to graph from say a text file - there is no such command that I know of.

However this is what I did:

"arraypolar"

change the number of lines to be 11

change angle between lines to 5 degrees

split the "arraypolar" group of lines into individual lines using "explode"

change line lengths using "lengthen" with absolute length from the excel table.

Re: Spur gear drawing using arraypolar

GrantsPirate — Wed, 17 Apr 2019 14:14:18 GMT

One last comment. Since this is being done in AutoCAD there is no need to use math to calculate the length of each arc, although it is a good thing to know how to do with math. What I did was draw the first arc from the top quad point to the first 5 degree line. I listed that and then used that length in lengthen command. For the the next line I used the lengthen command and when it is time to input the length I used (* N.n 2), and for the next one while in the lengthen command again I used the up arrow key to find the previous little formula and changed the 2 to a 3 (* N.n 3), and so on for each line.