Cogs and shapes 3

[Wedwin]

I feel the need to design a module or two of a cogwheel.

While I have seen many a nice picture on the net, I haven't come across a good description of how to actually design a cog tooth.

My aim is a standard metrical spur cogwheel.

This problem has bested even "Karl Bjorks lilla gula"

 

[MikeHalloran]

When you say 'design', do you mean 'produce an image of'?

If you have access to AutoCAD, there are only about a thousand "gear.lsp" programs available for it.




Mike Halloran
Pembroke Pines, FL, USA

 

[Wedwin]

I am drawing it with Qcad, and I have no libs.

But I would like to make a drawing with all the data so as to be able to produce the modules.

 

[MikeHalloran]

Your objective is still ambiguous to me, because the phrase "all the data" means different things for producing images of gears or physical gears.

You don't need a drawing of a gear in order to get a physical gear produced. A drawing would help you communicate any special requirements, but gearmakers don't need a drawing to produce the gear per se, and in particular they don't need an accurate drawing of the tooth profiles, which are generated by machines. The traditional convention for pencil drawings is a single simple tooth with trapezoidal flanks, and phantom circles to imply that there are more than one. AutoCAD can, with help, generate an accurate image of a tooth profile, and can array that to give an exact representation of a gear, but the effort is not necessary to procurement of an actual gear.

If your objective is production of an _image_ of an actual gear, you can do that, with a little more difficulty. You could, for instance, use Excel to calculate the coordinates of the transverse section of a single tooth, enter them and connect them with a spline or polyline, and array that to produce a full image, or whatever you want.



Mike Halloran
Pembroke Pines, FL, USA

 

[Wedwin]

I suppose a professional gearmaker would know far batter than me how to make the teeth. It is not likely though that a pro gets to make these ones. Not even dead certain it is going to be made at all.

I guess I want to produce a image of an actual gear by your definition. It is the measurements I lack. Failing searching the net for two hours made me turn to Eng-tips.

I want to do it, partly to be able to, and partly to understand it.

If only I can draw one tooth, I can then copy it all the way around.

 

[MikeHalloran]

I couldn't find a simple "how-to". Here is an overview which is rather more broad and opaque than I would like it:

http://www.du.edu/~jcalvert/tech/trochoid.htm

If you can't find an add-in for your CAD program, it may be easier to do it with pencil and paper.

This would be a good time for you to snag a copy of "Machinery's Handbook", which will tell you all you need to know, and then some.



Mike Halloran
Pembroke Pines, FL, USA

 

[Wedwin]

Thanks!

I appreciate it. It is closer than what I succeded with. Not perfect, but it's a step in the right direction.

 

[TheTick]

MikeHalloran's link says most of it (yet I will continue blathering)...

The key word is involute. That is the curve made by the end of a string as it unwinds from a spool.

If you know the pressure angle and pitch diameter, you can graphically solve for the base diameter from which the string "unwinds".

I have a SolidWorks model which uses this process to generate a geartooth. Ask, and I will post it.

I could be the world's greatest underachiever, if I could just learn to apply myself.

http://www.EsoxRepublic.com

-SolidWorks API VB programming help

 

[Wedwin]

I will gladly recieve any help, so please feel asked.

Know though, that I use neither Solidworks or Windows, but Qcad (

http://www.ribbonsoft.com)

on Linux

 

[MikeHalloran]

Perhaps a friend with AutoCAD could export a 2D model of the geartooth you want as a DXF file.

It might have been done already, had you told us the gear module and number of teeth.



Mike Halloran
Pembroke Pines, FL, USA

 

[dimjim]

Can Qcad use lisp files?
You might want to check the
acad forum.
Are you trying to draw external
involute spur gears?
Dudley and Buckingham both have
some excellent texts to show you
how to design these.
You will find some help too on the
gear and pulley engineering forum.

 

[Wedwin]

Qcad saves to dxf, but autocad won't recognise the format.
Lisp? I can't say, but I am willing to try.
Acad? I'll check it out. Got an address? I tried Google, but found everything but CAD forum.
External spur, yes

 

[Wedwin]

Here is an image of the gadget in question so far

 

[rb1957]

are you trying to show the motion of the various linkages ? (which sounds way different to trying to draw a gear)

 

[Wedwin]

It is a robotic finger.

 

[rb1957]

and i guess you're trying to draw the gears, shown where you've got the double lines ?

ok, why ? i'd have thought that gear geometry was something machinists understood, so all you should need to do is to specify a "X-Y-Z" gear geometry; no?

 

[Wedwin]

I am not a machinist, and I want to understand it, as previously stated.

 

[rb1957]

sorry to labour the point, but do you wnt to understand how to draw a gear, or how it works (in particular, how the attributes of the gear tooth design affect its performance) ?

 

[Wedwin]

I suppose understanding them would aid me in drawing them, which is the goal.

 

[dimjim]

You might want to check out the stub form
rather than the full depth form for your design.

Also check out thread406-103836 for a lisp
program and simply accept all of the defaults
the first time thru to get some idea of what
it does. The thread mentioned above in in the
Gear & Pulley Forum which is part of this
general Forum.