Re: Lasercutting Spur Gears?
 

How does Ed Sparks draw those on firstcadlibrary? Is that a real involute, or is it a closely approximated constant-radius curve (i.e. segment of a circle)?

Here's how to calculate and draw an involute curve in Pro/E, and put it on a part--I didn't want to go through all of this, but maybe it's a better thing in the end. If you wanted to try it, maybe Inventor can do this too. (Then again, probably not.... :p )

Re: Lasercutting Spur Gears?
 

It's the top picture. Sometimes an involute is approximated with a radius, which is apparently what was done there. But a real gear has an involute.

ChrisH

Re: Lasercutting Spur Gears?
 

I believe Ed Sparks' model is a closely approximated constant-radius curve (i.e. segment of a circle). Now understanding what an involute curve is, I have a new question. Will gears work with either an approximated constant-radius curve or a straight face tooth?

Re: Lasercutting Spur Gears?
 

For those that are interested:
http://www.metalworking.com/DropBox/GearHobb1.JPG

And I have a question. What radius circle is used to generate the involute curve? Does it change from gear to gear? I'm assuming the circle shares it's center with the gear, but it seems like it should have a smaller radius as the tooth face is not perpendicular to the face it intersects on the gear.

Greg

Re: Lasercutting Spur Gears?
 

The involute , or ever increasing radius surface, maintains the proper angular contact of the two mating gears as they mesh with each other to drive your robot. As the one gear tooth drives the other, the point of contact will move along the surface of the gear tooth involute surface. The inner area of the driving gear tooth will engage the tip area of the driven gear and as it rotates through, this contact point will be eventually be at the tip of the driving gear. As all of this is happening the gear teeth that are about to mesh have a certain amount of clearance that grows smaller as they too begin to completely mesh.

It is the geometry of the gear teeth that allows the smooth transmission of power without binding and conflict in the case of a too tight condition, or backlash and high impact damage in the case of a sloppy fit between the teeth.

I would be happy with line segments as small as .001" by the way, thanks for checking that out. I have been using Anvil 1000 (CAD Program) for the stuff that goes to the waterjet and as I zoomed on the tooth detail, I could tell it was far in excess of the precision we needed for our applications. Maybe seat of the pants, but there you go.

Re: Lasercutting Spur Gears?
 

Now that you mention it: for anyone intending to use this shape, I think I should make exactly clear how I got the 0.001" segments--I said something about it earlier, but I'm not sure if I was explicit enough. So, to replicate my 84 tooth, 0.7 module gear (with 6 mm bore), do this (these instructions assume you want an "accurate" model for CAD work--if you're setting a toolpath, you'll need to eliminate some excess geometry later, and make adjustments for your CNC machine):

1. Go to the DXF/SVG Gear Generation Utility v0.98, and set the following options:
   • Gear Type to Spur Gear
   • Diametral Pitch to 0.03628571 (i.e. 0.7^-1*25.4/1000, which converts 0.1 module to the diametral pitch--its inverse, then converts to inches, then divides by 1000 for the fun of it--this is where the precision comes from; we're tricking the applet into giving us lots of line segments, then scaling them to meet our needs!)
   • Number of Teeth to 84 (or whatever you like, really!)
   • Pressure Angle to 20 (that's what these PIC gears were, but Boston, Martin and others make both 14.5° and 20° pressure angle gears--they aren't compatible, so be sure what you've got)
   • Addendum to 1.00 (a guess, because I don't have the exact spec on hand--but this is a logical value)
   • Dedendum to 1.20 (again a guess, for the same reason--this uses the fine-pitch AGMA Imperial spec)
   • Shaft Diameter to <cleared> (don't bother--it's made of segments too--unless you're feeding this straight into a CNC machine, and have no CAD capability, in which case, this approximation will have to do)
   • Mark Center [sic :p ] of Gear with Crosshair to <checked> (because it's useful for drawing--when you set the toolpath, don't include it, though!)
   • Format for Result to DXF (if you can use SVG, go ahead...)
   • Offset Amount to 0.000 for drawing purposes, or some appropriate value for your machine--this varies, and you'll have to find out for yourself.
   And press Go!
2. Save the file that comes up for download somewhere.
3. Open it in your favourite 2-D CAD program (AutoCAD 2004 in my case) and go to View | Extents, which shows everything.
4. Select everything, and type in the command window the following
   Code:

   ---

   scale
0,0
0.001

   ---

   This shrinks the drawing (scaled about the centre point) by a factor of 1000, which is what you're looking for--remember we made the whole thing 1000 times too big by dividing the pitch by 1000 (small pitch number=big teeth).
5. Now type this:
   Code:

   ---

   c
0,0
0.11811

   ---

   This puts in a 6 mm bore circle.
6. Now draw a circle centred at the centre, and with radius snapped to the endpoint of the crosshairs (which were drawn in automatically by checking the checkbox!). The circle's diameter is your pitch diameter--and if it isn't, you messed up! Don't cut that!

Otherwise, you now have a gear like mine! If you measure the individual segments that make up the gear teeth, you'll find them to be around 0.001" long, as opposed to between 0.028" and 0.036" long if you hadn't done the factor-of-1000 thing. Of course, you need to know whether your CNC machine will cough and die if force-fed about 13411 different line segments at once! (That's really how many there are.)

Of course, you may be able to model a gear with a simple pitch circle (or cylinder in 3-D); maybe this is wasted effort? (It depends on why you want a gear-shaped thing--if it's cutting, of course you need the shape; if it's drawing, usually, the pitch circle is enough.)

And for the Pro/E users out there--use that formula thing (that I mentioned earlier) instead, if you can get your head around it! Don't mess with an approximate .dxf, if you can help it, especially because most of those 13411 segments will spawn an attached surface on the model (which makes everything ludicrously slow), if it doesn't annihilate your sketcher, resulting in one of Pro/E's endearing (and sudden) crashes. (And if you use Pro/MECHANICA for anything, use a plain old cylinder--or load a simplified rep!!! :eek: )

Re: Lasercutting Spur Gears?
 

I dont know about much about making gears but I can tell you a good place to purchase spur gears!
http://www.martinsprocket.com/
We used 20 degree pitch spur gears from this company in our 3 speed shift on the fly transmission

This year we made a sprocket for 35 pitch chain using our high schools CNC mill. We used a drill bit and put holes all the way around the aluminum disk. Once all the holes were made we toik a mill bit all the way around the outside leaving half circles for the chain to catch

They can be seen here: http://www.tahsroboticsteam.org/Pict...n/100_0716.JPG

Re: Lasercutting Spur Gears?
 

If you want to learn alot about gears, then check out this link:
http://www.sdp-si.com/D190/D190cat.htm

You can get a free hardcopy too.

Re: Lasercutting Spur Gears?
 

Sandrag,

We use a laser here at my facility. For the robot, we have made structural channels (arms/masts), pulley discs chassis side plates and drill motor mounts to good effect in the past couple of years.

I'm told the cutting tolerance on our Trumpf laser cutting machine is about +/- .002 inch in thin material (say under .25 inch thick). Given that, and making a quick judment call, I would avoid cuttting any gear teeth with a tooth thickness less than say 50x to 100x that (so minimum .10 or .20 inch chordal thickness at pitchline)

#25 chain/#35 chain sprockets are a natural for lasers, especially handy are large diameters for arms etc, . Cut 25 chain sprockets from .100 inch thick aluminum sheet, just be sure to use a strong hard alu. alloy.
Large diameter sprockets can be sandwiched against a stiffener disk if needed for stability. (stifferner disc could be 1/4 inch plywood, alu, lexan)


Drop me a line if you are still gathering info on this gears/sprockets/laser cutting etc. A short phone conversation would probably be worth an hour of working through postings.
Jesse