Gears in engineering drawings

Ste_Wilson

Hello, 
I had a quick search and couldn't find quite what I wanted to know.
Back in't day gears, as per BS8888 were often shown in engineering drawings as a circle with a PCD, possibly with an area showing two or three teeth.
Much the same way threads aren't drawn, but tramlines are.  Setting up 'gear trains' would just be a case of matching up the pitch circles.

For modeling of the teeth there is the wonderful Spur Gear Feature Script (THANK YOU) but is there a 'cosmetic' gear option that could be attached to a disk?

Or am I just being set in my ways?  Just model it, show it, and attach a note?

Ste

S1mon

I prefer to just see teeth. I also like screw threads most of the time, but I know that slows things down.

Typically there will be a detailed table on a gear drawing with all the numbers like PCD, contact angles, AGMA class (or equivalent), etc... 

The 3D CAD is really for reference in your CAD assemblies. It's helpful to check things like clocking of features relative to the teeth. 

If the gear is machined metal, they aren't going from your model, they're using special gear cutters. They'll inspect it with special tools, so the 3D CAD of the teeth likely won't be used even for a CMM inspection. If it's molded, you need a tooler that specializes in gears, and they will modify the tooth profile from what the final part is supposed to look like, not just for simple overall isotropic shrink, but they will tweak the profile from root to tip so that the as-molded shape is correct.

Ste_Wilson

Thanks for the informative response.

So just show the teeth on the drawing, as it's easy to generate in the model, and refer to the table!

nick_papageorge073

What is your end goal? Are you 3d printing a gear? Are you having a gear commercially made in production qty's?

I designed a few transmissions for children's products over the years. The gears were injection molded, and powder sintered metal. In my design, I drew the gears as simple circles at the pitch circle diameter. That's what went in the assemblies, and what drove the parametric design of the transmission case. On the 2D drawing of the gear, I used tables like shown above. The gear vendors have their own gear specific software and machines that forms the teeth. They are not following a 3D CAD model that has teeth modeled, at least in my experience.

Ste_Wilson

End Goal?  Good question!

I've asked my students to do a small project that might involve a simple gear train, probably 3d printed, BUT I'm also trying to get them to do best practice on engineering drawings, WORK TO RECOGNISED STANDARDS as the qualification spec repeatedly tells me. So while I WILL need the teeth for printing, I'm also thinking about the workflow for the engineering drawings. Versions with plain circles/ teeth showing, similar to thread/ cosmetic thread versions?  Just put the teethed version on the drawing? (What is going to make the examiner smile the most?)  Hence wondering about 'cosmetic teeth' analogous to cosmetic threads.

I may be overthinking this.

nick_papageorge073

In that case I’d probably model the teeth for real since you will be printing. And because it’s so easy to do these days with the gear FS. On the 2D, put a note pointing to the teeth saying “tooth geometry for visual reference only”. Then have a chart with all the parameters the gear vendor needs. 

The commonly available gear teeth generators are missing profile shifting, which the vendors all do when the gear pair includes one gear with a small number of teeth. It prevents undercutting the tooth. This is for both molded gears and machined gears. 

The gear vendors on their own internal drawings often have the gear pair, which includes the center distance of the pair, and the tooth info of each individual gear. The profile shifting works as a pair. One gear is shifted in one direction, and it’s mate in the opposite. 

NeilCooke

Don't forget, showing only some teeth on a drawing was to make it easy for the draughtsperson - now you have CAD you can show everything.

Ste_Wilson

@NeilCooke Very true, the counterpoint to this is that educators, or at least the systems we engage with, are very resistant to progress!

shawn_crocker

I think its too late to resist the progress that has undeniably been made.  Gears have teeth, drawings have gears, therefore, drawings must have teeth.