Spur gear script OK?

risto_alanko

I tested the spur gear script, I'm planning to print a gimmick gear train in 3D. Is the script working OK with large numbers of teeth? The shape of the gear flanks seem s OK, but the gap between the teeth is strangely deep and the fillets disappear on around 100 teeth .

I tested a gear pair with module 2 mm, number of teeth 20 and 80, with the theoretical center distance of 100 mm they seem to mesh OK, but the teeth of the large gear are too deep and the fillets are tiny.

NeilCooke

You could try a different pressure angle or offset the root diameter (if I remember right it does accept negative numbers). 

Don't forget as well that these are theoretically perfect and 3D printers are quite rough so you may have to modify the module slightly on one gear to introduce some backlash (or move the gears further apart). 

risto_alanko

I couldn't get the root offset accept negative numbers, some millimeters positive worked, but it only made the gap still deeper. I extruded a circle with the correct root diameter and filled the gaps that way. Many fillets to add, though. I may have to adjust the center distance on the 3D-printed frame parts, I have to drill the holes anyway to get correct diameter.

NeilCooke

Why not change the pressure angle to 14.5 or other lower setting? That would get you the profile you need with no adverse affect on performance (since you do not need high power transmission). 

risto_alanko

Because of my degree in mechanical engineering in my metric world... my mind is locked in 20 deg pressure angle and metric module gearing
My gears will be quite small, the whole gadget will be hand cranked. I'll try to adjust my thinking if I can.

joseph_dick

Neil,

There is something inherently wrong with the Spur Gear Feature Script.

Increasing the number of teeth to large numbers should approach a involute rack.

Here is a link to a 144 tooth gear:  http://geargenerator.com/#200,200,100,6,0,1,0,2,1,18,3,6,20,-60,0,0,144,24,6,20,-60,0,0,2,781

Compare that to your result for a 144 tooth gear.  Something is not right with the Script.

Please feel free to contact me if you would like to collaborate on getting this Script fixed so that it produces correct results.

Thank you.
Joe

Dan_43

The problem is in the dedendum section. The addendum section is correct and is simple to check in the module mode as module equals addendum.

A search for gear whole depth led to several versions 
http://khkgears.net/gear-knowledge/gear-technical-reference/calculation-gear-dimensions/
http://www.micro-machine-shop.com/module_gear_data.pdf
http://www.engineersedge.com/gear_formula.htm

The formula varies from 2.157 to 2,25 times the module for whole depth. With large numbers of teeth say 100 the whole depth is clearly too large as shown in the visual example in this thread.

I see the offset can be used to correct this but it would be very handy if the zero setting was set to a given formula.

Thanks
Dan

billy2

Watch for the printing resolution, set it to high tessellation count to prevent it from being the inaccurate means in your process.

NeilCooke

Feature updated, thanks for your help @Dan_43

https://forum.onshape.com/discussion/6179/updates-to-spur-gear-feature/p1?new=1