Worm/Wheel Gear Straight to Real Parts?

[MikahB]

Working on a new design that will require taking a worm/wheel setup that I'm modeling and use the 3D models to go directly to prototype and then injection molded (PA6) parts.

Just curious if anyone has taken output gears from the Gear Generator and produced parts directly from the models.  Are they accurate enough to work properly?  I've only ever done it by providing specs to a gear hobber, but this time it's going to be a straight shot and misses on my end will be costly.

[JDMather]

You can easily check this yourself - have the gear generator create the gears and check for interference.

I don't think you can use the worm gears generated by Inventor can be used in a real assembly.

[MikahB]

Your response demonstrates exactly my question - I am perfectly comfortable checking the 3D parts for interference/fitment, I'm just not sure how accurate they are for making pieces that will actually work (and be durable!) in real life.

[JDMather]

This result would indicate to me that the mechanism cannot work.

[rmerlob]

Even for normal gears I think that if you want accurate geometry you have to ´´export tooth shape´´ and then model manually, do your worm gears have that option as well? I haven´t checked.

If that option isn´t availabe then you might be in for a ton of gear profile research.

BTW I´m just curious, how do they machine the mold for injecting the plastic, using a CNC center with very small tools and intricate pathing or do they do something like a ´´negative gear´´ with a hob (if that makes any sense).

RM

[MikahB]

Yeah, when I've done traditional (spur and bevel) gears in the past, all they want are the specs on the gear train and they hob to their shape.

I do not know exactly how the shop is going to prototype and then make production molds for these, but I will post back when I find out!

Makes me nervous, though, that this place (an injection molding shop) doesn't want my gear train information, just the 3D parts. Hence my apprehension!

[mrattray]

I wouldn't trust it.

[rmerlob]

I think it is totally possible that, since they are a plastics shop and are used to machining complex shapes using intricate paths and they probably don´t have as much experience using hobs, that they are going to use the 3D model to machine the mold exactly as the model.

That of course depends on a lot of things including the size of your gear.

I believe that this is one of those moments were ´´in doubt, ASK!!!´´

If you do need to do accurate 3D models we would need the gear train specs to see if we can help a bit more. Anyways, be sure to post back since I find these things quite interesting.

RM

[LT.Rusty]

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@rmerlob wrote:

BTW I´m just curious, how do they machine the mold for injecting the plastic, using a CNC center with very small tools and intricate pathing or do they do something like a ´´negative gear´´ with a hob (if that makes any sense).

RM

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At a guess - and this is only a guess, and it depends on the size of the gear a lot here - but I'd say EDM.  Machine an electrode that's a positive copy of the gear, then cut the mold with that.  Based on the .003" shown in the image above, and how visibly large the interference is, that's probably the most feasible way to do it.

[coreyparks]

FYI, Inventor does not create true gear profile geometry using the default parts from gear generator only a simplifed version to minimize the overhead on the software since gear teeth have such complex geometry.  Once you generate a gear I believe you can right click and "export tooth profile"  and then model the gear using that.  This exported geometry is accurate for spur gears and we have EDM'd gears from this profile that have been running for a year at least.  Not sure how this would work with a bevel or helical gear.

[graemev]

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@MikahB wrote:

Working on a new design that will require taking a worm/wheel setup that I'm modeling and use the 3D models to go directly to prototype and then injection molded (PA6) parts.

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Your best bet might be to contact a gear manufacturer and explain what you're trying to do.  They may be willing to generate a 3D model for your purposes if they don't produce injection molded gears themselves.  Be aware that they may request additional gear train data that is not common, such as shift and backlash.

[rmerlob]

This might help you study the geometry a bit, you can drive the angle constraint and see it move, all geometry for both parts is in a single sketch.

Regards,

BTW all parts have EOP rolled up

RM

[MikahB]

Thanks!!

[rmerlob]

No problem, I was reading somewhere that straight flanks and involutes are acceptable profiles for worm and crowns respectively, but I might have misunderstood, can anyone confirm this?

RM

[rmerlob]

BTW, what are your gear specs?, just to see if I can be model them with this.

RM

[graemev]

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@rmerlob wrote:

No problem, I was reading somewhere that straight flanks and involutes are acceptable profiles for worm and crowns respectively, but I might have misunderstood, can anyone confirm this?

RM

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Not a confirmation, but it seems likely.  The worm is geometrically similar to a rack, which would have straight sides - an involute gear of infinite diameter.

[MikahB]

I have not finalized the gear specs yet - still in early stages. As soon as I get those, I will share here.

[lakewillson12]

Worm Wheel Hobs are manufactured in bore type and shank type in single or multi-start, topping or non-topping as per DIN, BS specifications or as per customer's requirement.