Threaded rod as a linear mechanism? (Maker Wheel)

[Wing Wong]

Just saw this on Hackaday: https://hackaday.io/project/25905-makerwheel?utm_source=Hackaday.io&utm_campaign=a011b42a8f-EMAIL_CAMPAIGN_2017_07_13&utm_medium=email&utm_term=0_880d5c0e21-a011b42a8f-159124765&mc_cid=a011b42a8f&mc_eid=b1786b7b14

(Wow, that was an ugly paste...) 

But basically, it's a design that uses a threaded rod and a matching wheel to do linear motion. Seemed interesting. Not sure how accurate it would be considering those threaded rods bend and flex so easily... but how well would they work as an alternative to say GT2?

Some pros:

* quieter than using the threaded rod as a screw drive

* their is no stretch or tensioning required vs GT2 belts

* No need for pulleys/etc.

Some cons:

* heavy. It's steel.

* still need a linear rod/rail for accurate movement since the typical threaded rods deflect WAY too easily.

* change in length means a different threaded rod and for longer lengths, can be expensive.

* threaded rods aren't exactly known for their awesome build quality... some parts of the rod might have different responses(skips, deflection, not mate with the wheel due to thread dent/damage/etc)

W.

[Whosawhatsis]

(Wow, that was an ugly paste...) 

For future reference, you could have cut off everything from the question mark on.

https://hackaday.io/project/25905-makerwheel

[Ryan Carlyle]

Huh. That's new. Interesting idea.

With hardware-store threaded rod, it's going to wear the plastic worm wheel like crazy. The surface finish on cheap threaded rod is god-awful, which is totally fine for fastening, but wear-tastic for sliding/rolling contact. 

I suspect the FDM printed wheels are not going to have precise enough tooth shape to produce the proper rolling contact that you get with involute-generated spur gear teeth and straight rack teeth. So there'll be a tiny amount of velocity variation (probably doesn't matter) and much more propensity to wear than a spur gear shape. 

Being able to use the same parts for worm drives and linear motion is neat. 

[Ryan Carlyle]

Oh, and load-bearing capacity will be very low. You're basically pressing a dull knife-edge in the root of the plastic worm wheel for your load-bearing contact. 

[Wing Wong]

Hmm... had forgotten to consider the threads cutting/eating into the FDM part.

What you said about worm gear is interesting though.... I wonder if something like this would be doable for a trapezoidal threaded rod. Though I suppose at some point, it's better just to implement a proper track designed for this. -_-;

W.

[Whosawhatsis]

Yeah, you'd really want to to be metal. I've thought about trying this with hobbed drive gears.

This would actually make the most sense for manual applications, I think. A microscope, for instance, could give you coarse adjustment by turning these wheels and fine adjustment by turning the screw the wheels run on.

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[Wing Wong]

Hmm.... your mentioning microscope adjustment just made me think of lenses focusing or perhaps tape library kind of applications....

For use cases where you want to be able to accurately pick something up(worm/screw drive) and then move it quickly(screw wheel? Worm wheel?) to another location and then switch back to fine movement, this would be kinda cool.

What if the wheel were made from something like acetal, nylon, or some other HDPE/UHMWPE? Would the low friction material help with the wearing away of the wheel assembly?

W.

[Jetguy]

Still needs better than off the shelf allthread. The gear wheel is one part of the equation and yes, better materials and machining the tooth profiles matters there, but you cannot negate the other side that how the spiral thread is cut on the shaft matters just as much if not more. Simply put cheap methods of production are cheap and sloppy. Allthread is really rough stuff. It's made fast and cheap and no precision whatsoever. The actual thread surface is about as sharp and abrasive as you will find.

Even a good and more precise thread from off the shelf hardware does not a worm gear make it.

Both sides need to be a custom machined smooth and tight finish tolerance purpose made gear tooth profile. Anything less is going to wear and slop in even the most limited use.

On Wednesday, July 19, 2017 at 5:24:37 AM UTC-4, Wing Wong wrote:

Hmm.... your mentioning microscope adjustment just made me think of lenses focusing or perhaps tape library kind of applications....

For use cases where you want to be able to accurately pick something up(worm/screw drive) and then move it quickly(screw wheel? Worm wheel?) to another location and then switch back to fine movement, this would be kinda cool.

What if the wheel were made from something like acetal, nylon, or some other HDPE/UHMWPE? Would the low friction material help with the wearing away of the wheel assembly?

W.

On Wed, Jul 19, 2017 at 1:11 AM Whosawhatsis <whosaw...@gmail.com> wrote:

Yeah, you'd really want to to be metal. I've thought about trying this with hobbed drive gears.

This would actually make the most sense for manual applications, I think. A microscope, for instance, could give you coarse adjustment by turning these wheels and fine adjustment by turning the screw the wheels run on.

On July 18, 2017 at 20:29:30, Ryan Carlyle (temp...@gmail.com) wrote:

Huh. That's new. Interesting idea.

With hardware-store threaded rod, it's going to wear the plastic worm wheel like crazy. The surface finish on cheap threaded rod is god-awful, which is totally fine for fastening, but wear-tastic for sliding/rolling contact. 

I suspect the FDM printed wheels are not going to have precise enough tooth shape to produce the proper rolling contact that you get with involute-generated spur gear teeth and straight rack teeth. So there'll be a tiny amount of velocity variation (probably doesn't matter) and much more propensity to wear than a spur gear shape. 

Being able to use the same parts for worm drives and linear motion is neat. 

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[Jetguy]

Again, everything from how the profile is cut to what the base metal is, to the coatings and surface treatments matter. A zinc plated hardware bolt is an abysmal thing to use as a linear motion device. Even more expensive stainless and other options doesn't mean the actual thread finish, tooth profile, base metal and surface metal hardness are suitable for a linear application load.

[Ryan Carlyle]

I think you could buy a 2mm trapezoidal thread tap and use a home-hobbing setup (https://www.thingiverse.com/thing:185448, https://www.thingiverse.com/thing:234155, etc) to cut a set of pretty nice brass worm wheels to ride on Tr8x2 lead screws. You'd need to use pretty big wheels I think. Not sure exactly how the trapezoidal thread tap will convert to gear teeth (actual gear hobbing cutters have distorted shapes to account for the rolling geometry factor), but I think it'll work reasonably well. Running a brass worm wheel on a lead screw should actually perform pretty ok. 

[Patrick Barnes]

What about trapezoidal thread, a 3D printed wheel wheel with an approximate thread pattern...
... then put the wheel in a jig where rotation / linear movement are controlled, heat the wheel up to the point the material becomes soft, and "mold" the correct inverse geometry into the surface?

-Patrick

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[Ryan Carlyle]

Good idea, I think that would probably work. 

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