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Whats inside the gearbox/motor?


#21

Does it have to have more than 1 thread start to achieve 2 teeth per turn?


#22

I believe so, but maybe @dlang has more experience on that question than I do. Double the number of encoder pulses per rotation could do thus as well, yes?


#23

you can’t really tell by looking at the side of the gear, you need to look at the end (I guess I need to open it again)

Yes, it would need multiple starts to get two teeth/turn

the encoder is pulses/rev, that gets multiplied by 4 (two sensors, each reporting the rise and fall of the voltage), there’s no easy way to double this without it being very obvious


#24

Ok, I am very surprised to report that I think it is a double start gear


#25

Sorry about that.

Yeah, my brief research agrees with you.

I agree, I can’t see a way in which this logically would be doubled.

Thank you so much for opening that three times to get us the answer!!

Alright that puts the gear ratio at 289.776234568:1 if there are two starts which results in 8113.7345679 steps/revolution. That matches what we see empirically. I certainly don’t see anything that gets us the 291:1 that was advertised at all.

It equates to about .27mm excess distance per revolution. This certainly causes a decent amount of error over long distances. Luckily when in use, the error is probably unnoticeable unless cutting something very large where you need good high accuracy.

Once we agree on the results, it is an easy fix to update the default values in GC.


#26

so this makes it:

worm gear 2:25
next stage 10:35
next stage 9:29
final stage 18:37

648:187775 (and it does not factor smaller)

factor in the encoder (7*4) and you get
162 revs per 1314425 encoder steps or 8113.7345679012 encoder steps/rev

so we have been off by 0.42% in our measurement of all chain lengths, or around 10-12mm in motor spacing measurements (and at about 1500mm of chain, one tooth off)

no wonder we have been going nuts trying to get things accurate

We owe a LOT to the people who reported the positioning error


#27

using 8114 results in an error of 0.00327%, which over 3000mm translates into an error of 0.098mm, which should be close enough (8113.7 would result in an error of 0.013mm over that same distance)

Can someone please change this and set their machine to spin for a while and validate this?


#28

Agree. This “small” of an error would have slipped through for a long time.

The learning experience is not to trust data sheets. The zaxis is probably wrong too, but its error has even less if an effect. And everyone buying stuff on Alibaba, me included, has to go take their gearbox apart now.


#29

I can run it tomorrow. I believe we can handle fractional steps per revolution if we cared. But I can set it to 8114 and let it do 100-200 revs. I doubt I could see that error even at that distance.


Calibration pattern for triangular kinematics
#30

at 200 revs there should be an error of about 0.2mm


#31

thank you @dlang @krkeegan for this


#32

JDogHerman,

Fantastic post,
I have correspondence from Bar when I needed to clarify the specification of the X&Y axis motors along with the encoder. I also confirmed this with the supplier.

The gearbox ratio should actually be 291:1 from the motor shaft to the output of the gearbox. This has been confirmed by both Bar and the supplier. But as they say fact can sometimes be stranger then fiction so we should send the findings to actually confirm the gearbox ratio with the supplier.

The encoder is an ET-MY37 and has a resolution of 7ppr (Pulses per revolution) and uses quadrature encoding.

Bar noted that from these two signals we can actually extract 7*4 = 28ppr because each transition is a distinct place in the signal. Multiplying that by the 291 reduction ratio give us a final output shaft resolution of 8148ppr

image

Kind Regards Stuart


#33

@JDogHerman I’m doing my best to tell the story of the community taking apart the motor and figuring out the real gear ratio, could I use your pictures in the newsletter? (giving you and everyone involved credit of course)


#34

To all,
I checked another thread “what is the correct chain pitch” which made reference to the followup with ETONM and found the response regarding confirmation that you all already know but thought it should actually be in this thread .

Question to Bar

Do you know if anyone has raised the issue of the gearbox ratio with the supplier yet? If not I would be happy to follow up.

Response from Bar

I did follow up with ETONM their response was:

“I understand your concerns. the gear ratio stated in catalog, not all of them are exact as the real, main for protecting gear motor technical information. Your caculation is correct, 289.77:1 is ET-WGM real gear ratio. and we always use this gear ratio for your order, never change. so don’t worry this.”

I’m not sure I believe the argument that it was to protect the technical information but at least now we know that they know what is going on and they know not to change anything without telling us.

Did we get confirmation regarding all the gear stages though??

Kind Regards Stuart


#35

see the pictures I posted of each individual gear, the only thing I didn’t get a
good picture of was the worm gear in a way that shows that it’s a two-start gear

and since the math and reality match, that’s good confirmation for me :slight_smile:

(and yes, anyone can use the pictures for anything)


#36

Fantastic work dlang.
I am certainly learning a great deal to get up to speed with the rest of the community but enjoying it.

Regards Stuart


#37

I was following up on the multi start worm and came across this useful video on youtube

They go over single and multi start worms

Regards Stuart


#38

@stuartri

Could you add this to the mechanics section of the Wiki?

Once this is complete can someone take lead to put all of this in the wiki?

Thank you


#39

Bee,
Do you mean add the video link to the mechanics section?

Let me know and I will add what you need.

Stuart


#40

Yes - A link to the video as an educational piece.

I think it’s worth keeping as at some point we should develop an education section aimed at schools to try to spark the students in to making other projects reusing the technology they already know how to use. This explains how the magic happens behind the curtains.

Thank you