Tap, what happens when a rail smaller than MGN9 is used

[Indeed]

Hi
A while ago I watched "Voron:LIVE! *Tap Tap Tap* Is this thing on?"

At some point it was mentioned that an MGN9 rail was used because a smaller rail, such as an MGN7, is constructed slightly different from its big brothers and it doesn't hold up in the Tap tests.

I'm really curious, what happened during the tests?
Such as:
* it wore out within a few hundred thousand probes
* unreliable probe results, either right from the start or within too few probe cycles
* it wasn't stiff enough and the hotend vibrated a lot when printing
* forces were too high, it broke and everything fell off
* something else

Edit: I forgot to set the "Question" flag and now I can't find out how to set it in edit mode. Ah well...

 

[NoGuru]

I don't have data but just a guess, the weight in combination with the torque from the gantry moving, it will twist out of spec.

 

[Badnoob]

The tricky part to tap isn't the moving up and down...its preventing it from moving in X and Y.
MGN7 didn't work for me. On a different printer in a different toolhead maybe it would, I don't know.

 

[claudermilk]

I swear I remember that being addressed in the linked video. My memory says they ran into rigidity issues like @NoGuru said.

 

[Indeed]

I swear I remember that being addressed in the linked video. My memory says they ran into rigidity issues like @NoGuru said.

Yes, but "rigidity issues" is quite broad.

It was narrowed it down a bit by Badnoob (Thank you!), apparently there was a problem with XY movement.
That could mean several things though.

XY movement while probing, movement while printing, both, immediately or not until some wear and tear, only while chamber above 70 celsius when printed parts begin soften, or something else.

If I were to guess I'd say that it was immediately, and only while probing.
Immediately, because of more flex in the printed parts due to the narrower MGN7 rail, with less contact area between rail and printed part. Same with the carriage.
Only while probing, because the magnets should have kept the toolhead stable during prints if they were correctly seated.

Disclaimer: I'm not trying to be a know-it-all. I'm just very curious...

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Edit:
About magnets being "correctly seated": It's very easy to twist the magnets when the adjustment screws are tightened. Even the smallest angle between magnet and the flat screw head can make the toolhead more or less wobbly. I know beacuse it happened to me. And haven't been able to get a perfect fit yet. Maybe I'll just remove the adjustment screws and instead put some epoxy on the magnet holders and then let it cure while the magnets are properly seated (yes, I'll probably regret it later).