Do you think M4 bolts will flex with jerks and changes of directions and accelerations enough to loose accuracy for the following XCarriage configuration? The belts catchers are, as seen from the pic, riding on the M4 bolts, one constrained at both ends but pretty long, also being the third post that positions the hotend mount, while the other is doing the LM10UU clamping as well as providing a support for the belt catcher.
Not the bolts but maybe the parts they hold together.
@Marcus_Wolschon Mmm, the plastic they are stuck into is quite thick, ~15.5mm including the gap, maybe 13 if the gap is squeezed until touching. It’s also about 12mm wide …not enough? I have room to make it a bit wider …15 or so.
I have a design similar, but without a top part. I used M5 and the were bending. What i end up doing, is using M5 40mm motor spacer to beef up the screw
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@Jean-Francois_Talbot If M5 were bending 
… I kind of considered M4 safe because the insertion point of the belts is quite low, basically in the first 10mm above the carriage. Now I am even more wary. I wouldn’t beef up anything there as it’s impeding the airflow behind and after the axial fan to cool properly the hotend…
Also it will be getting quite heavy with two hex couplers if I were to beef up. I might make them M5 though…
@Florian_Ford don’t forget my design only has 1 anchor point, and my carriage being all metal carrying a geared stepper motor and four metal bearing block is quite heavy. The forces of back-forth are greater in my case than it yours.
And as it should be in any 3D project, do accept the iterative concept… try, fix, try again until perfect
@Jean-Francois_Talbot
Right, I won’t touch anything right now. Just about to send my parts to 3dhubs to be printed, as soon as I get home next week i guess. I just checked if I can do M5 in the same parts and it’s feasable.
Are you using the intake sides of the part cooling fans to cool the hotend?
@Daniel_Kruger
Nope, the blower fans just provide the non-intake side to make a sorts of a funnel for the axial fan that sits in the center. Pic below:
It looks like the fans will add a little to the stiffness of the assembly.
@Daniel_Kruger
Nope, they are decoupled from the HotEnd Mount as I want that to be able to be adjusted in height and also by 3points leveling if necessary, so the upper holes of the blower fans are not used. I am still pondering how to only fasten the blowers to the base carriage…
I could use m4 hex couplers to strenghten the posts.