In my opinion the thread on pulleys is too short; or, in other words, the pulley collar has to be thicker.
The grains have a conical tip, so they engage only one or two threads of the pulley. Very easy to strip, and not easy to re-thread.
Considering that the pulleys have to be moved sometime to find the good belt play, I strongly suggest a redesign of the pulley collar. And, possibly, with two screws.
What other think? Am I the only one with three stripped threads on pulleys?
I found the same problem, which leaves my printer useless until I find a way to fix the threading on the pulley.
Any suggestions?
at least 3 Pulley threads are worn out.
At least I’m not alone 
Agree , bad engineering had to tap to for 4mm for temp fix and then make a sleeve on lathe to reinforce they need to supply better pulleys
It looks like I also have pulleys with stripped threads, or ones that aren’t gripping the rods.
http://forum.velleman.eu/viewtopic.php?f=58&t=13224 (final post is when I realized it was probably a pulley issue)
I haven’t finished to built the K8400… but I’ve the same problem with one pulley :-(. I’ll probably use e 4mm screw.
It’s a glimpsy 1.73mm wallthickness. ith the M3x,5 thread and the 1mm ‘no-thread zone’ on the setscrew you can consider yourself lucky when 2 threads hold it in place. I pressfitted some aluminium collars on mine and drilled them with 2 M3 holes.
Well, i would suggest to make the thread collar bigger in diameter so the screws got mor “flesh” instad og just another “breaky” thread hole.
@to3dornottobe:
I would be very interested in what type of “collar” you did put on.
Can you tell about the diameter. Did you take some special rings or what was your source material ?
Sorry, no widely available ring; used my lathe and some 7075T6 rod I had; while some of them where 11.27 mm, 3 of them where 11.33 Can’t seem to attach pics here…
Perhaps you can print them from PLA?
You can upload pics with Tinypic without having an account.
Cheers,
Dylan
Hi Dylan, i’ve used that before, thanks for pointing me to that site again.
Here is the ‘upgrade’ i performed. Nothing fancy, just turned to the right diameter and cut-off, not end machined or anything.
Looks nice. unfortunately not everyone has a lathe …
I don’t understand why Velleman chose that pulleys.
The look brittle right from the distance …
That looks great!
Unfortunate, I don’t have a lathe :’(
Cheers,
Dylan
I agree, i think it shouldn’t be needed to use these collars in the first place… You can print them. Thickness is 5mm, id is 11.27 or 11.33. I can draw them and upload an STL, if anyone needs it? (it would be better than the 1.5 threads in the aluminium) I’m pretty sure that 1 screw would do in combination with the plastic colar. Too bad you need to take it apart again though 
Anyone else noticed that the Z stepper sits too close to frame? It needs a spacer behind it to align it with the acme rod.
How did you get to this conclusion? What is the difference in mm? Can it be calculated from the drawings released by Velleman?
[Edit] Actually I measured the distance between the rod and the back wall and I found a difference of about 1 mm between the bottom and the top. So this indeed means that the threaded rod is not parallel to the back wall…
I find it super disappointed to read this thread. First post is 2 years old and i have the same problem on my K8400 bought 1 week ago. “The Fix” from Velleman is another bag of set screws which isnt pointy, but with a flat nose. that doesnt help a lot because the walls on the gooddamn pulley is 1mm. So i got 7 stripped out holes and a K8400 that are not functional.