RE: Z Axis trapezoidal nut is difficult to thread onto lead screw
In inspection I've found it's not so much the nuts that are an issue as there being burrs on the end of the leadscrews where they've been ground off...
RE: Z Axis trapezoidal nut is difficult to thread onto lead screw
I also had difficulty with the Z-axis trapezoidal nut, specifically when lowering the X-axis onto the two lead screws.
In my case, despite suspecting the threads, I found that the position of the trapezoidal nut in relation to the X-end hole (i.e both centred and in line) is easily misaligned by over-tightening/uneven tightening of the two M3x18 screws. If I left the two screws just tight enough, I could get the nut to slide easily down the lead screw.
RE: Z Axis trapezoidal nut is difficult to thread onto lead screw
The more experienced forum members will tell you it is best to attach the T-nuts AFTER threading them onto the Z-screws. The Prusa sequence of attaching to x-carriage first increases the risk of cross-thread damaging the nuts.
Use a pair of empty filament boxes standing upright to hold the x-carriage.
Lower the x-carriage WITHOUT the T-nuts onto the rods and z-screws.
Individually thread the T-nuts down to the x-carriage.
Finally, bolt the T-nuts to the x-carriage.
We repeatedly read the woes of first time assemblers damaging the nuts. Threading them separately from the x-carriage reduces damage risk and lets them auto-align.
RE: Z Axis trapezoidal nut is difficult to thread onto lead screw
I had the same experience as the OP. Support claimed it was "User Error".
And they were right. Those nuts come shipped on the shafts. They spun easily at the time.
Cross threading one of them was all my doing.
Prusa shipping costs are prohibitive. I ordered new ones from China.
While waiting, I manually twisted my bad one on the shaft for a long time. Until it moved smoothly but tight.
I put it back in service and it pretty quickly wore in. Still in service - and my new Chinese ones sit in my box of spares.