I finished the assembly of my K8400, and here is the summary of problems I had :
1- the manual is very good, but there is a small mistake : in the chapter 11 (Assembling the extruder), the M5 screw must be 16 mm, not 12 mm.
2- A little risk of mistake : in the chapter 11 (Assembling the extruder), the distance between the pulley an the motor must be measured from the “mounting face” of the stepper motor (meaning the square face). Be careful not to measure from the round centre part of the motor.
3- As the printer will vibrate a lot, it’s necessary to put thread lock glue on screws without a locking nuts. Use only a specific light thread lock glue, not a strong one, and no cyanocrylate glue ! Thread lock glue is necessary on : LCD panel nuts, printer head nuts, motor mounts screws, filament spool mount. There is no information about that in the manual.
4- When you switch on the printer the first time, the print head plug must be removed, and then, it’s very important to check the motherboard : there should be no red light switched on ! If a red light is on, the motherboard is faulty. I had the problem, and destroyed the isolator guide of my print head (you can read it in french in the post “Problème de chauffe du deuxième Extrudeur (K8402)”).
5- The filament spool mount support might not rotate well. You can read my solution in the post “Filament Spool Mount Wobbly”.
6- In the chapter 17 (tightening belts and pulleys), the alignment method to obtain parallel rods is very bad (impossible to measure a distance from the side of the printer). The alignment tool should be included in the kit. I will have to print it.
7- Last but not least, all the screws are too short !!! The Nylstop lock nut is designed to stop nut and bolt disengagement during use. The internal nylon ring deforms around the threads as the nut is twisted on to the bolt, ensuring a tight, vibration resistant grip on the bolt. But for that, the screw needs to be long enough. All screws should exceed at least one thread over the nut. I tried to tighten hard some screws to obtain that, but broke some plastic parts… I will have to change all the screws on my printer… This is a very big fault from Velleman !