One of the most critical parts on a Rostock style printer are the diagonal rods. All rods must have the same exact length.
I asked the experts over at the reprap forums and got some fast and informative answers. You can read the whole discussion here. The short answer is given by Wildcard:
It is extremely critical. If they are not exact, the machine will suffer from mechanical effects. My first build had this problem and if you moved the effector/platform near the tower of the arms that were shorter, you would see the effector start to tilt up. If you moved it away, it would tilt downwards. So it effects how flat the printer will print and with changing angles like that, the prints will suffer from incorrect measurements and other print issues that will be unable to fix in the firmware unless corrected.
Too bad I expoy-glued the rods to the Traxxas u-joints before I sought advice on forum. Now epoxy can withstand forces of up to 1900Nm/cm² (that is a lot I guess) but the glue becomes brittle if you heat it up to 120-150 °C. So I removed the rods from the printer and put them in the oven at 150°C for 20-30 minutes. It turned out that this actually works really well and with a plier I could easily remove the u-joints.
Well, on my first try on connecting the rods with the u-joints I already did build a jig to aid the process of getting the u-joints in parallel to each other. This really work out well but as you can see from the picture it has one major drawback. You cannot reliably set the distance of the two u-joints. Every time you glue a pair together it has a different length.
Also the epoxy glue I used is a fast hardening 2 component glue. The processing time is only 5 minutes which means you must be very quick and certain on trimming the length of the rods.
So I bought an epoxy glue with a processing time of 90 minutes. Plenty of time to mess around with. I made a new jig using 2 M3 bolts, 2 nuts and 2 washers. The pictures below should explain it.
The washers on top have a big enough surface to help to get the two u-joints in parallel.
That’s it. Perfectly aligned and all six rods with the same length.