If you are referring to aligning the rails so that the rails are perfectly parallel:
It is important to make sure one rail (a rail at one side of the table) is perfectly aligned first. Do this by fastening one end of the rail to the table (flush with the table edge), then fasten the other end (flush with the table edge). The rail may not be perfectly straight at this point, but that's ok. This process includes the top and bottom rail, fastening and positioning simultaneously.
You are going to fasten the midpoint of the rail. Get a clamp ready so that you can freeze the rail in place. Look down the rail from one end. This is a good way to determine if the rail has a curve, or is straight. If there is a curve, move the rail midpoint laterally until the rail is straight while looking down the rail from one end of the rail. This may take a couple of people to do, one looks down the rail, the other moves it slightly until it is straight and then clamps the rail. Drill and fasten the midpoint when satisfied. Repeat this process, fastening at the midpoint between screws.
Now place the gantry on the table with the v-groove bearings resting on the one rail side. Add the next rail to the other side of the table. Position the rail so that it is directly under the other bearings. Make sure the gantry is at the extreme end and fasten the rail at that end. Move the gantry along the and fasten the rail as you move the gantry so the rail auto-positions itself.
Thanks for your answer. So the next step is making certain that the gantry is exactly perpendicular to the x axis. Do you just place a square on the rail and adjust one or both chains as needed?
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Is there a specific process for aligning the x axis on the greenbull? There is mention of it at the end either the rail or gantry installation but I could not find it anywhere.