Some fab questions......

[Kaizen]

Starting work on my long awaited sliding gate in front of my garage. This is the kind that sits on wheeled base plates. I did not have anyone near me that supplies this type of thing and because shipping over ten feet is ridiculous i had them cut the track into ten foot sections. Going to weld it back together is leading me to questions how i make sure this 20 plus foot assembly will be straight? i have 2.5" tube that i thought i could use as a backbone but i'm not sure that is straight either. Looks like it might be bowed or the floor is.
Do i just run a string line? Do i need two so i have the x and the y straight end to end?

 

[jack stand]

What is the function of the track? Does it simply guide the top of your gate?
Is it above the door and out into the "air" ?
I'm just wondering if you might not need super tolerances or
Could you fabricate a 2' long "plate" or something to bolt the pipes together allowing for some minor adjustment.
Catch us up on your vision.

 

[sick467]

I'm not exactly sure how this track will be mounted, but I would consider using two pieces of 3 or 4 foot angle iron (or channel, or whatever you have that is straight to the eye) and clamp one piece on the bottom and one on the side prior to welding. I would block up the overhung portions of the track so that the whole length is as straight as your eye (or string) can get it. You could also position and clamp one piece on a stable and the other on another and let the clamped section hang over mid air. The key is to have the ends to be welded as straight in-line as possible. I straight edge would be handy to have here.

Once you are satisfied with the straightness, especially with in a few feet of the weld, Tack one corner and let it cool. Check the straightness, adjust if necessary, and then tack the opposite corner. Let cool, check again, tack weld adjacent corner and repeat for the last corner. This will help lock in the two pieces and you can perform the stitch welds to finalize it.

Any broad curves can be taken out upon installation so long as it will have multiple sturdy connections that allow for some adjustment. I recently did this with pipe sections to make long hand rails out of several pieces of short pipe.

Whatever you do, don't blow through the joint and create weld boogers on the inside that cannot be ground back off.

 

[Kaizen]

Sorry probably should have included this in the first place. The below trolley is bolted to concrete piers and the slotted track sits on them and slides. You are correct i don't need perfection but If i am off or have a bowed base it will telegraph as i build on top of it. The frame of the gate will be 1/8 x 2.5 tube and sit on top of this trolley track. It will be welded to this base so when its assembled it will be fine. These parts are stupid expensive so trying not to make any serious errors. Also below is a picture of an approximation of what i will be doing. the pickets will be most likely welded into 2 panels and then attached to the frame of the gate.

 

[Kaizen]

Appreciate everyone's help. Stringline worked great. Used two clamps at either end and some shims to get it straight and level. This thing is going to weigh a ton