I'm faced w/ laying out some centerlines on my x axis. This normally wouldn't be a huge issue on a smaller machine, however in my case the centerline is 240" long and must be pefectly straight. I'm laying out the center line for mounting my linear rails and I've found that these rails aren't nesc. perfectly straight right out of the box. Mine have ever so slight a curve to them. This isn't really an issue as they are bolted down every 60mm and this can be taken out of them. But how do I get their mounting holes [in the sub structure] straight before the install? I'm not going to D&T their mount holes but rather just drill oversize mount holes and then add a nut to the bottom of the mnt bolt. I've got holes in the side of the mnt structure to reach through and put the nut on. This will allow me some lattitude to get things parrellel and sqr w/ the opposite rail [ which is 10' away]. However I do need to start w/ a master rail which is perfectly straight, and the second rail will then be slaved/matched to it. I thought about using fishing line and feeler gauges, any other suggestions?? I don't think a heavy pc of CRS would be straight enough would it?? Is there some trade secret out there that someone could pass on?? My sub-structure [ which the rails mount to] is composed of welded HSS, its very straight, but there are no garrentee's that there isn't a 1/32" [its small if there is any] bow in them [side to side] so measuring off the side of them isn't an option. I'm potting the rails down w/ Moglice so the elevation isn't an issue [ if there is slight warpage from welding]. I've purchased a master precision level [ 0.0005" level accuracy over 10"] which when used in conjuction w/ jig blocks that fit the rail and two set screws per jig block, I can get the rail as flat and level as it needs to be, I just don't know how to get the mount holes laid out perfectly straight. And once the rails are mounted there is still going to be the need to bolt them down straight w/out and curves in them.
Suggestions and opinions welcome as always.