is there a way to join long pcs of linear guide ways ie from hiwin
i know alignment is key.... has anyone had any success with it .. im guessing if you miter the joint at 45 you have smoother transition.

I've got a factory set that is ready for butt joining:

I've got a factory set that is ready for butt joining:

how well do they travel over the seam

Hard to say as I haven't bolted the rails. Table test reveals some grinding noise and slight hesitation when crossing.
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I don't know if you're trying to make a perfect machine, this will probably hinder it. Alignment making CNC machines is critical, but trying to joing linear rails so the bearings glide without misalignment or a stutter would be hard. What are the rails made out of? If they are easily welded I would build some accurate jig and align them so the bearing rides perfect, then Tig weld some small tacks, so as not to heat up the rails or warp them, keep tacking until it's stable and slowly weld, then grind. I wouldn't weld the whole thing and stay away from the areas where the bearings contact. You know tack weld in the top recess and on the base.

Drill holes at both rail cross section. Insert a dowl. Do No Weld

A possibility if he has a mill or something, it would have to be a very accurate hole.