Use the Allen wrench to get the 5x12mm bolt in place in the centre hole of the tensioner housing. Fasten it to The end of the pen rail, so the slots in the housing are on the side of the rail, and the nubs slide into the extrusion on the top/bottom.
https://cdn.masto.host/mastodonart/media_attachments/files/112/615/828/319/134/288/original/026d9a2872f615b3.jpeg
https://cdn.masto.host/mastodonart/media_attachments/files/112/615/828/580/267/979/original/03c8bfea3109416b.jpeg
 

 Assemble the tensioner, idler pulley and printed axle as before. Screw on the tension nut and pop the result it into the tensioner housing. 

Next time we’ll square, size and align the frame and rail, and add the y axis belts. Monday, maybe!
https://cdn.masto.host/mastodonart/media_attachments/files/112/615/838/789/964/490/original/2e8066c3ce5bec6a.jpeg
https://cdn.masto.host/mastodonart/media_attachments/files/112/615/839/042/825/341/original/09fa9432064fe780.jpeg
 

 One of the design goals for the machine was to standardize on bolt sizes and lengths to make is easier to shop for. All the axle bolts are the same, 30mm. Every other bolt is 8, 12, 16 or 20 mm in length.