The basic kit uses two 900mm MakerBeams for the long rails of the frame. A simple way to extend the frame is to replace the 900mm MakerBeams with 1500mm MakerBeams. The timing belt in the basic kit is long enough for such an extended frame.
Depending on the load you want to put on the cart you might need an extra pair of legs in the middle of the extended frame to support the weight. For one extra pair of legs you need the following parts:
From the basic kit you should have enough 60mm MakerBeams, 6mm bolts and lock nuts leftover for one extra pair for legs. You should only need additional MakerBeam Corner Brackets, L-Brackets and 3D printed end caps.
This extra pair of legs is similar to the original legs of the frame but more H-shaped. It consists of three 60mm MakerBeams connected by two MakerBeam L-Brackets to form an H-shape that is connected to the frame with two MakerBeam Corner Brackets. For the upright MakerBeams:
Repeat these steps for the second upright MakerBeam.
For the crossbeam:
Connect the three MakerBeams to form an H-shape:
The cart has bearings that run on all four sides of the long MakerBeams. But now with the extra pair of legs about to be connected to the middle of the frame, the bearings that run on the bottom of the frame don't have a free path anymore and have to be disconnected from the cart:
Connect the extra pair of legs to the frame:
Two 1500mm MakerBeams can be connected to form a 3m long frame with three extra pairs of legs. For three extra pairs of legs you need the following parts:
From the basic kit you should have four 60mm MakerBeams leftover. You should only need one additional pack of 60mm MakerBeams and the other parts from the list.
Finally, you need the 1500mm MakerBeams, straight brackets and an additional timing belt:
Two 1500mm MakerBeams get connected with a straight bracket:
Repeat these steps for the second pair of 1500mm MakerBeams.
Build three extra pairs of legs and connect one to the middle of each 1500mm MakerBeam section and one beside their junction. Then replace the long MakerBeams in the frame with this 3m long assembly.
Finally, the timing belt has to be extended:
The camera slider frame is now 3m long.