I am after building the frame of a model gantry crane out of 12mm diameter brass bar and for that reason I can not use my rack and pinion idea and am really stuck in trying to find another way to make the hook move over and back. Also the hook goes up and down using a motor and the motor is resting on a piece of plastic which also which is really all that moves like one big block to move from one side of crane to the other the distance from end to end of crane is about 350mm really appreciate any help

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I was involved in building a Gantry Crane which was featured in a magazine. Here are the pics of the Gantry. It was the second generation. This was before the IBM PC. It was computerized with a SBC (Single board computer)

The gantry had 4 fixed positions and potentiometer feedback for the up/down of the electromagnet. This was turned into two positions with the electronics. The black gizmo in the car is a 10 turn pot. Under it is a gear reduction motor.

There is two rows of a sliding contact on a gold plated circuit board. One of the traces was not continuous.

It connected via a 10 conductor coiled cord.

The other part of the gantry had 9 positions. This thing was about 4' x 6'.

It was capable of loading and unloading 27 pieces of cargo to a loading dock at the push of a button. Dynamic breaking was also employed.

The larger gantry used a precision rod and a couple of linear bearings. It only had to rest and ride at the extreme edge, so it just needed and idler.

The long Gantry has to really consist of at least one pair of linear bearings and one piece of precision shafting. See: LM76 Linear Bearings & Linear Shafting & Linear Motion Slides

Precision shafting is held to tight tolerences. Friction can be an issue with hobby motors. The far end just needs an idler because gravity holds it down depending on how much the short gantry weighs.

Movement can be done with ladder or bead chain with the ends secured to the gantry.