Hi
The Item units look cool but might prove expensive as this is a system that would enable a number of industrial production line machines to be mocked up and quickly altered.
You will need to check on the availability of timing belts that are long enough for your application and that the tooth form is of the low backlash form. The tooth form has to match the pinion and Item have this sorted with their tension unit and 'reversal' unit. Yes they are listed as 50 m being the max length.
In general timing (toothed) belts have a core of steel or kevlar strands on to which are moulded the teeth. The belt is placed under tension to take out the free movement and then to stretch the belt a little. With the 'reversal' unit there will be around 180 degrees of wrap which will place several teeth in full mesh with the pinion. With a wide gantry you will have a similar drive on both sides and a shaft through the 'reversal' units this links the gantry legs and prevents, or reduces, racking to a minimum. The quoted pre load is 1% elongation which makes me think the cores are not a strong as one would like for a precision milling operation.
There are plus and minus features when comparing metal R&P with timing belts.
I general timing belt drives will be less noisy (I am assuming that they are not slobered with thick grease) and more tolerant of dirt than their metal counterparts. The accuracy of the toothed timing belt will probably be inferior to a precision ground metal R&P but the cost will be far less and the setting up of a toothed belt drive is a lot easier than for a metal R&P. I suspect you would have to spring load the rack into the pinion as there would be flex in the frame of the machine that would cause the mesh of the pinion into the metal rack to be less than perfect - spring loading would mean extra friction and wear. It appears that the reversing units are used as a neat way of providing the gantry drive and integration with the alloy section used by Item.
The timing belt is made to an international standard and this will lay down the minimum pitch variation tooth to tooth but the individual source of supply would need to be consulted to find out the tolearance in absolute position of each tooth with respect to a fixed datum over the length of travel - also you would need the length to langth variation and this could be lower if you by as one length. This is a gut feeling as I would go with ball screws!
This link will give you a clue on costing:-
http://www.ondrives.com/commercial-g...gth-belts.html
I don't have any information to hand about the positional accuracy.
Good luck with the design
Pat