again, a few very important bits of info between you lot there.

I see the sence in the more expensive cards, i will pick one of them. and its seems that there on it with linuxcnc and homing double axis, and there are some work arounds, I like mach3, but hate windows, although i have to use it sometimes, but it is worth using a windows machine just for mach3/4.

I found a local company that can supply me the b-screws and rails and barings ect. and there are a few high precision engineering company's near me as well, so am looking at some alternative mounts for the Y gantry to the x axis. I have been reading with interest on here the posts for R&P over ballscrews. I will put the designs in sketchup in the next day or so.

2 rails per x axis side, one above, one below, 2 barings per rail, seems to make the most sense on an equal loading/force approach. Maybe its not a bad idea to have the barings as the week point, Because they are cheeper to replace in time and money, compared to unbending a machine gantry. If, from the design stage, you consider the barings as consumables ( not to short a life time though ). it free's up some design options.

R&P suits a heavy gantry, and i can see why most heavy machines use it, but, when your building your 80k to 100k + to sell to a customer, the customers not going to acsept thaty every n100 hours of running, they will have to replace the barings, and every n1000 hours , the ballscrews, so you design a machine thats heavy enough to absorb the forces, to give your barings and ballscrews some help. but the extra weight requires bigger motor or servoes, and causes other problems, or pushes you design in certan directions.

wereas, if you specificaly design your machine for the barings to be the week point, you can make you Y gantry a box, that contains a Z Axis, thats a box iteslf with rails on both insides of the Y gantry. The hole Y ganty, clamped by rails and barings on the top and bottom of the X axis, with the ballscrew in the gap between them (between the edge of the Table frame/X axis and the Y ganrty box ).

Done this way, every axis forces are contained within the box frames, and takes out any levers if you get what i mean, all the above gantry designs are big leavers that you have to compensate for. Whereas, if the only leaver is the projection of the bit from the Z bottom, and that is within the X frame box. What this gives you is equal forces/loading in X and Y both directions, and a very strong Z Boxwith heavy down force capability, but, very heavy loading X and Y, if the Z is retracted and only the tip of the bit is projecting, Like for surfacing a large piece of stone.

Also, a box within a box makes sence because you can bolt certan cruital parts inplace ( Y gantry ), on the machine, make sure there all square and level, before you weild, and you can use the bolt points to strengthen up the metal near the weilds, to help ofset any twisting/pulling your frames wants to do while its weilded.

My main reason for designing like this, apart from equal forces in all directions, is machine down time if a mistake is made, This is through experiance of owning/running a Gocarting track 97-2001. Go carts have lots of expencive barings in them, and after 2 years of large parts bills, we redesigned and refitted the frames, to eas of on the barings, What a mistake, we spent a lot less on barings that season but down time per cart frame due to bending was horrendus, each frame had to be stripped and bolted in the jig for 2 days, heated and coolded, then brazzed, then rebuilt, That job went from once after the end of each season, to frames queing up to be placed in the jig.
My point being, make a mistake and bend your machine, your down time is a lot more (maybe days ), and you dont know what the cost of repair might be, whereas, if you pop your barings, its costly, but you know how long it will take and how much it will cost before your machine is running.

I will try get it in sketchup later