first off i guess i should say hello.
my name is Adrian and i am new here. recently i have found myself needing something like this and after some googling i found that it is doable.

this is going to be a tight budget build mainly because i was recently made redundant. i got some sound advice from a few guys here and what i realise is that i really needed to make some realistic decisions about what i want this machine to do. after some thought i feel that routing mdf or plywood is what i need, milling alloy would be nice but not a requirement.

the design will be a moving gantry with a workable area of roughly 2' by 4'. that is not set in stone and may be changed later as i work out the details.

the bulk of my budget will go on a controller/stepper package. i do have some electronics training but i wouldnt be confident in the reliability of anything i could build. since this is an important part of the machine i guess i should get this part right. i dont want to spend hours trying to get the machine working only to find the controller isnt working right. i am looking at a kit from http://www.diycnc.co.uk/html/cnc_systems.html
it is the three axis kit for £207 i am looking at.
if anyone has any feedback on these products, good or bad, i would be interested in hearing it.

the machine will be primarily constructed from mdf. i do have a table saw, router table, pillar drill and some other stuff so i shouldnt have much trouble making the chassis. i will keep an eye out for some second hand linear bearings and lead screws but for the moment the good stuff is out of my league so i will attempt to make my own. i will design the chassis with a view to maybe upgrade it if i get the chance.

my idea at the moment is to use threaded rod and make a delrin or nylon nut, use some ball bearings at either end and see how that works out. i understand that direct drive of a threaded rod will limit my feed rates but i am thinking i might try to gear it somehow, anyone any experience with this method? also i do understand that ball bearings will not handle the thrust loads and that is something i will look into.

on the linear end of things i am thinking of getting some nice straight angle stock, probably alu, and making a linear bearing like i have seen on some builds here. this idea basicly has some mdf chamfered on either end, angle fixed to the chamfered ends. on the gantry side it has some more angle with bearings fixed to it, one assembly fixed top and bottom of the rail. one problem i see with this design is possible loose or tight fit. i intend to make the bottom bearing on the gantry spring loaded with a bolt to adjust the tension. i can see it in my head but its a bit hard to put in words. i think it should work well, in theory at least.

i have gotten hold of solidworks and i am trying to model my ideas so i can see how the various parts go together. it is a steep learning curve though as i have only ever used CAD electronic design before, not mechanical.
anyway i guess thats it for now. i will continue to think through the design and try to model it, might post some pics of the model to get some feedbcak. it will be well into the new year before i put blade to wood because i want to have a clear plan.

i would like to thank all the guys for their advice so far and i hope to get some feedback on these ideas as i will probably keep second guessing myself.