My first time welding, using a stick welder that cost a fraction of the price I was quoted for the aluminium profile I had planned to use just for the gantry, was very ugly but completely effective. But it will not be dead accurate and as Jazz says you will need to design in adjustment for everything. One thing to be careful of is to design the machine so that you can actually access all the nuts and bolts you will have to get at once the whole thing is assembled.