Deciding what I need, and what I could do with it

[coldpenguin]

For a couple of years I have been thinking that a CNC*machine would be a good tool to have. My main tasks for it at the moment would be PCB production, and cutting ribs/formers from balsa/ply for model aircraft. So I reckon that 6" X, and ~2" Z (vertical) would be enough (minimums) for this task?*Their are rare occasions where formers could be larger than this, so I would have to cut these by hand again.

I am making the assumption that the ?spindle? for cutting the material could be the same in these tasks, but possibly would need to be variable speed, and capable of (manually) changing the bit (between wood types and copper metal). I assume that for these 2 tasks, 3 Cartesian axis would be enough?

1) How long should I get the Y axis?
My guess is that you cannot do a partial cut, then turn the piece around to do the other side! Most woods come in either 3' or 4' lengths. Should I get a system to accommodate a 3' length and then move the wood for the rest of the cut, or should I aim for the longest length possible (i.e. 4').

2) For a 3 axis machine above, what else could it do?
If the spindle was variable speed, could it handle doing (what I believe is called 2+1/2) wood carvings, things like picture frames etc. if they fitted within the dimensions?

(hopefully up to this point things are still making sense)

These devices seem to be quite expensive on the whole. I guess that although the materials might be 'relatively' cheap in themselves, they need to be precision made. My metalwork/woodwork skills on this scale are probably not up to the accuracy required, so I would be looking at getting some 'kit's of axis sets [if such a thing exists, a different kit per axis to make the shape you want?] rather than building from scratch scratch, or a completed unit. What worries me, are people who state that it has taken them 3+ years to get them working. If I had a device working, I know that I would use it, but I am concerned that I would get down-hearted after 3 years if it still didn't work! Is this length of time usual?

3) The wishful thinking question
Is it usually possible to add the 4th axis to a machine? What would be required for example to be able to machine pieces suitable for a model helicopter? Would this also require the 5th axis, and would the 'spindle' need to change?


Edit:
One last question. I am in the UK, are there /good/ and /cheap/ places in the UK to obtain materials from, or am I best importing items?

[TarHeelTom]

Generally speaking, you want the X axis to have the longest travel. There probably are some exceptions, but almost every machine out there using a moving table has the X axis the longest. There are, however, some gantry type machines where the gantry moves in the X and Z directions, and the table causes the material to move in the Y material. Such a table could handle quite long cuts.

In my opinion, for making picture framing stock, a shaper table would be much quicker and more economical than a cnc machine.

Always try for more Z axis than you think you need. You're going to have to be able to change bits, chucks, and collets, and they take more room than you think. The Sherline, for example, has 7" of Z travel, and many guys find it necessary to install a spacer under the column to increase the Z height.

Building a router is very commonly done, but the cost of the materials may surprise you. I find that my projects always come in well over my estimated budget.

There is a "plotter" type device called "Klic N Kut", which is essentially a small mill using a knife for a cutter. These are made for Arts and Crafts applications (think ladies decorating) but seem to be excellent for cutting out parts from balsa.

Don't know how close you are to any industrial areas, or how much room you have, but industry is rapidly changing from the old manual machine tools to the very advanced automated "machining centers". The result is that there are a large number of Bridgeport vertical mills (and clones) out there for sale, at very reasonable prices. I shopped for a couple of months before buying one, and there seemed to be a new one up for sale almost every day. If you buy an older cnc machine, expect to replace all the electronics. The older stuff is hard, and expensive, to maintain. The newer stuff works much better, and costs a lot less. The trick here is to find out where these used machines are advertised for sale, and be ready to pounce when you find the right machine.

There are a lot of low-priced controllers out there which only offer three axes. Many of them offer a fourth axis. If you think you will need to add a rotary table, by all means make sure you can add a fourth axis.

The controller software is already equipped to handle more axes. EMC2, for example, is ready for 9 axes. Mach3 handles 6 axes, which is still more that most people will ever need.

My advice is to buy something ready to run, or almost ready to run, and go with it now. After you've had it a year or so, you'll have a lot better understanding of how it works, and what your needs are. Then decide if you still need/want to build something yourself.

[coldpenguin]

Thank you both for your responses.
It has given me some areas to think about.
Generally in the past when I have thought of CNC machines, the devices which are more often called routers came to mind, rather than mills. The subtlety between the two had previously been lost on me.

On a previous trip to the states, the Kick n Kut machine did interest me, but I wasn't sure at the time how extensible it would be, or whether I could keep it running back in the Uk. If I haven't taken the plunge before my next trip, I will look further into it.

Changing the spindle bit is not something I had previously considered either. I (naively) thought that perhaps the part rotates to an easier position, but now realise that this would destroy any calibration on position.
Thanks,

[M250cnc]

You won't lose position by changing the bit you will need to set the z height position though.

Cad/Cam makes this painless, apart from the pain to the wallet.

TarHeelTom made a valid point about a Bridgeport machine, in that they are very cheap to buy on the well known auction site. Downside is they weigh about a ton, so a bit difficult to move around.