Re: Small CNC for Steel - 30 x 40 cm table area. Worth attempting?
You will be far ahead to buy a machine designed to cut metal. Benchtop routers do a fine job cutting wood and plastics, a lesser job of cutting metals. A small milling machine is an order of magnitude better for cutting metal than a benchtop router, and it will also do a fine job of cutting plastic and wood. Mass is your friend.
Your idea of using a moving table with a fixed gantry will give you a much more rigid machine and would be the way I would build one. For cutting metal the machine frame and gantry needs to be massive compared to the size of the machine for rigidity.
For metal, the spindle needs a speed range of about 100 to 6,000 RPM. 18K or 24K spindles will not run in the correct speed range. The exception to this is for engraving where you do need high speeds for use with tiny tip size cutters. I use an auxiliary 50K RPM air powered spindle for this.
Overall it is probably more cost effective to buy a used bench mill and convert it to CNC. If you have the floor space, buying a used full size CNC milling machine and upgrade the controls would be my first choice. You can pick up used ones with bad controllers for really cheap, for scrap price.
Jim Dawson
Sandy, Oregon, USA