Hi Colin,
We have a couple of desk top routers at the school I work at and I use them regularly.
I'll try to answer your questions as best I can.
How to hold down the material being cut.
Vacuum beds are ideal but are not usually available for these small machines.
The most common way of holding material down on the bed is with double sided tape. You'd be surprised how strong this can be even when cutting parts out from a sheet of material. I like to screw the sheet to the bed outside the cutting area when cutting metal just for extra security.
The only metal I would recommend you machine is hard brass as it can be done dry. These machines are not powerful enough to cut steel, and aluminium really requires a cutting lubricant to cut well and this is not good for the machine.
For machining in 3D a self centering vice is a useful accessory as it enables you to machine both top and bottom of an object accurately.
When machining objects in 3D it is usual to leave uncut 'tabs' to be cut by hand after the machining is finished. A bit like cutting parts from the moulding sprue of a plastic kit.
Milling slot car wheels.
I assume you mean cutting the spoke detail etc on the wheel as the best way to actually make wheels is with a lathe (either CNC or conventional centre lathe).
It would certainly be possible to do this.
You would have to fix the wheel to the bed in an accurately known position. Probably best done with a screw through the axle hole.
Most of these small machines are not really designed for cutting metal though and you may run into problems with vibration and chatter.
Small milling cutters are very fragile and easily broken.
See also the comment above about machining aluminium.
Cutting supports for track.
Don't quite know what you mean by this but remember that most of these machines are limited to an area of about 300mm square, some much less.
They will happily cut plastics, MDF and wood within the limits of the bed area.
Cutting boarders. Cutting out picture frames.
Same constraints as above.
You biggest problem with any CNC machining is the design process.
You will need a vector drawing package that can be converted into the 'G' codes that run the machine.
Most of my experience is with Roland machines which can be run directly from Techsoft's 2D Design package.
This is relatively simple to learn but it's 3D capability is limited.
If you want to do anything clever in 3D you will need to become proficient with a 3D drawing package such as Solidworks.
What else can it do?
Limited only by your design capability and imagination.
Examples.
Printed circuit boards.
Engraving on trophies etc.
Parts for slot car chassis in plastic or metal.
Full 3D body mould for producing your own slot car bodies by resin casting or vac forming.
The parts for these brass and wire chassis were cut out on a Roland MDX20 machine.
Hope this is helpful.
Cheers.
Mick.
