design input for a CNC 'engraving' machine

[postvmvs]

I have been toying with the idea of making a CNC machine to engrave dies for making coins/medals and would be grateful to get some input from more experienced forum members on the overall design. Basically I am envisioning a system able to create very small feature size over an X-Y plane of less than 2"x2" with a Z-axis distance (depth) of less than 1/4". The main question I have is that given the limited range requirements, how can I get the best precision on a budget (a few thousand dollars)? I don't care at all about speed (if it takes hours or even days, fine). The materials involved are hard (steel or bronze) and in order to get small feature size, small diameter cutters will be involved (with the problem of easily breaking).

Some thoughts:
Would using stepper motors with very high gear ratios be an option? Or would the belts create too much backlash?
I was thinking about using an off-shelf rotary tool (e.g. a Dremel) for the spindle but I am not sure it would be good enough (run-out, rigidity, etc.)
Are there already commercial machines that do this, and if so, how?

Any help or suggestions would be greatly appreciated!

[Zumba]

I'd look into finding a dentist type rotary tool... something with high precision and high RPM. The thing about dremels is that they're really not made to be able to be run for hours on end. So even if they were very accurate and vibration free (which they're not), they wouldn't be suitable.

Hopefully someone else will shed some light on this topic. I've never done any coin engraving myself, but I've seen it done at trade shows using lasers and EDMs. I'm curious as to whether a rotary tool is capable of the same surface finish without a lot of post machining cleanup work.

[BobWarfield]

Check out the Widgitmaster's Mini-Router thread. There is a guy on there who is engraving coins with a Dremel on the mini-router. See top of page:

My NEW Mini CNC Router Design!

Quite suprising what can be done. Engraving, in particular, will have low cutting forces.

Best,

BW