Source for RepRap basic requirements

I am in the process of adding stepper motors to my small metal working lathe with the idea that, as well as acting as a CNC lathe, it will provide the movement framework for a 3D printer, by adding an extruder.

In spite of a lot of Googling I haven't been able to find any website with information about the basic requirements for 3D printing, which must have been at the root of the original RepRap design. Things such as ...
What is the minimum and maximum speed for movement of the extruder head while it is extruding plastic?
What is the minimum size of features that can be printed, or what governs this limit?
What precision of positioning is required for the different axes?

If anyone has a link to a source for this data I would very much appreciate it.

On a slightly different topic,
I have also been unable to find any website or forum that explains or discusses the principles underlying the RepRap firmware, such as Sprinter. For example what are the essential features for the operation of the stepper motors and what "gotchas" must be avoided. It's almost as though the firmware design appeared on earth fully formed and ready to go.

Again, if anyone has a link to a webpage where this sort of thing is discussed I would appreciate it.

Thanks

...R

Robin2 Wrote:
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> What is the minimum and maximum speed for
> movement of the extruder head while it is
> extruding plastic?

Minimum: about 5 mm/sec. Any slower and the hot nozzle keeps too much heat on the parts.
Max: depends on the filament, but about 200 mm/sec. In practice, this speed will be much slower due to accelerated moves for all but the longest straight lines.
Average: about 30-90 mm/sec is what you want to shoot for to achieve a nice balance of speed and quality. Closer to 30 for the perimeters and 90 or higher for the unseen infill.

> What is the minimum size of features that can be
> printed, or what governs this limit?

This is governed by three things:
1) and the mechanical accuracy of the machine
2) the height of the layers
3) size of the nozzle opening
A mechanically accurate machine that is able to achieve perfectly repeatable positioning, prints small layers (0.1mm and below) and uses a 0.3mm nozzle or smaller is capable of astonishing levels of detail.

> What precision of positioning is required for
> the different axes?

The Z axis requires extremely high precision in order to repeatably move in increments measured in microns. The X and Y axes need high precision, but not quite as high as the Z-axis. Obviously more precision is always better.

Thanks for that information.

Because all my steppers are driving leadscrews I don't expect my maximum table speed to be much above 5mm/sec.

I don't understand why such high precision would be needed for the Z-axis. As I understand it that is the axis that lifts the extruder to do the next layer. I would have thought that would be the least demanding axis? I would be very interested to know the reason.

I would still appreciate it if someone could direct to to somewhere that has a more extensive discussion of the underlying design principles.

...R