Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "hjf2k2" <drgenio@h...> wrote:
> Hi everyone, it's me again. I'm starting to work on a homebrew CNC
> machine project. My question is about the motors. I know I need a
> stepper motor, but what else? Steps, torque, voltage, etc.
> 
> I had an idea for the Z axis. Most of the use of this machine will be
> drilling holes, so I thought of a spring mechanism. That is, a
> solenoid pushes a spring, which in turn pushes the arm holding the
> bit, which may be cheaper than a Z motor and also will let the bit do
> the drilling on its own, not forced by a motor. Is this a good idea?
> 
> hjf

Bad idea,

You will have no control over the force - with a spring, you would 
simply get a single feedrate for everything.  Believe me, a .010" 
diameter drill requires a much different feedrate than a .125" 
diameter drill.

Stick with a stepper drive Z axis.  As for what size steppers, that 
depends on the overall system design.  Here are few examples of why 
you have to consider the system as a whole...

Ball screws are 95% efficient
Lead screws are about 40% efficient
A smaller motor can move a bigger load with ball screws.

Linear bearings have low frictional and slip-stick losses
Metal on metal ways have higher frictional losses and higher slip-
stick.
A smaller motor can be used to move a bigger load when the way system 
is using linear bearings.

There is no quick and easy answer on motor sizing,  

Chris