I was just wondering how you would calculate the friction torque for a DC motor? and how the calculations would be different when finding the current, EMF, torque etc, when this is neglected.
You don't calculate it, you measure it. It is a purely mechanical effect, depending on things like bearing quality (both intrinsic and due to aging), and aerodynamic effects. In general, motor frictional losses are ignored - if you have to take them into account you're working too close to the edge.
Well, you have static friction when the motor is stopped. When the motor starts turning you have dynamic friction. Since most parameters of the motor are measured, they include the friction.
All frictional coefficients are measured, there is not a good way to say this is A material with X surface roughness rubbing against B material with Y surface roughness and predict a frictional coefficient, it has to be measured.
If one really wanted to find the friction there is a way to calculate it from the values listed in tables, its messy and you would get an answer that is off from the real world value by some degree but it can be done. It would be better to measure it.
"Frictional resistance to the relative motion of two solid objects is usually proportional to the force which presses the surfaces together as well as the roughness of the surfaces. Since it is the force perpendicular or "normal" to the surfaces which affects the frictional resistance, this force is typically called the "normal force" and designated by N. The frictional resistance force may then be written"
So to find the frictional force in the motor (presumably the shaft), you would have to know the coefficient of friction between each material between the shaft and the motor and the normal force of the shaft on that material.
The bearing will come into play and create frictional torque around the shaft, to do that you will have to know the frictional coefficient of the bearing.
Measuring the friction of the shaft and bearing could be done by using a force or torque gauge and some knowledge of how to compute moments from force. Keep in mind the static friction will be different than when the motor is in motion (kinetic friction).