Upgraded to metric rods

In the wikipedia i couldn't find anything about more friction...

It is actually a difference, as i said it depends if we talk about normal situation, is extreme situations. Friction is normally not depending on the contact area (just force and friction coefficient). But if the pressure is high (or very low) things change.
A loose nut is therefore not the same as a really tightened one.

http://www.katonet.com/article/coarsevsfine.html

i think they know also something about screws

and they say nothing about more friction, but about less torque for the same pressure (in our case lifting) force.
Maybe their english is better so they can explain it better.

 

The link works fine for me...

The longer because the nut is heavier...
But this also matches the situation very badly, 30 to 6.5 is not the same as 1.41 to 1.25.
I say both require the same amount of force, because the pressure (force per area) is smaller in the longer nut, but the nut is longer, but we will only know if someone measures it...

But actually this is only one point why it should be the same
it's also about geometry, if you have a force that pulls down an inclined plane (like we have in screws) then you have a normal force perpendicular to this plane, and a resulting force "downhill". The torque from the motor has to overcome this force, and this is force is bigger when the angle is bigger. http://upload.wikimedia.org/wikipedia/commons/thumb/8/85/Free_body.svg/212px-Free_body.svg.png

Also the torque is perpendicular to the orientation of the screw, so it doesn't just push parallel to the plane, it has an angle into the plane, so a bit of the force presses against the plane, so the total force (adding the force from the weight pressing against the plane) is bigger when the angle is bigger.

Just for example 45° and 30° inclined plane and a coefficient of friction of 0.3 with a force of 1N, this is the net force that has to come from the torque if you want do push something upwards (not taking into account that it has to be more, because from the torque is a force on the plane which causes even more friction)
45° plane = 0.92N (along the plane downward)
30° plane = 0.76N (along the plane downward)

So even if for whatever reason the coefficient friction should be a bit higher, there are other points which reduces the torque needed.
The real angels are about 3° and 3.36°
Normal force 0.9986 0.9983
friction at 0.3 coeff. 0.3 0.299
"downhillforce" 0.052 0.0586 (this force multiplied with the distance per turn gives the gain in potential energy)
total 0.352 0.3576

torque required 0.3525 0.3582
pressure from torque 0.0185 0.021 (to add to the normal force and friction, so actually friction gets increased about 2%)
torque with pressure 0.3576 0.365 (so the imperial rods require about 2% more torque)

this difference will be bigger for 1mm per turn fine threaded rods (angle 2.39 °), something less than 0.35 (about 4-5% less torque)

So then the question is, will the coefficient of friction on a 13% longer distance (41% for a 1mm pitch) be more than 2% higher?
and anyhow the effect is in either case should be very small. A different lube will make more difference than any of these effect.
But the motor has to turn 13 to 41% faster for the same Z-speed, and this is a real difference for a stepper... (especially when they are on the power limit (12V and amp limit of the driver = constant power), if they can't get more power but have to turn faster, then the torque is reduced bi 1/(rotation speed) or 10-30% less)

I find it far more interesting than talking about little changes in friction, to talk about the fact, that one stepper on the driver can rotate 3 times as fast as two.
Or that one stepper motor can lift almost the whole weight on one side (the right side with the extruder on the right side), with twice the speed, than both together can.
Or the strange thing that i can move the X-axis faster with higher acceleration...
So for me it obvious that the answer can't just be "more friction is the cause of problems", of course is low friction a good thing, but it can't explain the whole thing.

 

sad you stuck on this one point, and never say anything to the others...
also you could give me some reason, if they are so much heavier to turn, there's a reason, a theory that explains this
and in our case we talk about some percent, and not 30mm long nuts, at least mine aren't
if this effect is that big, then the best would be change to small nuts, that are not 6 but only 3mm high, that should solve the problem also...

 

So let me get this straight.
Metric rods would require these changes in firmware ?
#define DEFAULT_AXIS_STEPS_PER_UNIT {80,80,2560,723.38} // default steps per unit for Ultimaker
and
#define HOMING_FEEDRATE {50*60, 50*60, 5*60, 0} // set the homing speeds (mm/min) //robo
and
#define DEFAULT_MAX_FEEDRATE {500, 500, 2, 25} // (mm/sec)

Like everyone else i ran into same problem with homing jerking and getting stuck buzzing.
This is due to poor assembly.
I had enough work done on that black base plate to see just how screwed up it was because of a person put it together.
Y motors and belt guides were torqued so hard the plate warped and motors and guides were tilted. Screws holding the Y motor are screwed into the base so hard, they went inside the darn thing.

As for the homing issue after metric rods went in place, i look at the printer from a distance and saw that smooth rods and threaded rods are simply NOT ALIGNED.
I dont know if its by design (stupid design forgive me) or it was a bad base plate that came with my printer only, but i do see that if smooth rod was placed right across the threaded rod, i wouldnt have any issues swapping standard with metric rods.

Fix to that is simple without the need of altering the code. (except for the 2560)
Get 3 of 4 screws removed from both Z motors letting it hang on one screw.
Loosen up the last 4th screw so that you can freely move the motor.
Move the motor aligning threaded rod to the smooth rod (DO NOT SECURE)
You need some washers to help it stay aligned. This is because assembler dude over torqued those screws that they went into the base plate, so unless you use washers, screws will go into exact same alignment.