About driver 8825

Hay how i can calculating Vref for two nema17 motors conect on one dr8825

This might help, around 37 minutes, 8825 is is similar, I think:

That's not possible. You need one drv8825 per stepper. Connecting two steppers to one driver will not work.

Yes it will, they will move together. My Prusser 3D printer has this arrangement for the Z axis.

You need to know the current for the motor, NEMA simply tells you the size of the motor housing, nothing about the coil consumption.

I wouldn't recommend to do so. The DRV8825 is a current driver, and it needs the inductance of the stepper to regulate the current. Maybe it will work if the two steppers are identical and you connect the coils in series, what ensures the current is identical in both coils. But it's very special and cannot be recommended in general.

i tried to connect two motors to one driver and it works but now i am interested Vref for one motor is 0.85 for two motors 1.7 V or is 0.85 V

How did you do so? In parallel or serial?

Do you have any idea how stepping motors work?
The current is determined by the resistance of the coils and the voltage applied to them.

The only thing the inductance determines is the rise time of the current.
The regulation of the current is done by detecting the current flowing and turning off the voltage when the set current is flowing.

i soldered the same colors of cables and connected them to the output for e.g. x axis

That's only true for voltage driven steppers with a high rated Voltage. For current drivers like the DRV8825 the resistance is nearly irrelevant.

That's true, but is only possible because of the inductance of the motor coils. The inductance and the applied voltage ( and the motor load ) determine how fast the rated current will be reached. With high inductance, low voltages and high steprates the current will not be reached within a steptime ( and this leads to less torque ).

If you connect two steppers in parallel, the driver cannot regulate the current in each coil, but only the total current. How the current is divided between the coils depends on the characteristics of the two motor coils and the load, and it is not determined. Only when the coils are connected in series is it ensured that the same current flows through both. But then a higher supply voltage is needed to achieve the same current rise ( and thus the same torque ).
With different loads, the motors will impact each other. I would never recommend this. The DRV8825 is a relatively cheap component, so you should always provide a separate driver for each motor.

At last you have something right, but why would this matter, the total current is a good thing to regulate.

No in parallel with identical motors is fine, that is the only way to ensure that the same voltage is applied to each motor. It is common practice.

Now please stop posting rubbish you are only showing yourself up. You seem to have learned an imaginary rule.

There is no point in discussing with someone who is so convinced of his wrong opinion. Not everything that is 'practice' is also technically useful. The coil current is the parameter to be controlled, not the coil voltage. I am out of here.

So you have got the motors in parallel therefore you need double the driver’s Vref to have the same current flowing through each individual motor as you had with only one motor.

Except in this case.

TFF that. Why is your profile hidden from view scared of something?

ok if it can't in that way can you suggest how best to duplicate stepper motors on the x axis ie two motors on the x ramps 1.6

What do you mean by that?
Do you mean you can’t set the drive to twice the Vref of one drive?

You have still not said what current your motor draws.
The driver has a limit to the current it can supply and if you are on that limit then you need to use another driver and simply wire the step and direction pins to the same Arduino outputs. However you have to position the two motors to the same starting position before you attempt to drive them otherwise they will be a permanent offset between the two motor positions.