Unity3D eulerAngles smooth change by one axis - unity3d

Question

I'm try to rotate cube for 90 degrees by one axis, all work fine if any axes in 0 position. But if any axis in any position, like a 90 or -90, he starts spinning endlessly.
Video how it looks: https://youtu.be/PE4YH19ndCc
public Vector3 targetEulerAngles;
public float spd = 0.1f;
GameObject mainCube;
public void Awake()
{
mainCube = GameObject.FindGameObjectWithTag("mainCube");
}
public void FixedUpdate()
{
mainCube.transform.eulerAngles = Vector3.Lerp(mainCube.transform.eulerAngles, targetEulerAngles, spd);
}
public void RotateToLeft()
{
targetEulerAngles = new Vector3(targetEulerAngles.x + 90f, targetEulerAngles.y, targetEulerAngles.z);
}
public void RotateToRight()
{
targetEulerAngles = new Vector3(targetEulerAngles.x - 90f, targetEulerAngles.y, targetEulerAngles.z);
}
public void RotateToUp()
{
targetEulerAngles = new Vector3(targetEulerAngles.x, targetEulerAngles.y, targetEulerAngles.z - 90f);
}
public void RotateToDown()
{
targetEulerAngles = new Vector3(targetEulerAngles.x, targetEulerAngles.y, targetEulerAngles.z + 90f);
}

Answer 1

What #joel64 said in his answer is definitely true, but it is not the problem here.
The problem is that you use Lerp incorrectly. The last argument to Lerp(a, b, c) is NOT SPEED, it is progress from 0 to 1, where c <= 0 means result = a, c >= 1 means result = b, and all other values are in between. Your code constantly rotates cube and never stops, just over time it rotates less and less.
With euler angles it is very visible because of conversion errors, using Quaternions it will be less visible, but still - rotation never stops.
To do the stuff correctly, you need to write some more code. You need some variable to hold current progress of rotation, and you have to remember where the rotation started. Then rotate cube from start position to target position given progress and increase progress each frame.
Oh, and don't forget about deltaTime if you want your rotation to be the same on any FPS.
Final code will be:
public Vector3 targetEulerAngles;
public float spd = 0.1f;
GameObject mainCube;
private Quaternion startRotation;
private float progress = 0;
public void Awake() {
mainCube = GameObject.FindGameObjectWithTag("mainCube");
}
public void FixedUpdate() {
if (progress == 0) startRotation = mainCube.transform.rotation;
mainCube.transform.rotation = Quaternion.Lerp(startRotation, Quaternion.Euler(targetEulerAngles), progress);
progress += spd*Time.fixedDeltaTime;
}
public void RotateToLeft() {
targetEulerAngles = new Vector3(targetEulerAngles.x + 90f, targetEulerAngles.y, targetEulerAngles.z);
progress = 0;
}
public void RotateToRight() {
targetEulerAngles = new Vector3(targetEulerAngles.x - 90f, targetEulerAngles.y, targetEulerAngles.z);
progress = 0;
}
public void RotateToUp() {
targetEulerAngles = new Vector3(targetEulerAngles.x, targetEulerAngles.y, targetEulerAngles.z - 90f);
progress = 0;
}
public void RotateToDown() {
targetEulerAngles = new Vector3(targetEulerAngles.x, targetEulerAngles.y, targetEulerAngles.z + 90f);
progress = 0;
}

Answer 2

sometimes even I get weird results when using Euler angles so I use quaternions instead
maybe try this in your fixed update
mainCube.transform.rotation = Quaternion.Lerp(mainCube.transform.rotation, Quaternion.Euler(targetEulerAngles), spd);