Units, Formula editor and multiplication

[peetervois]

This falls between possible bug or "know how to make the trick". I write it in this topic. I was trying to make some more (maybe advanced) part design tricks: I made 2 planes, defined a height with a sketch between the planes, and then I wanted to define a rotation of a object between the 2 planes for lofting. For clarity

SketchFirst - is one of the 2 planes
SketshHeight - is the plane that has a line on it and the length of the line defines the distance to ...
SketchSecond - is the second of the 2 planes

For that on my plane there is Constraints.a that defines this rotation angle and everything is calculated around it. On the SketchSecond I would like to make Formula for Constraints.a:

SketchFirst.Constraints.a + 0.3 * SketchHeight.Constraints.h

And this fails because the SketchFirst.Constraints.a is in "deg", the SketchHeight.Constraints.h is in "mm" and it seems I can not specify the units for 0.3 to be "deg/mm".

It might be possible that the units checking can be turned off (I do not know it yet) or is there something else that I am missing? Maybe a function that will define the unit for the result regardless what is the unit inside that function? I did not find any hints for existence of the function. The function would then be like:

SketchFirst.Constraints.a + deg( 0.3 * SketchHeight.Constraints.h )

[chrisb]

SketchFirst.Constraints.a + 0.3 * SketchHeight.Constraints.h

And this fails because the SketchFirst.Constraints.a is in "deg", the SketchHeight.Constraints.h is in "mm" and it seems I can not specify the units for 0.3 to be "deg/mm".

SketchFirst.Constraints.a + deg( 0.3 * SketchHeight.Constraints.h )

You cannot switch the units checker off. Without the file or a handmade doodle it is hard for me to understand your model. But what I can see is that you of course cannot add a length to an angle, what should the result be? And also you cannot convert a length to an angle. You may invoke some trigonometric function, see the Expressions page.

If you want to remove a unit, e.g. mm then you can divide the value by 1mm.

[peetervois]

SketchFirst.Constraints.a + 0.3 * SketchHeight.Constraints.h

And this fails because the SketchFirst.Constraints.a is in "deg", the SketchHeight.Constraints.h is in "mm" and it seems I can not specify the units for 0.3 to be "deg/mm".

SketchFirst.Constraints.a + deg( 0.3 * SketchHeight.Constraints.h )

You cannot switch the units checker off. Without the file or a handmade doodle it is hard for me to understand your model. But what I can see is that you of course cannot add a length to an angle, what should the result be? And also you cannot convert a length to an angle. You may invoke some trigonometric function, see the Expressions page.

If you want to remove a unit, e.g. mm then you can divide the value by 1mm.

Yes I can not add length to angle, but I can convert length to angle through conversion formula. One of the easiest conversion formula is multiplication y=a*c, where the conversion factor a has unit [angle * length^-1]. Since [angle * length^-1] * [length] === [angle]. The formula of-course depends on physics and finding that formula is where physics ends and mathematics starts.

I change the topic a very little, since I see the units system has probably very related issues. Maybe my issue is more like that the units system is unable to correctly figure out units of the constraints. I attached a picture that shows the error. I can not calculate atan(Constraints.h/Constraints.w) but I can calculate atan(2mm/3mm) I also have seen that the multiplication of units works in some explicit cases but fails when Constraints are used. (FreeCAD 0.18). The project is also added.

[chrisb]

Now I understand, and the problem comes from a completely different corner: Using the names "w" and "h" conflicts with some internal names. This may be already fixed, there is a pull request waiting to be merged.
Rename them to width and height and use atan(Constraints.height / Constraints.width) without issues.

In your special case I would recommend something completely different: Use a geometric solution by adding a point-on-line constraint placing the top point on the diagonal line. It will be placed on the infinite prolongation of the line.

[peetervois]

Super ! Thanks !