Constraining Concave and Convex Geometries

[Patrick Dees]

Hello all;

I am attempting to constrain two simple geometries together, one which has a concave shape and the other convex. Specifically, these are two tanks, with cylindrical portions and elliptical domes. One, the forward tank, has its aft dome reversed (concave). Second, the aft tank, has no reversed domes (convex). In rocketry this is called a "common bulkhead" design. I am able to get close to properly aligning the objects, but am having difficulty properly tagging and constraining the two. Any advice would be appreciated!

python Version: 3.12.2

CadQuery Version: 2.4.0

Code:

---------------------------------------------------------------

import cadquery as cq

# dimensions of the tank

thickness = 0.1

dt = 10 - 2*thickness    # diameter of tank less thickness of the shell

rt = dt/2                # radius of tank

ht = 20                  # total height of tank cylindrical section

hht = ht / 2             # half the tank height

hd = 3 - thickness       # height of dome

# colors

green = cq.Color("Green")

red = cq.Color("Red")

def make_tank(reverse_aft_dome: bool = False) -> cq.Workplane:

    # create the starting plane from which to work

    wp = cq.Workplane("XZ")

    # trace one half of the perimeter then rotate

    # start at lower-left corner above aft dome

    tank = wp.move(xDist=rt, yDist=-hht)

    # create the vertical wall of the cylinder

    tank = tank.line(xDist=0, yDist=ht)

    # create the forward dome. this ends at the center

    # line of the tank

    tank = tank.ellipseArc(

        x_radius=rt,

        y_radius=hd,

        angle1=0.,

        angle2=90.,

        sense=1, #  clockwise (-1) or counter clockwise (1)

    )

    # create a vertical line at center of the tank

    tank = tank.line(xDist=0, yDist=-(ht+2*hd))

    # create the aft dome, starting from the center line

    if reverse_aft_dome:

        tank = tank.ellipseArc(

            x_radius=rt,

            y_radius=hd,

            angle1=0,

            angle2=90.,

            sense=-1 #  clockwise (-1) or counter clockwise (1)

        )

    else:

        tank = tank.ellipseArc(

            x_radius=rt,

            y_radius=hd,

            angle1=-90,

            angle2=0.,

            sense=1, #  clockwise (-1) or counter clockwise (1)

        )

    # mark the 2D shape as complete

    tank = tank.close()

    # revolve the shape 360 degrees to create the 3D geometry

    tank = tank.revolve(360)

    # finally, apply a thin shell

    tank = tank.shell(thickness=thickness)

    # tag EVERYTHING

    tank.edges(">Z").tag("topedge")

    tank.faces(">Z").tag("topface")

    tank.vertices(">Z").tag("topvert")

    tank.edges("<Z").tag("bottomedge")

    tank.faces("<Z").tag("bottomface")

    tank.vertices("<Z").tag("bottomvert")

    return tank

# create the forward tank with a reversed aft dome

fwd_tank = make_tank(reverse_aft_dome=True)

# create the aft tank with no reversed domes

aft_tank = make_tank()

# create the assembly to house both tanks

assy = cq.Assembly()

# create a center line to align along Z axis

center_line = cq.Workplane("XZ").vLine(0.1, forConstruction=True)

assy.add(center_line, name="CenterLine")

assy.constrain("CenterLine", "Fixed")

# add the forward tank

assy.add(fwd_tank, name='Forward_Tank', color=red)

# add the aft tank

assy.add(aft_tank, name='Aft_Tank', color=green)

# constrain the forward tank to be fixed

assy.constrain('Forward_Tank', "Fixed")

# constrain the aft tank to the center line

assy.constrain('Aft_Tank?topvert', 'CenterLine', 'PointOnLine')

assy.constrain('Aft_Tank?bottomvert', 'CenterLine', 'PointOnLine')

# constrain the two tanks to overlap at the

# forward_tank:aft_dome - aft_tank:forward_dome

# interface

assy.constrain('Forward_Tank?bottomvert', 'Aft_Tank?topface', 'PointInPlane')

assy.solve()

# visualize

display(assy)

The tagging of the tanks within make_tank and the final constraint are my issues. 

If you have any advice on how to 

1) Tag the vertex at the center of a concave dome

2) Properly constrain the items

I would be most appreciative!

Thanks,

Patrick

[Adam Urbanczyk]

You tagged the wrong vertices. I also simplified your constraints. You might want to take a look this section https://cadquery.readthedocs.io/en/latest/assy.html#constraints

    if reverse_aft_dome:
        tank.faces("<<Z[-2]").tag("bottomface")
        tank.vertices(">Z", tag="bottomface").tag("bottomvert")
    else:

        tank.faces("<Z").tag("bottomface")

        tank.vertices("<Z", tag="bottomface").tag("bottomvert")

    return tank

# create the forward tank with a reversed aft dome
fwd_tank = make_tank(reverse_aft_dome=True)
# create the aft tank with no reversed domes
aft_tank = make_tank()

assy = cq.Assembly()

assy.add(fwd_tank, name='Forward_Tank', color=red)

assy.add(aft_tank, name='Aft_Tank', color=green)

assy.constrain('Aft_Tank','FixedRotation',(0,0,0))
assy.constrain('Forward_Tank?bottomvert', 'Aft_Tank?topvert', 'Point')

assy.solve()

# visualize
show_object(assy)

[Patrick Dees]

Thanks for that, works perfectly!

I am confused by the lines such as

" tank.vertices("<Z", tag="bottomface").tag("bottomvert")"

Do these tag the vertices as both "bottomface" and "bottomvert"?

[Adam Urbanczyk]

That selects "<Z" vertices from the "bottomface" tag and tags them as "bottomvert". See also: https://cadquery.readthedocs.io/en/latest/classreference.html#cadquery.Workplane.vertices

[Patrick Dees]

Thank you very much for the clarification!