Roof challenge

BIW ROOF DESIGN FOR HEAT DISTORTION AND SNOW LOAD CRITERIA

To design an SUV Roof from the given class A and meet the required conditions:

• The design and functional requirements for the Roof and its components.
• Methods to increase the strength of the roof.
• Joining Techniques like Mastic and Welding.
• Meet the Gap and Flushness requirements.

PROCEDURE

STEP 1: INPUT

• We get the Class A surface with predefined edges from the Styling Team.
• We get a Master Section as well, which is a predefined sketch that is adaptable to different situations.
• As Design Engineer, we check for the initial feasibility and whether the part has to be raised/lowered.

Note: We don't have reference data, so we can go ahead with designing the engineering features.

STEP 2: DRIP AREA REQUIREMENTS

• We extend the front edge of the roof from the styling point to provide clearance for the drip area.
• The clearances are given with respect to the visibility criteria, for the front passenger and driver.

Draft

• An optimum draft angle of 3 degrees is given to facilitate deep drawing conditions.
• An increase in the draft would affect the clearance for welding and sealants and therefore, we'd have to increase the flange length (beyond 21.5 mm) which indirectly affects the visibility criteria.
• A decrease in the draft angle would cause complexity for producing the part.

Drip Flange Length

• The maximum length is 21.5 mm to facilitate clearance for:
  • Welding (8mm)
  • Rubber Sealants (5mm)
  • Assembly (2mm)
  • Remaining to extend the draft

Drip Flange Height

• The Flange height is extended to give clearance for:
  • Mounting Windshield
  • Rubber Sealants (Thickness = 3.7 mm)
• Law extended by the Trial and Error method to provide clearance and pass the other 2 parameters mentioned above.

FRONT ROOF RAIL

• An additional commodity that acts as reinforcement with features supporting:
  • Bracket Map Lamp
  • Room Lamp
• Though it is a reinforcement component, it is made from the Outer Body Scrap having the same material and thickness (0.75 mm).

FR Rail Depth

• The Rail is offset by 32 mm to facilitate clearance for visibility criteria for the driver and passenger.
• The offset value depends on the average citizen's height, from region to region.
• No other reference data is given so we can go ahead with the conditions.

Draft

• An optimum draft angle of 3 degrees is given to facilitate deep drawing conditions.
• An increase in the draft would affect the clearance for welding and sealants and therefore, we'd have to increase the flange length (beyond 21.5 mm) which indirectly affects the visibility criteria.
• A decrease in the draft angle would cause complexity for producing the part.

Headroom Clearance

• The Headroom clearance is 80 mm from the given reference data.
• Again, the value varies from region to region depending on the average citizen's height and seating position.

Mastic Clearance

• A clearance of (2.7-3.7) mm is given to facilitate the positioning of mastic sealants.
• Mastics provide additional strength to the local region and an adhesive for joining the parts in the roof assembly.

REAR ROOF RAIL

• The Rear Roof Rail also has a Drip area to provide clearance for mounting the Back door Hinge.

Drip Area

• Due to lack of reference data, we law extend the flange by trial and error to pass the Rear Roof rail criteria conditions.
• The Flange Length and extended Flange Height are given as 15 mm to provide space for the Back Door Hinge assembly and opening.

Rear Visibility Criteria

• The Rail is offset by 70 mm to facilitate clearance for visibility criteria for the rear passengers.
• The offset value depends on the average citizen's height, from region to region.
• No other reference data is given so we can go ahead with the conditions.

Rear Headroom Clearance

• The Headroom clearance is about 50-60 mm from the given reference data.
• Again, the value varies from region to region depending on the average citizen's height and seating position.

STEP 3: Bow Roof Design

Design Rules to be followed to meet Manufacturing standards:

Draft Angle	Facilitate Deep drawing conditions for the sheet metal. Minimum 10-15 degrees outer draft.
Emboss spacing/clearance	To facilitate Tool actuation while drawing. Minimum 5mm gap between the embosses lying on the same surface.
Clearance for Material removal	To facilitate Blanking for material removal of the sheet metal. Minimum 3mm gap to give clearance for the Punching tool.
Minimum Radius	Steel has the capacity to be extended only up to a minimum of 3mm at corners/edges. Lesser than that would result in tearing(tension)/crowding(compression) of the material atoms in the local region.
Corner Reliefs	To avoid stress concentration at sharp edges and corners, we provide relief. The CAE team studies the stress concentrated region, which is indicated as Red/Dark zone.

Functions performed for creating the Emboss:

Offset surface	It creates an Offset of the selected surface and projects it in the vector direction perpendicular to the surface. Note: The Offset dimension is limited up to the Radius of the surface (if curved). But if the surface is complex, it gets distorted from a far distance, well within the limit.
Offset curve on surface	Creates an offset of the edge onto the corresponding surface. Fillets are automatically provided to avoid sharp corners. Note: Ensure the curve gets, Trimmed to face edges, if it is extending beyond. Extended to face edges, if it is falling short.
Trim sheet	Lets us trim the Target-face/body with respect to a cutting boundary(curve/surface/plane).
Through curves	Helps to fix small patches by providing smooth transitions.
Law extension	Creates an Extended surface/sheet from a section and a reference plane/direction.
Emboss	Creates an offset of any desired shape/section from the corresponding plane. This command is vital for providing reinforcements for the sheet metal component.
Face blend	Joins 2 faces/bodies having separate edges.
Edge blend	Joins 2 faces of the same body/object bound by a common edge.
Sketch Tools	Helps us draw a line/spline/arc/rectangle in the Datum plane/flat surface, about the geometry features. The Result is a Curve on a specified datum Plane.
Project Curve	Projects the sketch features onto any Datum plane or Surface (even curved). Note: We cannot sketch on a curved surface. Therefore, we sketch on a datum plane and project it onto the respective surface to get an accurate section of the sketch.

Draft Analysis

Draft Analysis is done to ensure the part follows:

• Zero Undercuts
• Minimum Fold requirement gaps
• Positive Draft for Ideal Deep Drawing conditions

Colour	Description	Value
Green	Passes for Minimum Deep Drawing Condition	Above 5 degrees
Yellow	Needs work on those surfaces	5 to 0 degress
Blue	Fails for Deep Drawing Condition	0 to -5 degress
Red	Fails crucially for Deep Drawing Condition	Below -5 degrees

Roof with Drip Area
Front Roof Rail
Rear Roof Rail
Centre and Side Roof Bows

ROOF CALCULATIONS

HEAT DISTORTION CRITERIA

Prediction Formula:

W = `(1.73xx10^-3xxL)+(1.85xx10^-8xxfrac{R^2}{t})+(1.10xx10^-3xxI)-2.68`

Judgement Condition: `Ok<2.7mm<=NG`

L :	Roof Length in X-Direction (Roof Dimension with OY)
R :	Roof Curvature `R= 2frac{Rx xxRy}{Rx+Ry}`
Rx :	X Curvature
Ry :	Y Curvature
t :	Roof Plate Thickness (mm)
I :	Bow Roof Span (mm)

Sl. No.	BOWs	Rx	Ry	R	I	L	t	R^2/t	W	OK/NG
1	FR & 1st	5731.5493	2669.5256	3642.514265	345.5058	2030.4972	0.75	17690546.9	1.540091654	OK
2	1st & CTR	5076.7408	5775.0001	5403.405573	398.5589	2030.4972	0.75	38929055.71	1.991362477	OK
3	CTR & 2nd	4078.1913	10074.8917	5806.13223	423.564	2030.4972	0.75	44948228.63	2.130222786	OK
4	2nd & RR	3327.4958	14278.6646	5397.224086	465.8073	2030.4972	0.75	38840037.11	2.063688873	OK

SNOW LOAD CRITERIA

Prediction Formula:

`Q=frac{Iyxxt^2}{alphaxxsxx[frac{Rx+Ry}{2}]^2xx10^8`

Where

`alpha=MyxxLx^2xx10^-12`

`My=Y(Ly-Y)`

Judgement Condition:

`Qr<=3.1`

t :	Roof Plate Thickness (mm)
Ly :	Distance between the front and rear roof rails on the vehicle along OY (mm)
Lx :	Distance between the left and right end of the roof on the roof BOW (mm)
Y :	Distance from FR roof RAIL Center to roof BOW (mm)
s :	Distance for which roof BOW bears divided load (mm) `s=frac{L1+L2}{2}`
Iy :	Geometrical Moment of Inertia of roof BOW along Y cross-section `(mm^4)`
Rx :	X Curvature
Ry :	Y Curvature

Sl.No	BOWs	Iy	y	Lx	My	α	s	Rx	Ry	Qr	OK/NG
1	FR & 1st	396.3197	440.7716	1134.1725	700705.8963	0.90135111	238.3582533	4496.1902	2650.379	8.126598057	OK
2	1st & CTR	1173.6795	474.2786	1087.8153	738081.1789	0.873402552	238.4321213	3754.781	5766.1216	13.98931899	OK
3	CTR & 2nd	403.4295	499.7993	1059.0138	765041.7389	0.858002136	300.0301101	4215.9291	10378.4125	1.655495274	OK

RESULT

We have attained the Roof Panel to meet the desired requirements:

Drip Area
Front Roof Rail	Front Visibility Criteria Windshield Clearance Gap and Flushness
Rear Roof Rail	Rear Passenger Head Clearance Rear Visibility Criteria Backdoor Hinge Clearance
Bow Roofs	Snow Load Criteria Heat Distortion Criteria Mastic Influence