ROOF DESIGN USING NX CAD

AIM : To Design Roof, Bow Roof Rail, Central Reinforcement and Calculations.

                                         

OBJECTIVES :

1. Designing the roof from the given styling surface with Front and Rear roof rails.
2. Creating the Bow roof rails and center Bow with reinforcement.
3. Create a curvature study on the roof and validate the design by applying the Heat distortion criteria and Snow Load criteria.
4. Performing the section modulus.
5. Performing the draft analysis.

THEORY : 

An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.

The roof is a styling surface part which is fixed with the BIW body through the rails and panels inside the upper roof to provide it strength and stiffness.

Ditch area plays an important role here to fix the reinforcement on the component.

The rails and reinforcements are joined to the ditch area with a spot weld where it's not the styling part.

The required tool opening angle is minimum 3 Degrees.

Factors which is considered before creating Ditch area,

• Headroom clearance
• Visibility criteria
• Back door assembly
• Windshield assembly

APPROACH TO WORK ON THE PROJECT :

(1). Designing the roof surface usings styling part.

(2). Creating Front Roof Rail.

(3). Creating Rear Roof Rail.

(4). Creating Front Bwo Roof Rail 1.

(5). Creating Rear Bwo Roof Rail 2.

(6). Creating Reinforcement Roof Rail with Bwo Roof Rail 3.

(7). Creating and Measuring all required arcs/curves/length for calculations.

(8). Heat Distortion Calculation.

(9). Snow Load Calculation.

(10). Draft Analysis.

 

DESIGNING THE ROOF SURFACE USING STYLING PART

• Creating 5mm of bridge curve to avoid sharp edge on the end corner of the roof stuyling surface.
• Law extending the edge with 15 mm and 3 degrees of draft angle.
• Law extending the edge with 21.5 mm at 75 degrees.

• Give a face blend of 2.2 mm radius.
• Bridge curve to the space between the flanges.
• Studio surface the bridge curves.
• Sew the surfaces

• Do the same procedure for the rear end roof surface aslo.

• Law extends to 20mm length with 25 degrees.

• Law extends to 20mm length with 15 degrees.

• Law extends to 10mm length with 90 degrees.

• Provide 5mm radius face blend on the lower side of the rear surface and 2.2 mm radius of face blend with styling surface.

 

CREATING FRONT ROOF RAIL :

• Offsetting styling surface to 32mm downwards.
• Law extends 20mm length and 25 degrees according to the master section provided.
• Offsetting flange of roof.

• Trim the surface to 80mm.
• Offset the upper roof surface by (0.75+0.75+5) and (4.7+0.75+0.75+5) cut it by 5mm and 10mm respectively.

• Join the trimmed sheets by studio surface.
• Provide 5mm radius face blend on the lower side of the front roof rail and 3 mm radius of face blend on upper sides.

• Sketch a pattern, project and trim the front roof rail from material removal
• Mirror the front roof rail and sew it to get a complete front roof rail.

CREATING REAR ROOF RAIL :

• For the rear roof rail I repeated the same procedure from the above along the master section of rear roof rail. 

CREATING FRONT BOW REAR ROOF RAIL 1 :

Bow roof rail:

• Bow roofs are used to give strength to the Roof.
• There are 3 bow roofs created with equal distance between Front and Rear roof rail.
• Bow roofs are created by offsetting the roof surface as mentioned below.
• Trimming the surfaces.

• Use studio surface to create surfaces.

• Provide radius of 2.2mm to all the edges

• Offset the flange of the roof to 0.75+0.75=1.5mm
• Intersect the one end of the bwo roof rail and offset it middle of rail
• Offset in face to 8mm or 10mm to both the sides and trim it.

• Create the bridge curves and join with studio surfaces.

• Sketch, Project and Trim for material removal and for mastic points.
• Sew, mirror geometry the front bwo roof rail 1.

CREATING REAR BOW REAR ROOF RAIL 2 :

• For the rear bow roof rail 2 repeated the same procedure we did to create bwo roof rail 1.

CREATING CENTRAL REINFORCEMENT ROOF RAIL WITH BOW ROOF RAIL 3 : 

• Central Reinforcement Roof Rail : For the center reinforcement follow the same procedure from the bow roof creation with bigger dimensions. 

• Bow Roof Rail 3

• Center reinforcement of the roof is going to be fixed after the bow roof by spot welding. 

• After creating all these surfaces, use the Thicken command and thicken it in proper dimensions in the proper direction.

 

CURVATURE STUDY ON THE ROOF :

After getting the final body it has to be checked and justify that it passes the standards.

The tests we are done on the Roof are which satisfies the standards and the Bow roof positioning and the Final design are

1.Heat distortion

2.Snow load

3.Roll-over

Heat distortion criteria: Heat distortion test is to check the strength of the roof. Heat distortion temperature is when the metal softens and deforms under specific loads.

• To calculate the heat distortion criteria by the bow roof position formula.
• We need to create a sketch , draw a line on the center of the roof on OY axis
• Draw a line from the center line by a 300mm distance.
• Draw a line from 100mm distance on both sides of the 300mm line
• Draw a line between the center of roof rails and draw the lines from the center line by the distance of 100mm on both sides.
• Atlast make an intersection on these lines from a coincident line to get points.
• From that point draw a three point arc and measure the Radius of the arc that will be our Rx and Ry values

Judgement Condition : OK< 2.7> 3.1

Where

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y) 

R = Roof curvature

R = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Snow load criteria:

Snow load test used to verify that roof assembly exhibited no permanent deflection due to distributed load representing snow load.

• To calculate the snow load condition, we have a snow load formula.
• To measure the required values.
• I created a sketch and drew a line on the OY axis from the center of the roof.
• From that line draw a parallel line distance of 100mm on both sides.
• Then make intersection points of these lines and draw a three point arc.

Measure the Radius of the arcs, the arc on the OY axis going to be our Ry and the perpendicular arcs will be Rx

Qr =  [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]

Where

α = My x Lx2 x 10-12 , My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

                250 ≤ s ≤ 380

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

        Length of Roof panel with the center point between Roof rail Front /Rear as the front and rear reference point.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

        Width of the roof panel exposed on the surface.

Y = Distance front Front Roof Rail to Roof BOW[mm]

s = Distance for which Roof BOW bears divided load [mm]

        s = L1/2 + L2/2

Iy = Geometrical moment of inertia of Roof BOW (Y cross-section )[mm4]

Rx = Lateral direction curvature radius of roof panel Y cross-section on Roof BOW [mm]

        Roof panel curvature Radius of the Length Lx in Front view 

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

        Roof panel X curvature radius of length s in Side view

SECTION MODULUS : 

Section modulus formula

S = I/y S = Section Modulus.

I = Moment of Inertia.

y = distance between the neutral axis and the extreme end of the object.

Front Roof Rail

By taking the minimum value from the two value,Unit is converted from meter to millimeter 

MOI =821.855873342

Y =61.6

S = (I/Y)

S=13.34181613

Rear Roof Rail

MOI =700.761343313

Y =59.45

Section Modulus = (MOI/Y)

S=11.78740695

Front Bow Roof 1

MOI =5.773499941

Y =15.5

Section Modulus = (MOI/Y)

S=0.3724838672

Rear Bow Roof 2

MOI =7.167542215

Y =18.5

Section Modulus = (MOI/Y)

S=0.3874347143

Center reinforcement & Bow Roof 3:

MOI =68.745500970

Y =30.5

Section Modulus = (MOI/Y)

S=2.253950851

DRAFT ANALYSIS :

 

• ROOF

• FRONT ROOF RAIL

• REAR ROOF RAIL

• FRONT BOW ROOF RAIL 1

• REAR BOW ROOF RAIL 2

• CENTRAL BOW ROOF RAIL 3

• CENTRAL REINFORCEMENT ROOF RAIL

 

FINAL ROOF DESIGN :

CONCLUSION :

• The Design of Roof, Front roof rail, Rear Roof Rail, Bow Roofs, and reinforcement roof center by following the Master section with the design parameter for the given class A surface is done.
• Thus from calculations made the heat distortion criteria and snow load condition criteria was satisfied and the positioning of the BOW Roofs were determined.