Roof Design

DESIGN AND DEVELOPMENT OF CAR ROOF AND ITS REINFORCEMENTS

Objective:

            The main objective of this project is to develop the car roof design and its reinforcements from the given styling surface. And to prepare a detailed report about the engineering standards for roof design, its design procedure, analysis and manufacturing techniques.

Description:

Introduction:

            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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

            In later years, many variations on the automobile roof developed. These include,

• Convertible roofs
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs.

Hardtop roof is shown in the above picture.

           In this project, the hardtop roof design and development going to be carried out using the “NX CAD” application. A hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal.

Design parameters:

            The styling surface of a hardtop roof is given by the styling team to develop it. Commonly the car roof consists of outer panel and its reinforcements. Those reinforcements are,

• Front roof rail
• Bow roof
• Centre roof and
• Rear roof rail.

            There are certain engineering standards to design the automobile roof. These standards are derived from trail and error method to reduce the material wastage, production cost and to improve the safety over the years. These standards may vary region to region according to the conditions, usage, rules and regulations of that particular region.

            The common considerations for roof design are head room clearance, driving vision, heat distortion criteria and snow load criteria.

            The common standards for car roof design were the draft angle should be minimum of 7º and the blends should be minimum of 3+t (t-thickness of the part).

Master section:

            Master sections are the 2D outlines which is taken at a particular area of a 3D model or it is an intersection of 3D model about a reference plane to study its outline (or boundary) in a 2D view. These master sections are widely used in OEMs and in automotive industries to convey the functional requirements of commodity to the design engineer.

            It gives the parameters like length, width, height, radius and clearance should be considered while designing the particular part or component.

            For this project it gives the parameters to be considered while designing the roof outer panel, front roof rail, bow roof, centre roof and rear roof rail.

1.Roof outer panel & front roof rail master section:

            According to this master section, the front flange should have the 7º draft angle and following this it gives flange distance of 21.5mm to the front flange. Then it gives the radius value of 2.2mm for blends.

• These are all the minimum values to be maintained in the roof outer panel design according to the engineering standards.
• Some of these dimensions are also considered to develop the ditch area at the sides outer panel.
• The following front roof master sections gives parameters, which Concerning the field of vision for driving and head room for the occupants.
• The front roof rail is going to be assembled with the outer roof panel by means of spot welding at the front flange area. So, the offset of front roof front flange is 0.75+0.2+0.75mm from the outer panel front flange.
• Then the offset for emboss is 32mm for the front roof from the roof outer panel and its length should be of 80mm from the outer panel front flange.
• The rear flange of the front roof is 15mm distance from the emboss end and it is further divided into 10mm & 5mm.
• This flange has the offset of (0.75+5+0.75mm) & (3+2*0.75mm) from the outer panel, those variations are to provide clearance for mastic sealant to expand, while assembling it with the outer panel.

2.Bow roof master section:

            According to this master section, the overall width of the bow roof should lie between 65-75mm and all the embosses should have atleast 7º of positive draft with respect to the tooling direction. Then the offset of bow roof outer flange is 0.75+5+0.75mm from the roof outer panel and the placement of bow roof should be in the flatter region rather than the curvy region of roof. These parameters are same for all the bow roof section.

3.Center roof master section:

            According to this master section, the overall width of the center roof should lie between 80-90mm and all the embosses should have atleast 7º of positive draft with respect to the tooling direction. Then the offset of center roof outer flanges is 0.75+5+0.75mm below the roof outer panel. The center roof is placed at the region were the B-pillar connects with the roof.

4.rear roof rail master section:

            This master section gives the dimensional parameters for outer roof panel rear flange and rear roof rail.

• The outer roof panel rear flange should have the negative flange of 25º-30º draft to accommodate the clearance for back door movements.
• The overall hight of the rear flange must lie between 70-80mm, following this the outer most flange should have the required dimensions for spot welds and back door placement.
• Then the parameters for rear roof rail are offset of (3+0.75*2mm) & (0.75+5+0.75mm) for front flanges, 10.7mm for the offset of center emboss and its width is 90mm. Then the draft angle for emboss connecting the front flange is 32º.

Design procedure:

            In this project, design and development of car roof starts from creation of an assembly file and inserting the styling surface to the assembly window. By using this method, assembly of roof and its reinforcements become very easy and the reinforcements of roof were created at the part level individually. The main parts of car roof consist of,

• Roof outer panel
• Front roof rail
• Bow roof-1
• Center roof rail
• Bow roof-2 and
• Rear roof rail.

Roof design example is shown in the above picture.

1. Roof outer panel:

             The whole car roof is made of sheet metal, so it is called as BIW. The roof outer panel is the aesthetic part of the roof and also it is the external part which covers the occupant or passenger compartment from the wind shield to the back door.

  Roof outer panel in different views is shown.

• The given styling surface is extracted to start the development of roof and the styling surface is kept untouched for future reference as an input surface.
• The extracted surface is then untrimmed to fill the gaps and holes in it, then trim command was applied to remove the extra surfaces over the boundaries of styling surface.
• With respect to the master section of front roof rail, the law extensions of flange is created at the front side.
• Then these extensions were sewed to apply edge blends to its edges and corners.
• After creating the front flange, the ditch area was created using the same law extension, sew and edge blends with respect to the dimensions used for the front flange.
• The corner gap between the front flange and the ditch area is filled using the bride curves and through curve mesh commands to have a smoother corner.
• Then, with respect to the rear roof rail master section, rear flange of the outer panel was created following the previous steps and here the corners were left untouched to give some reliefs, but the sharp corners were trimmed off.
• After this the sheet is extended at the 0y section and trimmed with xz plane to avoid gaps while mirroring it.
• The completed outer panel is mirrored about the respective plane and sewed them together.
• Finally, the whole outer panel surface was thickened to 0.75mm to form a solid part and its sharp edges were blended.

2. Front roof rail:

             Front roof rail is one of the reinforcements of roof which is present at the front and provides required strength to the roof.

Front roof rail in different views is shown.

• The reference surfaces for front roof are taken from the roof outer panel using the wave geometry linker command. And their links were broken and kept as the input surfaces for front roof rail.
• With help of these surfaces, the front roof rail creation starts by offsetting it to the required dimensions according to front roof rail master section.
• After the offsets, using the ‘offset curve in face’ command, required curve was offset to the required positions to remove the unwanted surfaces.
• Those unwanted surface were removed using the ‘trim sheet’ command and the law extensions were created to connect the surfaces in different heights.
• Now, the overlapping surfaces were trimmed with ‘trim and extend’ command to make corners, then these corners are blended using the ‘edge blend’ command.
• Then some embosses were created, to improve the strength of front roof rail using the ‘emboss’ command.
• Once, the surface creations were done, then the sheet is extended and trimmed at 0y region.
• Finally, the front roof rail surface is mirrored about the xz plane, then sewed them together using ‘sew’ command to thicken it using ‘thicken’ command to create a solid part and its sharp edges were blended.

3. Bow roof-1:

             Bow roof is one of the reinforcements of roof which is present at the flat regions of roof to with stand heat distortion and snow load conditions. By using the bow roofs permanent deformation of roof at the flatter region is avoided by stress distribution.

              Bow roof-1 in different views is shown.

• The reference surfaces for bow roof are taken from the roof outer panel using the wave geometry linker command. And their links were broken and kept as the input surfaces for bow roof.
• With help of these surfaces, the bow roof creation starts by offsetting it to the required dimensions according to bow roof master section.
• After the offsets, using the ‘offset curve in face’ command, required curve was offset to the required positions to remove the unwanted surfaces.
• Those unwanted surface were removed using the ‘trim sheet’ command and the surfaces in different heights were connected using the ‘through curve’ command.
• Now, these surfaces are sewed together using ‘sew’ command and then its corners are blended using the ‘edge blend’ command.
• Then mastic regions were created at the outer flanges of bow roof using the sketch created on the plane normal to the surface by projecting it to the surface and trimming the unwanted regions.
• Once, the surface creations were done, then the sheet is extended and trimmed at 0y region.
• Finally, the bow roof surface is mirrored about the xz plane, then sewed them together using ‘sew’ command to thicken it using ‘thicken’ command to create a solid part and its sharp edges were blended.

4. Centre roof rail:

               Centre roof rail is one of the reinforcements of roof which is present at the centre region of roof or at the region where the B-pillar connects with the roof.

Centre roof rail in different views is shown.

• The reference surfaces for centre roof rail are taken from the roof outer panel using the wave geometry linker command. And their links were broken and kept as the input surfaces for centre roof rail.
• With help of these surfaces, the centre roof rail creation starts by offsetting it to the required dimensions according to centre roof rail master section.
• After the offsets, using the ‘offset curve in face’ command, required curve was offset to the required positions to remove the unwanted surfaces.
• Those unwanted surface were removed using the ‘trim sheet’ command and the surfaces in different heights were connected using the ‘through curve’ command.
• Now, these surfaces are sewed together using ‘sew’ command and then its corners are blended using the ‘edge blend’ command.
• Then mastic regions were created at the outer flanges of bow roof using the curves which is created by ‘curve on surface’ command and with help of these curves, the unwanted regions were trimmed off.
• Once, the surface creations were done, then the sheet is extended and trimmed at 0y region.
• Finally, the bow roof surface is mirrored about the xz plane, then sewed them together using ‘sew’ command to thicken it using ‘thicken’ command to create a solid part and its sharp edges were blended.

5. Bow roof-2:

             The same procedure of bow roof-1 creation is followed to create the bow roof-2 also, but in different position which is between the centre roof rail and rear roof rail.

Bow roof-2 in different views is shown.

6. Rear roof rail:

              Rear roof rail is one of the reinforcements of roof which is present at the rear and provides required strength to the roof at rear side.

Rear roof rail in different views is shown.

• The reference surfaces for rear roof are taken from the roof outer panel using the wave geometry linker command. And their links were broken and kept as the input surfaces for rear roof rail.
• With help of these surfaces, the rear roof rail creation starts by offsetting it to the required dimensions according to rear roof rail master section.
• After the offsets, using the ‘offset curve in face’ command, required curve was offset to the required positions to remove the unwanted surfaces.
• Those unwanted surface were removed using the ‘trim sheet’ command and the law extensions were created to connect the surfaces in different heights.
• Now, the overlapping surfaces were trimmed with ‘trim and extend’ command to make corners, then these corners are blended using the ‘edge blend’ command.
• Then some embosses were created, to improve the strength of rear roof rail using the ‘emboss’ command.
• Once, the surface creations were done, then the sheet is extended and trimmed at 0y region.
• Finally, the rear roof rail surface is mirrored about the xz plane, then sewed them together using ‘sew’ command to thicken it using ‘thicken’ command to create a solid part and its sharp edges were blended.

Designed Roof and its reinforcements is shown.

Conclusion:

            The car roof design and its reinforcements were developed from the given styling surface. And a detailed report about the engineering standards for roof design, its design procedure, its analysis for tooling feasibility, heat distortion & snow load conditions and its manufacturing techniques were prepared with the necessary pictures and calculations.