Hood design-Week 2

TABLE OF CONTENT:

• A. OBJECTIVES
• B. DEFINITIONS
• C. MANUFACTURING PROCESSES
• 1. HEMMING
• 2. FORMING (EMBOSS CREATION)
• D. DESIGN CONSIDERATIONS
• E. COMMANDS USED IN SIEMENS NX CAD
• F. DESIGNING USING THE SIEMENS NX TOOL (STEPS, SNIPS, AND RESULTS)
• G. CONCLUSION

A. OBJECTIVE

• The aim of this project is to Design a hood assembly with the input Class A Surface of the Inner panel, through SIEMENS NX CAD Software, considering all the industrial standards, manufacturing feasibility and methods, assembling the hood inner & outer panel, hinge assembly, and striker.

B. DEFINE HOOD

• A car hood also referred to as a bonnet in some other countries is the hinged cover that rests over the engine of a front-engine vehicle. Its purpose is to provide access to the engine for repair and maintenance also to cover the engine compartment to not to directly expose to external dust and debris and also to give an aesthetic look to the vehicle.
• A concealed latch is typically used to hold down the hood. On vehicles with an aftermarket hood and on racecars, hood pins may be used to hold down the car hood. Hoods sometimes also contain a hood scoop, wiper jets, power bulge, and/or hood ornament.
• Car hoods are typically constructed from steel and sometimes from aluminum. Aftermarket car hoods may be constructed from various other materials, including carbon fiber, fiberglass, or dry carbon. The hood design may vary from one make and model to another, including SUVs, MUVs, Sedans, hatchbacks, or heavy vehicles.

C. MANUFACTURING PROCESS

1. Hemming:

Hemming is a forming operation in which the edges of the sheet are folded or folded over another part in order to achieve a tight fit. Normally hemming operations are used to connect parts together, improve the appearance of a part, and reinforce part edges.

In car part production, hemming is used in assembly as a secondary operation after deep drawing, trimming, and flanging operations to join two sheet metal parts (outer and inner) together. Typical parts for this type of assembly are hoods, doors, trunk lids, and fenders. 

The accuracy of the hemming operation is very important since it affects the appearance of the surface and surface quality. Material deformations, which occur during the hemming process, can lead to dimensional variations and other defects in parts. Typical hemming defects are splits or wrinkles in the flange, material overlaps in the corner areas, or material roll-in. This is why it is important to use simulation tools in order to, on the one hand, better understand the hemming process and, on the other hand, significantly reduce the number of “trial and error” loops during tryout and production.

There are two types of hemming processes:

a) Conventional die Heming b) Roll Heming

a) Conventional die hemming:

Conventional die hemming is suitable for mass production. In die hemming, the flange is folded over the entire length with a hemming tool. Normally the actual hemming is a result of a forming operation in which the flange is formed with a hemming tool after the drawing and trimming operations have been completed. Production plants for conventional die hemming are very expensive, but the cycle times are very low.

b) Roll Heming:

Roll hemming is carried out incrementally with a hemming roller. An industrial robot guides the hemming roller and forms the flange. Roll hemming operation can also be divided into several pre-hemming and final hemming process steps. Roll hemming is very flexible to use and tool costs are significantly lower as compared to those of conventional die hemming. However, the cycle times are much higher since the hemming is realized using a hemming roller that follows a defined path.

2. Deep Drawing: 

           Deep drawing is a sheet-forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered a "deep" drawing when the depth of the drawn part exceeds its diameter. This is achieved by redrawing the part through a series of dies. The flange region (sheet metal in the die shoulder area) experiences radial drawing stress and tangential compressive stress due to the material retention property. These compressive stresses result in flange wrinkles (wrinkles of the first order). Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius.

Deep drawing processes are divided into three types:

• Deep drawing with tools
• Deep drawing with active means
• Deep drawing with active energy

In the automotive industry, deep drawing is usually carried out using rigid tools.

The rigid tools consist of a punch, die, and binder. In deep drawing, the plate holder closes after the metal sheet blank has been inserted.

Next, the sheet is clamped between the die and the binder. This process slows down the flow of the sheet while it is being drawn and thereby prevents wrinkles from forming under the binder. The punch stretches the sheet over the die radius and forms it in the die. The amount of punch force necessary for forming is thereby continually increased up to the lower dead center of the punch.

Whereas in pure deep drawing, there is no reduction of sheet metal thickness, forming is achieved in stretch forming purely as a result of a decrease in sheet metal thickness. Stretch forming is extensively used for the forming of only slightly curved parts with low depth of draw (e.g., roofs, doors).

D. DESIGN CONSIDERATIONS IN HOOD

• The opening angle of the hood
• Support system for the hood
• Pedestrian safety rules etc.…

1. Hood head zone for striker locking: While designing the hood this parameter should be taken into consideration because when we open the hood to get access to the engine compartment to perform any operation while the hood is in the open condition the minimum height of the hood should be such that it does not collide or hit to the head of the person who is operating. So, the height of a hood in open conditions should be more than the average height of humans in that region thereby no human gets hurt.

 

2. Hood Stay: 

This is to support the hood when we open the hood and needs it to be kept in an open position & this can be done by placing it over something or by providing support to it. While designing this one should keep in mind that the support system should be self-sustainable to take load of the hood even if any external force is been applied to it due to wind or any other reason & it should facilitate easy opening and closing of the hood.

This can be done by using two ways 

1. Support Rod 
2. Gas stay

3. Pedestrian safety:

While designing the hood, it is very important criteria to consider the safety of pedestrians as when your car hits a pedestrian during a crash, it is much required to study the areas over the hood where different body parts of pedestrians are going to hit. we should also consider the average height of adults and children while designing our hood. So, whenever a new car is been launched, all these safety parameters are checked by taking a crash test and studying the data obtained from it to make changes accordingly to follow pedestrian safety.

4. Mastic Data:

We know that the hood comprises two main parts: a hood inner panel and a hood outer panel. The hood's inner panel is enclosed below the outer panel and thereby provides rigidity to the hood. When these parts are assembled to form a hood, a small gap remains between these two parts as they are both metal parts this may lead to noise and vibrations when the vehicle is in running condition which is undesirable to avoid this mastic sealant being filled between this small gap.

Mastic sealant is a paste-like material that is applied on a surface and is hardened by heating.

The Avg. diameter of this sealant is 10mm and its effective zone is up to 80mm.

5. Force Distribution:

A good designer should always consider the safety parameters as a top priority, In the case of hood design, one should design the hood in such a manner that if the vehicle gets a frontal crash, then all the forces must be distributed towards the hinge of the hood or sideways, as it is important to reduce the impact of the crash by diverting the force towards the hinges and thus safeguard the passengers in the vehicle.

E. COMMANDS USED IN HOOD ASSEMBLY

• Offset surface: This feature is available in surface modeling commands by using this we can offset the surface by the required distance along a vector or selected plane.
  
  
• Offset curve in face: This feature is very useful as it allows us to offset the curves or edges on the face by the required distance & in the required direction.
  
  
• Trim body: These features enable us to cut off the portion from a solid body or surface body to be removed along a vector or along a plane.
  
  
• Law extension: This feature extends the selected curve or edges of a base in a controlled manner i.e. we can change the length, angle of the extended surface, and also the law with which it gets extended.

• Through Curves: This feature enables the creation of a body through multiple selected sections by following the path of form by sections and thus creating a body underlying between these sections.

• Trim & Extend: This feature extends or trims the set of faces & edges which get intersected with another set of faces & edges as shown below. by using this feature one can easily create an embossed surface without using emboss feature tool in NX-CAD.

• Face Blend: Adds tangent blend faces between a selected set of faces, the shape of the blended face may be circular, conic, or law controlled. There are various types of face blends such as two face blend, and three face blend it depends on our application to use the type of blend.         

• Emboss: Modifies a body's face by projecting a section along a vector direction, with options such as end-cap location or shape for the emboss created.

• Mirror Assembly: This tool is very useful with aid of this tool we can easily make a mirror copy of our assembly this tool is present in the assembly workbench only. We have mirrored the hinge assembly by using this feature tool.

• Thicken: This feature is used to create a solid body from a thin face by providing a thickness value to it.

• Clip Section: Gives the cross-sectional view at the selected plane, helps to keenly observe the overlaps/ and embossments, etc.…

• Geometry Analysis
  
  
  
  

F. DESIGN USING SIEMENS NX CAD TOOL

COMPONENTS REQUIRED

1) Hood outer panel:

This part of the hood acts as a covering or casing for all the other parts of the hood which are enclosed inside this part. The outer surface of this part is provided with a good surface finish, painted with corrosion-resistant paint to prevent the inner parts from corrosion and also to give an aesthetic look to the car from the outside. This part forms the shape of our hood, and it is given different styling and shapes to enhance the look & performance of the vehicle to reduce the forces exerted by air when the vehicle is in motion.

2) Inner panel:

This part is enclosed in the outer panel it forms an integral part of the hood and provides rigidity to the hood, it is the part on which hinges and striker are to be fixed to make our hood fully functional it also consists of embossed sections for proper force distribution and mastic sealant points are also located on this part with reinforcement for hinge & striker. To reduce the weight of our hood some material is removed from the hood's inner panel as shown below.

3) Hinge assembly:

It is an important part of our hood because by use of this part only we are able to open and closed our hood whenever required with ease. This part should be strong enough to take the load of the hood and also the forces which act upon the hood in case of a crash. So, this part should be designed with utmost care so that it should not fail during its operation.

4) Striker:

This part is used for providing locking to the hood it consists of a latch-type mechanism and it is located on the inner panel of the hood to decide the position of our striker, the trajectory method is used with the help of this method we can easily locate our striker. This part holds our hood in closed condition unless it is being opened by pulling the latch lever to open our hood.

Master section:

This data is used for the design of our hood inner panel it gives all the information about the standard dimensions to be followed for the inner panel also information about radius values, and the maximum height of the hood inner panel from the top.

Deciding placement of Striker:

• To decide the position of the striker on the hood inner panel, primarily we have to fix the position of the hinges on the hood inner panel. Once it is done then take construct two points one on each hinge by taking the center point of the hinge. Then draw a line starting from one point to another point of the hinge, this line forms our hinge axis along which our hood will rotate when we open or close the hood.

• Now, by taking the midpoint of this line draw a circle normal to the line and take the approximate diameter of the circle initially, after that draw a line that is tangent to the circle also draw a vertical line whose one end gets coincident with the endpoint of the tangent line. This forms the trajectory path of our hood.
• Furthermore, construct a similar line on the striker taking the midpoint and constraint this line with the trajectory path as shown below.

 G. Conclusion:  Hence, All the values are within the limit and as per the standards, we have successfully completed our hood assembly by following all the standards and considering the given input data of the hood, also learned about the hemming process and manufacturing process of hood components i.e. deep drawing, etc.