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Ring and Pinion Gear Blank Forging

With the ideal steel alloy selected, the next step in the gear making process is forming a gear blank and that can be done in one of three ways. The easiest and cheapest method is called casting, where molten steel is poured into a mold and left to cool. Another option is CNC machining the blank from a solid block of steel with high speed cutting knives and drills. The third method involves forging the steel into shape by heating it in a furnace and forcing it into a mold using extremely high pressures. Each process alters the internal structure of the steel in ways that affect the strength and toughness of the final gear, so selecting the right process is crucial.

Metal Casting

Casting is the cheapest metal forming procedure because it is fast and easy to perform in large quantities. A high temperature furnace is used to melt the steel so it can be poured into a mold and left to cool. When steel is heated to its melting point, its internal grain structure is allowed to expand freely, resulting in a courser and less organized grain after the metal solidifies. Unfortunately, casting also has a high risk of porosity defects, where gas bubbles form inside the metal when it cools. This formative process works very well for creating intricate shapes where strength and metal quality is not important; however, it should never be used for automotive gears.

CNC Machining

CNC machining uses computer controlled mills to cut a block of steel, called a billet, into the desired shape. Because this process does not alter the grain structure of the base material, it is significantly better than casting, although it is very expensive and time consuming. CNC machining also requires the billet to already have the desired properties of the end product, which significantly increases the cost of the steel. While a consistent grain structure is highly desired, cutting away material interrupts the grain flow and can reduce toughness while also increasing the likelihood that stress fractures will form on those surfaces. CNC machining works well for prototypes and is an excellent choice for final shaping and finishing, but automotive gears should not be machined directly from billet steel.

Metal Forging

Forging uses high heat and extremely high pressures to force steel into a new shape. The process begins by cutting down a piece of bar stock to the proper size billet and heating it up to 2,000°F in a furnace, where it becomes malleable but is still safely below its 2,600°F melting point. Once heated, the billet is placed between two dies in a forging press where thousands of tons of force is applied to mechanically push the steel into the new shape. In some cases, several sets of progressive dies are needed to achieve the final shape.

Unlike casting and CNC machining, the forging process does not interrupt the internal grain flow of the material but instead reorients it to match the new shape. Additionally, the high heat and pressures used in the forging process will break up and redistribute inclusions (gas pockets and impurities) throughout the metal and helps to create a more uniform grain. The end result is a gear blank that has significantly improved directional toughness, impact strength, fatigue resistance, and can be further improved with heat treatment after machining.

Grain Structure

The molecular structure of steel resembles the properties of wood where slow growing hardwood with a tight grain is stronger than wood from a fast growing tree with thick grain. Similarly, steel with small grain crystals is stronger and tougher than steel with large grain crystals.

Casting can be compared to the manufacture of particle board, where wood is shredded and pressed into a mold with an adhesive to create cheap and weak boards. CNC machining is like carving a sculpture from a tree stump where material is removed exposing the internal grain that, if left untreated, will crack and split over time. Forging would then be compared to using steam to soften wood so it can be bent into a new shape (like a skateboard) and retain the original grain structure.

Protecting and preserving the internal grain structure of steel is critical to retaining strength, durability, and fatigue resistance in gears, which is why forging is the preferred method of gear blank forming. Moreover, when done correctly, forging can even improve the overall grain structure of the gear blank over the original billet.

Gear Forging Process

Crown Race Gears are hot forged in a progressive multi-stage process that creates a gear blank with the ideal properties for gear tooth cutting and heat treatment. Perfecting this process requires a steel alloy with properties that allow the metal to be molded while retaining the desired properties for the end product. Billets are heated in the forge to a specific temperature before being pressed through a series of precisely machined dies at highly controlled pressures and speeds by a master forge press operator. The red hot gear blanks are then left to air cool before moving on to gear tooth cutting.

Due to the scale of this operation, forging is not a process that Crown Race Gears can perform in-house, and neither is it something we plan to integrate into our operation. We have sourced one of the premier forges in the world to create our gear blanks, with the highest quality control standards and a stellar reputation for precision and consistency.

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Removing Internal Stresses (Normalizing)

During the forging process, residual stress forces remain inside of each gear blank that need to be removed in a process called normalizing prior to moving on to gear shaping.

Crown Race Gears Forged Blanks

After being forged and normalized, the gear blanks go through an initial quality control inspection before making their way to the machining process. Any defects or imperfections found at this stage will result in the entire batch being scrapped as Crown Race Gears holds some of the tightest quality control standards in the industry. Making tough, strong, and efficient gears requires gear blanks with virtually no imperfections, a tight and constant grain structure, and no residual internal stresses.

How do They Make Gears? - Gear Forging Process Detailed Introduction

-02-21 15:47:11

Gear forging processes include blank forging, normalizing, turning machining, hobbing on shaper machine, gear shaving, heating treatment, grinding machining. Following shows the detailed introduction of each forging process.

Gear Forging Process Introduction

Blank forging

Normalizing

The purpose of this process is to obtain the hardness suitable for subsequent gear cutting and to prepare the structure for the final heat treatment, so as to effectively reduce the heat treatment deformation. The material of gear steel used is usually 20CrMnTi, and the normal normalizing is greatly affected by personnel, equipment and environment, it makes it difficult to control the cooling speed and uniformity of the workpiece, resulting in large hardness dispersion and uneven microstructure, which directly affects the metal cutting and final heat treatment, making the thermal deformation large and irregular, and the quality of the parts can not be controlled. Therefore, the isothermal normalizing process is adopted. The practice shows that the use of isothermal normalizing effectively changes the disadvantages of general normalizing, and the product quality is stable and reliable.

Turning machining

In order to meet the positioning requirements of high-precision gear processing, CNC lathe is used for the processing of gear blank, and the machining of hole diameter, end face and outer diameter under one clamping is completed synchronously by using mechanical clamping without regrinding turning tool. It not only ensures the perpendicularity requirements of the inner hole and the end face, but also ensures the small size dispersion in the production of large batch of gear blanks. In addition, the high efficiency of CNC lathe processing also greatly reduces the number of equipment, good economy.

Hobbing and gear shaping

A large number of gear hobbing machines and gear shapers are still used in the processing of gear parts. Although it is convenient to adjust and maintain, the production efficiency is relatively low. If a large production capacity is completed, multiple machines are required to produce at the same time. With the development of coating technology, the re coating of hob and insert blade after grinding is very convenient. After coating, the service life of the tool can be significantly improved, which can generally be increased by more than 90%, effectively reducing the number of tool changes and grinding time, with significant benefits.

Gear shaving

Radial shaving technology is widely used in the production of automobile gears with its advantages of high efficiency, easy realization of the requirements of tooth shape design and tooth direction modification.

Heat treatment

Carburizing and quenching are required for automobile gears to ensure their good mechanical properties. Stable and reliable heat treatment equipment is essential for the products that will not be grinded after heat treatment.

Grinding machining

It is mainly to finish the inner hole, end face and outer diameter of the shaft of the gear after heat treatment, so as to improve the dimensional accuracy and reduce the geometrical tolerance.

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