Introduction
Due to its relatively high cost and degree of difficulty for machining, most silicon carbide applications call for relatively low-tolerance or geometrically simple components, but precision components are very achievable with proper machining strategy.
Silicon carbide, known for its exceptional hardness, is widely used as an abrasive in cutting and grinding applications. Before the final sintering stage, silicon carbide ceramic parts can be machined in their “green” or presintered “bisque” states using conventional methods like milling, drilling, and turning. However, once fired, its hard-wearing nature necessitates diamond-grinding or lapping techniques, which are both time-consuming and costly. Consequently, while most applications involve low-tolerance or geometrically simple components, precision components are achievable with the right machining strategy.
Application of silicon carbide ceramics
Silicon carbide ceramics are high-quality materials with many excellent characteristics, such as low density, high hardness, extreme wear resistance, strong thermal conductivity, acid resistance, low thermal expansion rate, and good corrosion resistance. Therefore, they have a wide range of applications:- Wear parts •Mechanical seals •Nozzles •Pistons •Bearings •Heat exchangers •Kiln furniture •Valve components
The manufacture of silicon carbide ceramics
The silicon carbide manufacturing process includes the following steps:
- Powder preparation
- Kneading
- Shape forming
- Computer Numerical Control (CNC) Machining
- Sintering
- Lapping or grinding
Silicon Carbide can be machined in green, biscuit, or fully dense states, while the machinability become increasingly lower. In the green or biscuit form, it can be machined relatively easily into desirable geometries.
When cutting green ceramic, most tool wear is caused by the abrasive nature of the ceramic particles rather than by the material temperature or cutting speed. This places emphasis on selecting the most abrasion resistant tool surface such as CVD diamond.
In the sintering process, Silicon Carbide shrinks approximately 20%. In order to control the final sizes in tolerances, fully sintered material must be machined /ground with precise diamond tools. Like both oxides and nitrides, silicon carbide is a very hard-wearing material which requires diamond-grinding methods to process once fired. This process is usually time-consuming and costly.
The sintered silicon carbide ceramics are very hard and wear-resistant, which cannot be processed by conventional tools and require diamond tools.
Differences Between Green Ceramics (Unsintered Ceramics) and Sintered Ceramics
- Green Ceramics:
- Unsintered State: Green ceramics refer to ceramic materials that have not undergone sintering or are partially processed. This means they have been shaped but have yet to undergo the final sintering process.
- Characteristics: Green ceramics typically exhibit lower mechanical strength and hardness compared to their fully sintered counterparts. They are relatively brittle and easier to process and shape.
- Processing: Machining green ceramics requires tools and techniques suitable for their softer and less dense state.
- Ordinary Ceramics (Fully Sintered Ceramics):
- Sintered State: Ordinary ceramics, also known as fully sintered ceramics, have undergone a complete sintering process where ceramic powder particles are bonded together at high temperatures. This results in a dense, hardened structure.
- Characteristics: Fully sintered ceramics possess high mechanical strength, hardness, and durability, making them more resistant to heat, wear, and chemical corrosion than green ceramics.
- Processing: Machining fully sintered ceramics necessitates specialized tools, such as diamond-coated cutting tools, due to their extreme hardness and wear resistance.
Key Points:
- Green Ceramics: They are in a softer, unsintered state, making them suitable for shaping and machining prior to sintering.
- Ordinary Ceramics (Fully Sintered): Possess greater hardness and strength, making them ideal for applications requiring high mechanical and thermal performance.
The choice between processing green ceramics or ordinary ceramics depends on the desired properties and the manufacturing stage (pre-sintering or post-sintering).
The sintered ceramic (on the left) has a 20% reduction in volume compared to the unsintered ceramic (on the right).
Green Ceramic Machining Methods and Key Points:
End Milling：
Cutting depth should not exceed one-third of the tool diameter. Increasing the cutting depth to half the tool diameter can cause chipping at the cut exit.
Tool configuration: Use end mills with corner radius (R) whenever possible.
For rough machining, use 2-flute end mills for better chip evacuation. For finishing and semi-finishing, use 4-flute end mills to improve surface finish and extend tool life.
Preventing chipping: Before starting the cut, mill a small section at the exit end of the part to avoid chipping, similar to chamfering the end of a turned cylinder. Reducing the feed rate can also minimize chipping but will directly impact production efficiency. For surface machining, using helical interpolation is more effective than circular interpolation.
Feed rate: If the feed rate is too low (less than 0.002 mm/rev), the tool may polish the part instead of cutting, leading to rapid tool wear.
Starting parameters for end milling green ceramic | ||||
Endmill dia. in. (mm) | Machine speed rpm | Cutting speed sfm (m/min) | Operation | Feed rate fpt (mm) |
1/64 | 6,000 to 10,000 | 25 to 40 (8 to 12) | Finish | .0002-.0005 (.005-.013) |
1/32 (1.0) | 6,000 to 10,000 | 50 to 80 (15 to 25) | Finish | .0005-.001 (.013-.025) |
1/16 (2.0) | 6,000 to 10,000 | 100 to 160 (30 to 50) | General Finish | .001-.002 (.025-.050) .0005-.001 (.015-.025) |
1/8 (3.0) | 6,000 to 10,000 | 200 to 325 (60 to 100) | General Finish | .001-.002 (.025-.050) .0005-.001 (.015-.025) |
3/16 (5.0) | 4,000 to 10,000 | 200 to 500 (60 to 150) | General Finish | .001-.002 (.025-.050) .0005-.001 (.015-.025) |
1/4 (6.0) | 3,000 to 10,000 | 200 to 650 (60 to 200) | General Finish | .002-.004 (.050-.100) .001-.002 (.025-.050) |
5/16 (8.0) | 2,500 to 10,000 | 200 to 800 (60 to 245) | General Finish | .002-.004 (.050-.100) .001-.002 (.025-.050) |
3/8 (10.0) | 2,000 to 10,000 | 200 to 1000 (60 to 300) | General Finish | .003-.005 (.075-.130) .001-.003 (.025-.075) |
1/2 (12.0) | 1,500 to 10,000 | 200 to 1300 (60 to 400) | General Finish | .003-.005 (.075-.130) .001-.003 (.025-.075) |
Drilling:
The best method is step “peck” drilling, with each step depth not exceeding one-quarter of the drill diameter.
Dust removal: Special attention should be paid to removing machining dust from the hole during drilling. Proper dust removal allows for higher spindle speeds and reduces drill wear.
Machining parameters: The table below shows the initial machining parameters for green ceramics. As with all applications, these conditions will vary depending on the grade of ceramic, setup, and dust removal practices.
Starting parameters for drilling green ceramic | |||
Drill diameter in. (mm) | Peck size in. (mm) | Cutting speed sfm (m/min) | Feed rate ipr (mm/rev) |
1/32-3/16 (1.0-5.0) | 1/128-3/64 (.25-1.25) | 200 to 1,000 (60 to 300) | .001-.003 (.025-.075) |
3/16-1/4 (5.0-6.0) | 3/64-1/16 (1.25-1.5) | .002-.004 (.050-.100) | |
1/4-5/16 (6.0-8.0) | 1/16-5/64 (1.5-2.0) | .002-.005 (.050-.130) | |
5/16-3/8 (8.0-10.0) | 5/64-3/32 (2.0-2.5) | .002-.006 (.050-.150) | |
3/8-1/2 (10.0-12.0) | 3/32-1/8 (2.5-3.0) | .002-.008 (.050-.200) | |
Profile Machining:
The table below provides the initial machining parameters for ball-end mills, flat-end mills, and reverse taper mills.
Starting parameters for profiling green ceramic | ||||
Cutting dia. in. (mm) | Machine speed rpm | Cutting speed sfm (m/min) | Operation | Feed rate fpt (mm) |
5/16 (7.94) | 7,500 to 16,000 | 640 to 1,320 (195 to 400) | General Finish | .005-.008 (.130-.200) .001-.004 (.025-.100) |
3/8 (9.53) | 6,500 to 13,500 | General | .005-.008 (.130-.200) .001-.004 (.025-.100) | |
1/2 (15.9) | 4,900 to 10,000 | Finish | .009-.015 (.230-.400) .002-.008 (.050-.200) | |
5/8 (15.9) | 3,900 to 8,000 | General | .009-.015 (.230-.400) .002-.008 (.050-.200) | |
3/4 (19.1) | 3,200 to 6,700 | Finish | .009-.015 (.230-.400) .002-.008 (.050-.200) | |
1 (25.4) | 2,400 to 5,000 | General | .013-.020 (.330-.500) .004-.012 (.100-.300) | |
1-1/4 (31.8) | 2,000 to 4,000 | Finish | .013-.020 (.330-.500) .004-.012 (.100-.300) | |
Turning and Milling with Replaceable Inserts:
Tool configuration: For turning and milling graphite, inserts with 1/64” to 1/32” nose radii are most effectively used for turning and milling graphite. A positive rake insert with a finish ground flank is preferred.
Surface finish: Surface finish can be improved by selecting appropriate tool geometry and feed rate. A larger corner radius will enhance surface finish but increase cutting resistance. A smaller corner radius reduces pressure but requires a reduced feed rate to achieve the same surface finish. Depth of cut (DOC) does not affect surface finish unless cutting resistance causes vibration.
Exit chipping: This can be avoided by machining a chamfer at the exit end of the part. Avoid using square shoulder parting tools. It is recommended to use tools with a radius or 20-degree chamfer.
Turning
Machining parameters: When machining long bars and cylinders, higher speeds and deeper cuts can be used, especially for high-strength graphite materials.
Depth of cut: Maximize the depth of cut to avoid workpiece deformation. If deformation occurs, adjust the feed rate and depth of cut. A lower feed rate will allow for maintaining a deeper depth of cut. For rough machining, a feed rate of 0.005 inches per revolution is recommended, while for finishing, a feed rate between 0.001 inches and 0.003 inches per revolution may be needed. Deeper cuts will always result in higher cutting resistance and larger chips, leading to a rougher surface. The table below shows the initial machining parameters for general turning and finishing turning.
Starting parameters for turning green ceramic | ||
Operation | Cutting speed sfm (m/min) | Feed rate ipr (mm/rev) |
General | 100-500 (30 to 150) | .002-.010 (.050-.250) |
Finish | ||
Milling
Workpiece Configuration: When milling large surfaces or volumes, higher speeds and depths of cut can be employed. Use higher strength ceramic materials when thin walls are involved.
Depth of Cut (DOC): Maximize DOC when possible to reduce the number of passes. Lower feed rates allow for deeper cuts. For roughing, use a feed rate of 0.004 inches per tooth per revolution, and for finishing, a feed rate between 0.0005 inches and 0.002 inches per tooth per revolution may be necessary.
Multiple Cutters: For multiple-pocket milling cutters, it is recommended to use axial alignment to align all inserts within +/-0.0002 inches for the best results. This improves surface finish and reduces insert wear, as all inserts will be cutting equally.
Machining Parameters: The table below shows the starting machining parameters for general-purpose and finish turning.
Starting parameters for milling green ceramic | ||
Operation | Cutting speed sfm (m/min) | Feed rate ipr (mm/rev) |
General | 500-1,000 (150 to 300) | .002-.006 (.050-.150) |
Finish | ||
Métodos de Mecanizado de Cerámica Verde y Puntos Clave:
Fresado de Extremo:
La profundidad de corte no debe exceder un tercio del diámetro de la herramienta. Aumentar la profundidad de corte a la mitad del diámetro de la herramienta puede causar astillado en la salida del corte.
Configuración de la herramienta: Utilice fresas de extremo con radio en la esquina (R) siempre que sea posible.
Para el mecanizado en bruto, utilice fresas de extremo de 2 flautas para una mejor evacuación de virutas. Para acabado y semiacabado, utilice fresas de extremo de 4 flautas para mejorar el acabado superficial y prolongar la vida útil de la herramienta.
Prevención de astillado: Antes de iniciar el corte, fresar una pequeña sección en el extremo de salida de la pieza para evitar astillado, similar al chaflanado del extremo de un cilindro torneado. Reducir la velocidad de avance también puede minimizar el astillado, pero afectará directamente la eficiencia de producción. Para el mecanizado de superficies, la interpolación helicoidal es más efectiva que la interpolación circular.
Velocidad de avance: Si la velocidad de avance es demasiado baja (menos de 0.002 mm/rev), la herramienta puede pulir la pieza en lugar de cortarla, lo que lleva a un desgaste rápido de la herramienta.
Parámetros iniciales para el fresado de extremo de cerámica verde | ||||
Diámetro de la fresa in. (mm) | Velocidad de la máquina rpm | Velocidad de corte sfm (m/min) | Operación | Velocidad de avance fpt (mm) |
1/64 | 6,000 to 10,000 | 25 to 40 (8 to 12) | Acabado | .0002-.0005 (.005-.013) |
1/32 (1.0) | 6,000 to 10,000 | 50 to 80 (15 to 25) | Acabado | .0005-.001 (.013-.025) |
1/16 (2.0) | 6,000 to 10,000 | 100 to 160 (30 to 50) | General Acabado | .001-.002 (.025-.050) .0005-.001 (.015-.025) |
1/8 (3.0) | 6,000 to 10,000 | 200 to 325 (60 to 100) | General Acabado | .001-.002 (.025-.050) .0005-.001 (.015-.025) |
3/16 (5.0) | 4,000 to 10,000 | 200 to 500 (60 to 150) | General Acabado | .001-.002 (.025-.050) .0005-.001 (.015-.025) |
1/4 (6.0) | 3,000 to 10,000 | 200 to 650 (60 to 200) | General Acabado | .002-.004 (.050-.100) .001-.002 (.025-.050) |
5/16 (8.0) | 2,500 to 10,000 | 200 to 800 (60 to 245) | General Acabado | .002-.004 (.050-.100) .001-.002 (.025-.050) |
3/8 (10.0) | 2,000 to 10,000 | 200 to 1000 (60 to 300) | General Acabado | .003-.005 (.075-.130) .001-.003 (.025-.075) |
1/2 (12.0) | 1,500 to 10,000 | 200 to 1300 (60 to 400) | General Acabado | .003-.005 (.075-.130) .001-.003 (.025-.075) |
Perforación:
El mejor método es la perforación paso a paso, con cada paso de profundidad no excediendo una cuarta parte del diámetro de la broca.
Remoción de polvo: Se debe prestar especial atención a la eliminación del polvo de mecanizado del agujero durante la perforación. Una adecuada remoción del polvo permite mayores velocidades del husillo y reduce el desgaste de la broca.
Parámetros de mecanizado: La tabla a continuación muestra los parámetros iniciales de mecanizado para cerámicas verdes. Como en todas las aplicaciones, estas condiciones variarán dependiendo del grado de cerámica, la configuración y las prácticas de remoción de polvo.
Parámetros iniciales para la perforación de cerámica verde:
Diámetro de la broca in. (mm) | Tamaño de la pasada in. (mm) | Velocidad de corte sfm (m/min) | Velocidad de avance ipr (mm/rev) |
1/32-3/16 (1.0-5.0) | 1/128-3/64 (.25-1.25) | 200 to 1,000 (60 to 300) | .001-.003 (.025-.075) |
3/16-1/4 (5.0-6.0) | 3/64-1/16 (1.25-1.5) | .002-.004 (.050-.100) | |
1/4-5/16 (6.0-8.0) | 1/16-5/64 (1.5-2.0) | .002-.005 (.050-.130) | |
5/16-3/8 (8.0-10.0) | 5/64-3/32 (2.0-2.5) | .002-.006 (.050-.150) | |
3/8-1/2 (10.0-12.0) | 3/32-1/8 (2.5-3.0) | .002-.008 (.050-.200) |
Mecanizado de Perfil:
La tabla a continuación proporciona los parámetros iniciales de mecanizado para fresas de extremo esférico, fresas de extremo planas y fresas de cono inverso:
Parámetros iniciales para el mecanizado de perfiles en cerámica verde | ||||
Diámetro de corte in. (mm) | Velocidad de la máquina rpm | Velocidad de corte sfm (m/min) | Operación | Velocidad de avance fpt (mm) |
5/16 (7.94) | 7,500 to 16,000 | 640 to 1,320 (195 to 400) | General Acabado | .005-.008 (.130-.200) .001-.004 (.025-.100) |
3/8 (9.53) | 6,500 to 13,500 | General | .005-.008 (.130-.200) .001-.004 (.025-.100) | |
1/2 (15.9) | 4,900 to 10,000 | Acabado | .009-.015 (.230-.400) .002-.008 (.050-.200) | |
5/8 (15.9) | 3,900 to 8,000 | General | .009-.015 (.230-.400) .002-.008 (.050-.200) | |
3/4 (19.1) | 3,200 to 6,700 | Acabado | .009-.015 (.230-.400) .002-.008 (.050-.200) | |
1 (25.4) | 2,400 to 5,000 | General | .013-.020 (.330-.500) .004-.012 (.100-.300) | |
1-1/4 (31.8) | 2,000 to 4,000 | Acabado | .013-.020 (.330-.500) .004-.012 (.100-.300) | |
Tornería y Fresado con Insertos Intercambiables:
Configuración de la herramienta: Para torneado y fresado de grafito, los insertos con radios de punta de 1/64” a 1/32” son los más efectivos. Se prefiere un inserto con ángulo de ataque positivo y flanco rectificado para un mejor acabado.
Acabado superficial: El acabado superficial puede mejorarse seleccionando la geometría adecuada de la herramienta y la velocidad de avance. Un radio de esquina más grande mejorará el acabado superficial pero aumentará la resistencia de corte. Un radio de esquina más pequeño reduce la presión pero requiere una reducción en la velocidad de avance para lograr el mismo acabado superficial. La profundidad de corte (DOC) no afecta el acabado superficial a menos que la resistencia de corte cause vibración.
Astillas en la salida: Esto puede evitarse mecanizando un chaflán en el extremo de salida de la pieza. Evite utilizar herramientas de corte de hombro cuadrado. Se recomienda utilizar herramientas con radio o chaflán de 20 grados.
Torneado
Parámetros de mecanizado: Cuando se mecanizan barras largas y cilindros, se pueden usar velocidades más altas y cortes más profundos, especialmente para materiales de grafito de alta resistencia.
Profundidad de corte: Maximice la profundidad de corte para evitar la deformación de la pieza. Si ocurre deformación, ajuste la velocidad de avance y la profundidad de corte. Una menor velocidad de avance permitirá mantener una profundidad de corte más profunda. Para el mecanizado en bruto, se recomienda una velocidad de avance de 0.005 pulgadas por revolución, mientras que para el acabado, puede ser necesario un avance entre 0.001 y 0.003 pulgadas por revolución. Cortes más profundos siempre resultarán en una mayor resistencia de corte y virutas más grandes, lo que lleva a una superficie más áspera. La tabla a continuación muestra los parámetros iniciales de mecanizado para el torneado general y el torneado de acabado.
Parámetros iniciales para el torneado de cerámica verde | ||
Operación | Velocidad de corte sfm (m/min) | Velocidad de avance ipr (mm/rev) |
General | 100-500 (30 to 150) | .002-.010 (.050-.250) |
Acabado | ||
Fresado
Configuración de la pieza: Cuando se fresan superficies o volúmenes grandes, se pueden emplear velocidades y profundidades de corte más altas. Utilice materiales cerámicos de mayor resistencia cuando se involucren paredes delgadas.
Profundidad de corte (DOC): Maximice la DOC cuando sea posible para reducir el número de pasadas. Las velocidades de avance más bajas permiten cortes más profundos. Para el desbaste, utilice una velocidad de avance de 0.004 pulgadas por diente por revolución, y para el acabado, puede ser necesario un avance entre 0.0005 y 0.002 pulgadas por diente por revolución.
Múltiples Fresas: Para fresas de fresado con múltiples cortadores, se recomienda usar alineación axial para alinear todos los insertos dentro de +/-0.0002 pulgadas para obtener los mejores resultados. Esto mejora el acabado superficial y reduce el desgaste de los insertos, ya que todos cortarán de manera uniforme.
Parámetros de Mecanizado: La tabla a continuación muestra los parámetros iniciales de mecanizado para el torneado de propósito general y torneado de acabado.
Parámetros iniciales para el fresado de cerámica verde | ||
Operación | Velocidad de corte sfm (m/min) | Velocidad de avance ipr (mm/rev) |
General | 500-1,000 (150 to 300) | .002-.006 (.050-.150) |
Acabado | ||
