PROCEDURES OF CNC MILLING MACHINING

To understand what is CNC milling. HESEM introduces the definition of CNC milling and related knowledge about CNC milling.

What is CNC Milling Machining?

– CNC milling is a machining process that uses a computer-controlled machine tool to gradually remove material from a workpiece by attaching a milling cutter, drill, etc. to the spindle. This process is suitable for machining. manufactures a wide range of materials, such as metal, plastic, glass, and wood, and manufactures a variety of tailored parts and products.
– This article focuses on CNC milling, outlining the basics of the process as well as the components and tools of a CNC milling machine. In addition, this article explores different milling operations and provides alternatives to the CNC milling process.

Definition of milling

– What is milling? It is a type of machining that uses a cutter to shape the workpiece, usually on a movable machine table, although some milling machines also have movable cutters.
– Milling was originally manual work done by humans, but most milling today is done by a CNC mill, which uses a computer to monitor the milling process.
– CNC milling offers greater accuracy, precision, and production rates, but there are still some situations when manual milling becomes useful.
– Manual milling, which requires more technical skill and experience, provides shorter turnaround times. It also has the added benefit that manual blenders are cheaper and users don’t need to worry about programming the machine.

CNC Milling Machining Process

– Like most conventional CNC machining processes, the CNC milling process uses computer controls to operate and manipulate machine tools that cut and shape the parent material. In addition, the process follows the same basic manufacturing stages that all CNC machining processes follow, including:

1. CAD model design

– The CNC milling process begins with creating a 2D or 3D CAD part design. Currently, most outsourcing companies use 3D models for CNC milling. This process is usually performed by mechanical engineers. Popular CAD software today: Fusion 360, Inventor, Solidworks, Top solid…

2. Convert CAD model into CNC program using CAM software

– The CNC programmer will use the 3D model to conduct CNC programming. Based on the shape, size and technical requirements of the part to be processed.
– The programmer will provide the machining technology process, design jigs, and make a list of the necessary equipment to prepare for the machining process. Then they will use the functional commands of the CAM software to create the toolpath for the CNC milling machine.
– After having the toolpath and the appropriate cutting parameters (cutting speed and spindle speed), they will proceed to compile the toolpath through a post-processor.
– This compiler software is designed specifically for each machine. It will translate these toolpaths into G code lines. The CNC milling machine understands these G-codes and executes the same toolpaths as those programmed in the CAM software.
– To ensure that the compilation is error-free and that errors are detected during toolpath generation. This G code program will be used to simulate cutting on cutting simulation software such as NC Simul, Vericut, CIMCO Edit, or online solution NC VIEWER
– Popular CAM software today: Fusion 360, MASTERCAM, CIMATRON … In which, Fusion 360 is the most outstanding solution today with the best cost-on-function approach. Very suitable for small companies that can use copyrighted software with the lowest cost and the most flexibility.

3. Preparatory operations on CNC milling machines

Before CNC milling is started, the Machine Operator must perform the following steps

Load fixtures and workpieces onto the table

– The fixture can be a vise or a metal block that holds a specialized workpiece that has been machined. After attaching the fixture to the machine table, we proceed to attach the workpiece to be processed on the fixture. If the machining plan does not need a fixture, we can place the workpiece directly on the machine table.
– The requirement of this step is to ensure that the workpiece and fixture are positioned and oriented correctly according to the programmer’s instructions. At the same time, this step must ensure that the workpiece is firmly fixed on the table and does not move during machining.

Load the cutter to the holder

– Based on the tool table that the programmer sent with the CNC machining program. The operator proceeds to attach the milling cutter to the knife holder.
– This step requires the operator to select the correct tool holder and the required tool type. At the same time, it is necessary to check the exact length of the tool holder according to the tool table, the tool holder is not reversed and is tightened carefully so that the tool does not drop during the machining process.
– After preparing the tool, the operator proceeds to attach the milling cutter to the milling machine insert. This step requires the operator to properly mount the tool in the magazine indicated on the tool plate.

Enter the tool length parameter into the memory cell of the CNC milling machine

– The operator calls each milling cutter attached to the main shaft and then moves the milling cutter tip to contact the specified surface (steel block with flat machined surface, gauge). After the milling cutter has made contact with this surface, the operator will record the tool length coordinate value relative to the machine’s original position and enter it in the tool length memory box.
– Perform this step sequentially for all tools.
– There are many methods of defining tool lengths for CNC machines. The above is just one of many methods.
Currently, in order to limit human error arising during the operation, take the tool length. Current CNC machines are equipped with automatic tool length takers. The operator just calls out the tool and runs the tool length subroutine. The CNC milling machine will automatically move to the tool position and enter the measured value into the machine’s memory.
– This is a very error-prone process, so the operator is required to enter and check the input value carefully.

Input the workpiece coordinates to the CNC machine following the setup book

– The operator determines the position of the machining standard and the programmer gives it and then proceeds to determine the actual machining standard on the machine. This machining standard value is stored for memory cells G54 through G59. Some machines may have additional memory expansion cells. Normally, each program has only one machining standard, and this standard is stored in memory cell G54.
– The operator attaches the centering probe to the spindle. For the mechanical probe, the main shaft must be rotated at a speed of about 300 rpm and then proceed to take the standard position according to the X axis and the standard position according to the Y axis. Enter these two values in the X and Y memory cells in the standard workpiece G54… 
– For the standard position in the Z-axis direction, we proceed to determine the distance from the position of measuring the tool length made in the previous step to the position required by the programmer. 
– This value is entered in the internal Z memory cell in the G54 standard. Note, depending on the position where the tool length is measured is higher or lower than the standard Z position, we enter a positive or negative sign accordingly.
– If the tool position is lower than the standard Z position, we enter a negative sign to bring the tool position to the standard Z position. Otherwise, if the tool position is higher than the standard Z position, we will enter a positive sign.
– This convention should be uniform and consistent across all machines to avoid confusion.
To determine the distance between the tool position and the Z reference position, we can use a tool holder containing a milling cutter or a milling cutter handle or use a 3D probe.

Double-check the above steps

Double-check all parameters one last time, and make sure no errors are missed. This step is very important because if there are errors, there is a possibility of crashing the machine causing great damage to the factory.

4. Perform the cutting process

– The CNC milling machine will read the instruction code and return all G codes to their default values.
– The CNC milling machine will read the workpiece standard and the selection program. Usually G54, the CNC machine will move the workpiece from the machine’s home position to the program’s standard position by the X, Y, and Z offset values entered in the G54 Workpiece standard.
– The next machine will read along the tool length offset instruction (usually G43) to get the tool length in memory cell H. Each tool has a memory cell H. Normally, the location of memory cell H coincides with the serial number. of the knife.
– After the machine recognizes the location of memory cell H. The CNC machine will add the value in this memory cell H with the value of the standard position Z in the standard workpiece G54. The sum of these two values is the offset distance Z from the machine standard to the machining standard Z.
– The CNC milling machine will continue to record cutting parameters such as spindle rotation speed, cutting speed, and functional commands such as coolant mode and the machine door status. These values will change based on the CNC program during the machining process.
– The CNC milling machine will sequentially read each command from top to bottom to determine the position to move to. Each command line will contain the position to move or the function commands and also include the position to move to and the function commands.
– After receiving the position to move, the CNC machine will send pulses to the servo motor to move. The CNC milling machine will finish moving the tool head to the position called in the program and then proceed to the next position.
– This process sequentially executes to create the machining motion that the programmer has previously created on the CAM software. 

5. Check the technical requirements of the processed product.

After the finished product is processed, it is transferred to the quality control room to measure and evaluate the processed product against the customer’s technical requirements.
If the product meets all the criteria, it will be packaged and delivered to the customer. If the product has a defect, it will be repaired or redone with new details.
To ensure production progress and costs, there will be an additional process check to detect errors as soon as the part is in the processing steps.
Product testers use measuring devices such as calipers, micrometers, altimeters, photometers, CMMs, and roughness meters.. to check product dimensions.

Epilogue

The above presentations, hopefully, will help customers understand more about the processing steps so that they can coordinate well with the finishing processing units to get the desired product.

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– Email: vannguyen@hesem.com.vn