How to Build the Process Flow of a CNC Machined Part
STCNC Machining by SINOTECH
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As a computer numerical control manufacturing process, CNC machining has revolutionized the production of complex parts and components. It is crucial to have a well-defined process flow chart outlining each step to ensure smooth and efficient manufacturing processes. From design and engineering to packaging and shipping, a comprehensive process flow chart can help streamline the CNC machining process and improve the quality and consistency of the finished parts. This article will examine the critical steps of a typical process flow chart to produce parts by precision machining.
STEP 1: Analysis of Technical Drawing and Its Manufacturability
When analyzing the part technical drawing for machining, there are two parts to consider: the part drawing itself and the structural processability of the part. SINOTECH's manufacturing engineers use CAD software and Solidworks, which can open your CAD model and 3D model in .stp(.step). igs(.iges), . prt, .sldprt, DXF, and more. The information from the drawings will guide the tool selection and cutting parameters for each machining operation.
1. Analysis of part dimension
When looking at the part drawing, we must consider how the dimensions are marked. Keeping them in a way that makes sense for machining is essential. We should use a single reference point or direct coordinates to make programming easier.
2. Integrity and correctness analysis of part drawings
We must ensure the drawing is complete and correct and pay attention to the geometric elements that make up the part, like lines and surfaces. It will be hard to program the machine cnc machines and make the correct parts if the drawings aren't right.
3. Analysis of part's requirements.
The technical requirements of parts mainly refer to the accuracy of dimensions, shapes and positions, surface roughness and heat treatment, surface treatment, package, etc. Only on this analysis basis of analyzing these requirements can we determine the correct and reasonable cnc machining processes, clamping methods, cutting tools and cutting process parameters, etc.
4. Raw material analysis
Under the premise of satisfying the function of parts, we should always choose raw materials which are cheap and easy to cut. SINOTECH usually processes cnc machining parts according to customers' drawings that indicate the materials. We constantly analyze whether the materials are costly and difficult to machine, discuss their performance requirements with our customers, and suggest materials with higher-cost performance.
5. Analysis of other component drawings from the same assembly and the assembly drawing
Sometimes we are entrusted to process one set of cnc machined parts instead of a single part. We need to consider the assembly requirements and try assembling the parts to ensure it is correct before we ship the spare parts, even though we are not required to send an assembled unit.
6. Selection of positioning reference
In CNC machining, locating with the same benchmark is crucial as the machining operations are often concentrated. It may be necessary to establish auxiliary benchmarks, particularly for parts that undergo CNC machining on both sides, to ensure the unification of the process benchmarks.
STEP 2: Determine the CNC machining content of the parts
In most cases, CNC machining operations cannot do all machining processes for a part, and only some parts of the process are suitable for CNC machining. Therefore, it is necessary to analyze the part structure carefully to choose the most appropriate and essential content and strategies for CNC machining. When selecting the content, it is required to consider the equipment of the machine shop and focus on solving problems, overcoming key issues, and improving production efficiency to take advantage of the benefits of CNC machining fully.
When selecting content suitable for CNC machining, we should consider the following in order:
Additionally, when selecting and deciding on machining content, one should consider factors such as production batch, production cycle, and turnover between processes. In short, one should strive for rationality and achieve more, faster, better, and cheaper goals while avoiding downgrading CNC machines to the level of general-purpose machine tools.
STEP 3:Determine the CNC machine capabilities
Next, you will need to identify the CNC machine used to manufacture the part and determine its capabilities. This includes the maximum spindle speed, tool capacity, and axis movements. Knowing the machine's capabilities will help you decide which cutting tools can be used, the cutting parameters that can be achieved, and the optimal machining sequence.
STEP 4: The Selection of Cutting Tools on CNC Machines
We must consider the factors below when choosing the cutting tools for cnc machined parts.
1. The commonly used part material category includes nonferrous metals, ferrous metals, composite materials, plastics, etc.
2. Material properties of the workpiece, including hardness, toughness, organizational state, etc.
3. Category of the cutting process. CNC turning, CNC drilling, CNC milling, CNC boring, etc.
4. The workpiece's geometric shape, part accuracy, and machining allowance.
5. The cutting amount that the blade (cutter) is required to withstand.
6. Conditions at the production site.
7. The workpiece's production batch size affects the blade's economic life (cutter).
Let's take CNC milling machines as examples to learn more about tool selection.
Selecting the appropriate cutting tool for CNC milling tools depends on several factors, such as the part's material and size, the machining operation, and the cutting conditions.
Here below is the chart for your quick look
STEP 5: Design of CNC Machining Process Route
The difference between designing process routes for CNC machining and general machine tool(Non-NC machining) processing is that the former only describes specific CNC machining operations rather than the entire process from blank to finished product. It is important to note that CNC machining operations are interspersed throughout the whole part manufacturing process and must be well connected with other machining processes. The common process flow diagrams below show how this works.
1. Division of machining processes
Based on the characteristics of CNC machining, the division of CNC machining processes can generally be carried out using the following methods:
① Treat one installation and machining as one process. This method is suitable for parts with relatively little machining content, which can reach the waiting-for-inspection state after machining.
② Divide the processes based on the content machined with the same cutting tool. Although some parts can be machined on many surfaces with one installation, the long CNC program length will be limited by certain factors such as the control system's capacity (mainly memory) and the cnc machine's continuous working time (i.e., a process cannot end within one shift). Additionally, a long CNC program length can increase the difficulty of error detection and retrieval. Therefore, the machine program cannot be too long, and the content of a process cannot be too much.
③ Divide the manufacturing process according to the processing area. We can divide the processing area into several steps for the workpieces with many processing contents based on their structural characteristics, such as an inner cavity, external shape, curved surface, or flat surface. Each step's machining process can be considered a separate process.
④ Divide the manufacturing process into rough and finishing techniques. Generally, the rough and finishing processes need to be separated.
Note: The processing of the previous process should be fine with the positioning and clamping of the subsequent process. We should also comprehensively consider the processes involving general machine tools.
2. Connection of CNC machining and general manual machining operations
In CNC machining, other ordinary manufacturing processes are usually interspersed before and after. If the connection is not good, it is easy to produce conflicts. Therefore, while familiarizing oneself with the entire machining process, one should be clear about the technical requirements, objectives, and characteristics of the CNC and the ordinary machining process. This includes considerations such as whether to leave machining allowances, how much to leave, the accuracy requirements and positional tolerances for locating surfaces and holes, the technical requirements for the calibration process, and the heat treatment state of the blank. Only in this way can each process meet the machining needs of each other and have clear quality targets and technical requirements, with a basis for handover and acceptance.
STEP 6: Design of CNC Machining Process
After selecting the CNC machining technology's content and determining the part's machining route, we can design the CNC machining process. The main task of the CNC machining process design is to define further the machining content, cutting amount, process equipment, positioning, clamping method, and tool motion trajectory of this process to prepare for the preparation of the machining program.
1. Determine the cutting route and processing sequence
The tool path is the movement track of the cutting tool in the whole processing procedure. It includes the work steps' content and reflects their order. The tool path is one of the bases for programming. When we determine the cutting route, the following points should be paid attention to:
(1) Selecting the shortest machining route:
To machine the holes on the part shown in Figure A, the tool path in Figure B first machines the outer circle holes before machining the inner circle holes. If we use the tool path in Figure C instead to reduce the idle time of the cutting tool, we can reduce the positioning time by nearly half, improving machining efficiency.
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(2) The final profile is done in one pass of cutting tool path
To achieve the required surface roughness, machine the final contour continuously in the last pass.
In Figure A, the zigzag cutting method removes all excess material without damaging the contour but leaves a height difference between passes, which does not meet the roughness requirement.
The cutting path in Figure B combines the zigzag method with a final radial cutting pass to smooth the contour surface and produce better results. Another option is the cutting path shown in Figure C.
(3)Select the in and out cutting direction
When considering the tool path for entering or exiting the workpiece, we need to ensure that the tool's entry or exit point is on the tangent line along the contour of the workpiece, which will help to ensure a smooth contour of the workpiece. We also need to avoid cutting vertically up or down on the workpiece contour surface, as this may cause scratches on the surface of the workpiece. Additionally, we should minimize pauses during the contour machining cutting process, as sudden changes in cutting force may cause elastic deformation and leave visible tool marks. The below diagram shows how this works.
(4) Choose a route that minimizes deformation of the workpiece after machining
When working with small cross-sectional or thin sheet parts, dividing the machining process into multiple passes or using symmetrical machining methods to remove excess material is essential. When arranging the machining process, it is advisable to prioritize steps that are less likely to cause damage to the rigidity of the workpiece.
2. Determine positioning and clamping solutions
When determining the positioning and clamping scheme, we should pay attention to the following issues:
(1) We should unify the design, process, and programming calculation benchmarks as much as possible to reduce benchmark misalignment errors and metrological workload in CNC programming.
(2) We should try to reduce the number of clamping times. After positioning and clamping as much as possible, we machine all or most of the surfaces on the workpiece to minimize clamping errors, improve the mutual positioning accuracy between the processed characters, and give full play to the efficiency of CNC machines.
(3) The clamping method is reliable, and the action point of the clamping force should fall on the part with better rigidity of the workpiece.
(3) Avoiding manual adjustment schemes that take too much time is highly efficient.
3. Determining the relative position of the cutting tool to the workpiece
(1)What is a tool-setting point? And where is it?
For CNC machines, it is crucial to determine the relative position between the tool and the workpiece before starting the machining process. We can achieve this relative position by confirming the tool-setting point. The tool-setting point is the starting point of the tool's relative movement concerning the part during CNC machining. We set it on the workpiece or the fixture at a particular position related to the part's positioning reference dimensions. The selection principles for the tool-setting point are as follows: it should be easy to find and determine the part's machining origin and a convenient and reliable position for inspection during machining.
Usually, we use the tool-setting point at the programming origin, and "tool touch-off" (also called "tool pre-setting") is required to determine the machining origin using the tool-setting point.
(2)What is a tool position point? and where is it?
Tool touch-off refers to accurately positioning the "tool position point" and the "tool-setting point." Each tool's radius and length dimensions are different, so after we install the tool on the machine tool, we set the primary position of the tool in the control system. The "tool position point" refers to the positioning reference point of the tool. For example, for a cylindrical milling cutter, the tool position point is the intersection point between the center line of the tool and the bottom surface of the tool; for a ball-end milling cutter, the tool position point is the center point or top point of the ball head; for a lathe tool, the tool position point is the tip or the center of the tip arc; for a drill bit, the tool position point is the top point of the drill bit. The tool-setting methods for different types of CNC machine tools are different. We should consider them case by case.
(2)What is a tool-changing position point? and where is it?
When changing cutting tools during the machining process, it is necessary to establish a tool-changing point. The tool-changing point refers to the position at which the tool holder rotates and changes tools. This point can be a fixed point (such as the position of a machining center or its tool-changing robot) or any point (such as a lathe). The tool-changing point should be located outside the workpiece or fixture to ensure the tool holder does not collide with the workpiece or other components when rotating.
4. Determine the amount of cutting
To efficiently cut metal with machine tools, the three main factors are the materials, the cutting tools, and the amount of cutting performed. These factors determine machining time, tool life, and processing quality. To achieve economical and effective cutting, it's important to select cutting conditions that make sense.
When programmers determine the cutting amount for each process, they should choose appropriate cutting depths and cutting speeds (including feed rate and spindle speed) based on the material, the hardness of the workpiece, the durability of the cutting tools, and the specifications provided in the cnc machines. Alternatively, they can use analogical methods based on practical experience to determine the cutting amount.
The principles for selecting cutting parameters during rough and finish machining are as follows:
Methods for selecting cutting parameters:
1) We determined the selection of the back-feeding amount based on the machining allowance. For rough machining (Ra = 10-80μm), the back-feeding amount can be up to 8-10mm. For semi-finishing (Ra = 1.xn--25-10m-0ze), the back-feeding amount should be 0.5-2mm. For finishing (Ra = 0.32-1.xn--25m-yyc), the back-feeding amount should be 0.2-0.4mm.
2) The feed rate selection should be based on factors such as the surface roughness of the workpiece, machining accuracy requirements, and cutting tools and workpiece material. The relationship between the feed speed "V", tool speed "n", number of teeth "Z", and feed per tooth "f" is V = fn = fz.
3) The cutting speed selection should consider the following points:
① We should avoid the area where chip nests likely occur as much as possible.
② When cutting intermittently, we should appropriately reduce the speed to reduce impact and thermal stress.
③ In cases where vibration is likely to occur, the cutting speed should avoid the critical speed of self-excited vibration.
④ When machining large, slender, or thin-walled workpieces, we should use a lower cutting speed.
⑤ When machining workpieces with the outer skin, we need to reduce the cutting speed appropriately
STEP 7: Verify the process
Verify the process by simulating the tool paths using computer-aided manufacturing (CAM) software, performing test cuts, and making adjustments as necessary.
STEP 8: Compilation of Technical Files for CNC Machining
Filling in particular technical documents for CNC machining is one of the process contents. These documents are the basis for CNC machining, the base for product acceptance, and the procedures for operators to follow and execute. The technical paper is a specific description of CNC machining; the purpose is to let the operator know more about the content of the machining program, the clamping method, the tool selected for each machining part, and other technical issues.
CNC machining technical documents mainly include:
1. Workpiece installation and origin setting card
2. Machining Process Flow Chart
Conclusion:
The process flow of CNC machining involves part designs,?programming the CNC machine, setting it up, machining the part, inspecting it for quality, finishing it, and packaging it for delivery. The machinist monitors the process throughout to ensure accuracy and efficiency. To build a process flow chart, we need to analyze the part drawings, determine the CNC machining contents, the machine capabilities, and cutting tools, and design the CNC machining process route and each machining process. Then verify the processes.? I hope this post will give you some ideas for your daily work :) Thank you for reading.