Dimensional Accuracy in Machining: Essential Methods You Need to Know
What exactly does the machining accuracy of CNC parts refer to?
Processing accuracy refers to how closely the actual geometric parameters (size, shape, and position) of the part match the ideal geometric parameters specified in the drawing. The higher the degree of agreement, the higher the processing accuracy.
During processing, it’s impossible to perfectly match every geometric parameter of the part with the ideal geometric parameter due to various factors. There will always be some deviations, which are considered processing errors.
Explore the following three aspects:
1. Methods to Obtain Dimensional Accuracy of Parts
2. Methods to obtain shape accuracy
3. How to get location accuracy
1． Methods to Obtain Dimensional Accuracy of Parts
(1) Trial cutting method
First, cut out a small part of the processing surface. Measure the size obtained from the trial cutting and adjust the position of the cutting edge of the tool relative to the workpiece according to the processing requirements. Then, try cutting again and measure. After two or three trial cuts and measurements, when the machine is processing and the size meets the requirements, cut the entire surface to be processed.
Repeat the trial cutting method through “trial cutting – measurement – adjustment – trial cutting again” until the required dimensional accuracy is achieved. For example, a trial boring process of a box hole system can be used.
The trial-cutting method can achieve high accuracy without requiring complicated devices. However, it is time-consuming, involving multiple adjustments, trial cutting, measurements, and calculations. It could be more efficient and relies on the technical skill of the workers and the accuracy of measuring instruments. The quality is unstable, so it is only used for single-piece and small-batch production.
One type of trial cutting method is matching, which involves processing another workpiece to match the processed piece or combining two or more workpieces for processing. The final processed dimensions in the production process are based on the requirements that match the processed precision turned parts.
(2)Adjustment method
The accurate relative positions of machine tools, fixtures, cutting tools, and workpieces are adjusted in advance with prototypes or standard parts to ensure the dimensional accuracy of the workpiece. By adjusting the size in advance, there is no need to try cutting again during processing. The size is automatically obtained and remains unchanged during the processing of a batch of parts. This is the adjustment method. For example, when using a milling machine fixture, the position of the tool is determined by the tool setting block. The adjustment method uses the positioning device or tool setting device on the machine tool or the pre-assembled tool holder to make the tool reach a certain position and accuracy relative to the machine tool or fixture and then process a batch of workpieces.
Feeding the tool according to the dial on the machine tool and then cutting is also a kind of adjustment method. This method requires first determining the scale on the dial by trial cutting. In mass production, tool-setting devices such as fixed-range stops, cnc machined prototypes, and templates are often used for adjustment.
The adjustment method has better machining accuracy stability than the trial cutting method and has higher productivity. It does not have high requirements for machine tool operators, but it has high requirements for machine tool adjusters. It is often used in batch production and mass production.
(3) Dimensioning method
The sizing method involves using a tool of the appropriate size to ensure the processed part of the workpiece is the correct size. Standard-size tools are used, and the size of the processing surface is determined by the size of the tool. This method utilizes tools with specific dimensional accuracy, such as reamers and drill bits, to ensure the accuracy of processed parts, such as holes.
The sizing method is easy to operate, highly productive, and provides relatively stable processing accuracy. It is not heavily reliant on the technical skill level of the worker and is widely used in various types of production, including drilling and reaming.
(4) Active measurement method
In the machining process, dimensions are measured while machining. The measured results are then compared with the required dimensions by the design. Based on this comparison, the machine tool is either allowed to continue working or stopped. This method is known as active measurement.
Currently, the values from active measurements can be displayed numerically. The active measurement method adds the measuring device to the processing system, making it the fifth factor alongside machine tools, cutting tools, fixtures, and workpieces.
The active measurement method ensures stable quality and high productivity, making it the direction of development.
(5) Automatic control method
This method consists of a measuring device, a feeding device, and a control system. It integrates measurement, feeding devices, and control systems into an automatic processing system, which automatically completes the processing process. A series of tasks such as dimensional measurement, tool compensation adjustment, cutting processing, and machine tool parking are automatically completed to achieve the required dimensional accuracy. For example, when processing on a CNC machine tool, the processing sequence and accuracy of the parts are controlled through various instructions in the program.
There are two specific methods of automatic control:
① Automatic measurement refers to a machine tool equipped with a device that automatically measures the size of the workpiece. Once the workpiece reaches the required size, the measuring device sends a command to retract the machine tool and stop its operation automatically.
② Digital control in machine tools involves a servo motor, a rolling screw nut pair, and a set of digital control devices that precisely control the movement of the tool holder or worktable. This movement is achieved through a pre-programmed program that is automatically controlled by a computer numerical control device.
Initially, automatic control was achieved using active measurement and mechanical or hydraulic control systems. However, program-controlled machine tools that issue instructions from the control system to work, as well as digitally controlled machine tools that issue digital information instructions from the control system to work, are now widely used. These machines can adapt to changes in processing conditions, automatically adjust the processing amount, and optimize the processing process according to specified conditions.
The automatic control method offers stable quality, high productivity, good processing flexibility, and can adapt to multi-variety production. It is the current development direction of mechanical manufacturing and the basis of computer-aided manufacturing (CAM).
2. Methods to obtain shape accuracy
(1) Trajectory method
This processing method utilizes the movement trajectory of the tool tip to shape the surface being processed. Ordinary custom turning, custom milling, planing, and grinding all fall under the tool tip path method. The shape accuracy achieved with this method primarily relies on the precision of the forming movement.
(2) Forming method
The geometry of the forming tool is utilized to replace some of the forming motion of the machine tool in order to achieve the machined surface shape through processes like forming, turning, milling, and grinding. The precision of the shape obtained using the forming method primarily relies on the shape of the cutting edge.
(3) Development method
The shape of the machined surface is determined by the envelope surface created by the motion of the tool and the workpiece. Processes such as gear hobbing, gear shaping, gear grinding, and knurling keys all fall under the category of generating methods. The precision of the shape achieved using this method primarily relies on the accuracy of the tool’s shape and the precision of the generated motion.
3. How to get location accuracy
In machining, the position accuracy of the machined surface relative to other surfaces mainly depends on the clamping of the workpiece.
(1) Find the correct clamp directly
This clamping method uses a dial indicator, marking disk, or visual inspection to find the position of the workpiece directly on the machine tool.
(2) Mark the line to find the correct installation clamp
The process starts by drawing the center line, symmetry line, and processing line on each surface of the material, based on the part drawing. Afterward, the workpiece is mounted on the machine tool, and the clamping position is determined using the marked lines.
This method has low productivity and precision, and it requires workers with a high level of technical skills. It is typically used for processing complex and large parts in small batch production, or when the size tolerance of the material is large and cannot be clamped directly with a fixture.
(3) Clamp with clamp
The fixture is specially designed to meet the specific requirements of the processing process. The fixture’s positioning components can quickly and accurately position the workpiece relative to the machine tool and tool without the need for alignment, ensuring high clamping and positioning accuracy. This high clamping productivity and positioning accuracy make it ideal for batch and mass production, although it requires the design and manufacture of special fixtures.
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