Precision in mechanical machining indicates the mechanic’s workmanship and skills whether done by CNC precision machining machine or directly machined by traditional techniques. So how is the accuracy of mechanics understood? We also learn a little bit about the concepts and related information.
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The concept of machining accuracy
The accuracy in machining of machine parts is expressed by the degree of similarity in size, geometric shape, correlation position of machine parts is machined compared to the ideal machine detail on the technical drawing.
In fact, machining any one machine detail with absolute accuracy is almost impossible – this means that all the parameters, technical properties put on the drawing will be absolutely accurate for machine details. But most of them can’t do this, so we’ve used another statement to measure the accuracy of the details, which is tolerance.
Tolerance and machining accuracy assessment factors
The concept of tolerance
Tolerances are defined as a permissible range of errors, the tolerance will be based on the difference of the size with the highest limit to the minimum limit size, and they are understood that the smaller the difference, the greater the accuracy in machining of the detail, The greater the difference, the more accurate the detail will be reduced, which directly affects the quality of the post-machining details.
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Accuracy assessment factors
- Size accuracy: Determined by the actual size of the processed product versus the size on the design is calculated by the tolerance of that size.
- Geometric shape accuracy: is their greatest relevance to its ideal geometric shape and is judged by taper, ozone, non-cylinderity, non-roundness … (cylindrical surface), flatness, straightness (flat surface).
- Correlation position accuracy: assessed according to the error of the rotation angle or the shift between one surface position and the other (used as the standard surface) in two coordinate planes perpendicular to each other and recorded as separate technical conditions on the design drawing such as parallelness, perpendicularness, concentricity, symmetry…
- Subtle geometric shape accuracy and surface layer mechanical properties: surface roughness, surface hardness …
Causes of machining errors
When we process a series of details under the same condition, we can see that the cause of the errors of each detail is the same but the total number of errors on each detail is different. There is such a phenomenon that is practically due to the different properties of component errors.We can find out some of the following reasons:
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Causes of constant system errors:
- Theoretical errors of the cutting method.
- Fabrication errors of cutting tools, accuracy and wear of machines, jigs, etc.
- Deformation of machining details.
Causes of system errors change:
- Cutting tools wear out over processing time.
- Deformation because of the heat of the machine, jigs, cutting tools.
Causes of random errors:
- Material properties (hardness) are not uniform.
- Uneven machining residues (due to errors of the workpiece).
- The position of the workpiece in the jigs changes (nape error)
- The change of residual stress.
- Because of the knife several times.
- By grinding the knife several times
- Due to changing many machines to process a series of details.
- Due to the thermal oscillation of the cutting mode.
- Types of vibrations during cutting.
Methods of achieving accuracy in machining on the machine
Most do not have anything absolute and the processed products can not be absolutely the same but how will we be able to achieve the highest precision machining required by the fabrication drawing. Each different form of production or a different type of material will have different technological methods and production organization. But there are two best methods to use:
Test Cutting Method
- This method is mainly machined in a traditional manual way using machines that need the detailed control of the machinist. With experience and eye observation, the machining engineer will adjust and cut until the completed reaches the required size, then proceed to cut the entire length of the machining surface. When processing the next detail, do the same as the aforementioned process.
- Before cutting the test, it is usually necessary to take the mark so that the worker can make the movement of the cutting blade that coincides with the mark and avoids producing waste due to the over-hands that the knife eats too deep at the first cut.
Automatic Method of Reaching Size
- In large mass production, mass row, in order to achieve the required machining accuracy, mainly using the method of automatic reaching size on pre-adjusted machine tools.
- In this method, the cutting tool has the correct position compared to the machining details. In other words, the machining details must also have a definite position compared to the cutting tool, this position is guaranteed thanks to the positioning structures of the jigs, while the jigs have a definite location, on the machine table also thanks to the separate positioning items.
- When machining according to this method, the machine and knife have been adjusted.
- Machining details are positioned by the positioning structure that comes into contact with the bottom and side.
Accuracy when processing details is very important, this is directly related to the quality of the product, so controlling and avoiding large tolerances is something that a machinist needs to pay attention to. At VCC MOLDS, we have a team of machining engineers with many years of experience, along with a high amount of modern and technical machinery to be able to produce the most accurate and quality detailed products.
Vietnam Capacity Precision Mold Joint Stock Company
Address: Lot B2-3-3b, Nam Thang Long Industrial Park, Thu Phuong, Bac Tu Liem, Bac Tu Liem, Hanoi
Phone: (+84)24 6687 8283
Google map: https://goo.gl/maps/u6TrPvLWSb3bXAwQ6