How to control machining errors? 4 types of process system errors + 2 major analysis methods, a complete summary of useful information
Be wary of congenital defects in machine tool equipment
When many machining masters encounter an out-of-tolerance situation, their first reaction is to change tools and adjust parameters. In fact, they have not identified the root cause at all.
The geometric error of the process system is an innate problem. There are deviations in the manufacturing of the machine tool itself, deviations in the manufacturing of the fixture itself, and deviations in the manufacturing of the tool itself. Errors will be introduced in the installation process, and wear will occur with long-term use. These problems will be transmitted to the parts.
In particular, we must pay attention to the repeated positioning accuracy of the fixture. The root cause of the dimensional instability that occurs during mass production in many factories lies in the deviation of the position of the fixture during repeated clamping operations.
Although the spindle rotation has jumps, it is a very hidden problem. It is often ignored during daily inspections of the equipment. The guide rails are worn and there are gaps in the transmission chain, which will directly affect the geometric tolerance. The method to ensure the accuracy of boring processing is to calibrate the accuracy regularly. This is the first line of defense for controlling errors.
Clamp cutting and don’t let your knives fall apart
When processing thin-walled parts, the "cutting tool" phenomenon is almost an inevitable problem that will be encountered. When processing slender shafts, how to ensure the accuracy of boring processing , the "cutting tool giving" phenomenon is also almost an inevitable problem. The clamping force will cause elastic deformation of the process system, and the cutting force will also cause elastic deformation of the process system.
Once deformation occurs, the actual position of the tool will deviate from the theoretical set position, which will inevitably cause the part size to exceed the tolerance range. This type of dynamic error based on specific conditions is particularly significant on structures with insufficient rigidity.
The key point of this solution is to improve the overall rigidity of the system, optimize the distribution of clamping points, and add auxiliary supports to share the force when necessary.
The adaptation of cutting parameters is very important. Excessive cutting amount will increase the deformation. Only by selecting the feed and depth of cut reasonably and using a stable clamping plan can the dynamic error be minimized.
Thermal expansion and contraction are the number one killer of accuracy
In the field of precision machining, the impact of thermal deformation on accuracy is often ranked first. When the machine tool is running, its friction will generate corresponding heat, and the cutting process is also continuously heating.
What is even more troublesome is that the ambient temperature of the workshop changes at any time. The superposition of these factors will cause the entire process system to expand and contract with heat, causing the relative position of the tool and the workpiece to change.
There are many operators who proceed directly to processing after turning on the machine and never preheat the equipment. As a result, the first piece inspection is completely a qualified boring machining accuracy guarantee method . However, when the second piece is processed in batches, deviations begin to occur.
The key reason is that the thermal deformation that occurs after the spindle temperature rises is not taken seriously. When performing precision boring, it is necessary to achieve a sufficient thermal balance for the equipment. The temperature control of the workshop environment must also keep up with the rhythm, and thermal deformation must be properly controlled from the root cause.
Internal stress relief is often forgotten
There are some parts that are completely qualified when measured immediately after processing. However, when measured again after being stored for a while, deformation occurs. Such problems often originate from the residual stress existing inside the workpiece.
After the blank is cast, forged, heat treated, and even in the previous cutting process, internal stress has accumulated to varying degrees, but it is only temporarily locked by the material structure.
After the processing is completed, these stresses will slowly be released and distributed again, and then the parts will be deformed. Most of the cases where box-like parts are out of tolerance after storage fall into this situation.
The solution is to strictly separate rough machining and finishing machining, and subject key parts to aging treatment to completely release the internal stress in advance so that subsequent dimensions can be stable.
Use sensitive direction method for single piece out-of-tolerance
When encountering a sudden out-of-tolerance situation in a single piece or small batch production, the single factor analysis method is the most practical troubleshooting method, and its core is to find the error-sensitive direction.
As far as the positive normal direction of the machined surface of the part is concerned, the error-sensitive direction is this. Any displacement made in this direction will be reflected on the final size without any reduction, forming corresponding results.
Taking the boring of internal holes as an example, the direction of the radial displacement of the boring tool is the sensitive direction. When encountering an out-of-tolerance situation in the hole diameter, you can follow this direction to check the tool wear, whether the fixture is loose, and problems such as spindle clearance.
This method can quickly identify key contradictions, prevent wasting time on large-scale inspections, and is particularly suitable for rapid response to unexpected quality problems on site.
Mass production must rely on statistics
When large-scale batch processing is carried out, it is no longer enough to rely on the experience of a single piece to make judgments. At this time, it is necessary to rely on statistical analysis to control the development trend of quality.
Record the dimensions of the workpieces produced in the same batch of processing, and then draw the distribution curve and control chart. In this way, you can clearly see whether the error is systematic or random.
If all the data tilts to one side, it means that it is a systemic reason such as machine tool debugging and tool length fixing, and it needs to be recalibrated.
If the data dispersion exceeds the tolerance zone, it is mostly due to random errors caused by uneven blank margins or fluctuations in material hardness. At this time, the quality of the incoming materials must be improved from the source.
