factor:
mechanical equipment:
If you have to recreate the settings on different machines at different times, the parts may be different.
For example, parts can be created as prototypes on 3-axis machine tools. When parts are reordered in larger quantities for production, and these parts are created on a more efficient 5-axis machine tool, this will introduce different tool paths and will slightly change the appearance of the parts.
Tool wear:
Tool wear or less sharp may cause slight changes in part characteristics.
Linear irregularities on the part surface may indicate tool wear and dimensional irregularities found during the process inspection period.
Manual sorting:
Manual processes such as sand blasting, coating and installation of inserts may vary due to differences in sand blasting particle size, coating thickness, etc.
Machinists may leave "thumbprints" when holding parts during sand blasting, and their thumbs will cover a part of the surface. Standards and best practices in finishing can help mitigate these differences and use the same supplier for post finishing.
How to realize consistent CNC machining tolerance
Manufacturers can prevent tolerance changes between parts in various ways. During the regular part sampling and measurement of Machinists, tool wear is often found. If the wear is serious and leads to deviation from customer specifications, the tool will be replaced.
In order to reduce the appearance and measurement deviation, the engineer can communicate the acceptable level of change through drawings and notes. The processing inspection plan selected by the customer, such as ASQC z1.4 2008 level II zero rejection, can help the manufacturer understand the acceptable tolerance limits.
Principle: should you use CNC machining to obtain repeatable parts?
When CNC machining is used, the tolerance change rate of parts is smaller than that of 3D printed parts. Due to the small variability in the pre-processing process and the ability to specify highly complex inspection options, the function and appearance of parts may be almost the same. To sum up, if the project requires very high precision, low tolerance and high consistency between parts of the same design, CNC machining is a good choice.