Originally developed to craft minuscule parts for Swiss watches, Swiss CNC machines have since become a staple in precision manufacturing for industries from the aerospace sector to the medical manufacturing field. Although the machining process itself can be quite complex, CNC Swiss machining’s capabilities in achieving tight tolerances are rarely matched. Such dimensional accuracy is not achieved with complete ease, however. There is a learning curve which comes with Swiss machinery operation, even amongst experienced machinists.
While CNC Swiss machines are well suited for precision manufacturing processes, a key factor in getting the desired results lies in the choice of material itself. As new machinists quickly learn, when working with such close tolerances, not just any old stuff will do. A material that lacks consistency- whether that be in composition or form- is bound to cause issues that stifle the machine’s true potential. When it comes to the form of round bar stock material, these following three are some of the most important factors in which variations should be minimized to accomplish optimal machining efficiency.
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The first factor of bar stock material which has a significant impact on the CNC precision machining process is straightness. Straightness refers to the lack of bends across the entire length of the material. As a result, the longer a material is, the more likely it is to experience straightness defects. However, short materials may also display straightness defects if they are of poor quality.
If a material is not straight across its length, alignment issues may come to light once the bar stock is loaded into the machine.
If a material is to be fed through a machine, poor straightness will negatively impact the efficiency of the feed rate and the accuracy of the operation.
In any case, rods which have not been properly straightened are subject to frequent adjustments when used for precision machining.
The next factor of interest is roundness. Roundness refers to how close to a circular shape the rod’s profile is. As an example, a rod with a perfectly circular profile would be said to display ideal roundness. Conversely, a rod profile which more closely resembles an ellipse or a jagged shape would be said to exhibit poor roundness.
Roundness is especially important in pieces which are meant to be part of a precision assembly. If a material with poor roundness is machined equally across its outer face, the profile may be off in a way that prevents the intended fit.
On the topic of fit, poor roundness can negatively impact the machine’s ability to get a secure grip on the material prior to machining. If the workpiece is not properly held in place as it’s spun at speeds of hundreds of thousands of RPM, there can be serious safety repercussions. At the same time, an uneven profile can also result in poor machining quality and lead to part rework or scrapping.
As the material is spun within the machine’s workpiece holder, the tool is carefully dragged along its surface to create the desired dimensions. In finishing operations it is imperative for the piece to exhibit uniform roundness, otherwise the surface will be ground unevenly and display poor results. In cosmetic parts this may not be a major issue, but in parts like hydraulic cylinder shafts and tubes, surface finish is critical to the final part’s performance.
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Finally, diameter is the measurement of the outer surface of the rod. While roundness refers to the circular shape of the rod’s profile, diameter refers to the size measurement across the profile of that shape, be it in inches, centimeters, millimeters, and so forth. Although not common, it is possible for a rod to show perfect roundness throughout but still have variations in diameter, and vice versa.
In most part designs, the straightness and roundness across any length of material are usually not defined; but the diameter of a round feature always has an accompanying measurement and tolerance to adhere to. With diameter being a discrete measurement, any variation across the length of the rod has the potential to cause issues with staying within a part’s intended tolerance. This ties into the next issue of machining accuracy.
CNC precision machining technicians can take one, five, ten, or a hundred measurements across a round bar to try and verify the diameter’s consistency. However, if the diameter of the material is just far enough off in any one place that they didn’t measure, it can throw off the entire machining operation. It could fall outside of the intended tolerance by being too small, rendering any work performed before discovering the discrepancy entirely moot. Or, if the diameter of a given section is larger than previously thought…
… it can cause undue wear on the cutting tool. In milling and facing operations, this isn’t as large of a concern because there are typically more than one cutting tool attached to the holder or spindle at once, so the stress is divided across the multitude of tools. However, in CNC precision machining there is usually only one cutting tool performing an operation at any given time. In this case, any undue stress on the tool can cause premature wear and even failure.
When tools aren’t performing as expected due to excess wear, naturally the quality of the finished part will reflect that poor performance.
In summary, easily overlooked factors like straightness and roundness can contribute just as much to machining challenges and final part quality as a simple feature like diameter can. To get materials that you can rely on for unbeatable straightness, roundness, and diameter, reach out to Boston Centerless today. With over 60 years in supplying centerless ground materials for CNC precision machining, we guarantee our quality with a historic track record of precision in everything we do.