The axis letter name for a rotary table or an articulated spindle on a machining center can be designated using one or two of the following letters: A, B, C. Determining which letter(s) to use is based on the directional relationship of the rotary axis to the linear Z (spindle centerline), X, and Y axes, which can be different from machine to machine, or even when comparing horizontal and vertical machining centers, for instance.
A 5-axis machining center will almost always include X, Y, and Z linear axes, plus two of the rotary axes mentioned above. There is such a thing as a 6-axis single-spindle machining center, though one of the rotary axes cannot be used if another one is used and vice versa.
I’m always very careful about using the letter designation when describing a machine’s functionality because the same letter can mean different things based on the relationship it has to other axes. I’ve even read articles that incorrectly apply the axis letter names and make definitive statements about these axes that are just plain wrong. The axis names should really just be used to refer to discrete slides or programmable motions on a machine that two or more parties are very familiar with. Be wary of someone who casually throws around axis letter designations when describing a complex machine tool. They’re probably just trying to impress you or don’t know what they’re talking about.
Both rotary axes can be configured within the spindle or within the table or with one on the table and one on the spindle. When the spindle has at least one rotary axis of motion, I refer to it as an articulated spindle. When a rotary table is configured with two rotary axes, it’s almost always referred to as a trunnion-style table.
There are many ways to skin this 5-axis cat. Some machine tool builders, such as Kitamura, offer field-retrofitable rotary tables for their own models. Several third party rotary and trunnion-style tables, such as from Haas Automation and Koma Precision, can be added to 3- or 4-axis machining centers in the field. Some machine tool builders build-in the 4th and 5th axes directly to the machine only as a factory-supplied option. This is especially true when the 4th and/or 5th axes are on the spindle.
As suggested above, axis designations per se are not inherently meaningful nor important without the full context (thorough understanding of the programmable elements of the machine). But the reason why a 5-axis machining center might have both rotary axes on the spindle as opposed to the table, or one on each, is very important.
Part clearance to mounting surfaces and other machine members plays into the decision regarding which style is appropriate for any job. Additional considerations include workload, power required to mill the parts, accuracy of the finished part, the number of sides of the part that need to be machined, the features (helical surfaces or flat) to be machined, and more. Many job shops think of 5-axis machines not so much in terms of the complexity of the part but rather the flexibility of the machine to handle different, less complex parts, job-to-job. Brad Branham of Haas Automation elaborates on this point in a white paper. He writes that a growing number of manufacturers are using 5-axis CNC machining to reduce costs, increase accuracy, and improve profits. (Read More.) The article focuses on applications that use the extra two axes, not to facilitate 3D complex shape machining, but rather to free up the operator, reduce total part cycle time, and eliminate multiple setups.
Of course there are plenty of applications that rely on true simultaneous 5-axis functionality and require CAM to generate tool path and “look ahead” controls to calculate rapid 3D tool-position changes. But it’s high time that we all stop fearing 5-axis as some unattainable, too complex, bogey-man machine, because there are many more applications that can justify and profit by using a 5-axis machine on a 2- or 3-axis part. It’s taken me too long to realize that multi-function machines aren’t just for complex parts. I used to think that only the super rich manufacturing companies, such as those in the medical or aerospace fields, could justify 5-axis or multi-function, hybrid-style machine tools. But as we know, so many more factors go into machine justification than just the feature to be machined. Once you do the math, you may find that labor savings, set up reduction, balanced cycle times, and elimination of scrap add up to to real savings that turn 5-axis from a concept to be feared, into a real option to be considered.