Many shops today have begun evaluating the purchase of multi-function turn/ mill machines, weighing factors such as cost, capability and productivity gains. These shops must consider what type of turn/mill machine tool best fits their individual needs and the parts they produce. Within the turn/mill machine category is the B-axis machine, a turning machine that includes five-axis milling capabilities. The acquisition of a B-axis machine requires a significant investment in both money and time. B-axis machines are approximately 20 percent higher in price compared to turn/ mill machines of similar capacity without B-axis features. However, for shops that produce parts that would benefit from B-axis machining, the productivity gains achieved by B-axis machines more man justify these expenditures.

Shops evaluating the purchase of a B-axis machine should ask these key questions: What does B-axis capability add to a turn/mill machine tool, and what kind of CAM system is needed to take advantage of this capability success fully? The answers are found by analyzing the physical characteristics of B-axis machines, evaluating machine configuration and setup time, and assessing the level of integration of the programming environment.

B-Axis Fundamentals

Methods Machine Tools Inc. (Sudbury, Massachusetts), a machine tool importer and the exclusive distributor of Nakamura-Tome machine tools in the United States, frequently discusses the fundamentals of a B-axis machine with prospective buyers to be sure they fully understand its capabilities.

Methods provides this standard definition to potential customers: The B-axis machine tool combines the turning capabilities of a horizontal/vertical turning machine with the milling and machining capabilities of a five-axis machining center.

Like traditional turn/mill machines, B-axis machine tools provide control over the Z-X (turning) and C (rotary milling) axes. The B-axis head is used as a milling spindle or a turning/boring toolholder, thus enabling the machine to complete all milling and turning with one setup. These machine tools also give users control over the Y-axis for off-center milling operations. However, it is B-axis capability that sets these machines apart. The B axis is defined as rotation about the Y axis, and and this fifth axis positioning makes cuts with compound angles possible. B-axis capability gives a machine full support for five-axis index milling and 3D / five-axis simultaneous freeform milling. The B-axis machine is literally one machine doing the work of two because it supports the entire range of milling and turning operations possible, with the advantage of doing it in one setup.

Whole Part, One Setup

B-axis machines increase part accuracy because they hold tighter tolerances between turned and milled features. For example, when a cutting tool body is machined on a B-axis machine, it is significantly easier to maintain geometric tolerances between milling and turning operations. As with traditional turn/mill machines, using one workholding setup eliminates errors that may be introduced by switching a part from a turning to a milling machine. For parts that require the turning and multi-axis milling operations that only B-axis machines are equipped to handle, keeping tight tolerances across multiple datums is especially important for maintaining geometric tolerances between features.

When outfitted with dual spindles, the extended machining range of a B-axis machine enables it to perform both milling and turning operations, on both the front and back of the workpiece. Each spindle (main and sub) acts as a workpiece holder, allowing simultaneous five-axis cutting on either the front or back of the part, thereby completing the entire part in one setup. A job that previously required four setups (front-turn, back-turn, front-mill, back-mill) can be reduced to one setup on a machine with B-axis capability.

A multi-spindle B-axis machine outfitted with a lower turret further increases capacity for producing parts in a single setup. See Figure 1. In this configuration, the B-axis head operates at the same time other turret-based cutting tools are being used on the opposite spindle, enabling synchronized simultaneous machining. In other words, cutting occurs on both the front and back of the part at the same time.