Turning the tables

As more and more machine tool builders take advantage of digital control technology and advanced CAM, multi-axis machine tools are no longer being confined to their traditional markets.

Multi-axis machinining has long been associated with such sectors as aerospace, power generation, and automotive where the cost of large ‘builtto- order’ machines could always be justified.

However, new demands facing the toolmaking and mould and die sectors, such as speed and cost-effectiveness, have driven a market for more general, yet highly dynamic, machines.

These come in all shapes and sizes, the main players including Mikron, Matsuura, DMG, DS Technology, Hermie, Giddings and Lewis, Heller, Nicholas Correa, Mazak, Mecof, Mori Seiki, Parpas OMV and Breton and Rye.

Some pivot the table, some pivot the head and others do both — but all have their own individual advantages. The principal advantage of multi-axis over conventional three-axis machining is the ability to cut five sides of a cube in a single set-up — which is quicker than cutting a job in a series of set-ups.

Also, multiple set-ups mean there is the possibility of incorrect alignment each time the part is moved. Less obvious advantages include an excellent footprint-to-work space ratio, reduction of idle times, improved machine dynamics, surface quality and contour precision.

However, before committing to any purchase you must consider CAM software, tooling, coolant systems, spindle requirements, the controller and the type of motion best suited to your requirements. Programming most continuous multi-axis jobs without a quality CAM system is near impossible.

Some workpieces require continuous five-axis machining capability to be produced. These include more complex contoured parts, such as mould cavities, blisks, impellers, and other turbine-type parts where tool orientation must be controlled to reach a surface to be machined. In all cases, the objective of multi-axis is to provide optimum tool positioning.

More specifically, it must to provide the fastest cutting tool access to individual workpiece locations, with the tool and part surface oriented at correct angles to each other for uniformly accurate machining of required features with the highest overall metal removal rate.

The idea of achieving this objective is to move the least amount of mass — whether it is the workpiece, spindle tooling or both. However, the drive towards lights-out machining and ergonomic use of workspace is also becoming a major factor when investing in machine tools.

A multi-axis machine is often capable of finishing jobs that would traditionally have been done by several machines. Some say this is putting all your eggs in one basket — if there is a breakdown, you are in trouble. However, the cost benefit of having only one machine to look after outweighs this argument.

Only time will tell, but it seems that multi-axis one-hit machines are here for the foreseeable future. So what are the options? The answer is anything but straightforward, given today’s demands for dynamics and rigidity. Some companies, such as DS Technologies, would justifiably argue that the high mass of the bridge or cross rail found on the typical gantry type machines makes them unsuitable as the basis of any multiaxis machine.

They would say that to meet today’s demands, a machine’s linear axes have to be extremely stiff, yet highly dynamic. In addition, they would argue that complex contours and radii couldn’t be provided by the conventional gimbal designs using rotary fork or swivel heads.

Fork type heads are very versatile but give poor surface finishes due to flexing, and require extensive setting up to ensure that the spindle is central between the forks. They also have poor positioning velocities and require either expensive slip rings to maintain a power connection to the AC spindle motor, or a bevel gear arrangement to  drive the spindle nose.

Further, the length from the pivot point to the tip of the tool is variable, depending on tool length, and must be accounted for in each case. Swivelling heads are stiffer generally, but often dwell as the tool waits for the C-axis to re-orientate before continuing along the desired cutter path.

Like fork type heads, they also require rotary couplings and either gears or slip rings, making both of these solutions relatively expensive and slow for today’s ‘highspeed’ applications. DS Technologies has overcome this with its Z3 head design, which is essentially a horizontal parallel-linked tripod using three parallel linear ways and ballscrews attached by linkage to a spindle carrier.

The unit is designed to achieve virtually any combination of linear and rotary motion within the head’s working zone and gives the spindle high stiffness characteristics in both dynamic and static modes.

Generally speaking, the fewer rotary axes moving the workpiece, as opposed to the tool, the better the machine is at accommodating larger parts. However, there is a trade-off. Any pivot removed from the table has to appear at the spindle head — and where a pivot at the table may limit the workpiece dimensions, a pivot at the spindle head makes the size of the tool more difficult to manage.

Accounting for the location of the cutting edge becomes much more challenging when the tool pivots. If the tool only moves in the linear axes, as with conventional machine tools with fixed workpieces, then the offset accounting for the tool’s length is a fixed value in Z, and the offset accounting for the tool’s diameter is a fixed value in X and Y.

But when it moves in one or more rotary axes, precisely where the cutting edge is located in every axis becomes a function of both the length and diameter of the tool, along with some combination of the sine and cosine of the pivot angle.

In other words, in five-axis machining with a pivoting spindle head, the tool offsets in X, Y, and Z must all change when the spindle angle does. Some more recent CNC controllers can accommodate these calculations automatically, and change the tool offsets independent of the program.

However, many cannot, meaning the operator/setter must incorporate the offsets directly into the code by limiting the process to ‘qualified’ tools. 

Finally, the main challenge facing continuous multi-axis users is to ensure that the head/tooling does not collide with the job as the orientation is changing. This can provide a major test when machining inside a small but complex cavity — and is just one reason why NC simulation and verification is so popular with the aerospace industry.

Companies such as Airbus UK, make CGTech’s five-axis machine simulation, Vericut, part of the quality process. The system is capable of detecting incorrect or misread drawings, inaccurate programming, incorrect tool path motions, rapid motion contact, collisions with fixtures and clamps, tool shank and holder collisions and CAD/CAM/postprocessor errors. It will also optimise cycle times by adjusting the feeds and speeds in accordance with the programmed cutting conditions. OptiPath, (which is a module of Vericut) mathematically subtracts the material being cut from the model on an iterative basis.