What is turn mill machining and Why Do We Use Them?

From medical implants to aerospace components, extremely precise parts are the key to keeping some of most critical aspects of our modern world running smoothly.

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Manufacturing these small, complex parts efficiently, and within demanding tolerances, requires the right state-of-the-art equipment paired with the engineering skills to exploit it.

Increasingly such parts are manufactured in a single set-up using mill/turn machinery.

This blog is intending to explain how a mill/turn works, how a mill/turn is used in high precision parts manufacturing, and the many benefits of the mill/turn’s combined functionality.

A mill/turn machine is a hybrid CNC (computer numerical control) machine that combines both milling (tool rotating) and turning (workpiece rotating) functionalities into one fast, powerful, and flexible machine that can complete complex operations faster and with greater accuracy than traditional machining technologies.

While other machines perform a single function, mill/turn machines can accomplish up to four operations at the same time.

Using mill/turn machines to combine operations, means less machinery – or less floor-space. Fewer machines in turn means reduced operators. It also means less power and compressed air is consumed. All of these reductions combine to deliver cost savings and improve profitability.

As the advances in user-friendly CAD/CAM packages with Mill/turn platforms and post-processors, mean it is also much more accessible and no-longer just the preserve of large tier one specialists.

And with more machine tool manufacturers beginning to produce Mill/turn technology so it becomes more commonplace and more affordable than you might think.

The benefits of this technology is increasingly being harnessed by forward thinking sub-contractors, looking for a competitive edge.

Turn-mill machines are a type of multi-tasking machine capable of both rotating-workpiece operations (such as turning) and rotating-tool operations (such as milling and drilling). They enable users to machine workpieces complete on a single piece of equipment—increasing accuracy by reducing setups and positioning adjustments, decreasing footprint compared to dedicated machines for each operation and reducing backlog waiting for different machines to become available on the shop floor.

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Adoption of these machines increased heavily during the s, a change that Modern Machine Shop’s Peter Zelinski credits to two developments. First, CAM software and postprocessors have caught up with these machines and can now reliably run them. Second, more shops have become interested in them as a means of facilitating automation.

Over the past decade, programming and control software advancements have gone beyond basic compatibility for turn-mills. Some systems use AI and machine learning in conjunction with digital twins, optimizing tool paths on offline testing grounds to reduce cycle times and eliminate potential crashes. Ongoing efforts to develop powerful postprocessors that limit the need for G-code editing should also lower the programming entry barrier for turn-mills.

As for shops becoming more interested in using turn-mills for automation, surveys from Gardner Intelligence’s Top Shops program show that the best predictor of whether a shop is likely to conduct light-out machining is whether that shop uses a turn-mill machine — followed by whether it uses a Swiss-type lathe, twin-spindle turning machine or twin-turret turning machine.

Lathe or Mill?

Many turn-mill machines are based on lathe designs, with the milling spindle more supplementary. This setup evolved from live-tool lathes, which use driven spindles that mount on a tool gang plate or turret to mill flats and drill holes.

The best predictor of whether a shop is likely to conduct light-out machining is whether that shop uses a turn-mill machine.

Another common form of turn-mill, B-axis turn-mills, more closely resembles a mill, but offers both five-axis milling and turning functionality. The turn-mill’s rotating milling head can machine parts held in the spindle or subspindle from many different angles. These parts don’t need to be the round parts of turning, either, with the B-axis flexibility enabling processing of fully prismatic parts or machine-contoured parts.

These B-axis turn-mill platforms can perform simultaneous milling and drilling operations, as well as more complicated maneuvers such as pinch turning, skiving, contoured turning work and “turning milling,” where instead of feeding a static, single-point turning tool down the Z-axis, a spinning milling tool is moved down the part to “turn” the diameter. These machines are also good at creating big, shaft-like components that need milled features, such as large crankshafts.

More traditional twin-spindle and dual-turret turn-mill configurations can also improve performance, but may require synchronization of separate part programs running simultaneously, lest workpieces or spindles collide. Swiss-type machines with B-axis milling spindles have also become larger players in the turn-mill field, especially after the debut of automatic toolchangers and full-power B-axis spindles expanded their audience from shops working with small-diameter barstock to shops working with larger parts.

Tips and Tricks

Turn-mill users should follow a few workholding and parts loading best practices to optimize productivity.

When using a subspindle in conjunction with a bar feeder, the subspindle grabs the end of the bar and pulls out the length needed for the next part. As the subspindle will be more precise in pulling out the stock than the bar feeder is at pushing the stock in, relying on the subspindle for positioning will improve the final results for high-tolerance parts. Also, once the parting tool severs the finished part from the barstock, have a parts catcher ready so the turn-mill’s subspindle can drop the workpiece and make room for the next part.

If using automatic subspindle transfer, use three-jaw or dead-length collet chucks. If a collet or chuck moves in or out slightly, pushing or pulling on the workpiece when clamping, this unwanted motion could easily mar the workpiece.

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