What is Virtual Machining?

A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine. A virtual machine was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real machine". Current use includes virtual machines which have no direct correspondence to any real hardware.

Definitions of Virtual Machining

Virtual is a term that originally came from optics, to understand objects in a mirror. Objects in a mirror are reflections of an actual physical object but mirrors are not actually that object. This means that the image looks exactly like the actual object. In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. Essential Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine.

(a) System virtual machine

(b) Process virtual machine

A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). Characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine—it cannot break out of its virtual world. System virtual machines (sometimes called hardware virtual machines) allow the sharing of the underlying physical machine resources between different virtual machines, each running its own operating system. The software layer providing the virtualization is called a virtual machine monitor or hypervisor. A hypervisor can run on bare hardware (Type 1 or native VM) or on top of an operating system (Type 2 or hosted VM).

Process Virtual Machines

A process VM, sometimes called an application virtual machine, runs as a normal application inside an OS and supports a single process. It is created when that process is started and destroyed when it exits.  Its purpose is to provide a platform-independent programming environment that abstracts away details of the underlying hardware or operating system, and allows a program to execute in the same way on any platform.

Advantages of System Virtual Machines

(a) Multiple OS environments can co-exist on the same computer, in strong isolation from each other.

(b) The virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machine.

(c) Application provisioning, maintenance, high availability and disaster recovery.

While the graphic simulation available in most CAM packages show only motions, the VM systems can depict the material removal as well. In a VM system, one starts with the virtual model of the work piece and as machining progresses, the geometry of the work piece gets updated by subtracting the volume swept by the cutter during each motion. It is possible to model even the machine tool and fixture elements so that even collisions can be detected. The user gets a realistic visual feel of the machining process in a VM system; it helps in predicting errors on the computer screen itself. This system can automatically do the verification both to ascertain the safety of machining (detection of collision etc.) and geometric conformance to design. It is also possible to optimize the technological parameters (spindle speed and feed rate) from the geometric characteristics of the material removal process.

Disadvantages of System Virtual Machines

(a) A virtual machine is less efficient than a real machine when it accesses the hardware indirectly.

(b) When multiple VMs are concurrently running on the same physical host, each VM may exhibit a varying and unstable performance, which highly depends on the workload imposed on the system by other VMs, unless proper techniques are used for temporal isolation among virtual machines.

After verifying the NC programs using a virtual machining system, the NC programmer can hand them over to the operator for machining. This also makes global manufacturing possible - model in one place, prepare cutter path in another place and machine it in yet another place. VM systems are too few compared to the CAM packages due to its high computational complexity. However, with the availability of powerful low cost computer hardware and software, VM systems are emerging. VM can also be used for machine tool simulation as well as optimization of spindle speed and feed rate. Standardization of NC procedures and documentation and careful customization of VM system will improve its effectiveness.