Operating modes of automatic machine tools
Automatic machine tools , unlike manually operated machine tools, are moved by an electronic control ( NC , CNC control ). To do this, the operator writes a program that he can save for the production of recurring workpieces. The control can move all axes of the machine independently using these programs.
Additional devices such as tool changers , pallet changers , chip conveyors and coolant devices are also coordinated by the control.
Since these machines involve a considerable safety risk for the operator in automatic operation, EN 12417, edition: 2001 + A2: 2009 "Safety of machine tools - machining centers" and DIN EN 13128 "Safety of machine tools - milling and drilling machines “Different operating modes are defined which try to maximize the safety of the personnel on the machines.
The modes of operation
Operating mode 1 - automatic mode
The automatic mode can lead to a particularly high risk for the operator, since all functions of the machine are fully available. In this operating mode, protective devices must prevent entry into the machining area. Effective non-separating protective devices (e.g. light barriers ) must put the machine or system in a safe state when the work area is entered.
Doors leading into the processing room must be locked with circuit breakers so that they cannot be opened.
The doors can only be opened and the processing room entered after the automatic mode has been switched off. Automatic mode can be restarted after the door has been closed.
Operating mode 2 - setup mode
The set-up mode is available to prepare the machine for automatic operation. To do this, set-up mode must be selected on the machine using a key switch. In order to be allowed to use this operating mode, machine operators must be specially trained.
The machine functions are significantly limited in this operating mode compared to automatic mode. The speed of all axes must not exceed 2 m per minute in setup mode; the movements must be actuated via handwheel or inching mode.
When a control element is released, the drives are switched off immediately. The spindle speeds are also limited in setup mode. The spindle must stop within two revolutions after releasing the enabling button.
The automatic tool change has no function in operating mode 2 and the chip conveyor may only function in jog mode.
Operating mode 3 - process monitoring in production
The process monitoring operating mode in production allows manual intervention under restricted operating conditions.
Operating mode 3 is used, for example, to machine a complex individual workpiece or when areas of the workpiece cannot be viewed. With this operating mode, the standard provides for additional manual intervention. Unlike in automatic mode, the user can observe and control the machining process when the protective devices (doors) are open.
For this it is necessary that the operator can take an "alternatively effective safety device" in the form of a hand-held control device or a swiveling control panel into the work area. This means that he can quickly shut down the machine in an emergency. In addition to an emergency stop button, these operator panels must also have an enabling button.
When this approval button is released, all machine movements are stopped immediately. The rotation of the spindle must come to a standstill within five revolutions. The speeds of one or more axes may not exceed 5 m / min.
Operating mode 4 - process monitoring in production without an enabling device
The European standard does not provide for operating mode 4 (process monitoring in production, without consent device). However, it is possible if there are special manufacturing reasons. These reasons can be, for example, individual items in production that make it impossible for the operator to continuously press the consent button during lengthy processing and if the resulting termination of workpiece processing could result in considerable, irreparable damage to the workpiece or the machine. Undercuts or the search for a zero point on complex workpieces can also be reasons for placing increased demands on process monitoring.
If this operating mode is to be made available on a new machine tool, the machine manufacturer must clarify safety-related details and procedures with the subsequent operator and have these confirmed in writing.
The standards "DIN EN 12417, Edition: 2001-12 Safety of machine tools - machining centers" and DIN EN 13128 "Safety of machine tools - milling and drilling machines" provide procedures for the manufacturer for these basic requirements. Essentially, it is stipulated that the manufacturer of the machine, if there is evidence of the "inevitability" of this operating mode, works out a safety concept in which protective measures and rules of conduct are specified, which offer the operator the greatest possible protection according to the current state of the art.
In this concept, foreseeable misuse must also be prevented by restricting the speeds of the required movements and switching off unnecessary dangers and movements. These include, for example, the tool changer or a high-pressure coolant system.
The spindle speed and axis feed rates must be limited in this operating mode to what is technologically necessary.