Collisions between a moving axis and other components like fixtures, tables or workpiece can cause longest machine downtimes and highest repair costs among all process disturbances. Consequential damage after a collision frequently amounts up to five-digit sums. Outage of machine tool is accompanied by unsatisfied customers, because delivery deadlines must be considerably postponed. Typical reasons for collisions are:
- Wrong definition of the applied tool.
- Wrong tool changed.
- Improper process preparation such as zero offset.
1ms Response Time
Control machine security systems (as circled) can help avoid
costly machine damage.
An essential property of machine tools is a safe and reliable process, and control machine security (CMS) systems are a simple but effective tool for reducing such damage to a minimum or to even avoid it completely. Against the background, that collisions have to be detected during the first milliseconds, integrated overload systems based on electric components are too slow to avoid damages.
Electrical fallback systems lie at the end of a chain, consisting of tool, spindle, ball screw and drive. Thus, it takes up to half a second to stop drives. The response time of a machine operator lies in the range of a few seconds. Depending on worker’s position, approximately one to 10 seconds will elapse before the emergency stop is actuated manually. However, response time is reduced to one millisecond with the CMS. Valuable time is gained, the stop command is executed much quicker and, last but not least, the flow of the deforming force will be interrupted at an early stage.
Collision With And Without CMS
Preventing major damage caused by changing a wrong tool means collision has to be detected during the first milliseconds of impact. Taking into consideration the high masses and the inertia of today’s machine tools, an emergency stop may not avoid further damages, if it is not initiated immediately after the first collision.
Selection Of Wrong Tool
For example, an end milling cutter was used instead of a countersink. The greater length resulted in sooner contact to material at high speed.
Without CMS, the wrong tool got in contact with the workpiece. The spindle is slowly jammed, but it continued rotating. The force rose suddenly. In the further course the spindle clamped and the motor current increased. The machine was switched off by the motor current overload switch.
A control generally interprets a collision as a resistance and puts more energy into the scenery to correct resulting contouring error.
Expensive machine damage could have been avoided!
Collision Detection Method
An in-house developed piezo-based sensor is the core of this system. The sensor is mounted to the tool carrier (spindle housing, body of turret) and is able to detect slightest μ-range deformations inside the structure of the machine, which are caused by the forces acting upon the axes.
Thanks to this method of measurement, the CMS not only detects hard hits but also “dull” collisions at lower speed, depending on system adjustments. This is a notable advantage compared to other solutions using acceleration sensors.
A system operation is intuitive by means of only a few buttons and can be run by operators of virtually all skill levels without complex training. It does not require special adjustment procedures or teach-in process.
The basic version of CMS is self-sustaining and independent from type of machine and control. As there are no requirements in terms of machine and control interface, it does not matter if equipment is about machining centres, milling/turning machines, or handling systems like gantry loader or measurement and welding robots.
In addition, there are advanced systems with Profibus interface available, which enable fieldbus communication and automatic tool related limit adjustment.
Upping Machine Availability
The benefits of installing a CMS system are summarised below:
- Immediate detection of rapid force changes
- Simple retrofitting on existing machines
- Minimising of subsequent damage caused by programming, setup and operating errors
- Reduces costs due to standstill
- Reduces costs of repair
- Increases the availability of machine tools
- Enables multi-machine-operation and operation through breaks
- Prolongs the expected lifetime of machine
- No PLC modification required
An effective production process is largely dependent on the timely and qualitative availability of machinery. Significant cost reductions are possible through the use of CMS systems, which of course, also have a positive impact on the competitive position of the company.
APMEN Metrology & Design, May/June 2017