Collisions that can damage a machine tool’s spindle are most likely to occur when a new job is being set up or first attempted. The total cost for spindle repair/replacement exceeds the price of the unit itself when one considers other machine components that might also be damaged by the collision, as well as lost production time while the machine is down for repair.
As a result, GF Machining Solutions developed the Machine and Spindle Protection (MSP) feature for its Mikron HPM 600U and 800U VMCs. Stefan Fahrni, a milling product manager for the company, says this $20,000 option is approximately half the cost of a replacement spindle, meaning it can pay for itself after just one crash.
The MSP feature is comparable to the Integrated Collision Protection (ICP) capability that has been available on AgieCharmilles wire EDM equipment for some time. The system for the wire machines has energy-absorbing, spring-loaded ballscrews on the linear axes and two types of positioning sensors for each axis—a rotary encoder and a glass scale. If the location detected by the rotary encoder and glass scale for a given axis differ by a predetermined amount (meaning a collision is occurring), the system stops machine motion within microseconds before damage occurs.
Similarly, the MSP feature uses a mechanical system that enables the spindle to deflect slightly in X, Y and Z axes during a collision. A sensor system detects this deflection and stops machine motion before the spindle/spindle bearings are damaged. The company guarantees that the MSP feature will prevent spindle damage at travel speeds of 12 m/min. or less. In fact, an M304 code was developed to cap the rapid feed of the linear axes at that rate. Once a new part program is proven out and it is determined that there will be no collisions during the operation, a subsequent M305 code disables the MSP feature and enables the machine to run at normal production speeds.
This video shows how quickly the system stops machine motion after a collision. Mr. Fahrni say the MSP feature does not affect spindle rigidity during normal machining operations. The HPM 600U and 800U machines can achieve the same material removal rates with the MSP feature as they can without it.