Original Date: 09/15/2003
Revision Date: 01/18/2007
Information : Machine Tool Probing
Machine part touch probing has significantly contributed to the set-up reduction and Lean Manufacturing efforts at United Defense, L.P. Ground Systems Division Aiken. This process allowed the company to utilize its numerical control machines with a higher degree of efficiency and accuracy.
Machine part probing is one means of establishing a relationship between the material clamped to a machine table or located on a fixture and the numerical control (NC) part program. It is necessary to know the X, Y, and Z zero points on the table for the NC machine to move the machine spindle to the proper position on the part for machining.
Prior to establishing part probing capability on its NC machines, United Defense, L.P. Ground Systems Division (UDLP GSD) Aiken’s operators would locate the material or fixture by using an edge finder or dial indicator and manually jog the machine tool or move the part on the table. This process was time consuming and dependent on the skill of the operator. Other problems associated with this method included requiring the operator to climb onto a slippery machine table (on large machines) to view a dial indicator or edge finder, creating an unsafe condition. Occasionally, the operator would jog the machine in the wrong direction or at rapid travel and cause the machine to crash. Additionally, this process did not allow the operator or Quality Assurance (QA) personnel to easily verify machine accuracy on a regular basis.
Machine part probing uses touch probes and probe routines in part programs. Probes are spring-loaded devices that are loaded into a tool holder which are then loaded into the machine spindle. The machine program moves the spindle to an approximate location, and then the probe searches for part presence. Most NC programmers build their programs to begin this search within one-half inch of the anticipated part or part feature location. When the probe touches the part or desired feature, an electronic signal is sent to the machine controller telling the machine where that point is in relationship to the machine “home” zero point. Numerous touch points are used to further define the true location of part features (e.g., edges, holes, bosses, slots, webs). Probing allows a part or fixture to be mounted onto a table, not mounted to a locating fixture that is precisely located on the machine each time. Once the machine controller knows the location of the X, Y, and Z zero points of the part, machining of the part can begin using the NC program.
Setting of the X, Y, and Z zero-points by the NC part program results in faster and more consistent set-up times. Touch probing also allows for irregular, varying, and inconsistent parts to be machined to the desired accuracy because the machine controller knows where each part is on the table, not where it should be. By using touch probing, the operators of large machines are no longer required to climb on the bed of the machine to accurately locate each part. This method of locating work pieces also allows the NC programmer to easily perform multiple work offsets which reduces programming time and allows for better automation of processes by testing for part presence. Another major advantage of touch probing is that simple and frequent monitoring of machine accuracy can be accomplished using a master artifact on a regular basis to check the machine. Machine part probing helps improve part quality and reduces total set-up and cycle times. With the use of touch probing and other initiatives, UDLP GSD Aiken reduced set-up times by more than 80% on three NC machines.
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