Rugged OD gages working with robotic grippers provide real-time measurement of part diameters during machine load and unload.
Take a bench gage, flip it over and send it through the factory at
3 meters per second. Can this be done and and still provide useful measurements from a tight tolerance part such a crankshaft or a camshaft? The answer is yes according to gage manufacturer Control Gaging Inc. (Ann Arbor, MI).
"What makes the lean gage system innovative is that the gages provide the measurement precision that one would normally associate with a bench gage, but they do it while mounted to a robot's end-effector or a gantry's gripper," says Robert E. Darrow, business development executive at Control Gaging Inc. "With the lean gage system, the gripper or end-effector becomes a dynamic fixture and measurement takes place as the part is moved from place to place. Replacing the bench gage with a dynamic fixture fits perfectly into the lean manufacturing paradigm saving both the time required to load and unload the bench gage and the physical space required to accommodate the bench gage."
What makes this possible? First, rugged, reliable, highly precise gages are requisite. The gages used on the lean gage system were developed for use in the extremely hostile measurement environment typically found on production grinders. In addition to being robust, the gages have a measurement repeatability of 0.000020 inch or 0.0005 millimeter. Second, the robot end-effector or gantry gripper must be modified so that the part is held in place in a consistent and repeatable manner. This is accomplished by integrating carbide contact measurement "Vs" into the part gripping mechanism.
This system can be seamlessly integrated into the automation that loads and unloads the machine. "Using automation to feed machines and remove parts from machines has increased in use as global competition has increased," says Rick Scholz, operations manager. "Today there are thousands of robots, gantry loaders and other types of automation used in Japan, Europe and North America."
The key reason for using automation is to reduce labor costs. Integrating the gages into the automation serves the same purpose while providing the additional benefit of reducing cycle time and freeing up the floor space formerly assigned to a bench gage.
Another important development in automation has been the dramatically improved flexibility. "In contrast to the dedicated part handling methods of the past, automation today can be easily reconfigured as production demands change," Darrow says. "The lean gage system integrated into the automation shares this characteristic. Part changeover time is kept to a minimum because the gage can be mechanically set up in less than 30 seconds and additional gage contacts can be used to extend gage measurement range."
The system works to ensure quality by measuring each and every part as it leaves the grinder, polisher, turning center or other metal cutting process. Part size and part size trends are used to monitor and control the machining process thereby ensuring the production of high-quality parts. In addition to process control, part size data can be used for part sorting, to generate statistical process control charts and to provide data for process analysis indicators such as Cpk.
Typically this type of system is used to measure parts right after the machining process is complete; alternately this gaging technique can be used to measure the part just before machining to avoid tool damage that can occur when attempting to machine an oversize part. Also, the measurements provided can be used to ensure that out-of-specification parts are removed from the part stream before the additional cost of subsequent processes is incurred.
A typical sequence of operations follows:
1. After machining, the part is picked up by the gantry gripper.
2. If installed, a proximity switch on the gripper provides an input to the gantry controller when it senses "part present."
3. The gantry control energizes a pneumatic valve to close the gage fingers, which are initially retracted for loading.
4. Part size data is taken from each of the gages and is stored in the Statistical Controller.
5. The part sizes are compared to pre-set tolerance limits. Relay outputs "good," "over" or "under" are sent to the gantry control depending on part size condition.
6. If used for machine tool feedback, a running average-operator set-able-of part sizes is compared to pre-set compensation limits, and relay outputs for "comp in" or "comp out" are provided when the limits are exceeded.
7. When the gantry control receives the outputs, it knows that the gage cycle is complete and de-energizes the pneumatic valve, retracting the gage fingers.
8. The part is delivered to the next station and the gripper releases it in place.
An important feature that makes this lean gage system easy to use is the quick setup feature of the gage heads. "The gage can be mechanically set up in less the 30 seconds," Scholz says. To set up the gage, the gage fingers-the part of the gage that contacts the part-are opened up wider than the part diameter. There is a small lever on the side of the gage called the setup lever. The operator holds the setup lever in the setup position while squeezing the gage fingers into contact with the part (master or on-size part) and then releases the setup lever. The gage is now setup and ready for gaging. The slip-clutch that provides the quick setup feature also prevents costly damage to the gage by allowing the fingers to move out of harm's way if the part is misloaded.
Also contributing to the usability of the lean gage system is the informative data display screen. The statistical controller has a 6- by 8-inch color screen, which displays measurement values, process parameters and statistical charts to show how well the process is running.
The current configuration for this system's product line is suitable for precise measurement of shaft parts such as crankshafts, camshafts, mainshafts and motorshafts. Future applications are for chucker type parts such as gear blanks and prismatic parts like housings and cases.
The gages are designed to withstand abuse. Each gage is factory tested to ensure watertight construction. The delicate portion of the gage is protected by a sturdy case. And the gage is protected from mechanical damage by a patented slip-clutch that allows the gage contacts to slip and move out of harm's way rather than transmit excessive mechanical force to the measurement mechanism inside the gage.
By comparison, the typical measurement device used in a bench gage is a linear variable displacement transducer pencil probe. These measurement tools are very precise but not very robust. Simply clamping them too tightly can affect measurement performance. Setup also can be time-consuming because the total measurement range is quite limited and positioning the probe correctly can take a fair amount of operator finesse. When gages are not factory tough, they often break too and are difficult to set up; both of these problems cause expensive downtime.
-With the lean gage system, the gripper or end-effector becomes a dynamic fixture.
-Part changeover time is kept to a minimum because the gage can be mechanically set up in less than 30 seconds.
-This gaging technique can be used to measure the part just before machining to avoid the tool damage that can occur when attempting to machine an oversize part.
For more information on the lean gaging system, contact:
Control Gaging Inc.
5200 Venture Dr.
Ann Arbor, MI 48108