Checking Press Consistency with Oscilloscopes
For open loop flying die processes, press consistency is critical. The ability of the press to perform its operation in a repeatable fashion is directly linked to length tolerance.
Press reaction time is the amount of time it takes for the press to receive the signal to begin its operation and make contact with the material. If reaction time is stable and consistent, the resulting part lengths should be stable and consistent. That’s because speed, time, and distance are in direct proportion with each other:
s · t = d
s = speed
t = time
d = distance
If speed is consistent, but time varies, distance must also vary. Because of this, open loop flying die applications rely heavily on the ability of the equipment to behave in a repeatable fashion. When a press’s reaction time is suspect, maintenance or engineering can test the press timing to verify its operation.
To check the timing of a press, the following equipment is required:
- 2-channel oscilloscope
- Magnetic base
- High-speed proximity sensor or accelerometer (sensor)
- Bracket to mount sensor to magnetic base
Begin by removing material from the press die. This test should be performed while the machine is standing still. There is absolutely no need to perform this test while the machine is in motion. Whatever time is required to fire the press is the same amount of time, regardless of the speed of the material.
Mount the sensor to its bracket, and fix the bracket to the magnetic base. Wire voltage to the sensor. Mount the magnetic base as close to the press as possible, and set the sensor so it will detect the press tooling at the bottom of the stroke.
Connect one probe of the O’scope to the press fire signal from the length control system. Connect the other probe to the output signal of the sensor. Test fire the press a few times to be sure the proximity sensor is picking up the tooling as close to the bottom of the press stroke as possible. Also, verify the magnetic base is not moving due to vibration from the press.
Once all the components are physically in place and verified, set the O’scope to trigger off the press fire signal of the length controller. This test will display two pieces of information – the true reaction time of the press (though this can be found in a much simpler way), and any variance in the reaction time.
Each time the length controller fires its output, the O’scope should display the transition of the press fire output, as well as the transition from the sensor. The time difference between the two signals is the press reaction time. Manually fire the press through the length controller at least 50 times to be sure the full range of reaction variance is displayed. Each time the O’scope updates the signals, record the time. The maximum and minimum times are the reaction time variance from the press, solenoid and any relays or boards between the output of the length controller and the press tooling contacting the material.
Many O’scopes have slider bars on their display that can be used to mark the trigger points of signals. They make measuring the time variance convenient. Once the total time variance is known, multiply the time in seconds by the maximum line speed in inches per second. The result should be the length variance from the machine. Any additional variance is probably attributable to encoder tracking issues.