Question: My Xermac CNC Machine vibrates during rapid positioning. The mechanical components are fine. We replaced the servo motor and still have vibration. What can we do next?
Answer: Given that the mechanical components are indeed fine, the next step would be to look at amplifier tuning. Xermac built CNC machines over a number of years. The majority of the machines were built using Baldor DC servo motors and amplifiers – specifically, Baldor UM series chassis. These amplifiers are analog and are “tuned” using on-board potentiometers. It is the nature of analog components to drift over time. Resistors, potentiometers, and capacitors integral to the tuning, change simply with the passage of time. Over a period of several years, it is possible that the components can change significantly enough to impart vibration. It is also possible that the change can be in the other direction, causing sluggish performance – which would manifest itself as a lack of stability and extended cutting times. The solution to the vibration problem may be as simple as re-tuning the servo amplifier.
This procedure should only be performed by maintenance personnel qualified to enter the electrical enclosure and work with high-voltage components. Your facilities safety manager should be consulted to make sure all appropriate safeguards are in place. EDM Zap can help you perform this procedure as part of a service call.
The first step is to determine whether or not your machine has a Baldor amplifier. See figure 1. This shows a typical Baldor UM chassis installed in a Xermac machine – dust and dirt included. Figure 1 shows the chassis with 3 boards populated. Depending upon the vintage of your machine, you could have anywhere from 2 to 4 boards.
Once you have confirmed that you do have this amplifier, the next step is to locate the adjustment potentiometers. Each board has 6 potentiometers located next to each other along the front edge of the board. Figure 2 shows the potentiometers, labeled A, S, T, R, C, and B. The amplifier is typically setup in velocity mode. This mode uses feedback from a tachometer to set motor velocity as a function of command voltage from the CNC motion control board. You can verify that your machine is setup in velocity mode by checking the motor nameplate. If it begins with either MT or MTE, you have a motor that has tachometer feedback.
The potentiometer that has the most effect on vibration is the “R” pot. During tuning, the Baldor manual calls for this pot to be rotated CW until the axis becomes unstable (vibrates), then turn back CCW 1 rotation. A simple test would be to turn the “R” pot 1 revolution CCW and see if the vibration disappears.
However, a better approach would be to go through a complete tuning of the servo system on an axis by axis basis. This will ensure that the system is performing optimally. For your convenience, you can download the complete 39-page UM Series manual from Xermac.com. Go to section 4, Adjustments and Start-Up. Follow the instructions for “Start-Up in Velocity Mode.”
Before performing the Start-Up procedure defined in the manual, you must confirm that the limit switches are functional in both directions for each axis. The procedure may cause some quick motion that will move the axis to the end of travel. It is vital that the limit switches are functional to protect the machine from damage that may result from excessive travel.
When performing a complete tuning, step 7 requires that a “step signal input be used.” This can be accomplished with an external potentiometer in line with a normally open switch. Disconnect the signal input into the amplifier card on the 18-pin connector. Put the switched input into the signal input terminal (pin 3) and the ground reference into the common terminal (pin 2). Adjust the balance pot so the motor is essentially stopped with the switch open. By closing and opening the switch quickly, a step command will be sent to the servo card.
Once the axis is tuned, the value of the POTs can be measured and recorded on test points TP1 through TP6 as shown in page 4-3 of the Baldor manual. These test points can then be added to an annual calibration to make sure that a valid tuning is maintained.
As a reminder, EDM Zap can provide this as part of a service call.