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.”

Download the Baldor UM Series Manual

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.

Question: I have a Xermac CNC machine and the main computer will not boot up. The error indicates low power coming into the drive, but when I throw a volt meter on the output of the power supply it appears to be working.

Answer: You may be facing two issues. That machine uses a solid state drive with TTL logic. TTL technology is very sensitive to high or low voltage and can be easily damaged by high voltage. To accurately test the power supply you need to connect an oscilloscope to the output and view the voltage output. Some older power supplies will produce a “ripple” voltage when they go bad. This ripple sends a fluctuating voltage that a scope can see, but a typical volt meter would read as normal voltage. Some solid state drives, like the drive in your machine have a maximum high voltage of 5.25 volts. If the ripple voltage exceeds 5.25V it can damage the drive. So it is possible that your power supply is actually bad and if so you may have to replace the drive as well.

Question: I have a Xermac CNC EDM. I am starting to see some slight position errors. Can this be related to backlash? How do I do backlash compensation?

Answer: This is a question that we get very often, especially with older machines. There could very well be backlash that has developed on the ball screws. But the backlash usually manifests itself as severe cut instability long before anyone would notice positional errors. This is particularly true with low power finishing. A more likely problem is a dirty linear scale. Most Xermac CNC machines are equipped with Acu-Rite scales. With time and exposure to EDM mist, the seals around the scale will harden and the glass scale will become dirty – leading to position variations.

The ball screw and fixed end support bearing are both preloaded. There should be no backlash or axial lost motion. If the EDM machine will be used for orbiting or cutting on angles, EDM Zap highly recommends not implementing backlash compensation. Unlike a CNC milling machine that stays in one direction for a long time, it is the nature of EDM to require a high number of direction reversals – often, many times per second. Adding backlash compensation will lead to instability and extended cutting times. However, for small amounts of backlash (less than 0.0003 inches), it is possible to add backlash compensation with reasonable results. Xermac CNC machines use the Delta Tau PMAC for motion control. Backlash compensation is handled inside the PMAC through parameters. Parameter IX86 determines the backlash amount. The units are 1/16 scale counts. The speed at which backlash is taken up is parameter IX85. The units are scale counts per millisecond.

Question: I have an old Xermac model HP power supply and I am getting more power than I should. My surface finish is rougher than normal and the overburn is greater than it should be. How can that be? How can I debug this?

Answer: It is of course possible that one of the peak current switches is stuck in the ON position, even though the toggle switch is physically in the off position. However, it is more likely that one of the peak current relays has failed. The Xermac model HP power supply used open frame 2-pole relays. Several different manufacturers have made this relay. The current replacement is Magnecraft model #199AX-14. This unit has 40A rated contacts. The original relays built back when the HP series was in production were Potter Brumfield with only 30A rated contacts. The Magnecraft relays are the preferred modern substitute. A common failure mode occurs when repeated cycling of the contacts under load degrade the contact surface. At some point, the contacts will weld together and you will see excessive power, poor finish and increased overburn. The relays are located on the side of the hotbox inside the rear door or mounted to the sides in the rear of the enclosure. To debug, turn power off to the machine (following proper lock-out/tag-out procedure) and physically inspect the relays. If there is no motion in the contacts, there are likely welded shut.

EDM Zap offers tuning and regular maintenance for Xermac equipment.

Question: What kind of grease should I put into my Xermac CNC Lube System?

Answer: This is not a simple answer. It’s like asking “should I use a 2-flute or 4-flute cutter on my CNC milling machine. The answer is; it depends. It’s not so much a question of choosing the correct lubricant. It’s more a question of choosing the best lubricant for a given situation. Xermac CNC models spanned several years and included both US built frames and frames acquired from Okomoto – sold as the “I” machines. The ball screw models and linear bearing models differed, but the recirculation technology and sizes are similar enough that the lubricant selection rules can apply to all Xermac CNC machines.

The lubrication system can supply either oil or grease. The decision of what’s best is based on the speed of ball movement. NSK calls it “DM” and it is a function of ball screw RPM, Pitch and Nut diameter. In general, the faster the balls move, the less viscous the lubricant needs to be. For the Xermac CNC, the speed of motion, ball screw pitch and ball screw diameter put the speed class into a low range. The best choice for lubricant is grease.

There are many different types of grease. The next selection criterion will be whether the base oil is synthetic or natural mineral oil. Synthetic grease will hold viscosity better as a function of temperature change. If the EDM is used in a typical temperature controlled U.S. or Canadian plant, traditional mineral oil based grease, such as Shell Alvania 2, is a good choice. If the EDM is in a region of the world such that it will be subject to temperature extremes, synthetic grease, such as Multemp SRL 2 would be a better choice. There is a bit of a trade-off here. The mineral grease is better for high-loads. So if temperature extremes are not a consideration, the mineral grease is the best choice.

There is one special situation in which different, special grease would apply; in situations where a chuck is used to hold the workpiece and long durations are spent with little or no motion in the XY axes, the lack of motion will prevent the balls from recirculating properly and could inhibit lubrication from getting where it needs to be. In these cases, special anti-fretting grease should be used to protect the uncirculated balls from damage. In these cases, NSK NF2 is a good choice. However, in a case where anti-fretting grease is added, the lubrication system should be separated so the anti-fretting grease is delivered only to the X and Y axes. The Z axis ball screw and Z-axis linear bearings should continue to get normal grease.

In addition to selecting the best grease, there are also considerations for frequency of lubrication and the volume of grease to add in each cycle. But that is a discussion for a different Q&A session.

Question: We perform annual laser calibration on all of our EDM machines. Does my Xermac CNC machine support cross-axis or volumetric compensation?

Answer: Yes, depending on the age of the machine. Xermac CNC machines use a Delta Tau Data Systems PMAC motion controller. Axis calibration data is stored on the PMAC board. Volumetric compensation is a relatively new industry term that is defined somewhat differently by different manufacturers. Essentially it implies that anywhere in the XYZ work cube, compensation will be applied to all axes so that position is accurate.
A simple example of this can be found on any of the Xermac ST machines. These stationary tank machines have the head attached to the Y-axis. As the Y-axis extends outward, it will dip down a small amount just under its own weight. Volumetric compensation will apply compensation to the Z-axis to correct for error in the linear scale. It will also add correction in the Z for the extension of the Y-axis, correcting for the sag in Y-axis when fully extended. Add to that a compensation for the Y axis as a function of X-axis saddle position and you have compensation everywhere within 3D space.

The very early Xermac CNC machines (Circa 1989) did not support the idea of cross-axis compensation. But by July of 93, with Delta Tau firmware V1.14, the system fully supported cross-axis compensation. Delta Tau called it a 2D leadscrew compensation table. By connecting the Y-axis to the X-axis saddle and connecting the Z-axis to the Y-axis arm, volumetric compensation can be achieved.

EDM Zap offers calibration services for your Xermac equipment.

Question: I just bought a 1996 vintage Xermac X-40B CNC machine. I got the machine back to my shop and got it powered up. It powered up fine and everything seems to be in order and I checked the cabinet to make sure nothing vibrated loose during transit, but the ram will not move. I am getting a general fault error. Do you have any suggestions of things I can check?

Answer: We once moved a machine that was working fine just 15 miles down the road and it had a host of problems when it arrived. It is not uncommon for boards to become unseated or weak solder connections to vibrate loose. Check the motion control card to make sure it is seated correctly in it’s carriage and the connections are good. If that is not the issue you should look for a position lock. Any Hydraulic machine of that size is going to have a hydraulic lock that prevents the Z axis from moving. Most are controlled by an electromagnet that overcomes a normally closed spring valve. Some of the larger Xermac machines of that era have pneumatically actuated position locks on the head instead of the traditional spring/electromagnet hydraulic locks. You may have to run an air line to your machine.