Q.

I started to have trouble holding size on my machining center, especially when using a circular interpolation. The ball screws seem to be running quietly and there are no strange noises when the slides move. Do I need to have the ball screws reconditioned, or is this a problem with the control?

A.

It could be a problem with the control, but it is more likely to be a mechanical issue. First, measure the part to determine which axis is mis-positioning. If it is both, proceed with one axis at a time using the following steps: Determine if the encoder (pulse generator) is connected directly to the ball screw, or attached to the servo motor. If the encoder mounted motor is connected directly to the ball screw with a couping, or if the encoder is mounted to the end of the screw, check for movement in the ball screw thrust bearings with an indicator on the end of the screw. If an encoder mounted servo motor has a belt, or gear drive to the screw, the adjustment could be incorrect. If these steps fail to account for the tolerance error, it is likely there is backlash in the ball screw. It is possible that with the servo disconnected and if the backlash is more than 0.001 inch for movement to be felt using a light touch. There could also be a problem with the spindle bearings if the metal being removed is not uniform, as with a casting.

Q.

I just purchased a turning center at an auction and there appears to be a problem with both the X and Z axis ball screws. They are producing noise, and there seems to be excessive backlash in both. Can they be reconditioned?

A.

In most cases, yes, but because turning centers are prone to catastrophic collisions with a revolving chuck, occasionally, they are damaged beyond repair. It won't cost you anything to send us the ball screw for a free evaluation, potentially saving you money on purchasing a new ball screw.

Q.

How can I be sure that the backlash shown on the control's readout is in the ball screws?

A.

With the machine assembled you can never be absolutely sure, but there are ways to improve on sheer guesswork. The encoder (a rotary pulse generator), or a linear scale (a linear pulse generator) feeds signals to the control. A linear scale is mounted directly to the slide and way, so it bypasses errors in the ball screw and thrust bearings. An encoder relies on the accuracy of the ball screw and thrust bearings, so control manufacturers invariably incorporate an electronic backlash elimination to alleviate any problems. Over compensated electronic backlash elimination can often cause jumping when slide direction changes.

Q.

When I move the Y-axis slide on my machining center close to the limit, the servo motor trips out. Is it likely to be the ball screw, or the servo motor?

A.

It is unlikely to be in the servo motor drive. Y-axis slides on many machining centers are equipped with heavy sliding way covers that often get damaged by falling parts and tooling, or just clogged with chips. When the slide moves towards its limit, the load on the servo motor exerted by the damaged cover can trip the motor. Similar symptoms can be caused by an incorrectly installed ball screw.

Q.

We did some work on the table slide of an older boring mill, but two weeks later the ball screw locked up. One wiper was pushed out and pieces of the ball could be seen in the end of the nut. A week after we installed a new ball screw the same thing happened. What could be the problem?

A.

Each ball screw is designed to operate up to a predetermined load. When the load on the unit is excessive, the ball screw fails. Your case is a prime example of this. The chances are either the slide is too tight, the ball screw is incorrectly installed, or both. This type of problem is quite common when machine tools undergo major rebuilding because the slide ways are reground and Turcite is replaced, changing the relative position between slide and way.

Q.

I clean the ball screw on my machine regularly, but the seals on the nut allow small particles to enter. The seals seem to be made out of some sort of plastic. Can I replace them to keep the chips out?

A.

The seals you mention are in fact, wipers, because it would be almost impossible to perfectly seal a ball nut. Manufacturers use varying materials to accomplish the task, but the results are rarely 100% efficient. Plastic, neoprene, felt, and brush wipers are used. Some ball screw designs are supplied without wipers and have their advantages, and disadvantages depending on the conditions of use. In your case, continue to clean, and lubricate the ball screw, but do not use an air gun to blow away chips. The use of compressed air to remove chips from a ball screw should be avoided.

Q.

There is backlash in one of my ball screws, but the screw itself looks to be in good condition. I was considering trying to replace the balls myself to keep the cost down, would you recommend it?

A.

Working on a ball screw is not rocket science, and anyone with reasonable mechanical abilities should be able to perform the task. It's a little like working on your own car; there are some jobs you know you have the ability to perform, and others you leave to the professional. The problem with do-it-yourself ball screw repair is if you make a mistake, it can damage the unit beyond repair and that can be expensive. In the final analysis, it's a judgment call you have to make for yourself, however, Express Ball Screw Repair & Engineering also provides re-balling services if you should decide that is the route you want to take provided that the ball screw assembly is a new unit.

Q.

I put a new ball screw in the table slide of my machining center, but I still have backlash. Why is that?

A.

It is likely that the ball screw thrust bearings were not replaced, or installed correctly, or the servo motor drive mechanism is not adjusted correctly. Many machine manufacturers use a servo motor with a toothed timing belt to drive the ball screw. It is important that this belt is adjusted correctly so that it doesn't cause errors in positioning. Also, if the electronic backlash elimination built into the control has been changed, it may need to be reset for the new screw. To do this, it is best to make note of the existing setting, then start from zero and gradually increase the parameter settings until the best result is achieved.

Q.

I have a laser cutter with a ball screw that is approximately 1.250 inches in diameter, and 12 feet long. It is time for me to have the unit reconditioned and I was thinking of taking it out of the machine and reinstalling it myself. Is there a specific way you recommend to approaching this, and are there any pitfalls to be avoided?

A.

Long ball screws that are small in diameter are notoriously difficult to install correctly. Contrary to common belief, ball screws are not perfectly straight, and long screws are the worst offenders. To work around this problem, it is necessary to install the unit in a predetermined order. If the thrust and support brackets are left in position (this may not always be possible) when the ball screw is removed from the machine, it makes the task a little easier. When installing the reconditioned ball screw, first assemble in the thrust and support brackets, then make all final adjustments to the bearings. With the ball nut all the way to the end, it will allow for tightening of the securing bolts. Move the slide until it comes gently into contact with the ball nut flange face, and snug the bolts down alternately until the flange is locked down. It is important to move the slide at its full travel, turning the ball screw by hand, noting any perceived tightness. Only when it is confirmed that the ball screw is installed correctly should the servo motor be attached.

Q.

I had a new ball screw installed in my machine two years ago and although it is positioning okay, it is starting to make noise again. How long should a ball screw last in a turning center?

A.

It is not unusual for a ball screw to still be functioning efficiently after 20 years of continuous use. If a high quality precision ball screw is well maintained, supplied with adequate lubricant of a suitable grade, and doesn't suffer mistreatment, then it should outlive the life of the machine it is in. Very few ball screws avoid the vigor's of abuse, tight production schedules, and poor maintenance, which is where we, Express Ball Screw Repair & Engineering, come into play. Aside from machines operating in extremely contaminated conditions, the turning center is the most known culprit for destroying ball screws. Some of the more powerful machines within the 10-12 inch chuck range sport 50 and 60 horsepower spindle motors, so, when turning at top spindle RPM, it stores an enormous amount of kinetic energy. When this energy is suddenly released, which happens when a tool rapids into the chuck, it is not difficult to appreciate the damage that can be caused to a machines components. This is why an otherwise premium quality ball screw is likely to have a shortened life in a turning center.

Q.

A ball screw on one of my machines locked up and upon inspection some of the balls have split in two pieces, why?

A.

When balls are split into two pieces like peas, it is invariably caused by extreme pressure. Usually, this pressure is the result of a misaligned ball nut. It is important when installing a precision ball screw to ensure that the ball nut flange face mates perfectly with the contact face of the nut housing, if not, a lateral twisting can occur, causing extreme pressure inside the ball nut. It is possible, especially on the nut housings that are bolted and pinned in position, that the alignment of the flange contact face was changed because of a collision.

Q.

I want to adjust the gibs for the slides on one of my machines. How will I know if the slide is adjusted correctly? Will I have to disconnect the ball screw and servo motor and push the slide, or can I do it while it is assembled?

A.

It is not necessary to disconnect the ball screw and servo motor to adjust the gibs of a slide, but some precautions should be taken before the task is attempted. Usually, the reason a slide needs to be adjusted is because it is worn. Unfortunately, slides rarely wear evenly which limits the effect of adjustment. With the ball screw, and servo motor attached to the slide it makes it difficult to feel the slides tightness, making it necessary to employ the use of a torque wrench to replace the feel. Before adjusting the slide, measure the torque required to turn the ball screw at various places along its travel, then slowly adjust the gibs one at a time until each starts to increase the torque. Keep in mind that the ends of the slides travel will be worn the least, and by adjusting the gibs it theoretically changes the position of the center line of the ball screw.

Q.

The turcite has started to come unglued on a machine I am trying to rebuild. I am considering replacing it, and wonder if doing so would cause problems when the ball screw is reinstalled. I know the ball screw should be aligned accurately, but just how precise does it need to be?

A.

It depends on many factors, but assuming it is a high precision machine with a quality ball screw, the alignment needs to be commensurate with the accuracy of the machine. Ideally, the alignment in all planes should be 0.001 inch, or better, but on some older machines it may prove impossible to achieve. It has been said that the single most critical alignment for a ball screw is the ball nut contact flange mating surface. You can always double check for tightness with a good quality dial type torque wrench.

Q.

What exactly is preload when associated with a bearing, or ball screw? How can I tell if the preload is correct?

A.

This is a difficult questions to answer because preload becomes increasingly difficult to measure accurately after it passes zero preload and when the variables become almost unmanageable. To give insight into these variables, lets consider a simple precision roller bearing preloaded at the factory. It is then pressed into a tool bored housing and onto a ground shaft, each with an upper and lower tolerance. The shaft is then spun, causing friction between the preloaded surfaces, and induces heat and expansion; this increases the preload even more. During a period of running, wear occurs, also decreasing the preload. Now, what preload do you have? There are ways to ensure that a preloaded device stays within efficient limits, but many of the variables must be known. In the case of a ball screw, consider that when the servo motor spins the ball screw from stationary to 1000 RPM, it requires the preloaded balls to instantly absorb the load exerted by the mass of the slide, but it is only the balls in the tracks responsible for taking the load in that direction that are affected. The balls in the opposite track momentarily become non-loaded, so, what preload do you have on them? To preload a device to perform flawlessly is like playing the violin; some can do it, but most can't, and for those than can, it take constant practice to play it well.