Axis Backlash is a loss of motion when a machine's Axis reverses direction. It is also referred to as lost motion, turn around, and reversal error. Reversal error is detrimental to accuracy. This article is about where it comes from, methods of measurement, and methods for correcting it. The article's application and terminolgy applies to modern CNC Machine Tools.
From The Drive Train?Lost motion coming from a drive train is defined as a loss of motion between the components of the drive train when the direction of motion is reversed. It is the result of things like loose drive train member bolts, gear backlash, loose gear belt, worn ballscrew assembly, loose ballscrew end support, and a loose drive key or keyway.
Less common lost motion from the drive train is that which occurs from excessive friction. It is the result of things like lack of lubrication in the drive train components or the guideway system, a component is set too tight, or components are rubbing that should have clearance.
Excessive friction in the guideway system and ballscrew nut can lead to ballscrew windup, or twist. Lost motion occurs upon reversal from first the unwind and then the windup again before the Axis can move. The further away the Axis is from the drive end, the more the lost motion.
Lost motion from excessive friction is more likely to occur when a rotary encoder in the servo motor is the position feedback device. There will be more on encoders later in the article. Also, excessive friction should increase the load on the servo motor which would be indicated by higher current, or amperage.
Measuring Backlash at the Axis LevelBacklash is measured at different locations for different reasons. The image to the right shows backlash measured at the Axis level. Backlash is commonly measured and corrected at the Axis level. However for highly precision machines, that would be a narrowed down location. Narrowed down for troubleshooting to find the source of the backlash or to see the results of corrections made at that level. The most meaningful measurement location on those machines would be in the Workzone discussed later in this article. The reason for that has to do with the machine's hysteresis.
Machine HysteresisMachine hysteresis concerns the rigidity of the machine when the structure of the machine is put under a load. The load can be any type of force, including friction. Any force opposing an axes direction of motion contributes to lost motion to some degree.
Lost motion from hysteresis can be the result of angular changes in the structure. The error is perportional to the distance from the origin of the angular change. The greater the distance, the greater the error. That's the reason why the Axis Level is not considered the ultimate measurement location for backlash other than for troubleshooting and repairs. What happens at the Axis Level is less likely to precisely represent the end product as well as the Workzone.
Precision Machine Tool, Coordinate Measuring Machine, and other equipment manufacturers are taking steps to measure, improve, and compensate for machine hysteresis to a much greater degree today than in the past. Another load or force that concerns machine hysteresis and lost motion is gravity.
GravityEverything on earth has a huge force applied to it all the time. That force is gravity. It's easier to go down than it is to go up. Of course that applies to vertical axes too! Reversal errors arise with vertical axes due to the effect of gravity. How much error depends upon a number of factors. One of the factors is the method used to compensate for gravity. That factor is the counterbalance system.
Counterbalancing a vertical Axis to reduce reversal error is a very tricky business because no method to date is perfect. The heavier the Axis the more complicated the effort. As an example, let's say the machine has a hydraulic counterbalance system. The type of system we are referring to has two pressure settings. One is the Axis holding and moving up setting either coming from a pressure compensating pump adjustment or a reducing valve. The the other setting is the Axis moving down pressure setting coming from a relief valve.
The downward pressure setting has to be higher than the holding/upward pressure setting due to the nature of the relief valve. The relief valve provides a path for the hydraulic fluid to excape back to the tank. The relief valve must have a higher setting than the holding/upward pressure or it will constantly relieve fluid creating a pressure drop over the valve. The constant pressure drop in turn generates heat which reduces the lifespan of the hydraulic fluid, components, and wastes energy.
The difference in pressure settings between moving the Axis up and down causes forces or loads which cannot exactly balance for gravity because gravity is one constant value not two. That load imposed against the machine structure can cause backlash due to hysteresis. It depends upon the rigidity of the structure.
Actual cases of this with one medium size and one large CNC Machine are, one with 0.002" and the other with 0.003" reversal error. Both manufacturers of those machines said, there is nothing that can be done about it, it is the normal amount for that machine.
Measuring Lost Motion in the WorkzoneAt some point, the accuracy lost with backlash could become a problem in the quality of the end product. Of course, improving it in a manner which will benefit the end product the most is optimum. The most beneficial backlash measurement location to the end product's quality is where the end product is being worked, in the Workzone!
The image on the right demonstrates the measurement being taken in the Workzone using a dial indicator and a magnetic base. The magnetic base is in the same location that the parts are machined at and the indicator probe is on the spindle where the work is performed on the part.
Measuring in the Workzone for accuracy relative to the quality of the end product doesn't just apply to backlash. It also applies to measuring for positioning errors, straightness, squareness, and performance evaluation.
Position Feedback DevicesWith the advent of CNC came positioning feedback devices. These devices feedback the axes positions to the CNC Controller and the controller adjusts those positions as dictated by the part program. With modern CNC Machines these are typically encoders. There are linear encoders sometimes called scales and there are rotary encoders. What does this have to do with backlash?
The relationship between encoders and backlash has to do with the type of encoder and how it's attached to the machine. Take a linear Axis for example. A linear Axis just travels straight back and forth. Commonly, this linear motion is generated by a rotary motion in which a servo motor turns a ballscrew. If the position feedback device for that Axis is a rotary encoder in the motor and there is any backlash in the drive train between that encoder and the Axis on the other end, then the CNC Controller can send out positioning commands unaware of the backlash. The Axis will come up short of the targeted position every time it reverses directions.
On the other hand, if the Axis uses a linear encoder for the position feedback device, the CNC Controller may still be completely unaware of the backlash in the drive train but not come up short upon axes reversals at all. This is because of the way that a linear encoder is attached to the machine. The scale part of the encoder is bolted to the Axis Base which is stationary, and the part that is reads the scale is attached to the Axis. It is not dependant upon the drive train's integrity at all because it is not attached to the drive train as in the case of the rotary encoder. The only amount of reversal error that will show up in the case of the linear encoder comes from either hysteresis or the amount of one resolution of the encoder. Modern encoders can have a resolution of less than one micron.
Software CompensationSoftware compensation is a feature of many modern day CNC Controllers. As far as backlash is concerned, it is used to compensate for normal hysteresis and minor wear. It simply involves inputting the appropriate value for the reversal error into the control's parameter for that. From then on the CNC Controller will make up for the lost motion upon Axis reversals without reflecting it as a position change.
SummaryThere's a lot more to Axis reversal errors than that mentioned here! There are different instruments for measuring it and more details of the causes and effects. It is a good size study all by itself and yet, it is just one tiny area among many in the world of CNC Machine Technology.
For a step-by-step procedure on how to measure machine backlash, see How-To Measure Machine Backlash.
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