How to tackle the effects of vibration on machining
There are many factors that make modern machining challenging. Some of these challenges are simply consequences of the nature of manufacturing in today’s technological landscape. However some, like machine vibration, can be controlled. This article will explain why vibration is the enemy of machining and what can be done to eliminate this problem.
What is machine vibration?
Vibration on a CNC machine originates from the various motors on the machine. These are the motors controlling the different axes. As the motor spindle rotates the cutting tool through the material, it generates vibration. This, coupled with the chatter generated from the motion axes, can cause the negative effects attributed to machine vibration.
Vibration in a CNC machine is often referred to as chatter. Chatter is a function of spindle speed and depth of cut. A stability lobe diagram can be created to visualise the stable zones and determine the optimum machining parameters for your machine. The areas under the peaks on the diagram are the stable zones, whereas the areas around the peaks are the unstable zones that produce chatter. There are various parameters that cause chatter on a machine, and each machine, tool holder, cutter and tooling combination produces a different lobe diagram.
What are its effects on machining?
Machine vibration, if left uncontrolled, can seriously reduce machining efficiency. The ways in which machining can be hampered due to chatter are listed below.
- Reduced machine life – Unchecked chatter can place undue stress on machine components, which can cause the spindle and key components creating the various axes of motion to wear out faster than normal.
- Reduced tool life – Cutting tools are not designed to handle the jarring impacts of the tool chattering against the part being cut. These impacts can chip and blunt the cutting tool which in turn ruins the surface finish and decreases cutting efficiency. The cost of replacing the tooling can be significant.
- Decreased dimensional accuracy – As a cutting tool chatters during machining, it deviates from its CAM-generated path. This deviation can result in an ill-fitting part (too loose/too tight, etc.) due to incorrect overall dimensions.
- Poor surface finish – One of the most visible effects of machine vibration is a poor surface finish, in the form of a wavy surface. This not only affects the aesthetics of the part but can also affect tolerances in extreme cases.
Machine vibration is clearly bad news for any manufacturer, however the good news is there are a number of methods to eliminate it, some of which are listed below.
- Maintenance – Machine maintenance is critical for reducing unwanted chatter. For example, a spindle’s bearings wear out over time, and if these are not maintained the vibration frequency of the spindle may change. This can then create vibration at machine settings that previously caused no chatter. A well-maintained machine will not introduce more unknown variables into your chatter woes.
- Hydrostatic guideways – Hydrostatic guideways can significantly dampen the chatter caused by the various motion axes. This is due to the natural damping effect of fluids. Reducing the chatter sources in a machine will in turn increase the range of speeds, feeds and depths of cut that can be machined without introducing chatter.
- Correct tool holder – A toolholder that incorporates a face and taper contact adds significant rigidity and reduces the chatter by improving stiffness as well as increasing the damping effect.
- Correct work-holding – Fixing the part with poor work-holding can cause the part to vibrate and introduce chatter. High precision and high clamping force work-holding are ideal for eliminating part vibration.
- Speeds and feeds – In order to eliminate chatter, the machining operation needs to be moved into the stable zones on the lobe diagram. This can be done by changing the speeds and feeds of the operation. It must be noted that the speed can both be increased or decreased, as either will move the operation to a more stable zone. Care must be taken to change the chip load with the speed change to control the cutting forces.
- Tool selection and set-up – A thicker tool shank will result in decreased chatter by reducing the tool deflection. However, another method of reducing tool deflection is, whenever possible, to reduce the amount the tool extends from the secured end. Doing this increases stiffness and also changes the vibration frequency, moving the operation into a more stable zone.
Machine vibration is definitely the enemy of machining, however there are a wide range of fixes available to eliminate this problem. These can be anything from set-up changes to machine changes. To learn more about which method best suits your unique manufacturing process, contact a Kingsbury representative today.