April 2014: Maintaining CNC Precision & Accuracy.
One of the hallmarks of CNC machining is consistent, repetitive precision – in other words how accurate the CNC machine can be, time and time again. This is the reason CNC is the industry standard technology that’s used for precision cutting of objects, parts and components for an almost endless range of applications & industries including aerospace, defense, automotive and marine to kitchens – and everything in between.
Virtually everything is CNC cut because it’s accurate, repetitive and economical.
Obviously the accuracy of the machine – how precise it is – is the essence of a CNC machines’ existence.
Accuracy determines how precisely a machine can turn the CAD object on a screen into a physical object, and is determined by a combination of factors.
Factors include calibration, tool selection and the material used for the job will, both individually and collectively dictate the level of accuracy the machine is actually capable of.
Let’s have a look at each of the factors and how they combine to determine accuracy.
CNC machine calibration is best summarized by Wikipedia as “a comparison between measurements – one of known magnitude or correctness made or set with one device, and another measurement made in as similar a way as possible with a second device.
The device with the known or assigned correctness is called the standard (these are the calibration tools we use - see right). The second device is the unit under test (the CNC machine), and is the device being calibrated” – Wikipedia.
Basically calibration is ensuring that 10mm in reality, is 10mm +/- 0.025mm as the machine sees it on all of the machine axes.
So what’s the reason for calibration?
What happens is that, over time, and through normal use, it's possible for a machines accuracy to erode to the point where it could start to affect the precision and finish of a job - in other words the machine starts seeing what’s 10mm in reality as 9.5mm (this is a huge exaggeration, just to illustrate the point!). When this happens, the difference between how closely a finished job reflects the CAD it’s based on starts to increase, thus you get a less accurate finish. This result brings with it numerous problems not least of which are fit (of the object into assemblies or with other components) and finish (the surface of the job becomes rough, or has machining marks on it).
For this reason, the machines are regularly calibrated to ensure that they are doing exactly what the computers and CAD models are asking. Calibration is carried out irrespective of if the machine appears to be out of calibration, or in response to the machine obviously being out of calibration.
Calibration is (as Wikipedia states) checking the machines measurement against a known standard. This standard comes in the form of a set of custom made calibration tools (left), in this case for our 5 axis machine. These calibration tools are fitted to the machine and, because they are of a known dimension, it’s possible to check that the machines measurement of a distance is the same as what’s a known quantity. By conducting this process it’s possible to determine if the machine is calibrated and, if it’s not, what correction needs to be made.
An important aspect of calibration is checking that all 5 axis of the machine are accurate to within 0.02mm when used in conjunction with one another. This is because there can be 'creep' of the tool when it's in use, especially when all of the axis are in use at the same time (in other words the spindle of the machine (the spindle holds the tool) is moving on a diagonal both up, down and across, in a curve - all concurrently).
This motion (when all of the axes are in use at the same time) is quintessential 5-axis machining – so ensuring that the machine is in calibration is critical to ensuring that it continues to deliver accuracy.
For the machine to cut something to within .025mm, it has to know exactly what tool it’s using, and what it measures in terms of length, diameter and cutting surface.
The diameter of the tool is measured using a digital veneer. If the diameter of the tool is incorrect, and the machine is told no different, then it’s possible to take either too much, or not enough material off. If the tool is longer than the machine thinks it is, then either too much material, or not enough is taken off – but the end result is the same - inaccuracy.
Another factor is the length of the tool. If the tool is long and thin, then there is a possibility that vibration will start to set in and, when this happens, it’s possible to get inaccuracies starting to appear on the job. For this reason a general guideline that ensures the tool is not too long for its diameter is that to multiply the diameter x 12 to get an approximate maximum length, for the particular tool, that will not vibrate.
The choice of materials will also have an influence on how accurate a result can be achieved. For example, polystyrene will not achieve the same level of precision as uniform density fiberboard (UMDF), which in turn will not deliver accuracy as well as tooling board (Ebalta) can and so on.
The material used will determine accuracy to a point; however environmental conditions and shrinkage rates can have as large, if not larger influence on an object.
For example, polystyrene is not a 'solid' – it contains a large volume of air and this expands and contracts according to environmental conditions. Because the material is not inert, it will change size after its cut (unless it’s sealed with fiberglass for example, like in a Surfboard).
The dimensional change is determined by environmental conditions (heat, humidity) and is cumulative – in other words the larger the object that’s being cut, and the number of pieces of polystyrene that are being joined will ultimately determine the amount of variation. It might be small – say 2mm shrinkage over 1m; but if the object is 5m long, then the variation might be as large as 10mm.
A material like Ebalta, which is a tooling board, is very dense and consequently is not as affected by atmospheric conditions. However it’s quite heavy, and expensive – so it’s more suited to smaller objects requiring a really high degree of precision. Polystyrene on the other hand is light and easy to handle, so it suits large objects, for example moulds for yacht masts, booms and keel bulbs or superstructures. Typically these objects require accuracy, however there is usually enough tolerance to allow for skilled fairing of the variations in size into a smooth surface.
Because different materials change shape at different rates, for example polystyrene and UMDF, it’s necessary to be aware of the potential variations and plan the changes into the object. In Solidworks (for example) it’s possible to incorporate material characteristics into a CAD design from the outset, meaning that it’s possible to reduce the affect that material dimensional changes have on the overall accuracy of an object.
The devil is in the detail...
Maintaining the precision and accuracy of Styrotech CNC’s machines is an ongoing project – it never really ends, as there are so many factors that can together combine to create inaccuracies.Through constant, proactive calibration, the accurate measurement of tools and a good understanding of the characteristics of different materials it’s possible to plan and make allowances for possible factors that will influence the overall level that can be consistently achieved on a CNC machine.