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moldmakingtechnology.com 25 Many machine tool builders have done fairly well in keep- ing budgets in check for three-axis machining centers, but when more capabilities are added to a machine to increase productivity, for example, adding two more axes, a more- capable high-speed spindle, a higher capacity toolchanger, a touch probe or a laser, part precision can quickly deteriorate. Similar to the case when multiple machines are used to pro- cess a part, each of the single machine's components has its own tolerance range. The more machine components there are, the more likely there will be tolerance lost at the end of the machine build. Therefore, the tolerance range for each of those components must be tightened even further during their respective parts of the manufacturing process in order to control the tolerance stack-up. This includes addressing spindle precision and laser capabilities. Obtaining a machine's achievable part accuracy is another challenge. Nearly all builders will provide some details about positioning accuracy and machine repeatability, with a single or multiple reliable standards. However, not all builders use the same standard, and that makes it difficult to produce a fair comparison. Standards for determining machine tool precision include NMTBA (U.S.), ISO 230-2 (Europe), BSI BS 4656 Part 16 (Britain), VDI/DGQ 3441 (Germany), JIS B 6336- 1986 (Japan) and ASME B5.54-92 (U.S.). Further, when capabilities are added to a machine, it becomes more difficult to get a clear picture of the machine's overall precision performance, particularly in a productive environment. The standards mentioned here only pertain to the machine and don't address what achievable or expected part tolerance the machine can hold. To overcome this, manufacturers typically resort to a "test cut" that they com- monly define themselves. Although this can be a great way to test the performance and precision of a machining center in a real manufacturing environment, it doesn't necessarily eliminate process tolerance due to machining methodology, as mentioned earlier. A goal is to define achievable part accuracy in the scope of the entire machining environment while simultaneously removing any influence from the actual machining of the part. Material type, milling strategy, cutting tool type, and size and amount of stock removal all influence the achievable tolerance. Limiting these influences can help reveal the true accuracy of the machine in operation in a real environment. For example, when machining a difficult material, the tool may deflect, causing a loss of tolerance. This doesn't reflect true machine accuracy, therefore, as it is influenced by the machining methodology used. However, dimensional part accuracy can be achieved. A part accuracy of 5 microns or less is plausible. The question becomes: Can this concept be used to identify the achievable productivity level on a multi-plat- form five-axis machining center in a real-world environment? A Multi Solution To answer this question, a shop should first revisit the machine tolerance budget with machine dynamics, mechani- cal design and thermal stability in mind. These are often- overlooked machine attributes that can have a large effect on five-axis precision when it is met with productivity. Machine dynamics. These are key to establishing a balance of what is possible across the largest machining area while holding the tightest tolerance. For example, some companies desire the best of every part size, from large to micro. This is not balance. It is not reasonable to expect to hold extremely tight tolerances on a large part and then hold tight toler- ances on a micro-part, particularly in five-axis machining. Companies should work towards a balance, with average or common part size. Machine design, mechanical design and the kinematics of the fourth and fifth axes in relation to the X, Y and Z axes become rather important to part size and are essential to tuning this balance to a suitable "sweet spot." Large compen- sation movements always lead to imprecise parts, and larger distances from the center of rotation, combined with heavier parts and higher feeds, can create instabilities in motion that can make holding part precision very difficult. The goal is to find that reasonable sweet spot in machine design that allows handling of large parts (greater than 12 inches in diameter), while also holding unprecedented tight tolerances (for Part tilting during surface milling can greatly improve surface fnishes on molds.