# The choice of machine tool precision

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01mm, athletic precision and treatment precision are in (0.

03 ～ 0.

05)mm above. Super- nicety use at special treatment, its precision can be amounted to 0.

001mm is the following. The machine tool of complete function numerical control with most application basically discusses here (give priority to with machining center) . Can divide by precision for common model with nicety model, precision of machine tool of ordinary numerical control examines the project has 20 ～ 30, but its have diagnostic project most is: Precision of odd axis fixed position, odd axis repeats fixed position precision and treatment of linkage of two axes above to give the circularity of test specimen, if express,1 is shown. Express project of feature of precision of machine tool of 1 numerical control other precision project and the corresponding concern with express 1 content to have particular. Fixed position precision and repeated fixed position precision to mirror this axis integratedly the integrated precision of each motion component. Repeat fixed position precision especially, it mirrorred this axis the fixed position stability that aleatoric inside the journey fixed position chooses, this is to measure this axis whether the main index of stable and reliable job. At present the software in numerical control system has substantial error to compensate a function, can go up to feed drive catenary the compensation that error of each link system has stabilizing. For example, transmission catenary is the clearance of each link, stretch be out of shape and contact the metabolic element such as stiffness, the report such as the speed that they often are apart from speed of accident, mobile fixed position as the laden size of workbench, shift gives different instantaneous motion to measure. In a few open loop and servo of feed of half closed circuit, measure component component of the following mechanical drive, suffer all sorts of accidental element effects, also have quite big random error effect, the workbench that if heat of ball guide screw is stretchy,causes locates actually positional drift. Anyhow, if can choose, so choose duplicate the facility with fixed position best precision! Precision of milling round cylinder or chamfer of milling space helix (whorl) it is to evaluate this machine tool integratedly to concern numerical control axis (two axes or 3 axes) servo follows the index of athletic character and function of interpolation of numerical control system, assessment technique is the circularity that measures treatment to give columnar side. Try in numerical control machine tool cut in still have mill rhombic machine a law quadrilateral, also can judge the precision when two can accuse an axis to move in linear interpolation. Doing this to try when cutting, establish those who be used at finish machining milling cutter outfit arrives on machine tool main shaft, milling places the circular test specimen on workbench, right medium or small test specimen of machine tool circle is taken commonly in Ф 200 ～ Ф 300, put the test specimen that cuts to circularity appearance next, measure the circularity that gives its to machine the surface. Mill gives cylinder there is Zhen Wen of apparent milling cutter to reflect rate of interpolation of this machine tool on the face flabby calm; The circularity that mill gives has apparently elliptical error, two when reflect interpolation campaign can accuse shafting all gain not to match; In the circle apparently each the point that can charge axial motion to change way on have mark of dot stopping a knife (in successive cutting motion, feed motion cutting tool stops in some position can machining apparently the trace that forms a paragraph small many to cut a metal) when, mirror this axis positive and negative to was not adjusted good to clearance. Odd axis locates precision is to show inside this axis journey random nods the error limits when fixed position, it reflected the treatment precision ability of the machine tool directly, it is numerical control machine tool so index of the most crucial technology. At present whole world each country is mixed to the regulation of this index, definition, measurement technique data processing differ somewhat, in introduction of data of example of machine tool of of all kinds numerical control, commonly used standard has American level (NAS) recommend standard, Germany level with Manufacturers Association of American machine tool (VDI) , Japanese standard (JIS) , international Organization for Standardization (ISO) with level of our country state (GB) . What set minimum in these standards is Japanese standard, because its measurement technique is to use only group to stabilize data to be a foundation, take out the value that use ± to reduce error cost again next half, the discrepancy that measures the fixed position precision that go out to often go out than be being measured with other level with its measurement technique so one times above. Other although a few kinds of standards are distinguished somewhat on processing data, but mirrorred should be analysed by error statistic rule measure fixed position precision, fixed position of the certain in can controlling axial route to some of numerical control machine tool namely nods an error, should mirror instead the machine tool is long-term after be in at this o'clock in using by tens of thousands second the error that locates here, and we can measure limited number only when measure (general 5 ～ 7) . For real report this fixed position is nodded all round one group nods distributinging limits of group of fixed position error dispersedly randomly, we used method of data processing of error statistic pattern. For example, recommend 3 σ of ± to come loose by old ISO standard poor processing method, we will measure precision of coordinate of the certain on machine tool of a machining center, if pursue,1 is shown. Graph curve of precision of 1 fixed position if we locate to among them some the dot is in, oppose directional approach dot of this fixed position, locate 7 times (N=7) , data of actual measurement of its every time is as follows: + 4 μ M, + 2 μ M, + 1 μ M, 0, - 1 μ M, - 2 μ M, - 4 μ M. Press ISO standard regulation, dot of this fixed position comes loose poor average is △ Xn=0, its come loose differ 3 σ to be about 7.

9 μ M. If the graph is shown 2 times,error of this bits of fixed position distributings. Graph distributinging range chart of error of 2 fixed position 2 show, on dot of this fixed position, when direction of positive and negative locates repeatedly, will have 99.

The possibility of 96% is in 3 σ of ± =15.

Less than of limits of 8 μ M. 5 σ set inside German VDI standard, will get bigger than this error. Accordingly, this repeats fixed position precision for 15.

8 μ M. From 1998 since, begin to try on international run new standard, by 4 σ processing will get precision is repetition fixed position 10.

5 μ M, but this algorithm mirrorred 95% or so fixed position to nod limits. Handle afore-mentioned cases by Japanese JIS standard, will get precision is ± repetition fixed position 4 μ M. See from here, JIS standard claimed accuracy is the most loose, and VDI standard requirement is most strict. The curve of fixed position precision that the graph shows 1 times, it is to use the fixed position error that a chain of fixed position nods inside whole journey to wrap line of subsidiary channels in the human body through which vital energy to make range of precision of whole journey fixed position actually. Be aimed at numerical control machine tool now, measure fixed position precision and repeat fixed position precision to use laser range finder commonly, the work out measures athletic program, allow part of machine tool motion every are removed (50 ～ 100) Mm shift is nodded, reciprocate 5 ～ 7, the computer application software that is linked together with range finder meets those who handle each level detect result. From precision of machine tool fixed position but the achieves possibly precision when estimation gives this machine tool to machine, if on odd axis shift machines the pitch of two aperture,precision makes an appointment with the axis that it is sheet to locate in this paragraphs the 1 ～ of the error 2 times (closely related specific error value and craft element) . The geometrical precision photograph of the fixed position precision of the machine tool and this machine tool matchs, the machine tool of precision of exact form fixed position asks the geometrical precision of this machine tool also cannot under congener coordinate boring machine. There is the treatment of typical spare parts that a few users produce to batch now, offer the assessment of coefficient of equipment technology ability, requirement CPK is worth > 1.

1 ～ 1.

33, this is precision of requirement machine tool essentially difference of allow of precision of opposite spare parts should have enough precision reserve, such ability satisfy batch to produce treatment precision stability to ask. Taller to locating precision asks machine tool, the feed servo that must pay close attention to it is to use means of half closed circuit, still be means of complete closed circuit, the precision that must pay close attention to use detecting element reachs stability. The precision stability when the machine tool uses means of drive of servo of half closed circuit should be affected by element of a few outsides, for example, because of working temperature change causes change of length of ball guide screw in transmission catenary, this makes workbench locates the position produces drift effect actually necessarily, affect the treatment precision that adds workpiece then. Graph 3 it is current the general composition of commonly used feed drive catenary. Below means of half closed-loop control, positional detecting element is put in other one aspect of the matter of servo electric machinery. Position of axial of ball guide screw basically relies on one aspect of the matter to secure, another end is OK and free stretchy, when guide screw is stretchy workbench is measured with respect to an add shift. Be in a few new-style in on machine tool of small numerical control, use reduce slideway negative charge (with linear scroll slideway) , raise guide screw to make precision, guide screw two end are added beforehand drawing and guide screw center connect constant temperature oil the measure such as refrigeration, also got stabler fixed position precision in system of half closed circuit. Graph catenary of drive of feed of machine tool of 3 numerical control (half closed circuit) CNC Milling