Preparation before acceptance According to the definition of mechanical capability, the influence of non-machine factors should be reduced as much as possible before acceptance. For example, the initial wear of the tool, the measurement of the gauge and the measuring instrument are not determined, the processing technology is optimized, and the appropriate fixture and cooling medium are selected. Here, an electronic inner diameter measuring instrument is taken as an example to highlight the measurement of the unreliability of the measuring instrument.

The measuring instrument consists of a precision measuring head, a mandrel, an electronic reading meter and a calibration gauge. The measured hole is á36.75mm. It is required to measure twice at the same measuring point and sample 24 pieces. The test pieces are numbered sequentially and will be 24 pieces. The magnitude is divided into 3 groups of 8 magnitudes and data.

First, find the range R$ of each group, and then find the range average R$ of the three groups, and calculate the dispersion degree SR$(Lm) according to the range method. After the measurement, the measurement of the meter is not The standard deviation of reliability, SRGer, where dn is the range factor, is the empirical value, which is related to the number of magnitudes in each batch of sampled values, generally using values.

∑R$=6.2R$=∑R$3=2.07(Lm)SR$=R$dn=2.072.847=0.727(Lm)SRGer=SR$2=0.7272=0.514(Lm) Finally, measurement unreliability from work The dispersion is subtracted. The processing method is SR=S2R-S2RGer where SR is subtracted from the standard deviation of measurement unreliability, LmS2R? is calculated from the measured data, and LmSRGer? is used to measure the standard deviation of unreliability.

The corrected average range Rkorr=∑RjKorrm=21.7610=2.176(Lm) The corrected standard deviation SR=RKorrdn=2.1762.326=0.936(Lm) The standard deviation of the measurement unreliability SRGer=0.514(Lm) The standard deviation after the measurement unreliability of the measuring instrument SR=S2R-S2RGer=0.782(Lm) It is seen by the X-graph that the tool wear caused by the continuous machining of 50 parts is an unavoidable system deviation TN=7Lm, this deviation It is caused by factors other than the machine tool (mechanical). It should be excluded according to the definition of mechanical capability. The data in the middle is calculated according to XiKorr=xi-(i-1)Ts, i=1~50. It eliminates the influence of tool wear and then obtains the corrected standard deviation SR. The measurement unreliability of the same meter should also be excluded. Finally, we obtain a standard deviation SR=0.782Lm after removing the main non-machine factors. Therefore, the mechanical capacity coefficient of the machine tool when machining the part is obtained. Cmk=X-uT4S=9.9-04×0.782=3.16 According to the calculation result of the above formula, we can judge that the machine tool has sufficient mechanical capacity when processing the above parts.

As mentioned above, the mechanical capacity coefficient has two kinds of Cmk value and Cm value. By calculating the formula, it can be seen that the magnitude of the Cmk value is not only related to the standard deviation S but also related to the X value. If the control is not taken care of, it is likely that the Cm value is very good when calculated by Cm=(OT-uT)/8S. However, the Cmk value is poor when calculated by Cmk=(OT-X)/4S or Cmk=(X-uT)/4S. We also take the part used above as an example. If the standard deviation is the same, only the cutting data is offset by 5Lm, that is, the first number is 14→9, the second number is 13→8, and the 49th. From 10→5, the 50th number is from 6→1, then we get X=4.9, at this time Cmk=(4.9-0)/(4×0.8)=1.53, and Cm=(OT-uT ) / 8S = (25-0) / (8 × 0.8) ≈ 3.9. This phenomenon is often present in actual work and should be avoided.

(Finish)

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