Graphite electrode thread measuring principle & design ideas

  In recent years, the use of thread gauges has gradually become popular in the machining of graphite electrodes. However, because most of the existing thread gauges are imported products and are expensive, most small and medium-sized carbon companies still use the original limit gauges as the main control method of the graphite electrode processing.

  Kaifeng Carbon Co.,Ltd., China Pingmei Shenma Group in 2012, a set of N/S317T4 and N/S374T4 thread gauges were purchased from Kuroda Seiko (Japan) and DECOM (Germany). The use analysis can effectively improve the accuracy of product processing control compared to the original limit gauges. However, with the expansion of the company's business, the coverage of product specifications has gradually expanded, beyond the control of the original gauge. The company in order to ensure the level of processing control and based on the consideration of cost control, after digestion and absorption of imported thread gauge technology, it has successfully developed thread gauge products that are similar to imported products and can meet actual use requirements.

  This article briefly describes the design principle and manufacturing process of thread gauges.

1 Measuring principle of thread gauge

  The essence of the thread gauge is actually a set of standard blocks with a known diameter that can be in close contact with the measured surface. When the thread gauge is used to measure the parameters of the graphite electrode, it can be calculated by comparing the relative positions of the same set of standard blocks The taper and diameter of the graphite electrode to be measured.

2 Design ideas

  The key point of the thread gauge is to determine the diameter parameter of the reference surface of the gauge itself, but due to the particularity of the graphite electrode connection method, the equivalent diameter at a certain position on the tapered connection thread cannot be directly measured. According to the JIS7202—2008 standard and the detection method of the thread room of the Chinese Academy of Metrology, a three-point measuring instrument can be used to measure and calculate the taper thread taper and the equivalent diameter of the reference surface, thus the equivalent diameter of the screw at a certain distance from the datum is calculated, but in the actual production process, this method cannot be used to control the processing parameters of tapered threads online.

  In order to avoid the problem that the taper thread is difficult to directly measure, the taper thread can be used to decompose the taper thread and then measured separately, thereby indirectly obtaining the diameter parameter of the taper thread.

  According to the characteristics of the taper thread, it is divided into two types: the inner taper thread of the graphite electrode body hole and the outer taper thread of the graphite electrode nipple.

2.1 Diameter decomposition of inner taper thread

  As shown in Figure 1, the diameter at a certain point on the inner taper thread can be decomposed into two parts: the distance between the thread tooth tips (small diameter) and the thread tooth height.

2.2 Measurement of the minor diameter of the inner taper thread

  The minor diameter of the inner taper thread can be obtained by comparing the height difference of the end surface with the standard gauge block of known diameter and taper. It requires that the taper of the standard gauge block should be greater than the measured taper of the inner taper thread. Based on experience and for the convenience of calculation, the taper of the standard gauge block can take the standard taper of 1:3. The details are shown in Figure 2.

  The diameter of the gauge block in the figure can be measured with an outside micrometer with a measurement error of ±0.001 mm; the end face height difference can be measured with a dial gauge to push and pull the flatness of the plane, with a measurement error of ±0.001 mm.

  It should be noted that the end face of the gauge block must be higher than the end face of the inner taper thread to be measured, otherwise the taper of the inner taper thread to be measured should be used when considering the influence of the taper in the calculation.

  At this time, the calculation of the minor diameter of the inner taper thread satisfies the following formula:

  Inner taper thread minor diameter = Gauge block diameter-Height difference/3

2.3 Measurement of tooth height of inner taper thread

  The tooth height of the inner taper thread can be obtained by comparing the tooth height meter with the standard tooth profile, but the measurement result obtained in this way has a large error and is affected by the operator's level of operation, so it is not repeatable. Therefore, Kaifeng Carbon Company uses a universal tool microscope to measure the tooth height of thread processing tools. When the thread processing tool has a complete cutting surface, the thread tooth height processed by it can be regarded as the tooth height of the processing tool.

  It should be noted that when using a universal tool microscope to measure a thread machining tool, the required measurement parameters should include the straightness of the tool to ensure the straightness of the tooth top and bottom of the inner taper thread.

2.4 Measuring the diameter of the inner taper thread

  According to the above steps, the diameter of the inner taper thread can be calculated according to the following formulas.

  Inner taper thread minor diameter = Gauge block diameter-Height difference/3

Inner taper thread pitch = Inner taper thread minor diameter + Full tooth height

Major diameter of inner taper thread = Minor diameter of inner taper thread + Full tooth height×2

  When the thread cutter is fixed, the minor diameter, middle diameter and major diameter of the inner taper thread can be regarded as a unified parameter. 

2.5 Measurement of outer taper thread diameter

  When the corresponding inner taper thread diameter parameters are known, the end face diameter of the corresponding outer taper thread can be calculated by the method of screwing and comparing the height difference, as shown in Figure 3.

It should be noted that when using this method to measure the height difference and calculate the diameter of the outer taper thread, the influence of the taper of the outer taper thread must be considered, but because the taper is an unknown parameter, the height difference can be zeroed, so as to minimize the impact of the taper on the calculation results.