Research on the machining of deep and small holes by small hole EDM drill machine with the ratio of depth to diameter greater than 1:100

The processing of deep and small holes with super large depth-to-diameter ratio has always been a difficult problem in the manufacturing industry. For small hole EDM drill machine, it is more difficult to process deep and small holes with a depth-to-diameter ratio of more than 100:1, especially those with a diameter of less than 1.0 mm. EDM deep and small hole machining is one of the more mature machining methods. Through a series of experiments (tests on brass/copper electrode tube), this paper expounds the feasibility of electric machining deep holes, making it possible to electric machining deep small holes and guaranteeing certain technical conditions.

1. INTRODUCTION

At present, the depth-diameter ratio of the injection holes of many EDM nozzle parts exceeds 100:1, and the diameter of the injection holes is relatively small (generally around φ0.1-φ1.0mm). When processing deep holes on a general lathe or drilling machine, the drill bit often floats and wears quickly; when using the drilling method to process small holes, the small drill bit is easy to break. EDM drilling has thus become a common alternative to making small and deep holes.

However, During EDM drill processing, a hardened layer is formed on the workpiece surface, and the surface roughness is poor, and the phenomenon of taper aperture often occurs when EDM drill deep holes are used. EDM holes need to be ground to improve their surface quality. In order to solve the problems caused by EDM, special research on processing technology is carried out.

2. EXPERIMENT

Electrode tube: TAGUTI brass electrode tube 1.0*400mm

Workpiece material: 1Cr11Ni2W2MoV

Total length of deep hole: 123.5mm

The experimental research on the electric discharge machining process plan is mainly carried out, in order to test and improve the electric machining plan.Increasing the depth-to-diameter ratio of small hole machining, improving the verticality of the machined hole, reducing the roughness, and improving chip removal, which can make the process plan for deep small hole machining by electric machining feasible. In addition, the processing and positioning method that can only be centered and punched by sight is improved, so that the equipment can automatically align and punch holes, and finally ensure that the equipment can process small holes with a depth of more than 100mm and a diameter of 1.0mm, and also make the cylindricity of the entrance and exit of the hole within 0.3mm.

In view of the special requirements of deep and small hole machining, based on the idea of improving hole machining accuracy, reducing electrode loss, and taking into account machining efficiency, chrome-copper alloy is the best choice; considering the machining accuracy and cost, we choose to use the common TAGUTI brass electrode tube in this experiment.

This experiment, LK-018A CNC EDM - small hole machining machine -  which is produced by TAGUTI INDUSTRIAL LIMITED is selected for processing. The equipment can process deep and small holes on workpieces for various conductive materials such as stainless steel, hardened steel, cemented carbide, copper, and aluminum. The processing hole diameter is between φ0.15-φ3.2mm.

The original CNC EDM small hole machining machine has only one flat circular worktable and no positioning system. Through the exploration of the clamping method, the method of using the center hole of the center of the worktable to perform the clamping datum is determined. Depending on the construction of the part, indirect positioning can be done by using a small chuck to connect to the device.

The principle of EDM is to remove excess metal based on the electro-erosion phenomenon when pulsed spark discharge is generated between the workpiece electrode and the tool electrode, so as to achieve the predetermined processing requirements for the size, shape and surface quality of the part. In order to achieve the purpose of metal corrosion through the phenomenon of electrical corrosion, the following basic conditions must be met:

a. It is necessary to keep a certain small discharge gap between the workpiece electrode and the tool electrode. If the gap is too large, the inter-electrode voltage cannot break down the inter-electrode medium to form a spark discharge; if the gap is too small, it is easy to short circuit, and there will be no spark discharge.

b. The spark discharge must be an instantaneous pulse discharge. After the discharge continues for a period of time, it needs to stop for a period of time. The discharge duration is generally 1 to 1000 μs, the pause is about 50 to 100 μs. In this way, the heat generated by the discharge can not be conducted and diffused to the rest part, and each discharge erosion point is limited to a small range.

c. Spark discharge must be carried out in a liquid medium with certain insulating properties, such as kerosene, saponified liquid or deionized water. Liquid media, also known as working fluids, must have high dielectric strength to facilitate the generation of pulsed spark discharges. At the same time, the liquid medium can also suspend and remove the galvanic corrosion products such as debris and carbon black generated during the spark machining process from the discharge gap, andalso has a good cooling effect on the surface of the electrode and the workpiece.

d. Using the hollow tubular electrode, and the high-pressure liquid to flush away the machining chips. The high-pressure flowing working liquid flows outfrom the hole wall of the small hole, so that the tool electrode tube is "suspended" in the center of the hole, which is not easy to cause short circuit, and can process straightness and roundness.

After analyzing the principle of electric machining test and the process of electric machining, the main reason for the above problems is due to the limitation of the external guide of the hole. When the machining depth exceeds a certain proportion, the hole begins to bend or show a bell mouth, which reduces the machining efficiency, and make it difficult to continue processing. As the processing continues, the depth of the hole increases, the length of the cantilever of the electrode tube increases, the stiffness of the electrode tube is insufficient, and vibration and jitter occur under the flow of the pressure medium. It can be seen that to improve the depth and accuracy of processing, it is impossible to rely solely on the external guidance of the hole, new methods and measures must be taken to make new breakthroughs in deep and small hole processing.

Regarding the problem of the metal core in the machined hole, we carried out the test of the dry running program. The machined workpiece was directly reprocessed 2 times along the original hole with the electrode, which effectively solved the problem of the metal core.

Regarding the problem that the taper of the deep hole is too large, in order to make the rigidity of the working part of the small electrode tube  independent of the machining depth, the electrode tube must be supported by the machined hole. Using the machined hole wall surface as the guide surface is undoubtedly the best choice. 

To guide the electrode with the machined hole wall, the following issues must be addressed: a) Insulation between electrode and workpiece; b) Slag removal; c) Circulation of spark fluid; d) Wear-resistant insulating layer.

Considering the characteristics of deep and small hole EDM without macroscopic force, it is necessary to choose an insulating material to realize the self-guidance in the hole of the deep and small hole machining electrode. The method we use is to use liquid material to coat the surface of the electrode to form an insulating layer and support it at the same time. At present, there are relatively few existing materials, and liquid glue is temporarily used, and the parts are processed after they are completely dried.In terms of aperture, certain effect has been achieved, and the diameter difference of the entrance and exit is controlled within 0.3mm, which has been further improved.