2.5-axis CNC EDM milling

EDM CNC milling machining is a new type of EDM machining process. This process uses a simple shape of the tool electrode, with the work table and spindle multi-axis CNC servo machining parts. With no macro cutting force. The electrode is simple to manufacture and the process preparation cycle is short. The advantages of low cost and easy implementation of flexible production are effective ways to realize EDM forming technology for product parts.
In the X, Y and Z coordinates of the 2.5-axis CNC EDM machine tool, any two coordinates can be used as linkage interpolation motion, and the third coordinate can be used as a single cycle feed motion. It can use simple tool electrodes to simulate Milling plane contour curves and simple solid surfaces. In actual production, the 2.5-axis type of CNC EDM machines owned by the company accounts for a large proportion, but at present these types of machines are still generally used “copy-form EDM” methods for machining flat cavities or Simple surface parts, equipment performance has not been fully utilized. Researching the use of 2.5-axis CNC EDM machine for imitating milling processing is undoubtedly important for excavating the potential of the equipment and improving the efficiency of the enterprise.

1 electrode design

One of the advantages of using the EDM milling process is that the electrodes are extremely simple to design and manufacture. For the processing of contour parts, cylindrical rod electrodes are usually used, as shown in Fig. 1(a). For machining three-dimensional curved parts, ball-head electrodes are usually used, as shown in Fig. 1(b). EDM milling machining is non-contact machining and has no macro cutting force. Therefore, the rigidity of the electrode is not high. For pre-machined workpieces with small residual amounts, direct machining can be performed directly. The electrode sizes L, D, and R are determined based on the workpiece's process dimensions. If no interference occurs, the L value should be as small as possible. There may be large D (or R) values.

Figure 1 Electrospin milling machining electrode

Preventing interference should follow the following principles:
L Hmax+Mmin+Gmax+Bmin
R rmin-Gmax
D Wmin-2Gmax-T
Where Mmin-electrode minimum clamping length;
Hmax - the maximum depth of the workpiece;
Gmax—the maximum discharge gap between the electrode and the workpiece;
Bmin—Minimum height of chip removal space;
Rmin—minimum radius of curvature of the machining direction;
Wmin - the narrowest width of the cavity;
T—safety constant (T=2 Gmax).

2 Pre-machining of workpieces
At present, the machining speed of EDM is still far from the machining speed of machining. Therefore, in order to shorten the electrical machining time and reduce the absolute loss of the electrode, it is necessary to perform pre-machining on the workpiece with a large blank amount before the heat treatment, so that the workpiece has an appropriate machining allowance before electrical machining. In principle, the smaller the better the electrical machining allowance can be, the better the discharge machining can be guaranteed. In general, the unilateral allowance for EDM milling machining may be 0.1 to 0.5 mm. For workpieces with small blanks or difficult pre-machining due to complex shapes, EDM milling can also be performed directly.

3 Electrode motion trajectory generation

3.1 Generating the trajectory of the electrode of the contoured part in the mimetic milling process. For a planar contour part without premachining, the typical process of the EDM milling process is to reserve the machining allowance along the contour boundary, and then use the flat cylindrical electrode to cut or line first. Cutting the knife, removing the excess material and finally cutting along the contour, finishing the contour of the boundary, for the workpiece with the appropriate machining allowance after the pre-machining, then directly along the contour of the knife for finishing. For the final finishing of the roots, the diameter D of the cylindrical electrode used shall comply with the requirements of the parts drawing. as shown in picture 2.

(a) Part model

(b) Rough machining electrode track

(c) Finishing electrode tracks

Fig. 2 The strength of spark contour milling in the outline parts

The tool radius compensation value should be calculated using the following formula:

H=r+

In the formula r-electrode radius;
- Discharge gap.

3.2 Facsimile Machining Simple Electrode Motion Trajectories of 3D Surface Parts Using the 2.5-axis CNC EDM machine to machine simple trigonal curved surface parts using the method of layered cutting. That is, a set of planes perpendicular to the axis of rotation of the electrode intersects the part surface and the blank body, and the intersection line obtained is processed using the planar row cavity method to calculate the electrode movement trajectory of the current layer. In the roughing process, the flat head cylindrical electrode is used for tool position calculation and processing. In the finish machining, the ball head cylindrical electrode is used for tool position calculation and processing, as shown in FIG. 3 .

(a) Layered processing (b) A layer of finishing tool tracks

Fig. 3 Spherical layered processing method

4 About electrode loss

For workpieces that are not pre-machined, coarse and medium machining must be performed first to prepare the proper machining allowance for the final finishing. The electrode loss is not compensated during the coarse and medium processing stages. Narrow pulse widths are used for workpieces with appropriate machining allowances after premachining (less than in Fig. 2 Planar contour parts EDM milling example 20 s), a small discharge current (less than 10A) directly finishing, because the discharge energy becomes smaller, the discharge gap becomes smaller, but the EDM milling process uses a rotating tool electrode, the electrode relative to the workpiece for interpolation motion, chip removal The conditions will not be significantly worse than this, ensuring stable processing. The existing process experiments show that the absolute value of the tool electrode at the finishing stage is very small and generally does not require compensation. If a small absolute loss still does not meet the machining accuracy requirement, the return is corrected when the electrode loss occurs. Reset the Z-axis height and restart milling. These operations can be set in the milling process. If the automatic exchange electrode device is used, the operation can be fully automated.

5 Calculation Examples

Figure 2 (a) shows an example of EDM milling of a mold part in a factory. The Archimedes spiral profile processed is not pre-processed, and it needs to be divided into two processes of rough machining and fine machining, and the unilateral allowance for finishing is 0.2 mm. The workpiece material is Cr12, the hardness is HRC50-55, the tool electrode material is copper, the diameters of the rough and finishing electrodes are all D=8mm, L=40mm, and the electrode rotation speed is 400r/min. Rough motion of the electrode is shown in Figure 2(b). The trajectory of the finishing electrode is shown in Figure 2(c). Mitsubishi M25C6G15 type CNC EDM machine processing, processing time 39h, including rough machining 11h finishing 28h, NC program is as follows q
G92 XYZC;
G90;
M88;
M80;
G10 F150;
E9958;
M84;

G00 Z2.0;
G00 X-15.349 Y28.570; (STP)
G00 Z-16.620;
G01 X-10.307 Y24.187;(V)
G01 X-5.265 Y19.804; (W)
G02 X3.799 Y21.780 R21.780; (X)

G02 X27.261Y6.390R25.579;(Y)
G02 X27.261 Y6.390 R25.579; (Y)
G03 X-10.307 Y24.187 R28.0; (V)
G01 X-5.265Y19.804;(W)
G01 X-7.805 Y2.527; (Q)
G03 X-8.077 Y0.0 R11.876;(R)

G03 X0.0 Y-8.077 R8.077;(S)
G03 X4.278 Y-3.799 R4.278;(T)
G03 X4.077 Y-3.397 R0.278; (U)
G02 X-7.805 Y2.527 R10.0; (Q)
G01 X-5.265 Y19.804; (W)
G01 X-12.685 Y29.737; (Z)

H01 = actual measured electrode radius Rt + roughing unilateral discharge gap
+ Finishing unilateral allowance
G42 X-12.685 Y29.737 H01; (Z) Right tool offset
G01 X-1.891 Y10.266;(P)
G01 X0.599 Y5.777;(A)
G03 X1.820 Y5.324 R1.0; (B)
G02 X3.799 Y5.680 R5.680; (C)

G02 X13.278 Y-3.799 R9.479; (D)
G02 X0.0Y-17.077 R13.278;(E)
G02 X-17.077 Y0.0 R17.077;(F)
G02 X-12.566 Y12.961 R20.876; (G)
G03 X-12.694 Y114.337 R1.0;(H)
G01 X-16.546 Y17.686; (I)
G03 X-17.974 Y17.559 R1.0;(J)
G03 X-24.177 Y0.0 R27.976; (K)
G03 X0.0 Y-24.177 R24.177; (L)
G03 X20.378 Y-3.799 R20.378; (M)
G03 X3.799 Y12.780 R16.579; (N)
G03 X-1.426 Y11.663 R12.780; (O)
G03 X-1.891 Y10.266 R24.177; (P)
G41 X-1.891 Y10.266 H01; (P) Left Cutter Compensation
G01 X-12.685 Y29.737; (Z)

E9960;
H02 = measured electrode radius Rt + finishing single-side discharge gap
G42 X-12.685 Y29.737 H01; (Z) Right tool offset
G01 X-1.891 Y10.266;(P)
G01 X0.599Y5.777;(A)
G03 X1.820 Y5.324 R1.0; (B)

G02 X3.799 Y5.680 R5.680; (C)
G02 X13.278Y-3.799 R9.479; (D)
G02 X0.0Y-17.077 R13.278;(E)
G02 X-17.077 Y0.0 R17.077;(F)
G02 X-12.566 Y12.961 R20.876; (G)
G03 X-12.694 Y114.337 R1.0;(H)

G01 X-16.546 Y17.686; (I)
G03 X-17.974 Y17.559 R1.0;(J)
G03 X-24.177 Y0.0 R27.976; (K)
G03 X0.0 Y-24.177 R24.177; (L)
G03 X20.378 Y-3.799 R20.378; (M)
G03 X3.799 Y12.780 R16.579; (N)

G03 X-1.426 Y11.663 R12.780; (O)
G03 X-1.891 Y10.266 R24.177; (P)
G41 X-1.891 Y10.266 H01; (P) Left tool offset
G01 X-12.685 Y29.737; (Z)

M85;
M81;
M89;
M02;
%

Figure 3(a) shows a semi-spherical part model. The workpiece material is 45 steel, the mechanical unilateral allowance is 0.2mm, the tool electrode material is copper, the electrode ball radius is R=8mm, L=50mm, and the electrode rotation speed is 400r/min. In layered processing, the trajectory of an electrode in a layer is shown in Figure 3(b). Mitsubishi M25C6G15 CNC EDM machine processing, processing time 6h, NC program is as follows.
G92 XYZC;
G90;
M88;
M80;
G10 F150;
E9960;
M84;

G01 Z-0.185;

G01 X1.378;
G01 Z-0.245;
G03 X-1.378 Y0.0R-1.378;
G03 X1.378 Y0.0R-1.378;

G01 X2.746;
G01 Z-0.425;
G03 X-2.746 Y0.0R-2.746;
G03 X2.746 Y0.0R-2.746;

G01 X4.093;
G01 Z-0.724;
G03 X-4.093 Y0.0R-4.093;
G03 X4.093Y0.0R-4.093;

G01 X5.409;
G01 Z-1.139;
G03 X-5.409 Y0.0R-5.409;
G03 X5.409 Y0.0R-5.409;

G01 X6.684;
G01 Z-1.667;
G03 X-6.684 Y0.0R-6.684;
G03 X6.684 Y0.0R-6.684;

G01 X7.908;
G01 Z-2.304;
G03 X-7.908 Y0.0R-7.908;
G03 X7.908 Y0.0R-7.908;

G01 X9.071;
G01 Z-3.045;
G03 X-9.071 Y0.0 R-9.071;
G03 X9.071Y0.0R-9.071;

G01 X10.166;
G01 Z-3.885;
G03 X-10.166 Y0.0 R-10.166;
G03 X10.166 Y0.0R-10.166;

G01 X11.183;
G01 Z-4.817;
G03 X-11.183 Y0.0 R-11.183;
G03 X11.183Y0.0R-11.183;

G01 X12.115;
G01 Z-5.834;
G03 X-12.115 Y0.0 R-12.115;
G03 X12.115 Y0.0R-12.115;

G01 X12.955;
G01 Z-6.929;
G03 X-12.955 Y0.0 R-12.955;
G03 X12.955 Y0.0R-12.955;

G01 X13.696;
G01 Z-8.093;
G03 X-13.696 Y0.0 R-13.696;
G03 X13.696 Y0.0R-13.696;

G01 X14.333;
G01 Z-9.316;
G03 X-14.333 Y0.0 R-14.333;
G03 X14.333 Y0.0R-14.333;

G01 X14.861;
G01 Z-10.591;
G03 X-14.861 Y0.0 R-14.861;
G03 X14.861 Y0.0R-14.861;

G01 X15.276;
G01 Z-11.907;
G03 X-15.276 Y0.0 R-15.276;
G03 X15.276 Y0.0R-15.276;

G01 X15.575;
G01 Z-13.254;
G03 X-15.575 Y0.0 R-15.575;
G03 X15.575 Y0.0R-15.575;

G01 X15.755;
G01 Z-14.622;
G03 X-15.755 Y0.0 R-15.755;
G03 X15.755 Y0.0 R-15.755;

G01 X15.815;
G01 Z-16.000;
G03 X-15.815 Y0.0 R-15.815;
G03 X15.815 Y0.0R-15.815;

G01 Z0.0;
G01 X0.0;
M85;
M81;
M89;
M02;
%
The workpiece and electrode after machining are shown in Figure 4.

Figure 4 Photo of the workpiece and electrode after processing

6 Conclusion

The use of 2.5 axis EDM CNC milling process, processing plane contour curve and simple three-dimensional surface, with good flexibility, wide application, tool electrode design and manufacturing simple, low cost advantages. These characteristics are even more pronounced when the shape and size of the object to be processed are changed or in small batches, multi-variety trial production and production. In general terms, as long as the NC program is properly programmed, the power standards are properly selected, and the discharge gap control is stable, the desired machining accuracy and surface quality can be obtained.