Method and apparatus for creating a starting hole for milling in a surface of a workpiece by a CNC milling machine

Abstract

A starting hole for milling in a surface of a workpiece by a CNC milling machine with a milling cutter mounted to a rotating spindle is created by milling a first hole at a first diameter to a first depth into the workpiece along the axis of the starting hole; and then successively milling a second hole at a second diameter to a second depth into the workpiece along the axis of the starting hole, wherein the second diameter is smaller than the first diameter. One or more additional holes may be successively milled to additional depths at successively smaller diameters.

Claims

1. A method of creating a starting hole for milling in a surface of a workpiece by a CNC milling machine with a milling cutter mounted to a rotating spindle, the method comprising: (a) milling a first hole at a first diameter to a first depth into the workpiece along the axis of the starting hole; and (b) successively milling a second hole at a second diameter to a second depth into the workpiece along the axis of the starting hole, wherein the second diameter is smaller than the first diameter, wherein the starting hole always includes milling the first hole and the second hole, and wherein the diameter of the milling cutter is less than the diameter of the first hole and the second hole.

2. The method of claim 1 wherein the CNC milling machine has a milling cutter, and wherein the first depth is about the diameter of the milling cutter.

3. The method of claim 2 wherein the starting hole is milled to a depth of at least twice the diameter of the milling cutter.

4. The method of claim 1 further comprising: (c) successively milling one or more additional holes to additional depths at successively smaller diameters.

5. The method of claim 4 wherein the successively milled holes have successively smaller depths.

6. The method of claim 1 wherein a ledge is formed in the workpiece at a boundary between the first hole and the second hole.

7. The method of claim 6 wherein the milling cutter creates chips when forming the starting hole, and wherein the ledge has a width greater than a thickness of the chips.

8. The method of claim 1 wherein the second hole is concentric with the first hole.

9. The method of claim 1 wherein the CNC milling machine is a vertical milling machine.

10. The method of claim 1 wherein the second depth is smaller than the first depth.

11. An apparatus for generating control code for a CNC milling machine such that when the control code is executed in the CNC milling machine, a milling cutter mounted to a rotating spindle of the CNC milling machine forms in a workpiece a starting hole for milling in the surface of the workpiece, the apparatus comprising a processor programmed to generate control code that: (a) mills a first hole at a first diameter to a first depth into the workpiece along the axis of the starting hole; and (b) successively mills a second hole at a second diameter to a second depth into the workpiece along the axis of the starting hole, wherein the second diameter is smaller than the first diameter, wherein the starting hole always includes milling the first hole and the second hole, and wherein the diameter of the milling cutter is less than the diameter of the first hole and the second hole.

12. The apparatus of claim 11 wherein the CNC milling machine has a milling cutter, and wherein the first depth is about the diameter of the milling cutter.

13. The apparatus of claim 12 wherein the starting hole is milled to a depth of at least twice the diameter of the milling cutter.

14. The apparatus of claim 11 wherein the processor is further programmed to generate control code that: (c) successively mills one or more additional holes to additional depths at successively smaller diameters.

15. The apparatus of claim 14 wherein the successively milled holes have successively smaller depths.

16. The apparatus of claim 11 wherein a ledge is formed in the workpiece at a boundary between the first hole and the second hole.

17. The apparatus of claim 16 wherein the milling cutter creates chips when forming the starting hole, and wherein the ledge has a width greater than a thickness of the chips.

18. The apparatus of claim 11 wherein the second hole is concentric with the first hole.

19. The apparatus of claim 11 wherein the CNC milling machine is a vertical milling machine.

20. The apparatus of claim 11 wherein the second depth is smaller than the first depth.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

(2) In the drawings:

(3) FIG. 1 is a diagram of a tool path for milling a starting hole in a workpiece in accordance with a preferred embodiment of the invention.

(4) FIG. 2 is a flowchart showing one preferred embodiment of the present invention.

(5) FIG. 3 is a schematic diagram showing one preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) Certain terminology is used in the following description for convenience only and is not limiting. In this application, the term vertical refers to a direction parallel to the axis of rotation of the milling cutter, collet and spindle of a CNC machine and the term horizontal refers to a direction normal to the axis of rotation of the milling cutter regardless of the actual orientation of the CNC machine. In terms of a Cartesian coordinate system, the Z direction is parallel to the vertical plane and the X and Y directions are parallel to the horizontal plane. The term descending refers to movement in the Z direction. The terminology includes the above-listed words, derivatives thereof and words of similar import.

(7) In view of the problems identified in the Background Section, the following experiments were conducted:

(8) EXPERIMENT 1

(9) Starting holes were milled in a titanium workpiece with a one-half inch diameter end mill using a circular helix tool path of constant diameter slightly less than the diameter of the end mill. A rumbling of the milling machine was observed to occur when the depth of the milling cutter in the workpiece exceeded the diameter of the end-mill. It was determined that the rumbling was the result of the chips cut by the milling cutter being re-cut due to the chips not being fully evacuated from the starting hole as the milling cutter descended into the workpiece.

(10) EXPERIMENT 2

(11) Starting holes were milled in a titanium workpiece with a one-half inch diameter end mill using a circular helix tool path of smoothly decreasing radius, with the notion that the chips would be evacuated from the hole due to the diameter of the hole above the actual cutting plane at the end of the end mill being large compared to the diameter of the hole at the cutting plane. It was determined that a starting hole cut with a circular helix tool path of smoothly decreasing radius did not allow for the starting hole to be cut substantially deeper than a hole cut using a helix tool path of constant radius before chip re-cutting occurred.

(12) EXPERIMENT 3

(13) Starting holes were milled using a one-half inch diameter end mill. A first hole was milled at a first diameter to a depth of about the diameter of the milling cutter using a circular helix tool path. The starting hole was made deeper using the milling cutter to mill successive concentric holes, each having a smaller diameter than the preceding hole. The amount that each successive hole was made smaller was varied to determine if a hole could be milled to a depth of twice the diameter of the milling cutter without substantial re-cutting of the chips. It was determined that if the diameter of each successive hole was made smaller by about twice the diameter of the chip thickness, successive holes of deceasing depth could be milled such that the starting hole could have a depth of at least twice the diameter of the milling cutter without substantial re-cutting of the chips. The results obtained in Experiment 3 are shown in

(14) Table 1.

(15) TABLE-US-00001 TABLE 1 Material Cutter 1.sup.st 2.sup.nd 3.sup.rd Total Diameter Depth Step-in Depth Step-in Depth Depth Titanium 0.5 in 0.55 in .005 in .33 in .005 in .22 in 1.1 in

(16) Referring now to FIG. 1, there is shown a tool path 10 of a milling cutter cutting a starting hole 22 in a workpiece 1 according to a preferred embodiment of the invention. The tool path 10 is generated by: (1) generating with a computer, a first plurality of coordinate values x.sub.i, y.sub.i, z.sub.i 14a, where the first coordinate values 14a represent a plurality of first connected line segments 18a, 1 to n, which surround a vertical axis 12, which gradually descend in the z direction to a first predetermined depth; (2) generating with a computer, a second plurality of coordinate values x.sub.j, y.sub.j, z.sub.j 14b, where the second coordinate values 14b represent a plurality of second connected line segments 18b, n+1 to n+m, which surround the vertical axis 12, and which gradually descend in the z direction to a second predetermined depth, and where the second predetermined depth is greater than the first predetermined depth. The tool path 10 is such that when the first coordinate values 14a and the second coordinate values 14b are converted to a code for controlling a CNC machine, and when the code is executed in the CNC machine, a milling cutter mounted to a spindle of the CNC machine forms in the workpiece 1, a first hole 22a based on the first plurality of coordinate values 14a and a second hole 22b based on the second plurality of coordinate values 14b such that a ledge 20 having a width preferably greater than a chip thickness is formed in the workpiece at a boundary of the first hole 22a and the second hole 22b.

(17) The chip thickness resulting from a helix tool path is a function of the pitch of the helix, the feed rate of the milling cutter and the rotational speed of the milling cutter and is determinable by known methods such as that described in the paper, Calculations Of Chip Thickness And Cutting Forces In Flexible End Milling, M. Wan and W. H. Zhang, International Journal of Advanced Manufacturing Technology (2006) 29: 637-647, which is hereby incorporated in its entirety.

(18) In the preferred embodiment, the first coordinate values 14a and the second coordinate values 14b are determined by the equations of a helix, where the first coordinate values, x.sub.i, y.sub.i, and z.sub.i, correspond to a first curve in space in accordance with:
x.sub.i=a cos t.sub.i+x.sub.0
y.sub.i=b sin t.sub.i+y.sub.0
z.sub.i=c.sub.1t.sub.i+z.sub.0; and

(19) the second coordinate values, x.sub.j, y.sub.j, z.sub.j, correspond to a second curve in space in accordance with:
x.sub.j=(as)cos t.sub.j+x.sub.0
y.sub.j=(bs)sin t.sub.j+y.sub.0
z.sub.j=(2pc.sub.1+c.sub.2t.sub.j)+z.sub.0,

(20) where 0t.sub.i2p, 0t.sub.j2p, 0sa, b, and s is greater than the chip thickness.

(21) However, the first and the second coordinate values 14a, 14b need not be computed from the equations of a helix as long as the coordinate values 14a, 14b result in smoothly connected line segments 18a, 18b which gradually descend in the direction of the Z axis.

(22) In the preferred embodiment, the first connected line segments 18a and the second connected line segments 18b are such that they form holes of circular cross section, as would be the result of the parameter a being equal to the parameter b. However, in other embodiments, the first and the second holes 22a, 22b need not be of circular cross section. For example, if the parameter a was not equal to the parameter b, the cross section of each hole 22a, 22b would be an ellipse.

(23) Also, the first and the second coordinate values 14a, 14b and the resulting line segments 18a, 18b need not be exclusively arcs. In another preferred embodiment, each line segment 18a, 18b comprises two arcs, and two straight lines which connect the ith line segment with the i+1 line segment and the i1 line segment so as to form a slot like cross-section of each hole 22a, 22b.

(24) Further, the cross-section of the first and the second holes 22a, 22b need not be geometrically similar. That is for example, the first hole 22a could be circular in cross-section and the second hole 22b could be triangular in cross-section. Such configuration would be suitable provided that at least a portion of the ledge 20 was greater than a chip thickness.

(25) While in the preferred embodiment, the number of successive holes 22a, 22b whose cross-section dimensions successively decrease are two, the number of holes having a decreasing cross-section dimension is not limited to two holes 22a, 22b but could be three, as for example in experiment 3, or could be greater than three.

(26) In the preferred embodiment, the depth of the first hole 22a is less than the diameter of the milling cutter, and the diameter of the second hole 22b and each succeeding hole (not shown) is made successively smaller. However, in some cases, the depth of the first hole 22a could be larger than the diameter of the milling cutter depending on type of material, the type of the end mill, the surface speed of the end mill flutes and the chip load per tooth. In practice, the optimum depth of the first hole 22a, the second hole 22b, and each succeeding hole would be determined by a machinist performing a test cut or cuts in the material to be used for the workpiece.

(27) FIG. 2 shows a self-explanatory flowchart of the steps for performing one preferred embodiment of the present invention.

(28) FIG. 3 shows a self-explanatory schematic diagram of an apparatus for performing one preferred embodiment of the present invention.

(29) Preferably the computer used for generating the control code is a programmable type of computer of a kind commonly called a personal computer. Preferably, the computer employs one or more arithmetic processor chips, a random access memory, non-volatile memory such as semiconductor read only memory, a hard disk, removable read/write memory drives such as a floppy disk drive and/or CD disk drive, a paper tape and/or a magnetic tape drive, a keyboard, a mouse, and a video display. Preferably, the computer utilizes the Windows software operating system manufactured by. Microsoft Corporation.

(30) Preferably, the code for execution in the CNC machine is transferred from the computer to the CNC machine using one of any well known wire or wireless interface standards. Alternatively, the code may be recorded on a removable media such as a floppy disk, a CD disk, a flash memory stick, a magnetic tape or a paper tape, for transfer to the CNC machine. However, the computer program is not required to be generated by the aforementioned hardware and software environment. Alternatively, for example, the computer program for generating the code for the CNC machine could be generated within the computer of the CNC machine.

(31) It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.