GB1578343A - Boring bars - Google Patents

Description

(54) BORING BARS (71) We, NATIONAL RESEARCH DE VELOPMENT CORPORATION, of P.O. Box 236, Kingsgate House, 66/74 Victoria Street, London SW1 6SL, a British Corporation, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - The present invention relates to overhung boring bars for accurately machining bores and in particular bores of non-uniform diameter along their length, for example stepped and tapered bores.

Problems arise in machining long bores since as the length of the boring bar is increased (for a given cross-section) the boring bar becomes increasingly prone to the build-up of chatter.

Thus conventional boring bars with diameters in the range 25 mm to 40 mm become unsatisfactory in this respect when their overhung ratio (i.e. ratio of the free length of the boring bar to its diameter) exceds 5: 1.

In one known attempt to increase the acceptable maximum overhung ratio, a boring bar was constructed with a tungsten carbide shank, which has a modulus of elasticity about three times greater than steel thus giving a boring bar of much greater rigidity than the previously known steel ones. Such a construction is expensive and was found to increase the maximum overhung ratio to about 7:1.

Furthermore, such a bar is brittle and thus fragile and so can only be expected to have a short life under practical machine-shop conditions.

Another arrangement is described by R.

S. Hahn in a paper entitled "Design of Lanchester Damper for Elimination of metal-cutting chateter" (A.S.M.E., 1951, p.

331). This paper described the inclusion, in a boring bar and adjacent its forward end, of cylindrical mass located, with a clearance, in a closed cavity within the bar. The clearance contains a gas such as air. Under vibratory conditions the assembly acted as a viscous-type Lanchester damper and if the clearance between the mass and the inner wall of the cavity was correctly chosen it successfully resisted the build-up of chatter in boring bars with overhung ratios up to about 7:1.

Further developments of boring bars including the damper proposed by Hahn are discussed in papers by Y. H. J. Au and R. W. New (Int. J. Prod. Res. 1974, Vol.

12, No. 2, 247-261) and K. W. Ng and R.

W. New (read at 3rd International Conference of Production Research, Amherst, Mass., U.S.A., August, 1975), both papers being published by Taylor & Francis Limited, 10-14, Macklin Street, London WC2B 5NF, England.

According to the present invention there is provided a boring bar comprising a unitary shank having a rearward end by which it can be mounted and, at its forward end, means for mounting a cutting tool and a damper, the damper comprising a damper mass positioned in a cavity with a damper clearance between the mass and the inner wall of the cavity, the shank having a solid portion adjacent its rearward end and a hollow tubular portion adjacent the damper and tool mounting means.

The boring bar may include, a second damper assembly which also consists of a closed cavity containing a second damper mass surrounded by a damping clearance, the tool holding means lying between the second damper assembly and the first mentioned damper assembly.

The second cavity, forward of the cutting tool, may be within a capsule which is detachable from the forward end of the bar in order that the second damper assembly may be removed should the boring bar be required to machine a bore, such as a blind hole, which would otherwise be fouled by the capsule.

A plurality of tools may be fitted to the boring bars of the invention.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which : Figure 1 is a longitudinal section of a boring bar; Figure la shows a detail of the forward end of the bar of Figure 1 on the line Ia of Figure 1; Figure 2 is a cross-section on the line 1I-II of Figure 1; and Figure 3 is a cross-section on the line III-III of Figure 1.

The drawings show a boring bar having a one-piece shank 1 with a diameter of 38 mm. carrying an annular mounting flange 3 which has bolt holes 4 (Fig. 2), the flange 3 being integral with the shank 1 and coaxial with both it and with a mounting spigot 5. The spigot 5 is adapted to fit snugly in a bore in a tool post (not shown) of a machine tool, with the flange 3 bolted tightly against a face of the tool post.

The one-piece shank 1 comprises a solid portion 36 and a hollow tube-shaped portion 23 defining a recess 23b.

The shank 1 is made of En 9 or En 24 steel in t (heat-treated, hardened) condition.

A damper assembly 6 is located in the forward end of the shank and comprises a cylindrcial sleeve 7 with an end wall 8 and a side wall 9 within which is located a cylindrical damper mass 10. The damper mass 10 is made of "Heavy Alloy" supplied by Osram G.E.C. Limited of East Lane, Wembley, Middlesex, England, and may for example be a tungsten-nickelcopper alloy having a density of 168 grams per c.c. or a tungsten-nickel-iron alloy having a density of about 18 grams per c.c. The damper mass 10 is 762 mm long and has a diameter of 27 mm, with a clearance 13 between the mass 10 and the side wall 9 of about 0'l0 to 0 13 mm.

The actual choice of length and diameter of the damper mass is not critical and best results are attained with a clearance in the range 0107 to 0 122 mm for the bar shown in Figure 1. With a clearance less than 0 107 mm, the performance deteriorates rapidly with reducing clearance but does not fall off so rapidly with clearances greater than 0 107 mm.

In all cases, however, of various bar dimensions, overhung ratios, and damper sizes, the optimum clearance is usually best determined by using vibration testing equip ment to determine the various combinations which produce minimum response to vibratory forces.

The forward end of the sleeve is closed by a tool mounting (Fig. 1) in the form of a steel plug 14 having a reduced diameter portion 15 which fits inside the open end of the sleeve 7.

The plug 14 is held in place by a ring of screws 16 (Fig. la) located in counterbored holes 17 in the plug 14 and screwing into internally threaded holes 17' in the forward portion of the lip 12. A sealing washer 18 is compressed between the plug 14 and the forward open end of the sleeve 7 to prevent the ingress of moisture and oil. It is essential for satisfactory operation of the damper assembly 6 that the internal surfaces thereof should be clean and dry when it is assembled, and that subsequently no moisture or oil should enter the cavity. The plug 14 has a bore 19 in which a cutting tool 20, having a cutting tip 21 is clamped by conventional means.

The space 23b between the forward end of the solid portion of the bar and the damper assembly 6 reduces the effective mass of the boring bar at its free end and thereby increases the effectiveness of the damper assembly 6. Although the space 23 has the effect of substantially reducing the effective mass of the boring bar at the free end, the boring bar is sufficiently stiff to maintain a good geometric form of bore.

The boring bar at its forward end carries a second damper assembly 24 which is detachable in order that it may be removed so as to allow the boring bar to be used to machine a bore, such as a blind hole, which would otherwise be fouled by the second damper assembly.

This second damper assembly 24 consists of a cylindrical capsule 25 containing a cylindrical damping mass 26, also of "Heavy Alloy", the clearance 27 between the damping mass and the inner surface of the capsule being of the same order as that for the mass 10 and having an optimum value which may be determined by experiment and calculation. Preferably the mass 26 has a length of from one to one-and-a-half times its diameter.

The capsule 25 has a cylindrical steel side wall 28 and a steel rearward end wall 29 integral with it, from which an externally screw-threaded spigot 30 projects rearwardly and axially to be located in an internally threaded hole 31 in the forward part of a steel plug 32.

The steel plug 32 is similar in design and function to the steel plug 14 of the boring bar 100 shown in Fig. 1 except that it has a reduced diameter portion 33 projecting forwardly in which the threaded hole 31 is drilled. When the capsule 25 is located in position its rearward surface abuts the forward surface of the steel plug 32.

The forward end of the capsule 25 is closed and sealed by a steel disc 34 having an internally threaded rim 35 extending rearwardly so as to surround the externally threaded forward portion of the side wall 28.

The damper assembly 24 is designed to assist the damper 6 in damping the primary mode of vibration of the boring bar, particularly when taking finishing cuts.

When taking heavy (roughing) and medium cuts in high duty steels (such as are used for gas turbine shafts), it is found that the boring bar may vibrate in a secondary mode at a frequency several times that of the primary frequency and with a mode in the central portion of the bar.

The second damper assembly 24 may be designed to have its maximum damping range effective to discourage this secondary mode. Since the second damper assembly is readily detachable, two alternative assemblies may be provided, one for the secondary mode for heavy and medium cuts and the other for the primary mode for light, finishing cuts.

Damped boring bars are also described and claimed in our copending application No. 4600/76 (Serial No. 1,578,342).

WHAT WE CLAIM IS: - 1. A boring bar comprising a unitary shank having a rearward end by which it can be mounted and, at its forward end, means for mounting a cutting tool and a damper comprising a damper mass positioned in a cavity with a damper clearance between the mass and the inner wall of the cavity, the shank having a solid portion adjacent its rearward end and a hollow tubular portion adjacent the damper and tool mounting means.

2. A boring bar according to claim 1, wherein the forward end of the shank is recessed to form the hollow tubular portion and to receive the damper.

3. A boring bar according to claim 1 or 2, wherein the tool mounting means is secured to the damper assembly.

4. A boring bar according to any of the preceding claims and including a second damper assembly, the tool holding means lying between the second damper assembly and the first-mentioned damper assembly, the second damper assembly comprising a cylindrical second damper mass located in a sealed cavity with a damping clearance containing a damping fluid betwen the said mass and the inner wall of said cavity.

5. A boring bar according to claim 4, wherein the second damper assembly forms a capsule detachably secured to the forward end of said bar.

6. A boring bar according to claim 4 or 5, wherein the second damper assembly is dimensioned to damp the second mode of oscillation of the bar.

7. A boring bar substantially as hereinbefore described with reference to any of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. an internally threaded rim 35 extending rearwardly so as to surround the externally threaded forward portion of the side wall 28. The damper assembly 24 is designed to assist the damper 6 in damping the primary mode of vibration of the boring bar, particularly when taking finishing cuts. When taking heavy (roughing) and medium cuts in high duty steels (such as are used for gas turbine shafts), it is found that the boring bar may vibrate in a secondary mode at a frequency several times that of the primary frequency and with a mode in the central portion of the bar. The second damper assembly 24 may be designed to have its maximum damping range effective to discourage this secondary mode. Since the second damper assembly is readily detachable, two alternative assemblies may be provided, one for the secondary mode for heavy and medium cuts and the other for the primary mode for light, finishing cuts. Damped boring bars are also described and claimed in our copending application No. 4600/76 (Serial No. 1,578,342). WHAT WE CLAIM IS: -

1. A boring bar comprising a unitary shank having a rearward end by which it can be mounted and, at its forward end, means for mounting a cutting tool and a damper comprising a damper mass positioned in a cavity with a damper clearance between the mass and the inner wall of the cavity, the shank having a solid portion adjacent its rearward end and a hollow tubular portion adjacent the damper and tool mounting means.

2. A boring bar according to claim 1, wherein the forward end of the shank is recessed to form the hollow tubular portion and to receive the damper.

3. A boring bar according to claim 1 or 2, wherein the tool mounting means is secured to the damper assembly.

4. A boring bar according to any of the preceding claims and including a second damper assembly, the tool holding means lying between the second damper assembly and the first-mentioned damper assembly, the second damper assembly comprising a cylindrical second damper mass located in a sealed cavity with a damping clearance containing a damping fluid betwen the said mass and the inner wall of said cavity.

5. A boring bar according to claim 4, wherein the second damper assembly forms a capsule detachably secured to the forward end of said bar.

6. A boring bar according to claim 4 or 5, wherein the second damper assembly is dimensioned to damp the second mode of oscillation of the bar.

7. A boring bar substantially as hereinbefore described with reference to any of the accompanying drawings.