CA2259018A1

CA2259018A1 - Tool holder

Info

Publication number: CA2259018A1
Application number: CA 2259018
Authority: CA
Inventors: Peter Bonac; Tomo Bonac
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-01-13
Filing date: 1999-01-13
Publication date: 2000-07-13

Abstract

A tool holder for high tolerance machining, especially for high concentricity requirements, characterised by four discrete clamping locations around the tool shank extending axially. The tool holder has stiff and flexible portions, which by using compression forces open the holder to change the cutting tool.

Description

TOOL HOLDER
FIELD OF THE INVENTION
The present invention relates to tool holder and, more particularly, to such device for centrally clamping cylindrical tools by elastic deformation of the holder body.
BACKGROUND OF THE INVENTION
The tool clamping devices used for more demanding application such as CNC milling are required to achieve very accurate concentricity, particularly if high rotational speeds are involved. Hydraulically actuated holder achieves good concentricity but lacks tool-tool holder stiffness. Heat shrink tool holders such as Lindex Corp Shrink Fit offer excellent concentricity and stiffness. Unfortunately, this system is rather cumbersome to use and requires heaters and coolers, which can be very expensive. Furthermore, time required for heating and cooling as well as for management of the procedure can be substantial. A new method based on cylinder deformation called Tribos was introduced recently by Schunk with the purpose to provide good concentricity and reasonable stiffness without the drawbacks of the heat shrink method. 'The method is based on deforming a cylinder with an original polygonal bore into a cylindrical shape. 4~Then the bore is forced into cylindrical shape a tool is inserted and when the deformation loading forces are removed the holder clamps the tool by trying to restore the non-loaded shape. A problem with Tribos concept is that the cylinder . CA 02259018 1999-O1-13 deformation from polygonal bore to th.e cylindrical bore provide relatively small range of radial clearance since it relies on compressive forces. This range is affected also by the wall thickness since larger wall thickness results in the increase of radial compressive stresses, eventually causing overall reduction of the inside diameter. Since, in general, a larger diameter tool requires proportionally larger wall thickness to provide adequate stiffness the Tribos concept appears to be difficult to apply to larger tool diameters. Another problem appears to be at the lower range of the tool diameter. For small tool diameters the radial clearance range is too small for practical use. Further disadvantage of Tribos system is that it requires special clamping fixture and collets to fit different outside diameters of tool holders.
SUMMARY OF THE INVE:I~TTION
The present invention overcomes the foregoing and other drawbacks by providing a generally cylindrical tool holder with relatively stiff areas, which are movable in the radial direction, by applying force to relatively flexible areas. Stiff areas of the holder are essentially two opposing thick wall portions of a cylinder. At tool clamping each portion contacts tool shank along two lines parallel to the axis. Therefore a tool is clamped at four equidistantly spaced lines. Stiff portions are integrally connected with two relatively flexible convex curved beams opposing each other. The beams clear the tool shank at all times.
At tool insertion and removal a force is applied at the centre of each beam thus bending it and spreading apart the stiff portions of the holder. To clamp the tool the forces are removed and the beams, which try to restore the unloaded shape, pull the stiff areas toward each other. The advantage of the present invention is an improved stiffness of the tool-tool holder assembly since . . CA 02259018 1999-O1-13 larger outside diameter can be used than allowed by Tribos system. Another advantage is that tool contact areas are relatively insensitive to compressive stresses at actuation of tool holder. Therefore much larger anal smaller tool radii than with Tribos system can be used. Additional advantage of the present invention is that simple devices can be used to open the holder. A hydraulic vise with load indicator can be used.
Alternatively, the outside geometry of the curved beam portion of the holder can be designed to prevent overloading of the holder.
In this case even a standard machine vise can be applied.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described further, by way of example only, with reference to the accompanying drawings, in which:
Fig. 1 is a section of the tool holder for clamping relatively large diameter tool according to the invention.
Fig. 3 is the same embodiment shown in Fig. 1. in open state to release/accept a tool.
Fig. 2 is perspective view of a tool holder according to the invention.
Fig. 4 is a section of the tool holder for clamping relatively small diameter tool according to the invention.
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
According to FIG.l the tool holder comprises a generally cylindrical body l, which has stiffer portions and relatively flexible portions aligned at 90 degrees relative to each other.
The stiffer portion 2 is the part of the cylinder having a thicker wall. At the inside diameter the stiffer part 2 is finished to a smaller diameter than the outside diameter of the tool 3. Also, the inner side of the part 2 is provided with the relief 4 which assures two contact locations 5 with the tool at ends of the part. Since the two stiff parts 2 are opposing each other the tool 3 is thus clamped at four equidistant locations 5.
Flexible portions of the tool holder are essentially curved beams 6 created by machining grooves 7 into the cylindrical body. The grooves are curved and extend toward the periphery of the cylinder to create ends 8 of the beams. The purpose is to improve flexing ability of the beams. As shown in Figure 2 this clamping geometry of the tool holder extends from the tool end along the axis of the holder and ends with cavity 9 and a thru hole 10. The purpose of the cavity is to reduce concentration of operational stresses at the transition of tool clamping geometry into cylindrical shank 11.
To insert tool 3 the tool opposing forces 12 are applied as shown in Figure 3. The forces 12 tend to flatten beams 6 thus exerting large forces 13 spreading apart stiff portions 2. A
clearance 14 is created allowing insertion of the tool. When forces 12 are removed the beams try to restore the non-load shape thus pulling together portions 2 and creating tool clamping forces due to interference fit at the four locations 5. To remove a tool the forces 12 are re-introduced.
To open the tool holder relatively simple devices such as machine vise can be used. For such operation it is advantageous to choose such outer radius 19 of beam 6 that the deformation X
provide desirable opening of the tool holder. Further loading in the direction 12 would not result in additional opening 13 but would increase load resistance thus preventing tool holder overloading in a manual operation.
For relatively small diameter tools a more suitable geometry of the tool holder is shown in Figure 4. Tool 15 is similarly clamped at four locations 16, however, the body 17 outside diameter is proportionally larger in relation to the outside diameter of the tool 15 than in the previous case of the larger diameter tool. The separation of stiff portions of the tool holder from the flexible portions is achieved by machining additional radial grooves 18. The purpose of the geometry shown in Figure 4 is to provide sufficient clearance for insertion/removal of small diameter tools.

Claims

1. A tool holder system for machining applications, comprising:
a cutting tool having a cylindrical shank portion defining a continuous, uniform outer surface of substantially constant diameter; and a tool holder having a generally cylindrical body, said body having a machine mount end and a cutting tool clamping end, said tool clamping end having two opposing equal stiff portions and two opposing equal flexible portions, both said portions extending axially therein, said stiff portions oriented axially at essentially right angle to the said flexible portions, said stiff portions partially separated from said flexible portion by essentially tangential grooves extending toward but not reaching the outer surface of the tool clamping end, each said stiff portion having a uniform inner surface of substantially constant diameter, the diameter of the inner surface of the stiff portion being slightly less than the diameter of the outer surface of the shank portion of the cutting tool, said flexible portions being essentially outwardly curved beams integral of the said body, said beams not contacting the outer surface of the shank portion of the cutting tool when flexed toward each other.

2. The system of claim 1, wherein each said stiff portion has a relief area of the inner surface, said relief area providing clearance of the shank portion of the cutting tool, said relief area located in substantially a symmetrical center of said stiff portion and extended axially therein.

3. The system of claim 1 or claim 2, wherein said machine mount end is a cylinder having substantially constant outside diameter.

4. The system of claim 1 or claim 2, wherein said machine mount end is a conical taper being releasable mountable to a rotating spindle of a milling machine.