WO1989003168A2 - Improved machining system - Google Patents

Description

IMPROVED MACHINING SYSTEM

TECHNICAL FIELD Background Art

The current methods of milling stone is with a three axis copy mill controlled by an hydraulic sensor probe determining depth of cut whilst a raster scan mill pattern is programmed into the other two axes. This requires a relatively hard material for the pattern to be copied and provides 1 to 1 scaling only. The tools used in the machining process are tungsten carbide or diamond plated milling tools and grinding tools. The process is characterised by high force of contact between the tool and the material being machined (up to 10 ton) and low spindle speed (below 10,000 RPM).

! SUBSTITUTE SHEET Spindle torque is also relatively low at 1 to 4 horsepower for tools up to 5 cm diameter. There is limited control of machine performance with only a coarse adjustment of hydraulic pressure driving the axes movement and a small number (1 4) switch selectable spindle drive ratios. Thus none of the machining characteristics - spindle speed (tool surface speed) , spindle torque (applied tangential force of tool), tool to work piece contact pressure and feed rate are accurately set and certainly not dynamically controlled.

The nature of the tool work piece material interaction is compressive abrasion machining.

The orientation of the machining could be vertical or horizontal there can also be turntable or index table mounting of workpiece or workpiece and pattern introducing an addtional polar axis for sculpture in the round or axis symmetric machining. The process does not produce a finished product rather only a "roughed out" piece of stone, which has milling ridges, poor fidelity and poor surface finish and requires extensive hand finishing. Disclosure of the Invention

The process introduces 6 distinct improvements.

(1) Computer digital control allowing greater flexibility of pattern information input, free selection of milling trajectories other than raster scan (e.g. contour), free control of machining area rather than limited to rectangular areas.

(2) Five axis milling with optional 6th polar axis allowing machining tangential to the desired final surface and eliminating steps in final milled surface.

(3) High spindle speed (tool surface speed 3000 6000 surface feet per minute) and torque (10 50 horsepower for a 5 cm tool diameter) with low contact

' S* . .??«Γ"S*I pressure (under 1 ton for a 5 cm diameter tool) so that the fundamental machining process is shear force cutting rather than compressive abrasion, (see note A) .

(4) Poly Crystaline Diamond "super abrasive" tipped milling tools, (see note B) .

(5) Real time adaptive control of characteristics of tool material interaction (see note C).

(6) Real time adaptive control of tool path trajectory to produce desired surface geometry and finish, compensating for inhomogeneous characteristics in material and compensating for tool wear and compensating for deflection of the tool due to limited stiffness in the machine tool structure. (see note D) . Anyone of these improvements could be introduced as a separate improvement to the process of machining stone. However these six improvements have a complimentary compounding influence.

The use in combination of these improvements allows high speed, low tool cost production of high fidelity, high quality surface finish, complex forms in a wide range of inhomogeneous very hard and abrasive materials.

Best Mode for Carrying out the Invention Note A

Stone materials tend to have very high resistance to compressive loads but very low resistance to shear loads and tensile loads, these characteristics are reflected in machining efficiency and throughput in that a plated diamond tool pressed against a piece of stone with great force and moved with relatively low velocity (compressive abrasion) is an inefficient means of removing material from stone, whereas the cutting edge of a milling tool moving with a high surface speed and

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Claims

force parallel to the surface removes material more efficiently and with higher throughput and only requires sufficient contact pressure to ensure the "bite".

Note B Materials widely used in cutting and abrasive machining operations high speed steel, tungsten carbide, aluminium hydroxide and silicon carbide (carbarundum) are no match for any but the softest of stone. Natural and man made diamonds in plated diamond tools and diamond impregnated compound tipped tools fare somewhat better but still experience substantial erosion while machining stone in grinding and sawing operations and are unsuitable for application on the cutting edge of milling tools. Tungsten carbide milling tools blunt rapidly in stone milling operations resulting in vast increase in the amount of force necessary to remove material. Current stone milling machines are effectively designed to work with blunt tungsten carbide tooling. Tungsten carbide also erodes more rapidly (measured against amount of material removed) as tool speed increases. Poly crystaline diamond has a far higher abrasive resistence than tungsten carbide (depending on test materials, speed, force etc. 30 1000 times higher) and its characteristics are such that it has a point of optimal performance at which speed and pressure the diamond itself sharpening but not excessively so whilst removing the maximum amount of material. The particular surface speed, depth of cut and contact pressure at which this optimal point of operation is attained varies from material to material and performance is relatively sensitive to proximity to this point of optimal machining characteristics. Optimal performance occurs at high surface speeds well suited to shear force cutting. The sensitivity of optimal performance to machining characteristics and the high efficiencies thus obtainable justify an investment in real time adaptive control.

Note C Optimal performance for machining operations is usually specified in terms of tool surface speed, depth of cut and feed rate for a particular tool and material being machined. These parameters assume the material is homogeneous and that the depth of cut can remain constant (true when machining simple geometric shapes but not so during final shaping of a free form) .

Conventional materials do not require nor reward close control of machining parameters hence conventional machine tools have only course control of spindle speed and no control of spindle torque.

Machining complex forms requires a wide range of tool sizes and tool shapes.

If tool size, tool shape, tool drag, the area of contact or the properties of the material being machined vary then the spindle speed, spindle torque, feed rate, force tangetial and force orthogonal to the zone of interaction may need adjustment if optimal performance is to be realised.

The adaptive machine tool has the means to sense and regulate spindle speed, spindle torque (that is torque required to maintain machining parameters rather than torque potentially available) the feed rate and direction of feed and the moments of force in the 3 axes about the zone of interaction. Once the mechanisms of sensing and regulating are in place the sensory information becomes input to the digital control system and the regulartory control is provided by output from the digital control system and the rules of regulation are contained in the control program.

SUDST5TUTΞ SHEET j Note D 6

Under variable machining conditions the parameters that may require adjustment include depth of cut and feed rate, consequently the amount of material removed in a single pass has to be considered variable. Hence tool path trajectory has to be determined in real time. This is readily achieved when the control system determines such varying trajectories and via control of the machines axes of movement has the means to effect such varying trajectories and also contains information describing the final form required.

SUBSTITUTE SHEET