VACUUM MACHINING OF SPHERICAL PARTS

Overview

In all machining processes there are forces that are present. These forces are needed to generate the action of the abrasive crystals against the material being removed. These can be very small or extremely high. In grinding, as in many machining processes, these forces are contained in a small area and they can generate a considerable amount of heat. In Superfinishing, these forces are much less, due to the type of abrasives used but there are still forces. In superfinishing, the accuracies far surpass what is usually obtainable by other machining processes. In this case any force, no matter how small, can pose deficiencies in the geometric shape or tolerances obtainable.

Current processing

Processing to obtain a spherical shape on a workpiece can be completed in several different steps. For this background we will only touch on the differences instead of delving too deep into the processes. The new process can be used in conjunction with most all the tooling and Workholding features currently in use.

Background

The Grinding Process (Grinding as described by SME)

Grinding is a material removal and surface generation process used to shape and finish components made of metals and other materials. The precision and surface finish obtained through grinding can be up to ten times better than with either turning or milling.

Grinding employs an abrasive product, usually a rotating wheel brought into controlled contact with a work surface. The grinding wheel is composed of abrasive grains held together in a binder. These abrasive grains act as cutting tools, removing tiny chips of material from the work. As these abrasive grains wear and become dull, the added resistance leads to fracture of the grains or weakening of their bond. The dull pieces break away, revealing sharp new grains that continue cutting.

The requirements for efficient grinding include:

●?????? abrasive components which are harder than the work

●?????? shock- and heat-resistant abrasive wheels

●?????? abrasives that are friable. That is, they are capable of controlled fracturing.

Most abrasives used in industry are synthetic. Aluminum oxide is used in three quarters of all grinding operations and is primarily used to grind ferrous metals. Next is silicon carbide, which is used for grinding softer, non-ferrous metals and high-density materials, such as cemented carbide or ceramics. Super abrasives, namely cubic boron nitride or "CBN" and diamond, are used in about five percent of grinding. Hard ferrous materials are ground with "CBN", while non-ferrous materials and non-metals are best ground with diamond.

The grain size of abrasive materials is important to the process. Large, coarse grains remove material faster, while smaller grains produce a finer finish.

The binders that hold these abrasive grains together include:

●?????? vitrified bonds, a glass-like bond formed of fused clay or feldspar

●?????? organic bonds, from synthetic resins, rubber, or shellac

●?????? metal or single-layer bond systems for super abrasives

Wheels are graded according to their strength and wear resistance. A "hard" wheel is one that resists the separation of its individual grains. One that is too hard will wear slowly and present dulled grains to the work and overheat, affecting the final finish. If too soft a wheel is used, it will deteriorate quickly, requiring frequent replacement.

Spherical machining

Generating a true sphere is not the most common of machining practices. Turning and line grinding are used to produce a form relatively close to true. The practice of lapping a sphere is also prevalent but this process requires a considerable amount of time. The process involving a cup style wheel on a spherical workpiece combined with counter rotation and allowing the wheel and workpiece to blend and conform is the most accurate way of producing the sphere.

The force behind this

As a start, in all grinding and metal removal operations, there must be an applied tooling load to generate the force needed for the cutting action to begin, in this case the abrasive grain. Each process and tool needs to have a certain amount of force which pushes the grain into the workpiece and thus causes a chip formation. In turn this chip is the particle with which the tool acts upon the workpiece.

Spherical machining

In spherical machining, the tool is a cup wheel that encompasses the workpiece. The angle of approach is determined by the cordial length from the pole at the rotational center-line to the lower full diameter of the sphere. This line effectively covers the entire face of the workpiece. There are different approaches to this layout as some would like to have the top of the wheel through the pole and the bottom slightly exposed. This is to release the coolant pressure and allow coolant flow out of the inside of the wheel. This approach will have the entire wheel covered. In spherical machining the, the abrasive action takes place when the workpiece and the tool rotate. This is typically done is opposite directions to enable the interaction of the process.

As illustrated, the cup wheel is in contact with the entire face of the workpiece. For any machining or cutting to begin there must be applied a certain amount of tooling force. In superfinishing the beginning stage is to open up the tool which may have been glazed at the end of the previous cycle by rubbing it against a course incoming workpiece. This action breaks open the tool and begins the cutting action again. This and any amount of machining will have a tool load against the workpiece.

The 4 stages of superfinishing

During the machining process there are 4 different stages in the cutting action. For simplicity, we will allow these stages to be defined as listed…This shows the four working states of one grit or grain.

Cutting

The cutting edge of the grit penetrates the material to a depth where a chip is formed and sheared off. On the sides of the grits path, material deformation takes place building a dam. This dam is sheared off by the following grits not shown here, thus removing stock.

Plowing

As less pressure is applied to the grit, it stops creating a chip in the front and in its path, it creates a ditch with a dam on each side which is then taken off by the following grit. The depth of the ditch is not as deep as in the cutting stage.

Grooving

In this stage dams of smaller height are created which are not sheared off but are pushed back by the following grits. Again, the depth of the path is not as deep as in the prior stage.

Rubbing

In this stage there is very little pressure on the tool, thus not creating a groove since all the material springs back to almost the same position as before behind the grit. This creates the smoothest surface, but on the downside, heat is generated.

Typical processing for a sphere

The process of a typical sphere starts as a turned workpiece is clamped and placed into the machining cell. Listed below is the typical steps and the times involved in producing a part. (the parts generated in this case are femoral hip joints for human implanting)

Step 1 C600-45V18T3

●?????? Approximately 30-40 seconds.

Step 2 C800-45V18T3

●?????? Approximately 30-40 seconds.

Step 3 C1200-45V18T3

●?????? Approximately 30-40 sec

As mentioned above, during the machining process there are forces the machine imposes on the workpiece and as in all of nature and action has an equal and opposite reaction. The force on the tool is applied to the workpiece. This load affects the pressure on the spindle and fixture. Any force will flex the fixture and spindle. This is what generates the inaccuracies of the final geometry. The stiffer and more accurate the system, the more geometrically accurate the end result is. The main issue is that all things flex no matter how small and in the case of the femoral ball or the spherical bearing, these inaccuracies can be measured in the sub microns...

This is the entire platform for vacuum finishing

The new concept on finishing the spherical surfaces of hip joints and other curved objects is without generating the typical forces. The use of pressure on a cutting tool is to create a force enough to cause the tool to cut. Instead of imparting a feed pressure on the cutting tool, this concept uses an inverse internal force of vacuum to develop a pressure between the cutting tool and the workpiece. With experience we know that having too much coolant pressure through the spindle and not having a release for it can reduce the cutting force of the tool. Reversing the flow will generate more of a pulling force. This force can be increased or decreased at will with the amount of vacuum as with any conventional system. The force will ultimately be decreasing as the part and the tool conform to each other. The force can also be reduced as the cycle approaches the end. The same gauging system can be used as in the present outline. The feed force on the part is negligible as the force is pulling the tool and workpiece together eliminating the pressure that is typically excerpted between the tool and workpiece to create the force required for the abrasive action to occur. This concept considerably reduces the need for rigidity of either the tool spindle or work spindle. With both spindles actually floating there would be no tool pressure that is off set or unstable. The only stability needed is to guide the tool to the workpiece.

Vacuum system overview

The figure shown above is a simple layout of the vacuum system that would be used. The components are a vacuum pump, whether an inline style or a motorized version. The vacuum canister is the key as this unit must be large enough to hold enough liquid used during a single work cycle. It can be evacuated at the end of each cycle or after the retraction of the tool and during the tool change cycle. The coolant being supplied by the coolant pump will be enough to generate the sealing effect of the tool to the workpiece. This is where the coolant is drawn through the interaction surface of the tool and workpiece. Of coarse the coolant must be held to a cleanliness level suitable for the final finish requirement of the workpiece.

Many different processes can be arranged during the cycle. For example, the first cycle of roughing can be accomplished by the conventional cup wheel grinding method. This is not as critical in the cycle as the geometric accuracies are needed later in the cycle. Every machining application requires a certain amount of interaction between the tool and the workpiece. This interaction is what generates the flex and deforming. Picture the push on an egg and how this would be different if the force was a vacuum instead.

The theory behind this is that introducing a vacuum from between the cup wheel and the workpiece will generate a force pulling these to components together. As with a suction cup on a bowling ball, the amount of vacuum and the size of the cup determines how much weight can be lifted. This weight is converted into an internal force pulling the tool into the workpiece.

This system and approach will not generate any of the forces or flex on the tool or the workpiece. The geometry of any machined part is based on several factors. Deflection of the part or tool is by far the largest. Locating and gripping the workpiece is accurately is another. Machining the workpiece with a process that is consistent generates and produces a finished part that truly will be as geometrically perfect as possible.

Conclusion

The vacuum generates a pulling force which eliminates the flex. The vacuum also causes the coolant to be drawn through the abrasive/workpiece machining area. In typical machining processes, getting the coolant into the “zone” is a very difficult design obstacle. High pressure coolant, contoured or point nozzles, directed flow, are used. All are fair at best. The vacuum system causes the coolant to be pulled through this zone and flushes the area with a good amount of coolant without the splash or spray. No high pressure pumps or any special design features are needed. A good supply of clean coolant is all that is needed. This action cleans and removes the finishing particles as they are generated, at the source. Having metal particles imbedded in the tool face is reduced as the coolant is present at all times. This vacuum also keeps the tool and the workpiece firmly attached. At last, one of the final critical issues is guarding the machine and the precision components from the contamination the fine grinding and superfinishing generates. This process draws all the swarf out of the machine at the point where it is generated and collects it in a separate tank outside the machine.