Recommendations in Wheel Dressing:

Linear dressing machine

Video dressing machine

CNC six-axis machine

Workflow and workplace organization, standardization, and waste reduction are not traditionally addressed in computer numerical control (CNC) grinding. Many medical contract manufacturers have implemented lean policies into their machining processes but few have extended this strategy into grinding. CNC grinding is sometimes looked at as an art rather than a standard process. Although it can be hard to apply lean manufacturing to CNC grinding, it is worthwhile for manufacturers to examine some of the areas where standardized process and continuous improvement can be used as tactics to get more out of their team and equipment. This approach applies in particular to the operations of orthopaedic instruments, where variables such as material deflection during grinding, burs on the cutting edges, softer and therefore less grindable stainless steels, variations in blank preparation, and bent blanks are prevalent.

New systems and tools are available for users of CNC grinding machines to apply some of the lean manufacturing philosophies. If lean thinking is applied logically from quoting through to final inspection, and adjustments in manufacturing are made to allow continuous improvement, then even small efforts will translate into a cost reduction and fast order throughput.

Request for Quotation: The Customer

In the initial quotation stage, the manufacturer needs to have a clear idea of the cycle time and the tooling needed for the job. Most medical contract manufacturers are familiar with the process of planning and manufacturing the part blank, and are therefore able to produce budgetary pricing quickly and reliably. However, on the grinding side, it is more complex. Desktop tool design software that is available with the grinding machine walks the user through the programming process. It then provides a realistic 3-D model of the part and an accurate total grinding time. An animation of the grinding process inside the machine, including all six axes and the tooling, helps the manufacturer identify the accessories and wheels that are needed. The pricing and availability of the tooling is essential to provide a quotation to the prospective customer with an accurate lead time. At the same time, the software allows an initial optimization in the grinding process, which will help the company estimate a competitive cost. If the quantity needed justifies automatic handling, then the additional time required for autoloading and unloading is added to the grinding time to arrive at a number of pieces per hour. In any case, shorter runs force a manufacturer to do setups effectively and productively.

Wheels Get Dressed

The wheel selection and the truing or dressing in CNC grinding is almost more important than the actual grinding process, because it determines the way in which the wheel interacts with the material. It also influences the amount of grinding pressure, potential material deformation, and possible surface damage.

After determining the wheel shape and finding the appropriate wheels, each wheel is assembled on the arbor separately and dressed both on the periphery and on the side. This process is essential for new wheels as well as worn wheels. The advantages of off-line dressing (on a separate machine) are that 

• Machine time is not wasted.
• Wheel packs can be prepared during working hours of set-up personnel.
• Silicon carbide or aluminum oxide wheels can be used, which work best to dress resin, polyamide, or metal-bonded super abrasive products.

These machines are usually custom-designed, and they use a silicon carbide or aluminum oxide wheel as a dressing medium. Some machines have capabilities for radius dressing, while others only provide linear dressing. Some machines incorporate a camera and monitor that enables the operator to dress accurately and produce consistent, reproducible results. The smaller the part diameter, the more care that must be taken to get the wheels dressed without any run-out.

Managing the Pack

Wheel pack management is an important component of lean grinding, because the level of organization in this area affects the bottom line of the operation. An experienced grinding operator can make a qualified selection of the grinding wheels. For orthopaedic instrument grinding, it is essential that the grinding wheels are superabrasive. In fact, only  cubic boron nitride wheels are suitable for stainless steel applications.

Depending on the size of the grinding cell, the operator determines how many wheel packs are needed to be on hand. Based on what types of products the company produces, the operator can then assemble selective wheels into packs by shape and grit size to fulfill all grinding operations in one chucking.

There is a general understanding in the tool grinding industry that 80% of the production should be manufactured with tooling on hand, and 20% is allowed to fall outside where tooling must be specifically ordered or made. Because superabrasive wheels are expensive, they generally represent the largest chunk of tooling cost in CNC grinding. Standardization of wheel packs for 80% of the production presents an enormous benefit to the efficient operation of a CNC grinding cell.

The wheel arbors must also be kept in good shape to provide repetitive positioning on the machine. Some machine builders deliver their grinding machines with special designs of arbor mounting. One arbor system is called PerfectArbor. These arbors work on the principle of a compression fit and provide a repetitive run-out of less than 1 µm.

Managing Wheel Pack Files

Adobe portable document format (PDF) forms lend themselves easily to creating a well-functioning management system for wheel packs. The machine tool builder supplies templates of these forms created in Adobe LiveCycle. The PDF form has drop-down menus and can be used electronically or manually. Experience has shown that manual data entry on the shop floor is usually  faster than typing the information into a PC. However, record keeping is difficult this way, and it is hard to implement lean procedures without having the data in electronic form. The preferred way is to build a data library of wheel packs with the relevant data saved in an electronic form.

The wheel data can then be traced throughout the grinding process and analyzed and used for testing new wheel bonds. This is part of the continuous improvement process, and it helps the manufacturer keep up with the latest wheel technologies, which reduces wheel cost.

The operator needs to run Adobe Acrobat Professional on wheel presetter, which is known as the PDF Writer software. Each form can be saved separately and can be identified with the corresponding wheel pack. A library of these files can be made available to all operators to share. These files can also be viewed or accessed directly from the CNC tool grinders.

Grinding Wheel Balance

A wheel balancing station can be customized to fit any CNC grinding machine.

Balancing the grinding wheels is another way to improve consistency and reduce set-up time. This is done most effectively with all of the wheels mounted on the arbor and after dressing or truing. Without proper wheel balancing the following errors can occur: 

• Reduced finish.
• Premature wheel wear.
• Loss of corner holding on wheels.
• Unnecessarily longer cycle times.
• More frequent need for sticking.
• Reduced length of unattended time during automation.

These points go precisely against the principles of standardization and lean manufacturing policies. However, it is also important to figure out the origin of the imbalance. Possible reasons include: 

• Undressed or improperly dressed wheels.
• Excessive diameter on wheel holes.
• Exceptionally heavy wheel bodies.
• Stacked up wheels on long arbors with poorly made spacers.

 The balancing equipment should be specifically customized to fit the arbor, which allows the balancing software to output relevant and correct information so the operator can adjust the weight distribution of the wheel pack (see Figure 1).

Quantifying Grinding Wheels: The Presetter

The most cost-effective way to qualify the grinding wheels is to do it away from the machine. This method saves machine time and can usually be done accurately and consistently, which is in line with lean manufacturing principles.

A wheel presetter that is available with the machine is usually a table-mounted unit that is used for external wheel scoping. This

3-D software enables operators to conduct part simulation.

unit is useful in cases when several machines are in operation or if the operator wants to avoid using machine time for wheel measurements. The conventional scope is replaced by a camera and the image is directly displayed in the setup part of the machine software. The captured picture is displayed in real-time and allows the operator to point and click the PC’s pointer to the various corners and radii of the wheel shape for measurement and data capture.

A well-qualified wheel pack reduces or eliminates wastage of blanks and machine time by obtaining a part within tolerance from the first blank. However, it requires that the presetter has glass scales so that the result is accurate and repeatable. Networking to the machine is also mandatory for efficient transfer of data and program management. After measuring the wheel pack on the presetter, the software prompts the operator to proceed to the stage of part simulation and eventually machine simulation. This process happens on the same PC with the same software, and it is completed before the program data is transferred to the machine.

With the increase of applicable CNC grinders, the manufacturing of surgical instruments has become competitive. The devil is in the details, and most of the topics discussed in this article relate to peripheral and preparatory processes rather than the actual grinding inside the grinding machine. A well-prepared and well-managed grinding process enables faster machining on any CNC grinder, which consequently reduces cycle time, machine downtime, and setup time.

Eric Schwarzenbach is president of Rollomatic Inc. (Mundelein, IL).

Find Rollomatic at booth #106 during OrthoTec 2011.

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