The Importance of Proper Fixturing

The quality and the performance of a dental machining center is heavily influenced by jigs and fixtures as they responsible for the placing and holding of a component.

Good jigs and fixtures provide dental manufacturing professionals the following:

• A higher degree of positioning precision and the repeatability thereof.
• A greater accuracy for the positioning of precise hole centers.
• Tighter tolerances at micron levels with higher-quality surface finishes.
• A faster time frame to achieve these tolerances.
• Increased cutting tool life.

Some of the advantages associated with a good jig and fixture include:

Productivity

Good jigs and fixtures are able to effectively increase productivity. This is because the part, during manufacturing, does not have to be frequently checked and eliminates the need for individual marking and positioning.

Costs

Because of an increased productivity, a reduction in waste material, a lesser use of manual labor and the easy assembly offered through using precision machining technologies, the cost of production is considerably reduced.

Skills

The need for a skilled work force to position tools correctly is severely minimized as the locating, clamping and the correct positioning of the work piece is done by tool guiding elements. Semi-skilled operators can be employed.

Quality

The variability of dimensions in mass production cycles are lower, as good jigs and fixtures limit vibration and are able to maintain a quality that is consistent.

Tolerance Stack

A tolerance stack is a calculation to determine how tolerances accumulate. Stacks help establish and optimize part tolerances, and ensure that parts are designed to promote maximum function and produce accurate repeatable results.

Tolerance Stack-Ups are vital to address mechanical fit and mechanical performance requirements. Mechanical fit meaning, do the parts that make up the assembly always go together and mechanical performance requirements would include the performance of mechanisms, like linear or rotary actuators (axis drives), alignments, latches etc.

High-quality fixtures account for certain sources of variation in the manufacturing process such as:

• Location of a surface or part on a fixture.
• Fixture placement on a machine.
• Non-uniformity or consistency in rough stock material.
• Internal stock stress relaxation.
• Repeatability of motions.
• Inconsistent operator set-ups.
• Rigidity of machine elements (twist, bend, expansion, etc.)
• Environment (temperature, humidity, vibrations, etc.)

Often Overlooked

There are many components to an effective high speed machining process for the fabrication of dental prosthesis. Although much has been written about the impact HSM (High Speed Machining) has had on CNC machine tools, spindles, tool holders, cutting tools, and machine controls, often forgotten is high speed machining impact on tool path programming techniques and the CAM software that creates them.

CAD/CAM technology is evolving today to meet the specific needs for new tool path strategies to suit the HSM environment - the use of higher spindle speeds and feed rates to remove material faster without a degradation of part quality. The goal is to finish dental restorations to net shape, to improve surface finish and geometric accuracy so that hand-finishing can be reduced or eliminated.

Basic Requirements

The challenge to the CAD/CAM system is to create smooth, error-free motion consisting of machining passes with very small step-overs at very high feed rates. This motion must be accomplished without overloading the cutting tool, forcing the tool to make sharp turns and with minimum entry and exits into the cut. A given toolpath for high speed milling has to satisfy a number of constraints.

Machining Strategies

High speed machining offers the opportunity to significantly increase quality, reduce machining times and minimize manual rework for complex shapes common to the dental industry. To achieve the best results it is necessary to apply HSM-suitable machining strategies to perform optimal high speed milling operations.

By examining the geometrical and topological information of the work piece’s virtual model, the feature elements can be evaluated. This covers several important high speed cutting aspects in the CAM system, like optimal technology, strategy selection and the process stability of different strategies.

For anything but the simplest parts, HSM will involve several machining steps. Choosing the right machining sequence is the most important stage of HSM programming and one where software capability is most valuable.

The level of automation is increasing steadily in dental CAM software, but in the end the quality of the final restoration is in a large part determined by the availability, use, quality and robustness of the available strategies provided by the software.

Corner Finishing

Pencil milling automatically cleans up corners or concave fillets so that there is uniform stock left at surface intersections. Relieving the larger volume of material allows for less tool deflection and noise when cutting corners.

Intelligent Stock Removal

Intelligently removes small islands of residual stock that can remain on the part due to a combination of tool radii and step-over, by inserting a smooth extension into the area.

Pattern Finishing

Depending on the part shape, Raster, Radial or Spiral strategies can be employed to provide maximum efficiency. Full control of the leads and links ensures smooth toolpath transitions, improving both tool life and surface finish.

3D Offset Finishing

3D offset machining delivers an excellent surface finish because the step-over is varied to give a constant cusp height, both on steep surfaces and shallow contoured areas.

Spiral Offset Finishing

Spiral offset finishing prevents ‘witness marks’, reduces machining time and improves tool life since the tool stays in constant contact with the model in one smooth spiral motion, and giving an excellent surface finish.

Optimized Finishing

3D Offset and Constant-Z finishing are combined to produce the best toolpaths for machining both the flatter and steeper areas of a part. Here the strategy automatically interleaves the toolpaths in the intermediate areas between the two strategies, ensuring high surface quality.

Parametric Offset Machining

Parametric Offset machining intelligently morphs toolpaths across a surface with a varying rather than constant step-over. This strategy covers the complete area without any sharp changes in direction, so improving surface finish and tool life.

High Efficiency Roughing

The main requirements of high-efficiency roughing are to keep the load on the cutter as constant as possible and to minimize any sudden changes in the cutting direction

Knowledge Base

High speed machining (HSM) contributes greatly in the manufacturing of dental prostheses. With high speed machining, complex contour shapes can be machined so accurately that little or no finishing operation is necessary. High speed machining offers a practical way to minimize cutting force and therefore minimize workpiece deflection.

It is the machining template library that ultimately maximize the effectiveness of high speed machining by creating the output to work within the CAM software’s core technology and the machine’s capability to produce the best possible restorations, in the shortest time possible with the minimum of post-machining bench work.

Our “knowledge-based” system is based on specific knowledge of various aspects about high speed machining, such as machining parameters, cutting tool materials, chip formation, stability analysis during high speed machining, etc. Cutting parameters, are selected to maximize machining performance based on cutting tool materials for different combinations of cutting speeds and workpiece materials. It takes into account stability analysis during high speed machining for the manufacturing of restorations fabricated from a wide variety of hard and soft materials.

Parameter Selection Criteria

Axsys Dental Solutions is unique as our templates incorporate a wide range of effective high speed machining techniques based on over 40 years of experience in working with small and large companies in the automotive, aerospace, die/mold, and machine tool industries. Selection criteria include:

• Workpiece specification (zirconia, PMMA, titanium, cobalt-chrome, etc.)
• Machine tool specification (spindle torque, machine and fixture rigidity,
          spindle speed and feed rate)
• Nature of the operation (face milling and ball nose end milling)
• Type of cut (roughing, semi-finishing, finishing etc.)
• Business criteria including machine time, cutting tool life, quality,
          reliability and manual labor.