In-House Grinding and Regrinding

Over the years, the need for stronger, tougher, and more corrosion- or oxidation-resistant materials has been the driving force behind much technological advancement. Particularly with widespread use of the jet engine, there has been a demand for materials with mechanical and chemical properties that can withstand higher temperatures. Originally, steels and stainless steel alloys were used in aerospace applications. But the drive for hotter, more powerful, more fuel-efficient engines led to the development of super-stainless alloys or super-alloys.

To answer these advancements, machining centers and CNC mills have increased in rigidity, accuracy and productivity. With this development, the cutting tool industry has also undergone massive changes in the manner in which tools are designed and produced. Recent developments in design and the use of rotary cutting tools in the aerospace and engine industries have given way to an entirely new line of cutting geometries.

Many mechanical and software adaptations have been made on tool grinding machines in order to accommodate the requirements of high performance cutting tools. Tolerance requirements, TIR (total indicator runout) on all cutting teeth, unequal index, variable helix, specific flute shapes and different point geometries are only a few of the features that have been added to what used to be known as a standard endmill or drill.

The regrinding industry is also experiencing changes. In-house production and regrinding tools are becoming more popular, and this is directly linked to advantages such as total control over geometry, the ability to test different tool geomteries (tweaking) and gather in-house knowledge of how to mill different and new types of materials, documented tool geometries, and predictability of tool life, among others.

Companies that can justify the cost of a CNC grinder often realize substantial savings on regrinding costs and prices for new tooling, especially if there is no outside vendor involved in designing and maintaining the tool geometry. If the company works with lean manufacturing and JIT procedures, in-house regrinding becomes even more financially attractive.

In order to ensure uninterrupted and consistent machining, the reground tool has to have equal performance on each single regrind like the new tool. Likewise, the new tool has to have the identical geometry as all future regrinding jobs. By implementing a formal process for the production and regrinding, similar to ISO procedures, the company can track the geometry and the performance of the tool. This also allows continuous improvement and the fulfillment of lean manufacturing.

Type of Machines

To fulfill the multiple high-tech features on endmills and drills, it is best to opt for a 6-axis CNC tool grinding machine. Six axes provide flexibility and freedom for producing cutting geometries without compromising quality.

Larger Tool Diameter

The accuracy demands on the larger diameter range, ¾" and up, can be ideally manufactured with a hydrostatic tool grinder that has increased stiffness and rigidity. Such a tool grinder has three linear and three rotary hydrostatic axes and a hydrostatic grinding spindle.

Due to the high damping ability, this powerful machine offers vastly better surface finish, consistent size and geometry tolerance, the best possible concentricity on endmills, and improved cycle times. With the hydrostatic principle, there are no mechanical contact elements such as balls or rollers in any of the guideways or the rotary assemblies. Much of the damping ability of the machine during grinding is obtained from the hydrostatic design of the grinding spindle. Vibrations are reduced, resulting in better wheel life.

Smaller Tool Diameter

Contour accuracy on ballnose or corner radius is an essential part of grinding or regrinding aerospace endmills. A 6-axis tool grinder must meet this challenge by including mechanical and software features that can guarantee a radius contour accuracy of 0.0003" throughout the radius, from the center to the OD, and throughout the grinding range of the machine.

Desktop Tool Design Software

A 3D tool simulator provides a realistic image of the tool. This is particularly useful to add features to endmills and drills that standard catalog items do not have, such as corner radius, corner chamfer, special gashes, step drills and form tools, S-spiral gash for large corner radius, super S-gash ballnose, and many more. Machine animation and collision tests enable the operator to perform a realistic 3-D simulation of the entire machine. It allows you to see the entire grinding process and simultaneously check for errors to observe potential machine collision before any blanks are ground.

Materials and Blanks for Endmills and Drills

For in-house tool production, it is necessary to have a competent material source. Both carbide and HSS blanks can be competitively purchased by companies specializing in blank preparation. These companies can advise you about the different types of carbide grades available and differently produced and hardened HSS blanks. For best results during in-house tool grinding, the blanks should be centerless ground and chamfered on one side. Drill margins should be finished to size and back-taper, and step tools should be plungeground to final shape.

Surface Finishing, Edge Preparation and Coating

These types of tool enhancements have become an integral part of cutting tool manufacturing and are particularly applicable for aerospace. Edge preparation and surface finishing can generally be done in-house, whereas hardcoating may have to be contracted out to the nearest coating center, since it is not cost-effective for end-users to have in-house coating. There are many companies nationwide that offer both of these services.

Edge preparation on carbide drills has been performed for many years. It is now becoming increasingly common for carbide endmills in aerospace to have an edge preparation. In general, carbide tools do not cut very well if the cutting edges are left sharp. Sharp edges are generally weak and subject to chipping. Brushing machines are used for creating a rounded hone, and hand-honing is used to generate a negative flat chamfer.

Edge geometry on cutting tools, particularly on high performance drills, affects tool performance in several ways. Edge condition influences the reliability and consistency of the tool. Properly honed tools can vastly improve the repeatability of machining operations, which helps to maintain lean manufacturing where consistency and continuous improvement is essential. It will also permit a higher level of unmanned manufacturing. Correct edge preparation will improve tool life by reducing the common causes of failure such as chipping, material build up on the cutting edge and other heat-induced problems. Total tool life is also a product of the number of regrinds, and properly honed cutting tools will generally allow more regrinds, since less stock removal is needed per regrind.

Some of the high-performance endmills have a flute polish that can be done by the CNC grinder in the same grinding cycle. Other polishing methods are more complicated and involve a process called drag-finish, which is done on a separate machine.

Tool coatings give distinctive advantages to the performance during machining and to the life of the tools. Traditional coatings (TIN and related coatings) are still being used for machining certain materials. PVD (physical vapor deposition) and CVD (chemical vapor deposition) are mainly used on carbide tooling for machining advanced materials. Coatings produce better surface finish, providing even better chip flow characteristics.

Whether CNC grinding of cutting tools should be done in-house or outsourced is an individual decision that depends primarily on the culture of the company, the availability of skilled labor, the size of the manufacturing business and the yearly consumption of cutting tools.