Cutting Tool Edge Preparation

Abstract: Proper selection and application of cutting tool edge preparation is considered one of the basic ingredients for a successfully manufactured and correctly performing hardmetal-cutting tool. This paper develops a basic understanding of the need for proper cutting edge preparation, specifically edge honing. The discussion is aimed at the process of nylon abrasive filament brush honing and how it effects the tool as an end product. It also identifies some of the manufacturing related pitfalls and the problems that can drive manufacturing costs. Edge preparation is a necessary process in nearly 100% of all hardmetal tooling.

Terms:
Edge Prep
Brushing
Waterfall Hone
Honing
Radius Hone

1. Introduction to Cutting Tool Edge Preparation:
(Evaluating "the need"). The tool edge preparation process, when administered properly, adds strength to the tool cutting edge, lengthens usable tool life, minimizes the propensity of the edge to chip, improves part quality and consistency, and enhances work piece surface finish. Some of the edge preparation options currently used by the tool manufacturer are up sharp (no edge prep after grinding), radius or waterfall hone shapes, T-Land (or K-Land), and T-land + hone. The most widely used edge preparation that exists in industry today is the radius and waterfall hone shapes. These edge preps are applied in a variety of sizes based on cutting tool size and application. Edge prep is not limited to application of indexable style tooling. It is required on most hardmetal round tools, "brazed on" tools, and single/multiple edge form tools.

2. Cutting Tool Edge Preparation, "The Beginning"
Edge honing was initially instituted to remove the rough edges produced in the powder metallurgical process of part manufacturing. (Nearly all companies produce the "green" blank using basically the same manufacturing method prior to the densification process of sintering). In recent years honing has become a necessary prerequisite for some methods of tool coating. Although times have changed and cutting tool manufacturing technology has improved greatly, the need for the controlled removal and smoothing of the edges of the cutting tool after sintering remains. During the last twenty years or so of modern carbide tool development, more emphasis has been put on honing to enhance tool life and tool performance. Both tool life and tool performance are greatly dependent on the correct size and geometric shape of the cutting tool edge preparation.

The "Original" Insert Honing Processes
Over the years edge honing on cutting tools has been accomplished by various methods. Vibratory honing, honing by hand with diamond stones, mass media honing, slurry honing, honing inserts with media impregnated rubber wheels, dry blasting, wet blasting, and tumbling are among the many processes still in use. Edge honing was (is) considered an "art", and many tool producers inventing their own processes had to protect them closely as guarded trade secrets. Edge honing was truly one of those competitive advantages that could help a company improve total tool performance over a competitor.

In the early honing processes, there were many variables that required controlling. Honing machine process variables and incoming part quality variability virtually required every batch of parts to be "learned" each time a new set up was made.

Edge honing in the early years was high in labor intensity and costly. But, being able to hone effectively is one of the keys to success in the cutting tool industry. "Effective honing" can be best defined as the most accurate, repeatable process that is lower in total cost than the competition.

Introduction of Brush Honing Processes
In the late 1970’s brush honing made its appearance in the cutting tool industry in two distinctly different processes. One brush application was the nylon abrasive filament process. The other brush process used a vegetable fiber brush with an oil and diamond paste. Due to process economics, nylon abrasive filament brush honing became the method more widely used for edge preparation. Considering the 20-year presence of abrasive filament brush honing in tool production, the technology connected with filament brush honing, even today, is still somewhat limited. During the initial years of use of the filament brushes, there existed a lag in the technological development of brush honing. This lag was driven in part by the insert manufacturers feeling the need to develop their "own" process utilizing the new brushing techniques. With limited standard process information readily available to the industry, a true technology gap developed and quickly widened between the filament brush manufacturers, hone machine manufacturers, and the cutting tool industry. Abrasive filament technology, not being completely understood, was cause for the brush honing process, in most cases, to be misapplied. The misapplication of abrasive filament brushes in tool edge preparation spawned many costly associated manufacturing problems, some of which still exist in the industry. This lack of understanding over the years has had a substantial impact on manufacturing costs.

Nylon Filament Brush Honing
As stated earlier, probably the widest used of all processes for edge preparation utilizes the Nylon Abrasive Filament (NAF) brush. Made of heat-stabilized nylon filaments impregnated with abrasive grain, the individual nylon filaments are designed to work like flexible files. Conforming to the part contours, the brush filaments wipe and file across part edges in an action that rounds and shapes the cutting edges. Nylon abrasive filament brush technology was invented many years before the application of the NAF brush for insert honing.

In the brush industry, there exist only a few manufacturers in the world producing the actual abrasive nylon filament. The filament is manufactured in several configurations. The filament can be round or rectangular in cross-section. The round filament can be straight or crimped. The filaments can be produced in different diameters as well as different media types, grit grades and percentage of media content.

In the filament brush community, there are many companies that assemble and market brushes. With all brush producing companies in the industry using basically the same source for nylon abrasive filaments you would assume brushes to be uniform in construction. However, the brush manufacturing assembly techniques differ and so do the finished products. Each assembly technique exhibits it’s own distinct characteristics when used in the application of tool honing. The difference in brush manufacturing techniques is fairly easy to see when the brush is used in the difficult application of hardmetal tool honing. The ability to hold microscopic edge hone sizes in the finished product can vary from manufacturer to manufacturer as well as the amount of dust emissions from each of their brush products. There is also a fairly significant difference based on brush life. These factors can drive the manufacturing costs up or down when brush cost per piece is calculated. With the tooling manufacturers producing millions of tools each year, even a fraction of a cent difference in per part brush costs is significant when applied to a total year’s production.

Today, the demands put on the cutting tool industry by their customers, using the new high-tech metalcutting equipment, and addressing the cutting requirements of exotic materials are high. With the new materials bombarding the industry and the ability for CNC machines to run in a "lights-out" untended environment, the quality demands are increasing on cutting tools. The need to produce more predictably performing tools is paramount to the industry and no one expects that trend to change.

3. Variability of Quality and Cost in the Honing Process
Since the arrival of NAF brushes for honing, the process of honing has become somewhat more predictable. (Nylon brushes are more robust in the application of tool edge honing and are not affected as much varying hardness tool substrate materials. The NAF brush has the abrasive media uniformly distributed within each filament. (There are always new, sharp particles of the media being exposed during processing as the nylon material in the filament wears away.)

In the process of edge honing, the most controlling variable that affects finished part quality is the in-coming part condition. Quite often the cutting edges within each cutting tool coming from the sintering operation are not consistent. They can vary edge to edge, corner to corner, or top to bottom, (in any combination). Another variable considered are the honing characteristics of different cutting tool materials and sometimes-even variability within batches of materials will appear. (Keep in mind the process of edge honing is "controlled" erosion of the tool edges, and carbide exhibits a very high resistance to wear). Consequently, the edge honing process needs to be tweaked as an "artsy", delicate balance of hone machine parameters (brush grit size, media grit size, part dwell time, etc.), set by the operator to accurately remove the sharp corner by wear.

The duty of the operator is to control this process of wearing the cutting edge before the wear exceeds the original hone specification. Depending on the honing process in use, if the media or methods of the edge erosion were too aggressive, uneven material removal on the cutting tool would result having specific sections of the tool being "over-honed" (beyond specification).

Variability in hone uniformity can also be linked to several components that exist in the manufacturing system. Sometimes "up-front" variables are built into the part making it difficult for an operator to make an "in-specification" product. Some of those process variables (in a non-ground as molded part) are as follows:

 Excessive die flash. This condition can be caused by press tooling that is either worn, damaged, or out of dimensional specification causing a "flash" or ragged edge to be formed around the cutting edges of the parts. Coming from the sintering process, the hone operator must consider the magnitude of this ragged edge and try to remove it during the hone process. The edge hone operator is required adjust his machine process parameters (if possible) to remove the flash and at the same time achieve the final size edge according to specification.

 Damaged edges caused by handling tools before sintering. This is a special problem consisting of the cutting edges of the insert being distorted. The inserts, as pressed, are chalk-like in consistency and touching the cutting edges can round them prior to sintering. Once sintered, these rounded edges need to be addressed in the hone process. Sometimes these rounded edges from handling can be greater in size than the specified finished hone size. (The parts then become scrap). The operator must assess the part condition and set machine parameters (if possible) to offset the effects of the distorted edge shape. (This problem is becoming less of a factor in manufacturing because it has been minimized with robotic unloading of tool producing presses.

 Most inserts require a process prior to sintering to remove the excess die flash. After the process of removing die flash in the "green’ state, final edge condition can vary greatly, depending on the skill of the person removing the flash. This uneven condition can vary from cutting edge to cutting edge and from top to bottom within the insert. This too is a condition the operator must assess prior to honing. The hone operator is then required to set the proper parameters of the machine (if possible) in an attempt to make the parts finish within specification. (Although still present, as in the case with damage from part handling, it is becoming less significant as a process variable with the improved technology related to manufacturing of press tooling).

 Insert honing can be an accurate and relatively low cost process if applied correctly. In the case of correct application of NAF brushing, using either flat or round filaments, a nominal hone size can be accomplished in a very short cycle time. The size and shape of the hone is determined by a combination of several machine set-up parameters. One set-up parameter that is generally held constant is dwell time. Dwell time is used to control production rates while other machine parameters are varied to maintain uniformity and shape.

Significant Factors that drive manufacturing costs are:
 Set up (Sometimes this process can consume excessive amounts of time obtaining machine settings for correct size and shape of the hone. This is based on in-coming part condition and operator ability)

 Average lot size. This variable will determine the number of set up changes required in a day.

 Nylon smearing This phenomenon was introduced to the cutting tool industry by NAF brushes. Incorrect set up and incorrect machine parameters basically cause nylon smearing. If nylon smearing goes undetected, a secondary, costly, non-value added process is required to remove the substance

 Process time/cycle time. Cycle times can also fluctuate and normally is driven by hone size and incorrect set up parameters.

 Producing large hone sizes, Hones above .005" can consume both cycle time, initiate increased brush wear, and introduce nylon smearing caused by elevated processing heat.

 Tool changes and brush changes, (These functions can drive up costs due to the time consuming process of tool and brush changing). This also necessitates a new set-up to keep the parts in specification.

Other process related problems that can increase costs:
 Controlling the inherent dust associated with dry abrasive filament brush honing.

 Disposing of coolant used during wet filament brush honing, slurry honing, or wet blasting processing

 Elevated temperatures of the cutting tool being honed dry can make handling difficult.

 Accurately measuring the hone size and subsequent size adjustments require the machine to be stopped, thus reducing output.

 Brush costs can be excessive if the brushes are misapplied. Misapplication can come in the form of nylon smearing, filament fatigue/breakage, high insert heat, and excessive dust.

 Improper training (since many processes were developed "in-house", training can be fragmented at best. Most of the time an operator will train another operators. The danger lies in the possible loss of process information in the unstructured exchanges between operators).

 Insert binder material leaching. (Leaching can contribute to the total scrap percentage. This problem is normally associated with any wet process used in the final production of carbide tools).

Extend brush life, reduce costs.
The rule is the same for any material removal process, the tool (in this case abrasive brush) must be matched to the job requirement. In the application of insert edge preparation, abrasive filament brush grit size is a key factor in obtaining uniformity in edge hone shape and size. Brush life will vary greatly with changing hone size requirements. That is why the brush application must be correct, otherwise, brush life will suffer and consequently, process costs will increase dramatically.

4. Proper insert edge preparation is key to consistent tool life
Most cutting tool experts understand the primary purpose of a honed cutting edge is to increase the mechanical strength and impact resistance of the insert edge. The properly applied hone adds value by adding predictable life to the tool. To get the proper results and to make the entire process function, the correct hone size must be selected based on insert size, geometry and the application of the particular cutting tool. The hone size/shape variations can range in size from less than .0005" to well above .008", depending on the insert size and cutting application.

Brush Honing Equipment
In many of the commercially used brush honing machines, a continuous rotating table moves the cutting tools through the (high speed, above 1,000 RPM) rotating nylon filament brush. There is an arc-like feeding motion of the rotating part. While the part is in contact with the brush and because of the arc motion through the brush, brush pressure will constantly vary. Therefore the brush pressure is reaches the highest point when the insert is at the tangency of the rotating table. The brush pressure begins to reduce as the part continues to move through the brush. This type of processing can possibly manifest in uneven edge hone sizes within each insert, even if the incoming quality of the insert is perfect.

Initially, when NAF brushes were introduced for honing, no machines existed that were specifically designed for hardmetal tool edge honing utilizing the filament brush process. Instead, existing machines designed for other applications, such as polishing and buffing machines were slightly modified and used for production insert honing. The use of modified existing equipment for brush honing filled the manufacturing need. A product of using converted machinery delayed machine development of specific machines and as an industry put development "on the back burner". Because of this, industry wide research, gathering of technology, and the growth and understanding of the process of insert honing was limited. Since that time, most work in brush hone machine research and new equipment development was patterned closely after the original modified machinery.

Along with the limited availability of specific process equipment, there also existed a lack of available equipment for accurately measuring the edge hone. Given the lack of standard machines for honing and measuring machine availability, most individual tool manufacturers developed their own hone processes and adopted their own quality standard for measuring hone size. (Accurate measuring is essential as the first step in developing an industry standard). Taking these circumstances into account, the true development of both the honing process and measuring became an R & D function of the cutting tool manufacturers.

An interesting comparison of technology growth can be drawn between the processes of tool edge honing and hardmetal tool grinding, two completely different processes. As you compare the tool edge honing process with an equally important tool grinding, (both processes involve controlled mechanical removal of material), you find that development of the grinding equipment technology and parameters are much more advanced. R&D of the grinding machine tool producers along with new grinding wheel technology have paced the advancements in hardmetal tool grinding. Because of these combined efforts the state-of-the-art in grinding is highly advanced. Grinding parameters in most companies are still held very "close to the vest" and considered secret, but at the end of the day with similar grinding machines, grinding a similar volume of material, grinding yields will normally be within 10%. The tool grinding process has become a science.

5. Where is the Tool Producing Industry Today?

In the process of hardmetal tool edge honing, this process still remains an art. Correct honing, for the most part, is completely dependent on the best fit of part condition and is limited by machine variability and operator expertise. Most of the time, the honing process is still the best educated guess of an operator experimenting with process variables to get acceptable results.

The "Good News":

Technology and equipment exist today using the nylon abrasive filament brush technique to overcome nearly all the current process problems:

 Machine set-up can be reduced to minutes.

 Hone uniformity can be greatly enhanced even in the inserts with complex geometries such as threading/grooving tools, full form threading and even wide form tools. Wavy edge geometric shapes and helical milling insert geometries should no longer be an issue for uniform brush honing.

 The ability exists to hone and exercise size control on each surface of the insert independently. This permits changing hone size within the insert to further enhance cutting ability.

 Machinery equipped with minimum numbers of insert stations/fixtures and with CNC controls for quick set up can better handle small lot sizes more effectively.

 Using small, low cost CNC honing machines presents the opportunity to combine processes to reduce lead time and reduce labor costs.

 Nylon smearing can be eliminated by correct application and set up of the NAF brush to keep processing heat low.

 Quick-change brush tooling allows the rapid change of brushes. This permits the operator to match the brush grit value to the finished hone size. (Application of the correct tool for the correct job).

 Dust associated with normal dry brush process can be reduced greatly if the correct brush is applied

 Cutting tool materials such as Carbide, Cermet, Ceramic, PCD and PCBN can all be effectively processed quickly and accurately using new brushing technologies.

 Proper training is available to further enhance the impact of the new machinery and technology.

6. Conclusion
Hardmetal tool edge honing, in recent years has been universally recognized as one of the four main ingredients required in successful insert manufacturing. These four ingredients include (1) tool substrate composition, (insert base material), (2) tool geometry, (3) proper coating (PVD and CVD among others), and (4) edge preparation.

The first three of the four ingredients are the end result of much research and development to make them more reliable, repeatable processes. The fourth ingredient, tool edge honing, needs to break away from being an "art" to join the other above mentioned three ingredients as a controllable mechanical process. Further advancements in cutting tools materials and the capability of new cutting techniques will weigh heavily on this technology quickly moving forward. The technology for better control of tool edge honing exists and the machinery is designed.