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.
1. Introduction to Cutting Tool Edge Preparation:
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.
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.
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.
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 its 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 years production.
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.
Factors that drive manufacturing costs are:
Extend brush life, reduce
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.
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.
The "Bad News":
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.
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.
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
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