Machining Cutting Tools - Industrial Engineering and Productivity Aspects

Lesson 52 of Industrial Engineering ONLINE Course

Lesson 9 of Process Industrial Engineering ONLINE Course (Module)

When Taylor started his productivity improvement studies and research, only carbon steel is the tool material. Taylor, developed high speed steel and did number of experiments to find the maximum cutting speed, feed and depth of cut combinations for various machining jobs.  This productivity improvement of machining by Taylor is the foundation for industrial engineering. Industrial engineering is engineering with productivity focus.

In the area of cutting tools, number of new materials were discovered and invented increasing the cutting speed, feed and depth of cut combinations to give very high material removal rate. Industrial engineers have to know the application of each tool materials and its best use in a machining job. They need industrial engineering knowledge base to guide them in this area of productivity improvement. They need to examine productivity aspect of each and every cutting tool element and develop ways to use the productivity potential in their process plans. Some aspects are covered in this note at the moment. They will be updated.

In area of cutting tools, there are engineering advances and developments every year. Industrial engineers have to obtain the catalogues of cutting tool manufacturers every year and have to note the capabilities of new tools. In industrial engineering there is a forward thinking which starts from the awareness of new engineering item or idea. The IE has to think over its application in the processes under his management. This is a screening process. If the idea or item is thought to be useful in the first step, the next step is to be undertaken and the process continues until a decision to use it in the process is taken. Some ideas or items will not prove feasible and they will be set aside at that stage.

Cutting-Tool Materials

High-Speed Steel (HSS) and Related Materials
Sintered Tungsten Carbide (WC).
Cermets
Ceramics
Polycrystalline Tools
Polycrystalline Cubic Boron Nitride (PCBN)
Polycrystalline Diamond (PCD)

Tool Coatings

Coating Methods
Conventional Coating Materials
Diamond and CBN Coatings

Tool  Geometry

New Cutting Tool Materials

Sintered Tungsten Carbide (WC)

Sintered tungsten carbide-based hardmetals are the most common tool materials for turning, 

milling, threading, and boring using indexable inserts as well as solid round tooling.

 Characteristics of tungsten carbides include high transverse rupture strength, high fatigue 

and compressive strength, and good hot hardness. The modulus of elasticity and torsional strength are 

twice those of HSS. 

Their main drawback is that they have low chemical and thermal stability at high temperatures, which makes them unsuitable for machining steels at high cutting speeds. For nonferrous work materials, WC tools will exhibit 2–3 times the productivity and 10 times the life of HSS tools; in steels 2 times the productivity and 5 times the life.

In the United States, WC grades are often classified into eight categories denoted C1 through C8. 

The grades are broadly divided into two classes (C-1 through C-4 and C-5 through C-8). As the number increases within each class, shock resistance (toughness) decreases, hardness increases, high-temperature deformation resistance and wear-resistance increase, and carbide grain size decreases. Therefore, an increase in cutting speed or the feed load should be followed by an increase or decrease, respectively, of the classification number for the carbide grade. 

The European classification has been adopted in ISO Standard 513. This system consists of six categories designated as P-(heavily alloyed multicarbides), M-(low-alloyed multicarbides), and K-, N-, S-, and H-grades. K-grades are suitable for use on cast iron, K01 is a wear-resistant, finishing grade suitable for finish boring with no shock, while K40 is a tough grade suitable for rough milling. The N-grades are for nonferrous metals (i.e., aluminum), the S-grades are for superalloys and titanium, and the H-grades are for hard materials. K25 is for  general purpose milling. 

Straight cemented tungsten carbide and alloyed WC-Co grades

Two basic classes of carbide materials are used for cutting tools: two-phase WC-Co (straight cemented tungsten carbide; grades K01 through K40, or C1 through C4), and alloyed WC-Co grades, in which part of the WC is replaced by a solid solution of cubic carbides, such as titanium carbide (TiC), tantalum carbide (TaC), niobium carbide (NbC), or a combination of these materials (grades P01 through P50, M01 through M50, and C5 through C8). 

Straight WC-Co grades are used for workpiece materials that cause primarily abrasive tool wear and generate a short, discontinuous chip; typical materials include cast iron, high temperature alloys, high-silicon aluminum, other nonferrous alloys, and nonmetals. When used with steels, which generally yield longer continuous chips, straight WC-Co grades often fail due to crater wear on the rake face of the tool. 

Alloying also improves cratering resistance; alloyed grades containing TiC or TaC were developed for improved crater wear resistance when machining steels. Compared to straight grades, alloyed grades have increased heat resistance, compressive strength, and chemical stability. TaC alloyed grades have a higher hot hardness and better thermal shock resistance than TiC alloyed grades.

The hardness, fracture toughness, and heat resistance of carbide grades depend on the Co, TiC, and TaC contents and on the carbide grain size. Increasing the Co content decreases hot hardness and edge wear, crater wear, and thermal deformation resistance, but increases fracture toughness. 

The compressive strength is affected significantly by the Co content, and increases with Co percentage to a maximum of about 4%. The abrasive wear resistance of cemented carbides increases with increasing TiC content and decreases with increasing TaC content. Substrates with Co content less than 6% are attractive for ferrous machining at higher speeds with smaller depths-of-cut and limited interruptions.

Grain Size Differences

The WC grain size affects the tool’s hardness, toughness, and edge strength. Typical WC grades have 1.5–5 μm grain size. Fine grain WC is used for inserts and for solid drills, reamers, end mills, and other rotary tools. Coarse grain WC (2.5–6 μm) has higher fracture toughness but less wear resistance than finer grades and is used primarily for roughing. 

Since hardness increases with decreasing grain size, micrograin carbides exhibit superior resistance to crater wear, notching, and chipping in semi-roughing through finishing applications and perform well in applications where normal carbide grades tend to chip or break. 

Micrograin carbide grades are classified as fine grain (<1.3 μm), finest or submicron grain (<0.8 μm), ultra fine grain (<0.5 μm), and nano grain (<0.2 μm). 

Ultra fine grain carbide is used in special cases such as rotary tooling for high speed and/or high throughput machining applications in ferrous materials and is becoming common for high performance rotary tooling. It improves cutting edge strength and stability, preventing premature chipping and material loading and allows sharper edges because ultra-fine grain carbide is 15%–20% tougher than common carbide grades (grain size 2–3 μm with 8% Co). Nano-phase carbides are being developed for very small tools and for round high performance tools for exotic materials. An improvement of carbide performance is obtained by providing a cobalt-enriched (binder phase) layer (about 13–25 μm thick) on the surface of the tool or the tool corner. This enriched layer contains two to three times the cobalt concentrations of the bulk material. This improves the toughness of the cutting edge and resistance to chipping.

Basic guidelines for selecting carbide grades are:

 (1) use the lowest Co content and finest grain size, provided edge chipping and tool breakage do not occur; 

(2) use straight WC grades when abrasive edge wear is of concern; 

(3) use TiC grades to prevent crater wear and/or both crater and abrasive wear; 

(4) use TaC grades for heavy cuts in steels.

Cermets

Cermets are TiC-, TiN-, or TiCN-based hardmetals often described as ceramic or carbide composites. The physical properties and application range of cermets generally fall between those of WC 

and plain ceramics. Cermets are less susceptible to diffusion wear than WC and have more favorable frictional characteristics.  They can be used with sharp cutting edges in many finishing applications, which enable achievement of smooth surface finishes.

Contemporary cermets consist of TiC and TiN particles sintered with a refractory metallic binder, usually composed of nickel (Ni), cobalt (Co), tungsten (W), tantalum (Ta), or molybdenum (Mo). 

Cermets are generally available in three grades—hard, tough, and (relatively) tough but 

hard. 

Hard cermets are used in applications requiring high resistance to wear and plastic deformation, such as semi-finish and finish cutting of steels, stainless steels, free machining aluminum, and 

other nonferrous alloys (brass, zinc, and copper) and some cast irons.  

Tough cermets are also used in semi-finish and finish applications (especially milling) and in some 

rough continuous cuts in low alloy steels, stainless steels, ductile irons, and hard steels. 

The tough but hard grade is used for turning and boring and for finishing milling operations. 

In some applications, cermets provide 20%–100% longer life than coated carbides. 

The advantage of cermets over carbide is operation at much higher surface cutting speeds, with longer 

cutting edge life. 

Cermets are generally used in semi-finish to finish applications and especially high speed finishing applications. The allowable cutting speed is lower than that attainable with ceramics. 

Cermets tend to be more shock resistant than ceramics. 

Coolant is often recommended for finish turning, threading, and grooving with coated cermets. 

Micrograin cermets have much better thermal shock resistance, allowing coolant to be used in all operations.

Ceramics - Ceramic Tools

Ceramic cutting tools can be divided into four categories

1. Alumina and Alumina mixed with zirconium oxide
2. Alumina-titanium carbide composites
3. Reaction-bonded silicon nitride (Si3N4, RB)

Si3N4 is the most appropriate ceramic tool material for machining cast iron at a speed up to 1200 m/min.

 4. Silicon carbide whisker-reinforced alumina, [SiCw-Al2O3]

Polycrystalline Diamond (PCD)

PCD, the hardest of all tool materials, exhibits excellent wear resistance, holds an extremely sharp edge, generates little friction in the cut, provides high fracture strength, and has good thermal conductivity. These properties contribute to PCD tooling’s long life in conventional and high speed machining of soft, nonferrous materials (aluminum, magnesium, copper, and brass alloys), advanced composites and metal-matrix composites, superalloys, and nonmetallic materials. PCD is particularly well suited for abrasive materials (i.e., drilling and reaming metal-matrix composites) where it can provide significantly better tool life than carbide.

PCD is not usually recommended for ferrous materials due to the high solubility of diamond (carbon) in iron. However, they can be used to machine some of these materials under special conditions; for example, light milling cuts can be made in gray cast iron at speeds below 200 m/min.

PCD tooling requires a rigid machining system because PCD tools are very sensitive to vibration.

In mass production operations, the attainable tool life may be over 1 million parts (e.g., for diamond-tipped drills or PCD milling cutters machining soft aluminum alloys). However,  tooling breaks due to vibration or rough handling might occur before wear becomes significant.

Grades of PCD  vary  between 1 and 100 μm.  Grades are grouped in several categories with average grain sizes of 1–4, 5–10, and 20–50 μm. The abrasive wear resistance, thermal conductivity, and impact resistance increase with increasing grain size, but finer grained tools produce smoother machined surface finishes. A coarse-grained PCD tool may provide 50% better abrasive wear resistance than a fine-grained tool, but produce a surface with 50% higher roughness. New laser-honing methods can reduce edge radii for coarse grained PCD and produce finer finishes with these grades. Because of their increased impact and abrasive wear resistance, coarse grades are preferred for milling and for machining high-silicon aluminum alloys and metal-matrix composites.

Multimodal PCD grades (made with bimodal, trimodal, or quadimodal distributions of PCD particles) provide the high abrasion resistance of coarse-grained unimodal grade with the high toughness and superior edge sharpness of medium-size grain tools. The PCD density increases with multiple particles sizes. Multimodal grades are less prone to chipping than unimodal grades.

Laser structuring has recently been applied to flat-topped PCD inserts to produce 3-D chipbreaking grooves and similar features, which have proven effective in ductile material applications where chip control has traditionally been an issue.

PCD-tipped HSS or carbide rotary tools (e.g., reamers, end mills, drills, etc.) are available in a limited range of geometries due to difficulties in grinding complex geometries, particularly on small diameter tools. More complex geometries can be used on carbide rotary tools by sintering the diamond into slots (veins) located at the point and/or along the flutes.

Issues to be resolved include identifying the optimal cutting edge geometry for the diamond tip and the best method of pocketing the polycrystalline blank for strength and manufacturability. The methods of brazing the polycrystalline/carbide substrate tip to the main tool body have been improving steadily, but one of the major failure modes is still the detachment of the polycrystalline tip or the wear and erosion of the braze joints intersecting the cutting edge. Wear and erosion of brazed joints is avoided when the diamond is sintered into veins within the carbide tool.

BASIC TYPES OF CUTTING TOOLS
The six basic types of cutting tools are solid tools, welded or brazed tip tools, brazed head tools,
sintered tools, inserted blade tools, and indexable tools

https://www.productionmachining.com/search?q=PCD

Case Studies

Production Machining - Cutting Tool Case Studies

Systems Approach to Tooling

Advanced Turning Insert Selection - Mitsubishi Course
http://www.mitsubishicarbide.com/permanent/courses/75/index.html


17.1.2020
Pocketing with high speed router RAL 90

The RAL90 aluminium milling cutter is designed for extremely high metal removal rates. The extra robust cutter body with optimized insert seats sets the standard for a new level of process stability in high speed milling - ideal for heavy roughing to semi-finishing pocketing of aerospace frames in aluminium alloys.

In applications requiring even higher metal removal rates, the new RAL90 Super MRR milling cutter can reach extra high spindle rotation, e.g. up to 33000 RPM for DC 50 mm compared to 23500 RPM for RAL90. This means a 40% productivity increase.

Machining aluminium for lighter and better recyclable vehicles
https://www.sandvik.coromant.com/gb/News/technical_articles/pages/machining-aluminium-for-lighter-and-better-recyclable-vehicles.aspx

Options, Benefits and Applications of Machining with Ceramic Turning or Milling Inserts
26/06/2019
https://www.cutwel.co.uk/what-are-the-benefits-of-machining-with-ceramic-turning-or-milling-inserts

12.2.2019

Kennametal’s KBH10B and KBH20B grades are designed for hard turning. They are available in double-sided inserts for materials as hard as 65 HRC.  The inserts are  for “high-volume production of hardened gears, shafts, bearings, housings and other drivetrain components. A ceramic binder structure and TiN/TiAlN/TiN coating provide extreme wear resistance even at elevated cutting speeds.  A gold PVD coating makes it easier to identify when an insert needs indexing, while the numbered corners ensure that a machine operator does not inadvertently switch to a used edge. Edge preparation in a “trumpet-style” hone, is  for heavier and interrupted cuts. 

A light hone edge inserts are for continuous turning. Both inserts give extending tool life and generate surface finish values as low as 0.2 Ra.

https://www.mmsonline.com/products/pcbn-inserts-from-kennametal-make-hard-turning-more-cost-effective

2014-07-29 (Reg Ral 90)
An optimally designed, high-precision insert seat with seat numbering ensures a maximum runout accuracy of 20 microns axially and 15 microns radially, a feed rate of 0.3 mm/tooth and cutting depths of up to 14 mm.
https://www.sandvik.coromant.com/en-gb/news/press_releases/pages/ral90.aspx

Sandvik Coromant 2020 catalogues
https://www.sandvik.coromant.com/en-gb/downloads/pages/default.aspx

https://www.sandvik.coromant.com/en-gb/downloads/pages/default.aspx

News of New Cutting Tools

Alternative Metal Cutting Tools - Productivity Engineering Applications

https://nraoiekc.blogspot.com/2020/09/alternative-metal-cutting-tools.html

Updated 22.7.2023,  6 January 2021, 8 July 2020