The key to boosting productivity is to remove more metal in less time. However, faster cutting speeds and feeds require tooling inserts that can take the heat and abuse without dulling. To achieve these goals, tooling suppliers have come up with various cutter shapes, edge geometries and surface coatings that significantly improve tool performance and life.
One way to achieve better tool performance is to use an insert that cuts easier so less horsepower is required to turn it, even at high speeds and feed rates. An insert that cuts cleanly and easily will produce a smoother finish and experience less vibration and chatter.
The angle at which an insert is held with respect to the work surface affects how efficiently it cuts. Positive rake tooling holds the insert at a slight angle so the edge of the insert cuts into the surface of the metal, sort of like a plow digging into soil. Positive rake tooling generally requires less cutting force and pressure, runs cooler and provides longer insert life.
But it can also increase edge chipping on inserts, so it typically works best with shallow cuts and lower speeds and feeds. Also, a positive rake only allows the upper edge of the insert to be used, preventing the insert from being flipped over when the edge becomes dull. Thus, a four-sided positive rake insert would have only four cutting edges (each side of the square on the top side).
Negative rake tooling, by comparison, holds the tool perpendicular or at a slight negative angle to the work piece so the edge of the insert drags across the surface with more of a scraping action as it cuts. This requires more pressure and horsepower to machine the metal, and it produces more heat. But a negative rake also provides more support for the insert and allows deeper cuts and high stock removal rates at higher speeds.
A negative rake also doubles the number of available cutting edges that can be used on a single insert, thus doubling tool life. A square insert can provide eight cutting edges, and a triangular insert can provide six edges. When all of the edges on one side of the insert become dull, it can be flipped over so the edges on the opposite side can be put to work. Negative rake tooling works well for machining hard, brittle metals.
The development of carbide materials with extremely fine grain size has improved the durability and performance of carbide cutting tools. Carbide valve guide reamers and carbide valve seat cutters are indispensable tools in most shops today. Carbide inserts are also great for high speed milling and boring operations. Newer style boring machines with spindle speeds that go as high as 1,200 rpm have cut the time it takes to bore a typical V8 in half. Coated carbide inserts usually work best with the higher speeds and feed rates.
According to a presentation given at a recent AERA gathering by industry veteran Ed Kiebler from Rottler Mfg., positive rake-1/64? radius black ceramic-coated inserts work well for through-boring and machining counter bores. The presenter recommends using a feed rate of .002? to .005? per revolution, and a surface-feet-per-minute (SFPM) speed of 800 to 1,200.
The surface feet per minute value can be calculated by multiplying the rpm of the cutter head times its diameter (in inches), then multiply by 3.14, and divide by 12. For example, a 12? cutter head rotating at 1,000 rpm would generate a SFPM value of 3142.
For heavy sleeve cutting and general machining, positive rake 1/32? radius coated carbide inserts are recommended. The 1/32? inserts should be run at a feed rate of .006? to .012? per revolution, with 800 to 1,200 SFPM in gray cast iron. The larger radius of the 1/32? insert leaves a smoother finish than the 1/64? insert, which will improve metal-to-metal contact for better heat transfer between the sleeve and block when the sleeves are installed in the cylinders. Kiebler also says you can run the 1/32? inserts up to 30 percent faster than 1/64? radius inserts, but it will also require more tool pressure.
For boring machines that use a negative rake, black ceramic-coated square inserts are good for removing .010? to .060? of material. Use 1,000 to 1,200 rpm and a feed rate of .008? to .012? per revolution when cutting four inch bores. The same inserts can also be used for sleeve cuts when a square step is not required. Use the same speed as before (1,000 to 1,200 rpm) but a slower feed rate of .005? per revolution with up to .200? of metal removal per pass.
Kiebler warns that speeds should be slowed down when boring compacted graphite iron (CGI) or ductile iron blocks. Machining CGI is more difficult than gray cast iron because of the increased hardness of the metal. Coated ceramic or CBN (cubic boron nitride) inserts work well for this type of job, but tool wear is accelerated and feed rates may have to be slowed to achieve the same surface finish. A speed of 200 to 400 SFPM on these applications is recommended, with a feed rate of .006? to .010? per revolution.
As great as coated ceramic is for boring and milling operations, CBN is far better, particularly on cast iron. CBN is second in hardness only to diamond. On the Knoop Hardness scale, CBN is rated at 4,700 compared to 7,000 to 9,000 for diamond, 2,400 for silicon carbide and 2,100 for aluminum oxide. As for abrasion resistance, CBN is nearly as good as diamond: 37 for CBN versus 43 for diamond, which is far and above the ratings for silicon carbide (14) or aluminum oxide (9).
CBN inserts for resurfacing applications are usually round and available in 3/8? and 1/2? sizes, single- or double-sided. CBN inserts will last much longer than carbide inserts, and are well-suited for applications such as resurfacing diesel cylinder heads with precombustion chambers. Some inserts may shatter when they hit the precombustion chambers, but a 1/2? diameter solid CBN insert will cut it cleanly without the chatter or streaking that often results when carbide inserts are used for this purpose.
Single-sided CBN inserts are recommended for resurfacing hard castings such as compacted graphite blocks, while the double-sided inserts are good for general resurfacing on ordinary cast iron.
When CBN inserts are used in a high-speed boring machine, they can typically handle rates ranging from 1,000 to 2,200 SFPM when stock removal is limited to about .040? per pass. CBN is recommended for machining cast iron, and it can also be used for aluminum, provided a lubricant is used to prevent the metal chips from sticking to the CBN inserts. However, PCD (polycrystalline diamond) inserts are usually the better choice for machining aluminum and should only be used for aluminum (not cast iron).
Though diamond is harder than CBN, it can’t take as much heat as CBN. At high cutting speeds, diamond gets too hot and reacts chemically with iron causing it to lose its cutting edge. This isn’t an issue with diamond honing stones because the cutting speed is much slower, and a coolant is used. But in a high-speed milling machine, diamond inserts will overheat on cast iron. CBN can withstand temperatures up to 2,500° F while 1,500° F is the limit for diamond before it starts to suffer adverse effects. CBN also dissipates heat about four times faster than carbide, which helps it retain its sharp cutting edge.
What makes PCD the best choice for machining aluminum is the fact that diamond can cut through the hard particles of silicon in the relatively soft aluminum matrix without dulling. CBN can lose its edge rather quickly if it is used to machine a high silicon alloy aluminum head or block.
The type of insert used will also determine how much metal you can remove with a single cut. CBN inserts typically have a honed edge, so the minimum depth of cut is usually limited to about .004? or .005?.
To cut efficiently with negative rake tooling, CBN inserts require plenty of speed. Cast iron can be milled with CBN at speeds ranging from 1,000 to 2,000 SFPM. By comparison, aluminum heads and blocks can be milled with PCD at speeds ranging from 1,000 to 4,000 SFPM. On a milling machine with a 14-inch cutter plate, that translates into a speed of about 900 to 2,000 rpm for the PCD inserts.
Higher speeds, slower feed rates and shallower cuts typically produce the best finish. When using carbide inserts to refinish a cast iron head, a typical spindle speed might be 140 rpm for an 11? cutter, 120 rpm for a 13? cutter or 110 rpm for a 14? cutter. With CBN or PCD inserts, the speeds would be much higher: say 1,040 rpm for a 11? cutter, 880 rpm for 13? cutter, or 720 rpm for a 14? cutter.
Troubleshooting Milling Problems
Regardless of what type of inserts you are using to resurface a cylinder head or engine block in a milling machine or CNC machining center, you may not be getting the finish you want. Here are some solutions for common resurfacing problems:
Chatter Causes include lack of rigidity (check the rigidity of the spindle and fixturing), excessive cutting force (reduce the feed rate, depth of cut or with of cut, or flex in the part.
Surface finish too rough Causes include worn inserts (use a coated insert that resists wear better), built-up edge on insert (increase speed, use a coolant and/or change to a coated insert), or wiper insert set too high (adjust to .0005? to .002? above insert).
Surface finish wavy Reduce feed rate.
Surface finish not flat Use a more positive rake cutter, or a larger diameter cutter head.
Various types of ceramic coatings are used on ceramic inserts to improve tool life, while titanium coatings are available on CBN inserts for the same purpose. Coatings improve tool life by conducting heat away from the cutting edge and by resisting wear. Some coatings can double or triple average tool life.
In addition to inserts, special coatings may be used on drill bits, end mills, saw blades, taps and other tooling. These coatings are usually designed to extend tool life or cutting performance, but some are for cosmetics only.
You’ve probably seen higher priced drill bits with a gold-colored finish. The gold color is not actually gold, but titanium nitride (TiN). The coating is formed by a physical vapor deposition (PVD) process, and is only about .0001? thick. But it forms a layer with a hardness at or near Rc 80! The Titanium nitride coating reflects heat and reduces friction, allowing the drill bit to turn at higher speeds and loads without burning up. Titanium nitride coated drill bits may last two to seven times longer than uncoated bits.
There are also gold-colored coatings that resemble titanium nitride, but are simply a cosmetic oxide coating that resists rust and makes the bits look more appealing. The gold colored coating does nothing for wear resistance or cutting performance. The gold coating may be used to identify cobalt bits, but if used on high speed steel is just there to dress up the product.
Black oxide coatings, by comparison, not only prevent corrosion but also aid lubrication retention and help the tool resist galling. The coating is produced by immersing the tool in a hot oxidizing salt solution. The elevated temperature created by the coating process stress relieves the tool to improve its toughness. Black oxide coatings are typically used on bits for machining steel and cast iron.
Another bright coating is zirconium nitride (ZrN). This type of coating is vapor deposited to for a protective layer about .0001? thick. Zirconium nitride is about 30% harder than titanium nitride, and provides increased lubricity and heat resistance at high temperature. Tools coated with this process typically last two to ten times longer than uncoated tools, and up to five times longer than titanium-coated bits.
Another high performance coating is aluminum titanium nitride (AlTiN). This is a hard, black coating applied by vapor deposition to a thickness of about .0001?. This coating has a very low coefficient of friction to reduce chip welding, which allows tools to be run speeds up to 40 percent faster than titanium nitride coated tooling, and was developed for machining hardened steels. The wear-resistant coating also extends tool life up to ten times compared to titanium nitride coated solid carbide tools.
Titanium carbonitride (TiCN) is a hard gray coating, also applied by vapor deposition to form a protective layer about .0001? thick. This coating also has a very low coefficient of friction to reduce chip welding, allowing tools to be run at speeds up to 25 percent faster titanium nitride coated tooling. The coating is often used on high strength steel (HHS) for machining cast iron and high silicon aluminum alloys.