Rants and Raves regarding carbide inserts, cbn inserts, ceramic inserts, cermet inserts, and inserts in general. Occassionaly will take about nothing, and sometimes other cutting tools. Most of all a resource for information on cutting tools.
Whether you’re making brake discs and flywheels in grey cast iron, or crank shafts and wheel hubs in nodular cast iron, there is always a successful solution.
Over the years there have been a number of CBN grades made available for machining cast iron with CBN. Most notably are DR-80 and DR-85. These grades are made for the following applications:
For rough and semi-finish turning, milling, grooving and boring of hardened ferrous and certain softer ferrous materials:-
Martensitic cast irons - Ni-hard - High chrome Chilled and heat treated cast irons
Fully hardened cold-work tool steels
Bearing steels
High speed steels (continuous cutting only)
Martensitic stainless steels
Cobalt and nickel based hard facing alloys
Fully pearlitic grey cast iron
DR-80 & DR-85 are high CBN content materials, diffusion bonded at the manufacturing stage to a tungsten carbide substrate. DR-80 is (80% CBN) has a greater particle size than DR-85, (85% CBN). DR-80 is better for hardened steel, but DR-85 has better wear characteristics when machining cast iron with CBN, particularly when the stucture contains Ferrite. For more information on machining with CBN Inserts please visit David Richards Engineering UK or David Richards Engineering US
Extend Tool Life In High Production Hard Turn Applications
Increase Thermal and Mechanical Shock Resistance to Handle the Most Demanding Contouring and Interrupted Cuts
Hard turning requires switching from carbide to CBN inserts.It’s easier and more economical than one would expect.The major adjustment is working with much higher surface speeds.
Earlier posts that deal with CBN Insert questions:
An important criterion for the economical application of cutting tools is their life in relation to the metal removal rates, especially with two materials such as ceramic and CBN. The latter is much more expensive. Thus, comparison of tool wear becomes most important.
The two materials show marked performance differences in both size and surface finish in roughing, interrupted, and finishing cuts. In comparison to grinding, however, both materials will achieve much higher metal removal rates.
CBN will out-perform ceramic in interrupted cuts. With depth-of-cut of 0.060 inch to 0.120 inch and a speed of 350 sfpm with 0.010 to 0.020 inch feed per revolution, ceramic often will fail when entering the interruption. With CBN, a tool life up to 20 minutes can be achieved with a half-inch round insert, under the above conditions. Flank wear in interrupted cuts is more irregular than in continuous cuts and can shorten tool life. Solid CBN inserts have greater flank wear resistance and are superior to ceramics for roughing, especially where the turning has interrupted cuts.
For finishing cuts where small depths of cut and low feed rates are required to achieve superior surface finishes, CBN inserts cannot equal the tool life of ceramic inserts. Under these conditions, the ceramic insert has less than half the flank wear of CBN after 30 minutes cutting time.
Considering that CBN is ten times more costly than ceramic for the same size and geometry insert, CBN economically can be justified only for machining heat treated steel in those situations where ceramic fails completely or breaks down before finishing the workpiece.
DR-100 has a high CBN content, 99%, and is supplied as solid I.S.O format indexable inserts for top clamp tool holders. PCBN cutting tools machine hardened steels with apparent ease because, using relatively high surface speeds, heat is generated at the point of cutting so the PCBN tool cuts locally softened material.
Although most of the heat essential for efficient cutting exits with the chip, any increase in the temperature of the component will make accurate measuring difficult. Coolant can be used to reduce this effect.
Coolant can be used to drive away any swarf which might interfere with the cutting edge, particularly in boring operations. Coolant can help reduce vibration, especially where rigidity is limited.
Where interrupted cutting with CBN inserts occurs, coolant should not be used as this will thermally shock the CBN tool as it comes out of the cut, resulting in premature failure. Although interrupted cutting with coolant will never improve tool life, it is possible to use coolant machining hardened steel with DR-450 CBN inserts and coolant can be used when machining cast iron with CBN, that is not hardened, under all conditions.
This blog entry focus is on machining the following:
Cemented carbide with a Co content of less than 17%:
1. First choice for machining is CBN 2. Should use a round cbn insert 3. The insert should have a chamfered cutting edge 4. Machining with coolant is recommended 5. Chamfer the workpiece at entry and exit point.
Should you require further assistance in choosing an insert or pricing. Please contact the experts at David Richards Engineering.
CBN cutting tools machine hardened steels with apparent ease because, using relatively high surface speeds, heat is generated at the point of cutting so the PCBN tool cuts locally softened material. The heat is carried away by the swarf, which becomes brittle and harmless, and the PCBN tool, which has a high co-efficient of thermal conductivity. If a light cut is required, however, a tool with a high CBN content conducts too much heat away from the shear zone and the conditions for efficient machining cannot be achieved.
DR-50 & DR-50N have a low CBN content and the individual CBN particles are isolated within a ceramic matrix. This gives the materials a lower thermal conductivity and greater wear resistance in finish cutting operations.
Low CBN tools can therefore keep sufficient heat at the cutting point to enable the optimum cutting conditions to be achieved when a light cut is taken. In most cases, even when very light cuts are required, low CBN tools employ negative rake geometry to provide a strong edge. Due to the nature of cutting, however, cutting forces are still very small. Low CBN can be used to provide a productive and cost effective alternative to grinding.Tolerances achieved are comparable but machining time can be dramatically reduced.
DR-50 has a low CBN content and is designed for finishing and semi-finishing applications where grinding with conventional abrasives proves time consuming or difficult. DR-450 is tougher and performs well over severe interruptions.
Hard part grooving can be a difficult process without the correct tools. CBN cutting tools for grooving can ease that process. During heat treatment, many features of machined components suffer distortion. If the position or dimensions of a groove are critical to the performance of a hardened component, David Richards can offer a simple solution to this problem - ‘Hard Grooving’. Based on the Top Notch system, David Richards supply grooving tools in a wide range of sizes from 0.5 mm width upwards. David Richards supply tools for circlip and ‘O’ring grooves with controlled corners or full radii, either full form for plunging or undersize for profiling.
Using such surface speeds typical for turning, grooves are machined at low feed rates (0.01 / 0.05 mm/rev). The low feed rate ensures that the swarf is very weak and brittle, decreasing the likelihood of it breaking the tool. Where there are no interruptions to the cutting path, coolant should be used to aid evacuation of the swarf and ensure that the component remains at a stable temperature for ease of measuring. If the cutting path is interrupted, an air blast will serve the same purpose without the risk of thermally shocking the PCBN tool.
If practical, it is better to finish a groove that has been pre-formed at the soft stage. This ensures uniform hardness around the groove and maintains the structural integrity of the component. However, if it is acceptable, satisfactory results are achieved grooving through a hardened layer into the core material. Successful applications include grooving case hardened (58 / 63 HRc) splined shafts and gear teeth, internal circlip grooving hardened EN31 (58 / 60 HRc) bearing components, and producing profiles in the face of D2 tools steel tools using Full radii face grooving tools. All operations that would be time consuming and difficult to grind.
Whilst the Top Notch system offers an ideal base for most grooving tools, other systems have been employed with excellent results.
It naturally follows that, since David Richards can produce tools for ‘Hard Grooving’ full and partial form cbn threading tools, both external and internal, are available for ‘Hard Threading’. Where the pitch of the thread is large, or the diameter of the component is small, it must be remembered that the feed rate required, at the required surface speed for ‘Hard Turning’ will be relatively fast. But, producing a 90 mm diameter internal stub Acme thread 150 mm deep, with a 6 mm pitch in 58 HRc Ni-Hard iron, at 120 m/min from blank bore, though somewhat exciting, proved a major cost saver.
Finding it difficult to machine soft steels with cbn inserts? I would suppose that soft steel is relative, so generally speaking, cbn inserts will machine steels from 45-65 HRc. For soft steels (the ones on the lower end of the 45-65 HRc), the lower you are on the hardness scale, the faster you will want to machine the material.
CBN needs heat to cut at its fullest potential. A quick lesson: CBN generates heat in the shear zone, thus locally anealling the material, cutting it as if it were soft.
If you are having trouble with turning using CBN inserts or PCD inserts, please call to discuss your application with David Richards Engineering or email to sales@drengus.com
The Problem: Inconsistent tool life and insert would chip after two parts
Solution:
I am sure that some may have already figured this out based on the radius size of the insert compared to the feed rate, but for those not in the know, here is the fix:
Hard turning is a developing technology that offers many potential benefits compared to grinding, which remains the standard finishing process for critical hardened steel surfaces. To increase the implementation of this technology, questions about the ability of this process to produce surfaces that meet surface finish and integrity requirements must be answered. Additionally, the economics of the process must be justified, which requires a better understanding of tool wear patterns and life predictions. An ongoing comparative study of wear rates and tool lives under varying cutting parameters is presented here. To date, the study has consisted of seventeen different machining conditions with four different cutting tool materials. Tool life results agree with previous research in this area, indicating that polycrystalline cubic boron nitride CBN tools with low CBN content have improved lives resulting from the benefits of the ceramic binders compared to the cobalt binder typically used for higher CBN content tools. More interesting is a resulting trend in flank wear patterns that could currently help to predict tool life under certain cutting conditions. Further work is being done to understand the wear process in an attempt to model this relationship for a larger range of conditions.
Ty G. Dawson and Dr. Thomas R. Kurfess The George Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA 30332-0450
Interrupted cutting D2 tool steel is very difficult and unpredictable. D2 contains up to 14% Chromium and was designed to be used at 50-56 HRc. If the material is hardened to +60 HRc and not tempered very carefully, Chromium Carbide formation at the grain boundaries makes the material impossible to machine with interrupted cutting.
A question I get asked quite often, is what sort of tool would be best for hard boring my x,y or z. Well that all depends on the answers I get for these questions:
What is the min. bore diameter?
What is the bore depth?
Are you willing to use a different tool?
Instead of sending me answers to these questions, here is a quick guide for boring applications:
If you are looking for a place to compare CBN grades from Sandvik, Sumitomo, Iscar, Dijet, Kennametal, Mitsubishi, Tungaloy, and Kyocera, then you have come to the right place. This chart covers CBN Inserts and PCD Inserts.
Generally speaking, a search on the web will net you quite a few results, however rather than click on all those links how 'bout just one?
For high-speed, dry, and hard machining, these cutting tools may prove the ideal solution
Ceramics' and cubic boron nitrides' (CBN) hardness, chemical stability, and high wear-resistance make the materials, even at elevated temperatures, well-suited for high-speed and hard machining that can achieve significant reductions in production time and cost. The ability of ceramic inserts and CBN inserts to run dry also allows cleaner machining processes with reduced environmental and health impact-saving coolant, maintenance, and disposal costs. In the high-performance cutting of cast iron, nodular or ductile cast iron, and in the turning of hardened steel, the materials enable productivity gains and cost efficiencies, which have resulted in recent market share gains by both ceramic and CBN cutting tools within the automotive industry.
CBN Machining Applications – Where is CBN Machining Cost Effective?
“Hard Turning” Tool Steels 45Rc to 65Rc+
Speeds of 200 SFM to 600+ SFM (Surface Feet per Minute)
Chip loads of .002 to .020 IPR (Inch per Revolution)
Depth of cuts .002 tp .200"
Finishes to 16-Ra+
Interrupted cuts – NO Problem
Solid-top CBN inserts eliminate the problem of Mini-tip Melt-Off
Chilled Cast Irons (as cast) & Grey Cast Irons
* 300% Increased productivity vs. carbide * Wear life 5 to 10 times TiN coated carbide * 200% wear life in finishing cuts vs. ceramics * Speeds start at 1600 SFM to 1400+ * Chip loads .004 to .020 IPR * Depth of cut .002" to .400" * Operations include turning, milling and boring
NiHard (White Iron) 450 Bhn to 750Bhn * Speeds range from 250 SFM to 400+ SFM * Roughing chip loads -.005" to .015" IPR * Roughing depth cuts -.010" to .400" * Finishing cuts, chip load starts at .002 to .020" IPR * Finish cuts, depth of cuts -.002 to .300" * Operations include turning, milling and boring * Acme threading both ID & OD
WHAT IS POLYCRYSTALLINE CUBIC BORON NITRIDE (CBN)?
Diamond, cubic carbon, is the hardest, most abrasive resistant material known to man. It is therefore an ideal tool material. Unfortunately, in the presence of heat and iron, nickel or cobalt, diamond transforms to hexagonal form, graphite. This is why ferrous materials are not generally machined with diamond. The second hardest material, Cubic Boron Nitride (CBN), is created by man, using temperatures and pressures similar to those for diamond synthesis, but does not have this inherrant weakness when it comes to the machining of ferrous materials. Used in the correct manner CBN inserts offer cost-effective rapid stock removal and finishing of hardened steels and certain softer ferrous materials.
CBN inserts incorporating reinforced, chamfered edges eliminate the edge breakout common when cutting materials harder than 50 RC.
Consider the entire application. Less expensive carbide inserts that can do the job in terms of tolerance and surface finish may be costly when the time spent indexing and replacing inserts is considered. Real productivity results from an understanding of the tradeoffs in throughput, cycle time and insert performance.
In one specialized, low-volume example, a sintered titanium carbide gas turbine blade was milled successfully with coated carbide cutting inserts. At 120 sfm, the carbide cutting edge cut well for just 5 to 10 minutes. Acceptable insert life is typically placed at 15 to 30 minutes in high volume production with difficult materials, but with a low-rate part, the short insert life and frequent tool changes are not major drawbacks. Longer insert life does become important in full production, however, to decrease tool-changing downtime and labor and to increase machine utilization and throughput. Carbide works well for the turbine blade for now, but should the part go to higher volume production, the application may justify harder, more costly inserts made of CBN.
Productivity with advanced material inserts requires adopting the right feeds and speeds. CBN inserts incorporate reinforced, chamfered edges to eliminate the edge breakout common when cutting materials harder than 50 RC. Yet even despite this toughness, CBN inserts demand cutting machine parameters held to tight tolerances. Cutting speeds 10 percent too low or 10 percent too high can dramatically hamper performance.
If faced with the need to machine a difficult material, consider contacting your cutting tool supplier. Suppliers can offer solutions based on how others have approached the same problem. When experimentation is required, careful trial-and-error generally starts with carbide inserts and moves on to harder and more costly cutters. Modern insert geometries, rigid toolholders and refined machining routines often make less costly carbide inserts suitable for tough jobs.
You can replace grinding with CBN Inserts and Hard Turning with a little bit of research. In the vast majority of cases hard turning with PCBN can replace a grinding operation. Finish turning of hardened components can achieve similar tolerances and surface finishes. Hard turning with PCBN also offers several environmental and cost benefits over grinding. These include greater process flexibility, reduced machine time, lower energy consumption, optional use of coolant and swarf recycling possibilities.
David Richards define “Hard Turning “ as machining hardened steels above 40 HRc, not hard in terms of “difficult”. Alloy Steels with a hardness below 40 HRc are not generally machined using CBN inserts because other tool materials work as well or better and cost less. Soft materials often stick to PCBN cutting tools causing “build up” on the cutting edge. This results in poor surface finish and tool life. The geometry of PCBN tools used for machining hardened steel is very blunt with no chip groove geometry to provide swarf control, not ideal for machining soft steels. However, some steels with a high alloy content and 30+ HRc are successfully finish machined with DR-50 because nothing else will do the job. If there is no adhesion, reliable size control and consistent surface finish can more than justify the cost of the tools.
Aluminum Alloy Machining Aluminium alloys cannot be machined with CBN inserts. PCBN has a trace content of Aluminium nitride. Aluminium builds up on the cutting edge very quickly causing rapid tool wear and poor surface finish.
Cast Iron Machining Cast iron and Iron based hard facing alloys with a significant ferrite content are not machined with CBN inserts. The soft gooey ferrite sticks to the CBN insert cutting edge causing rapid wear and poor surface finish.
D2 Machining Interrupted cutting D2 tool steel is very difficult and unpredictable. D2 contains up to 14% Chromium and was designed to be used at 50-56 HRc. If the material is hardened to +60 HRc and not tempered very carefully, Chromium Carbide formation at the grain boundaries makes the material impossible to machine with interrupted cutting.
HSS Machining Interrupted cutting of High Speed Steel – HSS is temperature resistant and does not soften in the shear zone. Interrupted cutting Nitrided steel is difficult. When continuous cutting, the super-hard surface is machined away by a part of the cutting edge that is not controlling surface finish and size. When interrupted cutting, the entire cutting edge impacts with a superhard surface resulting in poor tool life.
Hard Facing Alloy Machining Hard facing alloys – Stellite (Cobalt/Chrome Alloys)and Colmonoy (Nickel/Chrome alloy) with more than 20% Chrome is not practically machined with PCBN – Tool life is too short. Chromium cannot be machined using PCBN. PCBN can be used to remove hard Chromed plated surfaces and expose a hardened steel base material, but it is not possible to machine within the Chrome.
High Temperture Alloy Machining Machining high temperature alloys – Inconel, Hastalloy, Waspalloy, Titanium, Nimonics etc are not machined with PCBN. Tool life is negligible due to chemical affinity.
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