CBN Speeds and Feeds

CBN Machining Parameters

Typical Machining Parameters using DR-50/450 Tool CBN Inserts

Typical Machining Parameters using DR-80/85 Tool CBN Inserts

Typical Machining Parameters using DR-100 Tool Solid CBN Inserts

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 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.

David Richards have wide experience in the application of these tools and will be delighted to discuss specific examples on request.

Internal Grinding is time consuming and expensive. For small bores David Richards provide a simple solution

Hard Boring

For bores down to 10 mm David Richards provide cost effective triangular cutting tool inserts with 3 PCBN edges in grades DR-50/450 and a range of boring bar tool holders to suit - steel shank boring bars for bore depths up to 2 times shank diameter and heavy metal for bore depths up to 4 times shank diameter.

For ‘Back Boring’, stepped bores and other applications which do not accommodate the standard products, David Richards manufacture tool holders to suit.

If distortion and size variation as a result of heat treatment places unreasonable constraints on ‘Soft Stage’ machining, ‘Hard Boring’ will provide a cost effective, scrap reducing alternative. Turn a lathe into a universal grinding machine and a machining centre into a jig grinder. Combine internal and external grinding operations - even grooving - and replace grinding minutes with Hard Turning’ seconds.

Boring Small Bores In Hardened Steel

For Bore sizes smaller than 8 mm in diameter, it is not possible to use PCBN tipped cutting tool inserts.  In order to put a PCBN tip into the carbide insert, material must be removed. This makes the insert weak and prone to breakage when it is clamped into the boring bar.

At the same time, clamping of small inserts into small boring bars becomes progressively more difficult as the bore size reduces.Top clamps trap swarf and, in order to fit inside the bore, become too small to exert enough clamping pressure. Screw locked inserts, while leaving space for swarf, require sufficient material under the insert for the thread of the clamping screw.

These problems are overcome by using brazed tools. The PCBN tip is brazed directly to a tungsten carbide shank providing a rigid boring tool capable of remarkable surface finishes and providing a highly productive alternative to internal grinding.

David Richards provide boring tools, supplied from stock in a range of standard sizes.

Diamond is the hardest, most abrasive-resistant, material known to man. These properties make diamond an ideal cutting tool. Within the crystal structure, however, fracture planes, used by the diamond cutter to produce the gem diamond from the rough, can cause catastrophic breakage of the tool edge, when subjected to impact.

DR-PCD tools incorporate Polycrystalline Diamond blanks produced under conditions of high pressure (1 million PSI) and temperature (1700 Degrees C), similar to those of diamond synthesis. Randomly orientated, carefully selected synthetic diamond crystals are grown together on a hard metal substrate.

This results in a material with the hardness, abrasive resistance and high thermal conductivity of diamond with the toughness of hard metal.

Using the hard metal substrate the PCD blank is brazed to a carrier, either steel or hard metal, and
machined by grinding or E.D.M to produce the cutting edge.

When compared to other cutting tool materials, there are three main reasons for using
DR-PCD tools:-

Increased tool life results in reduced tool cost per component and less idle machine time.

Increasing cutting speed improves productivity through reduced cycle times.

Grinding and other less productive machining methods can be replaced by DR-PCD milling and turning.

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 offers cost-effective
rapid stock removal and finishing of hardened steels and certain softer ferrous materials.

PCBN can provide a cost effective and highly productive alternative to hard metal or ceramic cutting tools for the machining of cast irons.

In general terms, the following factors should be considered when applying PCBN to an ironcomponent. Cast iron is not generally very hard (less than HRc.30) but tends to be abrasive. PCBN is therefore employed because of its abrasive resistance. Unless the iron has been chilled or deliberately heat treated, the cutting action will be such that the PCBN will not be required to anneal the material being cut in the shear zone. Grey cast irons are often fully pearlitic in structure - Grade 14 & 17 are common.

In this case, the best results are gained using DR - 100/85 at surface speeds above 400 m/min. if the machine tool or component limit the speed available to less than 400 m/min, DR - 50 becomes more cost effective. Tool life and component quality improvements are often dramatic and more than justify the increased cost of the PCBN tools. if the grey cast iron is not fully pearlitic and more free ferrite is present within the structure, the machinability of the iron becomes more difficult to predict. As the level of soft and sticky ferrite increases, the tool is more likely to suffer adhesion pick up which will alter the cutting action, resulting in premature wear of the tool.

DR-50 becomes more likely to provide good results as the level of free ferrite increases and a short trial will
soon determine the most suitable PCBN material.

Fully ferritic grey cast irons are not generally cost effective PCBN applications. S.G. irons are usually soft (less than HRc.30) and fully ferritic. PCBN is therefore not often as efficient as other cutting tooltool materials.

There has, however, been a tendency to produce S.G. irons (nodular irons) with a pearlitic structure.This has
resulted in some nominally S.G. irons machining surprisingly well with PCBN, DR-50 tending to be more successful.

Low alloy irons commonly used in the automotive industry can be machined with PCBN, but the performance is again dependant on the level of free ferrite in the structure.

Hardened irons, either heat treated or alloyed and heat treated, are machined in the same manner as hardened steels and are therefore more predictable. Cutting speeds recommended are lower - less than 200 m/min.

It must be remembered that cast iron of a given specification produced by a foundry on one day may have a different machinabilty to that produced the next day.

Once the benefits of machining with PCBN have been determined, it may be considered worthwhile ensuring that castings supplied to the machine shop have a suitable microstructure to guarantee consistent PCBN tool performance.

If the structure or grade of cast iron is unknown, a simple trial will soon determine the suitability of PCBN as a cutting tool.