Machining M2 - Speeds and Feeds - Boring Tip

Recently came across an application machining M2 with CBN Inserts. Thought it was worth sharing. Here is the breakdown:

Job: Finish Bore M2

Material: M2

Hardness: 58-62 HRc

Insert: CDCD51 CBN Insert - Circle $150/ea

Depth of Cut: .002" per side

Feed: .005" per revolution

Surface Feet per Minute: 400 SFM

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:

Change to:

Insert: CDCD 51 T DR-50 Insert $50/ea.

Surface Feet per Minute: 300 SFM

Feed Rate: .003" - .004" per revolution

Depth of Cut: Keep the same

Result: Increased tool life by more than 15x to 30 parts per edge and saved over $100 per insert.

Disadvantages of Diamond Coated Inserts

Probably the main weakness of thin-film diamond-coated inserts is lack of toughness. CVD diamond films are brittle when deposited on low cobalt substrates. These inserts are not always robust enough to endure high mechanical shock. They are not always the best choice for heavy metal removal and interrupted cutting.

As with most coated inserts, thin-film diamond-coated inserts are "throwaways." They cannot be reground.

Predicting the performance of CVD diamond inserts is still somewhat uncertain. Vagaries in cobalt distribution in the tungsten carbide substrate makes wear difficult to predict. Nevertheless, compared to non-diamond inserts, a tool-life increase of 10 to 50 times is common--depending, of course, on machining conditions and material.

At this time, diamond tools are limited to cutting only non-ferrous materials. However, experiments involving super-cold gases blown into the interface between a steel workpiece and a diamond tool are being conducted with encouraging results. It may be practical to cut ferrous materials with diamond in the near future.

By Chris Koepfer

Anatomy of a Diamond Coated Insert

The base material, or substrate, for most thin-film coated-diamond inserts is tungsten carbide (grade C-2). It's basically the same substrate material used for many carbide inserts coated with titanium nitride or titanium carbide.

A carbide insert is made from two main ingredients, carbide and cobalt, which are mixed together in various ratios and sintered. Carbide manufacturers guard their ratios and sintering process details the way chefs protect their recipes.

The carbide used in an insert is granular, although individual grains are very smallmany formulas call for grains in the single-digit micron range. By using different grain sizes specific cutting performance can be selected. For diamond coatings to adhere more firmly to the substrate, a larger grain carbide is usually specified so the surface has some roughness.

Cobalt is the "cement" that bonds the carbide grains. It gives the carbide its cutting characteristics of hardness and toughness.

The mixture of carbide and cobalt is pressed in a mold, giving it the shape of an insert. Chipbreakers and other performance enhancing bumps and valley are formed as part of the molding process. These pressings are then sintered. At high temperature, the cobalt flows around the carbide grains creating a strong and very hard bond.

By Chris Koepfer

Types of End Mills

Flutes - Spiral cutting edge on the end mill. 2 and 4 flute end mills are the most commonly used.

• 2-Flute - Allows maximum space for chip ejection. Used for general milling operations.
• 3-Flute - Excellent for slotting. Used for general milling operations.
• 4, 5, 6, and 8 Flute - A greater number of flutes reduces chip load and can improve surface finish, if feed rate remains the same.

Ball End - Used to mill die cavities and fillets, round bottom holes and slots.

Carbide End Mills - This tool material combines increased stiffness with the ability to operate at higher SFPM. Carbide tools are best suited for shops operating newer milling machines or machines with minimal spindle wear. Rigidity is critical when using carbide tools. Carbide End Mills may require a premium price over the cobalt end mills, but they can also be run at speeds 2 1/2 faster than HSS end mills. For best results mount in a hydraulic type holder.

Coatings - The use of Titanium coated tools will increase the surface hardness of the tool to near 85 Rc. This will allow for greater tool life at increased cutting speeds & feeds (15 - 25%). Melin offers Titanium Nitride (TiN), Aluminum Titanium Nitride (AlTiN), and Titanium Carbonitride (TiCN).

Cobalt - Type of high speed steel tool which has a 8% cobalt content (M42). This material has excellent abrasion resistance for improved tool life over standard high speed steel (M7).

Corner Radius - Conventional end mill with radius ground on the tips of the flutes to help reduce chipping on the tip. For mold applications, radius tools can remove more material faster than ball ends.

Corner Rounders - Cutters having form ground radius with relieved clearance.

Double End - An end mill that has teeth on both ends of the cutter. End mill holders must have sufficient clearance to allow for the use of a double end cutter.

Drill Point - Multipurpose tool which can be used for drilling, milling, or chamfering.

High Helix - Usually a 40 to 60 degree spiral on the tool's flutes. Effective for rapid chip ejection in milling of aluminum and other materials.

HSS - A baseline tool steel. In the past, a majority of end mills were made from standard High Speed Steel (M7). Usually inexpensive, but do not offer the tool life or speed and feed advantages of Cobalt and Carbide end mills.

Left-Hand Spiral - Used for milling multiple layers of thin sheets where chip flow is directed away from work.

Rougher (Hoggers) - End mill with interrupted shape on outside diameter to remove large amounts of material quickly. Typically can remove material up to three times the rate of conventional end mills with different types available to achieve the desired finish on the material. Melin offers roughers in Coarse Pitch, Fine Pitch, Rougher/Finisher, and 3-Flute for Aluminum styles.

Single End - Teeth on one end of the cutter only. This style is the most common available.

Stub Length - Used for milling of shallow slots in all types of materials where heavy feeds are required and tool deflection is minimized.

Wear of CBN in Hard Turning

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

NFL Playoff Football

Its the start of another playoff season in the NFL and much to my disappointment the Detroit Lions have someone once again found themselves on the outside looking in.

As a long time fan of the inept Liedowns, I found myself drinking the Kool Aid this season. At 6 and 2 I was in the Club at Ford Field screaming playoffs just like everyone else. I mean even if the Lions went .500 the rest of the way they were going to make the playoffs, but 1 and 7? Are you kidding me? However, in my heart I knew this was coming. I mean it isn't as if the Lions have ever given me reason to think they would do anything but rip the hearts out of long time Lions fans everywhere.

It has gotten so bad that we have this flawed logic, that according to Murphy (Law that is) we can't pass on our season tix, because someone else might buy them and that's the year the Lions will make the playoffs. How sick is that logic.

But we may just be a symptom of the no fun league as with profit sharing and the such, it really doesn't matter if we don't show up, or renew our tickets, as the team still makes money. That's right folks in the NFL it doesn't matter if you put a good product on the field, you gets paid 4 sho.

I'm not exactly sure where I am going with this rant except I think by putting in words on 2007's version of paper, I am getting self therapy. So of course I will renew my tix, wait for the draft and be extremely excited for the hope of playoffs when we draft another receiver.

Machining D2 with CBN Inserts

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.

Using PCD and its APPLICATIONS

Machining Non Ferrous Metals and Alloys including Metal Matrix Composites.

Machining Plastics, Rubber, Fibre, Reinforced Plastics and CompositeS

Machining Chipboard, MDF and Natural Woods

Wear Pads, Gauging Points and Probes

PCD can offer dramatic improvements in both efficiency and product quality, but the following points should be considered:-

If conventional tools fail prematurely due to breakage, DR-PCD tools are unlikely to fair any better. If excessive vibration, inclusions in component material, or similar problems cannot be avoided, then it may be preferable to continue breaking relatively cheap conventional tools.

However, the benefits offered by DR-PCD tools can often provide the incentive to overcome these problems. DR-PCD tools may give a tool life improvement of 100 times, paying back any extra costs in improving the machinability of the material, improving the work holding or optimizing the cutting tool geometries.

Although DR-PCD is both tough and extremely hard, the cutting edges can be extremely fragile. Care must be taken to avoid chipping the cutting edges.

It should be remembered that diamond and graphite are two forms of the same element, Carbon. When diamond is produced synthetically from graphite, a catalyst is used to reduce the reaction time to a commercially acceptable level. Suitable catalysts are iron, nickel or cobalt.

Unfortunately, these elements also catalyze the transformation of diamond into graphite. Diamond is not therefore generally effective for machining ferrous, cobalt or nickel based materials.

WHAT IS PCD (POLYCRYSTALLINE DIAMOND)

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.

For more information on PCD Inserts please visit:

David Richards Engineering