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How Boring Bar Geometries Impact Cutting Operations

Boring is a turning operation that allows a machinist to make a pre-existing hole bigger through multiple iterations of internal boring. It has a number of advantages over traditional drilling methods:

  • The ability to cost-effectively produce a hole outside standard drill sizes
  • The creation of more precise holes, and therefore tighter tolerances
  • A greater finish quality
  • The opportunity to create multiple dimensions within the bore itself

 

Solid carbide boring bars, such as those offered by Micro 100,  have a few standard dimensions that give the tool basic functionality in removing material from an internal bore. These include:

Minimum Bore Diameter (D1): The minimum diameter of a hole for the cutting end of the tool to completely fit inside without making contact at opposing sides

Maximum Bore Depth (L2): Maximum depth that the tool can reach inside a hole without contact from the shank portion

Shank Diameter (D2): Diameter of the portion of the tool in contact with the tool holder

Overall Length (L1): Total length of the tool

Centerline Offset (F): The distance between a tool’s tip and the shank’s centerline axis

Tool Selection

In order to minimize tool deflection and therefore risk of tool failure, it is important to choose a tool with a max bore depth that is only slightly larger than the length it is intended to cut. It is also beneficial to maximize the boring bar and shank diameter as this will increase the rigidity of the tool. This must be balanced with leaving enough room for chips to evacuate. This balance ultimately comes down to the material being bored. A harder material with a lower feed rate and depths of cut may not need as much space for chips to evacuate, but may require a larger and more rigid tool. Conversely, a softer material with more aggressive running parameters will need more room for chip evacuation, but may not require as rigid of a tool.

Geometries

In addition, they have a number of different geometric features in order to adequately handle the three types of forces acting upon the tool during this machining process. During a standard boring operation, the greatest of these forces is tangential, followed by feed (sometimes called axial), and finally radial. Tangential force acts perpendicular to the rake surface and pushes the tool away from the centerline. Feed force does not cause deflection, but pushes back on the tool and acts parallel to the centerline. Radial force pushes the tool towards the center of the bore.

 

Defining the Geometric Features of Boring Bars:

Nose Radius: the roundness of a tool’s cutting point

Side Clearance (Radial Clearance): The angle measuring the tilt of the nose relative to the axis parallel to the centerline of the tool

End Clearance (Axial Clearance): The angle measuring the tilt of the end face relative to the axis running perpendicular to the centerline of the tool

Side Rake Angle: The angle measuring the sideways tilt of the side face of the tool

Back Rake Angle: The angle measuring the degree to which the back face is tilted in relation to the centerline of the workpiece

Side Relief Angle: The angle measuring how far the bottom face is tilted away from the workpiece

End Relief Angle: The angle measuring the tilt of the end face relative to the line running perpendicular to the center axis of the tool

Effects of Geometric Features on Cutting Operations:

Nose Radius: A large nose radius makes more contact with the workpiece, extending the life of the tool and the cutting edge as well as leaving a better finish. However, too large of a radius will lead to chatter as the tool is more exposed to tangential and radial cutting forces.

Another way this feature affects the cutting action is in determining how much of the cutting edge is struck by tangential force. The magnitude of this effect is largely dependent on the feed and depth of cut. Different combinations of depth of cuts and nose angles will result in either shorter or longer lengths of the cutting edge being exposed to the tangential force. The overall effect being the degree of edge wear. If only a small portion of the cutting edge is exposed to a large force it would be worn down faster than if a longer portion of the edge is succumb to the same force. This phenomenon also occurs with the increase and decrease of the end cutting edge angle.

End Cutting Edge Angle: The main purpose of the end cutting angle is for clearance when cutting in the positive Z direction (moving into the hole). This clearance allows the nose radius to be the main point of contact between the tool and the workpiece. Increasing the end cutting edge angle in the positive direction decreases the strength of the tip, but also decreases feed force. This is another situation where balance of tip strength and cutting force reduction must be found. It is also important to note that the angle may need to be changed depending on the type of boring one is performing.

Side Rake Angle: The nose angle is one geometric dimension that determines how much of the cutting edge is hit by tangential force but the side rake angle determines how much that force is redistributed into radial force. A positive rake angle means a lower tangential cutting force as allows for a greater amount of shearing action. However, this angle cannot be too great as it compromises cutting edge integrity by leaving less material for the nose angle and side relief angle.

Back Rake Angle: Sometimes called the top rake angle, the back rake angle for solid carbide boring bars is ground to help control the flow of chips cut on the end portion of the tool. This feature cannot have too sharp of a positive angle as it decreases the tools strength.

Side and End Relief Angles: Like the end cutting edge angle, the main purpose of the side and end relief angles are to provide clearance so that the tools non-cutting portion doesn’t rub against the workpiece. If the angles are too small then there is a risk of abrasion between the tool and the workpiece. This friction leads to increased tool wear, vibration and poor surface finish. The angle measurements will generally be between 0° and 20°.

Boring Bar Geometries Summarized

Boring bars have a few overall dimensions that allow for the boring of a hole without running the tool holder into the workpiece, or breaking the tool instantly upon contact. Solid carbide boring bars have a variety of angles that are combined differently to distribute the 3 types of cutting forces in order to take full advantage of the tool. Maximizing tool performance requires the combination of choosing the right tool along with the appropriate feed rate, depth of cut and RPM. These factors are dependent on the size of the hole, amount of material that needs to be removed, and mechanical properties of the workpiece.

 

Green Manufacturing: Lasting Environment & Shop Benefits

“Green Manufacturing” has become a common phrase used by many in America’s largest industry. It is defined by Goodwin College as “the renewal of production processes and the establishment of environmentally friendly operations within the manufacturing field.” Taking the time to rethink dated processes can save you time, money, and help build your reputation as a state-of-the-art business. The establishment of environmentally friendly machining processes is a huge leap in the right direction of creating an eco-friendly business.

Green Manufacturing is the next logical step forward for the industry.

How to Get Started with Green Manufacturing

The first step you should take on your march toward a more sustainable machine shop, and green manufacturing, is an evaluation of your facilities environmental impact. The most common method for environmental impact assessment of the manufacturing process is Life Cycle Assessment or LCA. ISO 14040 defines LCA as the compilation and evaluation of the inputs, outputs, and the potential environmental impacts of a product system throughout its life cycle.

4 Questions to Ask Yourself:

  1. Goal Definition and Scoping – What am I trying to achieve in this investigation?
  2. Inventory Analysis – What are the quantified inputs (energy, water, materials, etc.) and outputs (air emission, wastewater)?
  3. Impact Assessment – How do these things affect the environment?
  4. Interpretation – What can I change to make my processes more efficient and eco-friendly?

There are databases available to help with the inventory analysis. These databases collect information such as feed rate, cutting speed, tool diameter, cutting time, coolant properties, and then calculate all the material, and energy inputs and outputs.

Understanding the Impact of Cutting Fluids

Cutting fluids are most likely the number one pollutant in your machine shop, and are getting in your way of achieving green manufacturing. According to Modern Manufacturing Processes, North American manufacturers consumed more than 2 billion gallons in 2002, and the metal working fluid market has only grown since then. Cutting fluids have a number of benefits to the machining process, mostly involving cooling of the cutting region, lubrication, and chip evacuation.

Contaminants

Getting the most out of your coolant is a key factor of cost efficiency in any machine shop. Therefore, one of the largest problems you are likely to face, or are currently facing, is the deterioration of cutting fluid performance due to contaminants. The most common types of contaminants include:

  1. Free oils (tramp or sump oil) – Liquid that lubricates the gears and equipment of CNC machine seeping into the cutting fluid
  2. Coarse Particulates – Relatively larger solid waste, chips, and swarf
  3. Fine Particulates – Extremely small pieces of the workpiece or cutting tool that usually consist of heavy metals such as cobalt, cadmium, chromium, aluminum, and lead
  4. Microorganisms – Bacteria and fungi that grow inside the walls and pipes of the CNC machine

Coarse particulates such as chips and swarf are generally easily extracted using an H-Chain, chain and flight, push bar, or a flume system. Fine particulates are more difficult as they require either separation or filtration in the form of setting tanks, foam separators, centrifugal separation, or magnetic separators. Free oil can also be removed through filtration but can also be taken care of quickly and easily with a skimmer.

The right filtration system can both save machine shops money and help the environment.

Microorganisms are the most difficult to remove as they can be a tenth of the size of fine particulates. “Monday morning stink” is a common side effect of anaerobic bacteria excreting hydrogen sulfide during their metabolic processes. These organisms are one of the main reasons coolant has to be drained and changed from a machine every few months. Advancements in technology over the past few decades have made it possible for membrane microfiltration systems to separate biomass, as well as oil from coolant fluids. With these types of systems, operators will no longer have to use problematic biocides as these have troublesome health effects for both the environment and employees. However, it is important to note that most of the microorganisms aren’t growing in the cutting fluid but on the walls/pipes of the machine or buried in the residue of chips at the bottom of the tank.

Some ways of reducing microorganism build-up in your shop include:

  • Keeping machines clean – sludge, swarf, and chips can be a breeding ground for bacteria
  • Reduce organic contamination – spit, sweat, tobacco juice, and organic matter are all a food source for microorganisms
  • Reduce the amount of tramp oil – This can also be a food source

A dirty pile of chips where bacteria will grow and stink up your machine.

Types of Cutting Fluids

A good cutting fluid should have a high flash point, good adhesiveness, high thermal stability, and high oxidative stability. A high flash point is necessary as the fluid should not catch fire at high temperatures (gasoline has a low flash point). Good adhesiveness allows fluid to stick to the surface of the workpiece. This creates a layer between the cutting tool and the workpiece, helping to separate them and thus, reducing friction. Greater amounts of friction leads to higher cutting force, which leads to higher cutting temperatures.

High cutting temperatures are problematic, as this causes the cutting tool to wear faster and can also cause your workpiece to workharden. High thermal stability means that at high temperatures, the fluid should have a low viscosity (an example of a fluid with high viscosity would be honey at room temperature). A lower viscosity in the cutting region allows for a lower amount of friction, and therefore lower amounts of heat and cutting force. A cutting fluids oxidative stability ultimately decides how long that cutting fluid can be used. After a while, oil begin to oxidize, which then causes its viscosity and amount of deposits of sludge to rise.

Excessive amounts of coolants can mean an excessive amount of waste.

For the purpose of this article, cutting fluids will be placed into two broad categories: biodegradable and non-biodegradable.

Non-Biodegradable Coolants

Non-biodegradable coolants are petroleum-based. They have high human and ecological toxicity, which results in occupational health risks. They also have a complicated disposal processes.

Biodegradable Coolants

Biodegradable coolants are plant-based. These are usually manufactured from vegetables such as soy, coconut and canola, or non-edible plants such as neem, karanja, and jatropha. This factor makes them a renewable resource and less toxic to humans, as well as the environment.

In recent years, some modified vegetable-based oils have surpassed petroleum based oils in performance in regards to surface finish, heat suppression and lubrication. One study, published in Science Direct, centered on turning in 304 stainless steel revealed that coconut oil with a boric acid additive was significantly better at combating tool flank wear and surface roughness when compared to two other cutting fluids. This was due to the vegetable-based solutions high thermal stability.

Another study, published in IOP Science, found that a combination of neem and karanja oil was superior to SAE 20W40 (petroleum based oil) in regards to lubrication when drilling mild steel. The results showed that the vegetable-based oil solution reduced the cutting force of the operation due to its higher viscosity and adhesiveness. This ultimately led to a better surface finish on the part.

Summary of Cost Efficient Coolant Changes You Can Make

  • Reduce microorganism build up in your machines by keeping the machines clean and reducing amount of outside contamination
  • Install a membrane microfiltration system
  • Switch to a more efficient and biodegradable cutting fluid

The Positive Green Machining Impact of Dry Machining

Dry machining should be utilized whenever the opportunity presents itself, as the costs and environmental issues associated with cutting fluid obtainment, management, and disposal are eliminated. Another benefit to dry machining is the absence of thermal shock. When the cutter exits the cut but coolant is still blasting, the large temperature fluctuation (thermal shock) will cause the cutting edge to break down quicker than if it were to run hot full time. Dry machining is most prevalent in machining operations with interrupted cuts or when cutting hardened steels. It is especially popular in milling operations with high speeds and feeds. Cutting with high running parameters allows for most of the heat to be dispersed into the chips rather than into the workpiece. This is also the case when machining hardened steels.

Ideal Tooling for Dry Machining

The ideal cutting tool should be more heat resistant, and less heat generative. Carbide is a good substrate as it is extremely hard and strong. Coated tools are the best option for dry machining as they have improved thermal insulation as well as improved self-lubrication.

The Positive Affect of Minimum Quantity Lubrication (MQL)

  • Significantly reduced fluid consumption
  • Safer cutting fluids and lubricants
  • Reduced health hazards for employees
  • Cleaner shop environment
  • Reduced maintenance

Because such a small quantity of fluid is used in MQL, this make it a perfect application to use slightly pricier vegetable-based oils. MQL has been found to be most effective in sawing and drilling operations.

The Benefits of Green Manufacturing

Taking a second look at your current machining operations through and environmentalist lens can save you time, money, and create a less hazardous work place for your employees. Using the techniques above, one can approximate shop efficiency and make appropriate changes for the benefit of current and future generations.