CNC MACHINING LUBRICATION: REDUCING WEAR ONE CUT AT A
TIME
Friction and Wear in Materials
RPI Hartford Class MANE –
Report prepared by
Nicholas Szwaja
660860258
Date 12/13/12
ii
Table of Contents
Introduction ........................................................................................................1
What is CNC Machining? ..................................................................................1
Tool Wear .............................................................................................................4
Lubrication...........................................................................................................6
Conclusion ............................................................................................................10
References ............................................................................................................11
List of Figures
Figure 1: CNC Image ..........................................................................................2
Figure 2: Tool Bit 1 ............................................................................................2
Figure 3: Tool Bit 2 ............................................................................................3
Figure 4: Tool Bit 3 with applied lubricant ..............................................4
Figure 5: Edge Wear ..........................................................................................5
Figure 6: Crater Wear .......................................................................................5
Figure 7: Flute delivery system.....................................................................9
Figure 8: CO2 delivery system .......................................................................10
1
Introduction
The purpose of this project is to investigate the various types of lubrication
Computer Numerical Control (CNC) machines use during machining, more
specifically the milling operations. A CNC machine is an automated system used to
machine components (metallic and non-metallic) with high degree of precision [1].
The CNC machine uses a variety of tools that can be operated at various rotational
and feed speeds to cut various designs resulting in minimal additional machining
due to the accuracy of the programing. Due to the metal to metal contact (tool bit to
material) and high rotational speed of the CNC operation, the tool bits need a
method to reduce wear to increase the longevity of each tool and maintain the tools
sharp cutting edge. To do this, lubrication is applied directly to the tool bit and
cutting surface during machining to keep the friction low and to keep the
temperature down [1]. The lubricant also assists in carrying away debris as to not
get caught between the tool and material [1]. This project will investigate the
various CNC machines lubrication and the advantages and disadvantages of each
lubricant for milling.
What is CNC Machining?
Before lubrication can be discussed it is important to understand what a CNC
machine is and what type of capabilities these machines can possess. A CNC
machine is an automated process for machining that uses computer-aided design
(CAD) and computer-aided manufacturing (CAM) software [1]. The benefits of using
CAD and CAM programs is the machining accuracy (tight tolerances), reduction in
labor, and the increase in material productivity / reproducibility [1]. Another
benefit to using a CNC machine is the potential to use multiple tools and drills on a
single part without having to stop the machining process and changing out the tools.
Figure 1 is one type of example of a CNC machines and it should be known that there
are multiple types of CNC machines.
2
Figure 1: Example of a CNC milling machine and the operator provided by
http://www.tormach.com/
The CNC machine has a computer (provided on the right) that the CAD program is
loaded in the by operator. The machine on the left is where the stock material is
machined. The CNC machine can be a variety of machining processes that are done
by CAD or CAM programs including drilling, lathe work, milling, etc. There are a
variety of machines that do not use tool bits to create the cuts such as plasma
cutters, water jets, and laser cutters but this report will not discuss these
applications because these CNC machines, although have great advantages over
tools do not require lubrication at the cutting surface. The following Figures 2 and 3
are examples of tools bits.
Figure 2: Milling tool provided by http://www.muenz-engineeredsales.com/cnc-machinetools.html
3
Figure 3: CNC drill bit that has the lubricant ejection system shown. Picture provided by
http://www.muenz-engineeredsales.com/cnc-machine-tools.html
During the machining process, the tools produce a significant amount of heat due to
the contact combined with high rotational speed [2]. The heat has a negative effect
on the material being cut but especially on the tool bit. This dulls the tool bit which
requires the tool to apply more pressure which in the end makes it more difficult to
cut the part with high precision [2]. As the tool bit dulls, it tends to take more force
to machine the part and remove large pieces of material than it originally intended.
This will affect the finished surface, especially if the tool bit scars the material.
4
Figure 4: Lubrication be applied to a tool and the component being machined.
Image provided by http://www.canstockphoto.com/cnc-machine-tool6079915.html
Tool Wear:
CNC cutting tools experience gradual failure due to regular operation [1]. The tool
looses sharpness with every pass of the machine because of the contact. CNC tools
experience various types of wear that including flank wear, crater wear, edge build
up, and edge wear[2]. Flank wear is wear in the tool bit in which the portion of a
tool in contact with the finished part erodes due to metal to metal contact, crater
wear in which contact with chips erodes the raking face of the tool [2]. Crater wear
is minute divots that develop on the leading edge of the tool which affects the
cutting surface [2]. The more divots, the more scars would appear on the machined
surface that will affect the surface finish. Building up the edge occurs when the
material being machined starts to build up on the cutting edge of the tool [2]. Some
materials, softer materials that have a low melting point like aluminum and copper
have a tendency to attach to the cutting edge of the tool [2]. Lubricant is one of the
methods used to reduce this because heat at the surface can be reduced and
removed from the contact area [2]. The final tool wear is edge wear which refers to
the outer edge of a drill bit starting to degrade over time due to overuse of
machining [2]. Figures 5 and 6 demonstrate flank wear in which a portion of the
tool starts to erode and crater wear in which a divot is produced at the surface due
to a chip.
5
Figure 5: Edge wear on the tool due to overuse. Picture provided by
http://www.visionxinc.com/
Figure 6: Crater wear on the tool in which a divot is produced.
http://its.foxvalleytech.com/
Lubricants play a critical role in CNC machining because they can reduce various
types of wear types, extends the life of the tool, reduces heat, and carry away debris
from the machining surface. The effects of tool wear can be detrimental to the final
product due to the increase force required to cut, increase cutting temperature that
can occur due to the increased force or speed required to cut, reduction in surface
finish quality, and decrease accuracy of the finished part [2].
6
The way lubrication works is it creates a very thin film of fluid between both cutting
surfaces due to the No Slip condition [2]. The no slip condition states that at a solid
boundary (example flat plate), the fluid will have zero velocity relative to the other
boundary [2]. The lubricant adheres itself to the surface creating a stagnate layer of
fluid between the tool and the part [2]. Obviously, there is metal to metal contact
because stagnate fluid cannot cut metal but this boundary condition surrounding
the cutting surface helps reduce the heat produced due to the contact [2].
LUBRICATION
Soluble cutting oil:
These cutting oils derive from emulsifiers and petroleum oils [3]. These oils are
used for cutting brass, copper, aluminum, and other ferrous metals [3]. The soluble
cutting oils can be formulated for light operations such as simple cutting or milling
and can be formulated for heavy operations which are better for providing
increased cooling, lubrication, emulsion stability, and tool life [3].
Straight Oils – 100% Petroleum Oil
Straight oils are basically petroleum or mineral oils that contain additives designed
to improve specific properties. Properties like viscosity, thermal conductivity, and
solubility can be manipulated by these additives [3]. Generally additives are not
required for easy tasks such as light duty machining for ferrous and nonferrous
metals [3]. For more heavy duty machining applications, straight oils may contain
wetting agents (~20% fatty oils) and pressure additives such as sulfur, chlorine, or
phosphorous compounds that improve the ability to coat the cutting surface [3].
The enhanced lubrication improves the oil’s ability to carry away large amounts of
metal fines and help guard against microscopic welding (metal adhesion) in heavy
duty machining due to the excess heat [3].
The advantage to the soluble oil is it can be used for high performance cutting
operations for high and low speed tool feeds. Oil based lubricants increase surface
finish because the lubricant between the cutting tool and the machined part reduces
scaring on the surface [3]. This reduction in impurities increases the quality of the
surface finish. Along with the increase in the surface finish, the oil lubricant
increases the tool life. By reducing the wear on the tool bit through the use of
lubricant, the edge will remain sharp for longer periods of time that will result in
cost savings and a smoother cut.
Tools machining at a high rate of speed will result in high friction between the
leading edge of the tool bit and the machined surface which will lead to damage the
tools or the machined part resulting in impurities to the surface of the tool or
machined part [3]. Lubricant is a great source to provide instant cooling to the tool
bit and part. When the part or tool experience excessive heat, the tool can be
damaged quicker due to the tool experiencing warping or the tool will chip easier
[3].
7
One disadvantage of using an oil based lubricant is the cutting oil is based on
viscosity which can affect the operation [3]. A high viscous oil will be suitable for
low speed operation (more of a packing grease) because if the rotational feed of the
tool is too high then the oil will not provide any protection against wear reduction
although it could still provide reduction in heat [3].
As the CNC machining technology is progressing with more complex capabilities (5
axis machines) lubricant is also advancing to more complicated formulas. Synthetic
cutting oils are being developed as an alternative to soluble oil solutions [3].
Synthetic oil provides various improvements to soluble oil in that it can be
formulated for extreme high pressures (between tool and hard material), increase
lubricant life, reduced heat generation, and can be formulated for different
machining applications (high speed vs. low speed) [3]. The advantage to synthetic is
it can be formulated geared towards your applications, will last longer, and has a
mechanical, chemical, and physical advantage over non-synthetics [3]. The
disadvantage is the cost and considering the formula, it can be significantly higher
than a water based lubricant.
Synthetic fluids contain no oil or mineral oils [3]. They generally consist of chemical
lubricants and rust inhibitors that are dissolved in water [3]. Synthetics are
designed to provide high cooling capacity, lubricity, corrosion prevention, and easy
maintainable. Due to their higher cooling capacity, synthetic oil tends to be
preferred for high heat, high velocity turning operations.
The advantages of using synthetics oils for CNC machines include nontoxic mixtures,
enhanced corrosion control, long service life, wide range of applications, superior
cooling qualities, easy maintained, and the operator can easily separate the
machined chips from the lubricant. The disadvantages of synthetic oil is a reduction
in lubrication because it is not petroleum based, could cause misting, potential to
leave residual, and can easily be contaminated by other fluids [3]. 100% oils are still
superior to synthetics for lubrication purposes only but for the other characteristics
of a synthetic, they are superior.
Oil_Water
We go from synthetics, one of the most complicated and formulated lubricant to one
of the simplest forms of lubrication. Water is one of the most basic forms of
lubricant and has been used for generation to reduce heat formulation on various
applications. Water use in CNC machines is not common because water is not an
efficient lubricant and does not possess some of the chemical and physical
properties of an oil lubricant [4]. An oil water mixture however does provide a
significant advantage over strictly water [4]. By adding oil to the water it increases
rust prevention, helps reduce heat, and more effectively removes particles on the
surface [4]. Water itself is an excellent, all natural coolant that has been used to
carry away heat for decades but as a lubricant it leaves a lot to be desired. By
adding oil the lubricity is increase and the use in CNC machines become relevant [4].
8
The disadvantage to the oil water mixture is trying to keep oil and water as a single
mixture. Water and oil tends to separate so there has to be an additive to the
mixture to ensure the water and oil remain a single fluid.
Pastes and Gels
Gels and pastes can be applied to the surface of the tool prior to machining which
will help protect the leading edge of tool bit. Gels and pastes are not typically used
in high speed cnc milling operations but are more commonly used for hand
operations where the lubricant can be applied with a gel stick [5]. Although these
products do protect the tool, they are commonly not used for cnc milling machining
due to the high speed of the cutting tool.
Aerosols, CO2
Some cutting process, such as cutting plastics and non-metallic, high quality
lubricant used for cutting metal may not be required. For this type of cnc milling
process the expensive synthetic or oil based lubricant could be deemed unnecessary
and does not need to be applied. Aerosols is a form of cutting fluid that form tiny
droplets of liquid in the air that can scatter though out the machining surfaces to
create a moist machining surface [5]. There are inherent disadvantages with the
aerosol in that it is air born product which depending on what is being cut can
create a hazardous work place. Also, since aerosols are airborne they cannot be
directly applied so the machining area has to be completely free of any breeze or
outside disturbances to be applied correctly [5]. To deliver aerosol as effectively as
possible, the product is applied directly through the flutes of the tool as shown in
Figure 7.
9
Figure 7: Aerosol delivery through the flute of a drill bit. Picture provided by
http://www.mmsonline.com/articles/the-many-ways-ford-benefits-from-mql
The aerosol is distributed at the end of the tool so it is directly applied to the part.
Compared to lubricant this is more economical because the aerosol is directly
applied but control of the aerosol once it exits the flute is less than desirable
because the end of the tool is not doing the cutting, the blades of the tool performs
the machining. The synthetic and oil is applied in mass quantities but can be reused
after it is filtered [5].
Aerosols do not supply much protection against carrying away the heat but if
applied correctly can prevent the heat from generating [5].
CO2 coolant is the use of carbon dioxide that can be used during the CNC machining
process [5]. To apply CO2, pressurized liquid CO2, is allowed to expand and follow
the ideal gas law, this is accompanied by a drop in temperature which is enough to
cause the CO2 to form water droplets [5]. These solid crystals of water are
redirected into the cut zone by an external nozzle or through a spindle delivery
system [5]. This will provide temperature controlled cooling of the cutting tool and
work piece.
10
There are significant advantages of using CO2 as a coolant. CO2 is pressurized so it
can be applied at a higher feed speed more effectively than an oil based or synthetic
lubricant [5]. Because moisture is formed when the CO2 is depressurized, water
droplets can be supplied directly to the cutting surface [5]. The last advantage is the
coolant effect of CO2 which can reduce heat generation significantly [5]. CO2 also
has no fluid waste, no staining, and could provide a better finish. The disadvantage
is the CNC machine requires a storage container for all the CO2 and a pressurized
coolant system to distribute the CO2 [5].
Figure 8 shows two images of a CO2 delivery system and how it is applied
effectively.
Figure 8: This high pressure process keeps the cutting surface free of debris but
also supplies an extremely cool fluid to prevent heat generation. Pictures provided
by http://www.tungstenandtool.co.nz/cleaning-and-lubricating.php
Conclusion:
The use of CNC milling machines has made machining of complicated geometries
much easier and with higher degrees of accuracy than human machining. But with
these complicated machining processes and the high speed at which the tools turn
it, the lubrication applied is important to the final outcome of the product. Tool
wear, reduction of friction, and wear on the parts is an important aspect of this
machining process. Flank wear, crater wear, edge build up, and edge wear can be
reduced by applying lubrication based on the machining application. All lubrication
has there advantages but to protect the part properly, the type of lubrication must
be applied to the right application.
11
References:
[1] http://en.wikipedia.org/wiki/Tool_wear
[2] S. Kalpakjian and S.R. Schmidt. Manufacturing Engineering and Technology. 2000,
Prentice Hall, Upper Saddle River, NJ
[3] http://www.amrolgroup.com/cutting-oil.html
[4] http://www.carbideprocessors.com/pages/machine-coolant/types-of-machinecoolant.html
[5] http://en.wikipedia.org/wiki/Cutting_fluid