author
STEVEN J. KENDJELIC
Senior Product Engineer
Radiac Abrasives
Springboro, Ohio
abstract
In the area of abrasive machining the use of grinding technology currently being
developed permits more economical metal removal rates. Work piece quality
improvements are made efficiently without thermal damage and stress, often
eliminating secondary operations. Quality improvements required do not define
what the correct grinding wheel, feed, cutting speed and or grinding method are
to be used in grinding processes.
In the area of abrasive machining the use of grinding technology currently being
developed permits more economical metal removal rates. Work piece quality
improvements are made efficiently without thermal damage and stress, often
eliminating secondary operations. Quality improvements required do not define what
the correct grinding wheel, feed, cutting speed and or grinding method are to be used
in grinding processes. Applications specialists need to evaluate process requirements
to efficiently optimize processes used in manufacturing. Machine requirements are high
rigidity, precise positioning, minimization of reversal backlash, and machine axes free
from stick slip. To achieve flatness and parallelism surface grinding is usually utilized.
Profile grinding can use the reciprocating grinding method and or creep feed grinding.
The partnership approach between the end user and tooling vendors allows changes to
be initiated to current operating systems to increase productivity, economically.
Surface grinding is the most common grinding application seen in manufacturing.
Surface grinding is most often used for grinding work pieces flat and parallel. The
grinding wheel is operated at a predetermined surface speed and the work piece
(mounted to the reciprocating table of the machine tool), passes directly under the
grinding wheel. At the end of the work piece stroke the wheelhead is downfed in a very
precise increment at which time the work piece passes under the grinding wheel again.
When this process is taking place there is also a third axis, utilized for cross positioning
at each table reversal. The process of surface grinding is extremely accurate and can
provide a very fine surface finish. Work surfaces comparable to surface plate flatness
and parallelism are quite common.
The use of sharp single point diamonds to produce an improved surface finish is
necessary. Typically the diamond is mounted at an angle and as it is worn the diamond
is rotated to wear more evenly. An easy comparison would be that it is easier to drive a
pointed nail into a piece of wood than it is to drive a blunted one. The sharpness and
quality of the diamond directly effect the wheel cutting action and subsequently the
work quality.
When an applications engineer must plan an abrasive process there are many avenues
open. New processes don't necessarily mean a capitol investment in new machine
tools until all avenues are explored. As an example if the application is die components
or turbine blades it is possible to use an existing reciprocating grinder that does not
have computer controlled axes for form dressing of the wheel. One type of application
would be to purchase a precision diamond dressing block manufactured to the desired form.
By placing the dressing block at the end of the table stroke at the finish part height, it
will continually dress the grinding wheel as the part is processed to finish size, assuring work piece geometry and finish.
Older surface grinding machine tools can be adapted for plated Cubic Boron Nitride
(C.B.N.) use (after evaluating spindle motor speed and spindle bearing capability) quite economically. However Plated wheel use can be predicted by following trends in wheel life. The constant deterioration or dulling of the grain must be anticipated due to the single layer manufactured on the wheel periphery.
The terminology used by the industry can be confusing. The industry term of Low
Stress Grinding (LSG) was replaced by Creep Feed (CF) grinding which some
companies have now changed to High Energy Deep Grinding (HEDG). All the
aforementioned processes are basically the same and all have been accomplished with conventional vitrified abrasives and also super abrasives.
Super abrasives can make significant improvements when used in HEDG applications by the increase in Knopp hardness of the grain and grain durability.
Diamond is the
hardness known abrasive (7000 Knoop hardness) followed by C.B.N. (4700 Knoop
hardness) typical hardened tool steels are 50 RHC. Due to the process of
manufacturing plated and vitrified grinding wheels increased wheel speed is possible.
When wheel speed is increased so can traverse speed. With the increase of wheel
speed smaller chips are formed during grinding producing a cooler cut zone.
Vitrified C.B.N. permits the use of rotary dresser or diamond disks to generate forms.
Preforming the abrasive wheel is usually the preferred method. Diamond dressers only
true the super abrasive grinding wheel. To condition the wheel an aluminum oxide
abrasive stick is used to relieve the bond material. Grit size is usually two times greater than the C. B. N. wheel being dressed. Wheel geometry is an important consideration in the grinding process. Flat grinding wheels present different problems than profiled wheels. On flat surfaces grain contact is limited to the wheel width size. Complex forms such as Fir tree forms on turbine blades present significantly more grain contact and help pull coolant into the grind zone.
Creep feed grinding is used to remove a large amount of material (vertical infeed) with
a slow grinding feedrate This process allows high metal removal rates (MRR) to be
accomplished in a very short time as opposed to surface grinding. Tangential forces
are much greater in creep feed grinding due to the increase in the arc of contact by the
deep cut. Creep feed grinding does require high horsepower machines that are
extremely rigid with a closed loop system to monitor the process.
Which Abrasive For Grinding Applications?
What type of material to be ground and the hardness of the material?
Different materials determine the abrasive used. As an example powdered metals
require finer grain size due to the composition of the material. Requirements of grain
types and friability for penetration of the material to be ground must also be considered.
Variable or fixed wheel speed.
Constant surface footage maintains wheel performance throughout the wheel life. Fixed spindle speeds can require hardness (bond) grade adjustments for loss of constant surface footage.
Surface or profile grinding
Flat surface grinding generally has different requirements. The vitrified grinding wheel
is self sharpening to a point. Profile grinding must retain the shape dressed into the
wheel periphery, possible by higher bond content or finer abrasive grit. Wheel contact
area during grinding in consideration with machine tool capability for rigidity and
horsepower is an important consideration.
Accuracy and surface finish.
By adjustment of grain size, minimization of thermal damage to the material being
ground can be achieved. The finer the grain size produces improved surface finish but
limits chip load without induced porosity being used in wheel manufacture.
Conventional abrasives utilize various types of friable aluminum oxide grains and blends of grains and grain treatments in creep feed grinding (2100-2500 Knoop hardness). The abrasive structure used in creep feed grinding differs from the surface grinding application in that wheel density is changed by artificial inducement with pore forming agents. The distribution of the pore formers is an important issue in direct relation to pore former volume. The bonds used in the manufacturing of grinding wheels, when fired, control the wheel hardness along with furnace heat. Grinding wheel bonds are engineered to the application of their use. Materials below 38RC require increased bond volume and grain volume for use. Typical creep feed wheels are 40% abrasive grain 10% bond, and 50 to 60% induced porosity. Conventional abrasive wheel grains can be, Regular, Friable, Modified Friable, Microcrystalline, Monocrystalline. Super abrasives can be Monocrystalline, Polycrystalline (CBN), and natural or synthetic Diamond.
The desired effect of any grinding process is achieving dimensional accuracy, part
geometry, and surface quality at an economical cost. These process characteristics
must be anticipated prior to grinding. To optimize an existing process one must use
empirical methodology to define and evaluate existing operating characteristics,
determine known wheel operating characteristics and unique part grinding
requirements to optimize grinding variables. These building blocks are necessary data
collection for interpretation of the various phases.
The development of continuous dress creep feed (CDCF) for the use of conventional
abrasives, such as Aluminum Oxide, and Silicon Carbide allowed higher stock removal rates than the previous intermittent creep feed method. This is accomplished by continuously feeding a rotating diamond roll into the operating grinding wheel periphery thus presenting a freshly dressed wheel surface to the grinding zone at every
revolution of the wheel. This greatly reduces the forces used in the process. Constant
surface footage is a important factor in both creep feed and continuous dress creep
feed. A variable speed spindle motor is required in order to accomplish constant
peripheral speed throughout the grinding wheel life. Having a constantly sharp grinding
wheel allows deeper cutting ability of the wheel. A variable speed spindle motor will
allow surface feet per minute to be changed to assist in making the abrasive act harder or softer as needed by the wheel applications engineer to optimize grinding processes.
The positioning accuracy of the overhead dressing system by use of servo motors and
ballscrews, and feedback ability of the linear glass scales to the C.N.C. allow the
end-user to monitor the grinding and dressing process. The rotary dressers rotation
also will affect the surface finish of the work piece. Dressing in the unidirectional mode
will permit better surface finish and the highest stock removal rate. Dressing in the
counter-directional mode produces a less aggressive grinding wheel. In the
uni-directional mode diamond roller dwell should be used with extreme caution as not
to damage the precision diamond roll by staying in contact with the grinding wheel for
too long a period of time and dulling the grinding wheel and burning the work piece.
The general practice in continuous dress creep feed is to use reverse plated diamond
rolls for their superior accuracy and sharpness. The general practice for grinding wheel
selection for tool steels, is silicon carbide and aluminum oxide. Silicon carbide is also
used for cast iron and titanium alloys. Aerospace alloys, containing nickel,
molybdenum, cobalt and high chromium generally use grinding wheels of the aluminum
oxide and super abrasives type.
Coolant application is also an important consideration. We know a grinding wheel
rotating at a speed of 6500 surface feet per minute or 72 MPH produces an air barrier
surrounding the grinding wheel. Coolant is not efficiently delivered to the grind zone
because of this air barrier. One school of thought is that a baffle should be used to
break the air barrier and when the barrier is broken the wheel speed sucks the coolant
into the grind zone which reduces thermal damage to the part. This baffle approach is
improved when incorporated with the use of pressurized coolant injected into the pores
of the grinding wheel, 220 p.s.i. are typical. Coolant velocity delivered to the work area
should meet or exceed the wheel surface speed at new wheel diameter with coolant
nozzle position tractability. It is obvious that nozzle shape and size must be calculated
to optimize process conditions with 120 to 140 p.s.i. quite typical.
Coolant temperature is another area that needs to be considered. Chillers need to be
utilized to provide a stable process. The use of chillers also prevents bacteria growth
by the close monitoring of the desired temperature below 80 degrees Fahrenheit.
Coolant temperatures above 80 degrees Fahrenheit reduce coolant effectiveness. The
high pressure pumps required for the process also induce heat into the coolant system
making coolant temperature an important consideration. As the coolant temperature
increases thermal expansion take place in the machine tool making it difficult to
maintain size. This temperature increase then can be transmitted to the hydrostatic oil
used in the guide ways or machine bed.
Partnerships in grinding benefit all parties involved. As modern technology improves
constant changes will allow a process as near to optimum as possible. The desired
effect of any grinding process is dimensional accuracy, part geometry, and surface
quality at an economical cost. When this process has been optimized grinding and the
associated costs become economical in manufacturing. The advantages of high
performance grinding are that parts can be processed from solid in the hardened state, with better form holding capabilities and minimal burrs to the part. The minimal burrs sometimes eliminate secondary deburring operations. Any application requiring premachining before heat treatment, and hard to grind materials of high density are an
excellent candidate for high performance grinding.
Reducing costs to become leaner and more efficient and the implementing of
just-in-time (JIT) production methods have driven companies to new equipment. Speed is the order of the day, and manual machining methods are not as competitive. Newer equipment has better technological capabilities, and produces higher levels of quality.
The replacement of machinists with CNC equipment does not automatically mean
increased productivity or reduced costs. Human supervision is still necessary in
engineering and on the shop floor to verify new programs and jobs, or misloaded parts
or other problems.
If a major investment in a new machine tool is made, proper planning to make this
installation trouble free is necessary. Program specialists are required to program the
machine tools computer numerical control. Tooling specialists are required to evaluate
the machining requirements. Part processing can sometimes require outside help to
effectively manufacture precision components. Button pushing operators cannot
replace experienced machinists because understanding basic machine practices are
not known by operators. It takes a learning curve to become familiar with new
processes and controls. Without the proper research the increased cost of automation
tends to deteriorate the bottom line. With the addition of the above specialists, labor
cost can become much greater than when using the past manufacturing process.
Support personnel are necessary to effectively increase output of product. An
investment in preventative maintenance must be made to avoid downtime while the
machine tool is still new. Preventative maintenance programs implemented on older
equipment need to be considered long before the machine tool starts to exhibit
repeated problems. A new machine tool down with untrained maintenance personnel is
worse than no machine at all, as this unit is not offering a return on investment and
production schedules cannot be met.
When updating equipment, consideration to the machine tools controller type must be a
factor. The idea of purchasing controls that cost less initially on a new machine tool is
often the poor judgment made. True planners know that if machining centers, lathes,
and grinding machines have the same or similar types of CNC controls that
standardization of programming and maintenance support people reduce the cost of
the necessary training courses required when using many different control types.
Bottom line costs of new equipment do not take into consideration that CNC processing
speeds, and servo loop closure times can add up to reduced productivity.
The machine tool builders that are responsible for the current lines being offered were
developed in Europe. The primary companies are ELB, Blohm, Jones & Shipman, and
Meagerle. The ELB company established in 1949 utilizes a broad, large flat and V-way
guideway design that offers extreme rigidity, additionally offered are hydrostatic
grinding axis and roller bearing guideway systems used in the Y & Z axes. Machine
bases are offered in cast iron and also an epoxy-granite type that permits greater base
thickness. The epoxy granite machine base offers greater vibration damping than the
cast iron type.
Blohm utilizes roller bearing elements in it's guideway design offered in weldments and
casting type machine bases. The low coefficient of friction in rolling-element bearings
allow the bearing to operated at high speed without excessive heat buildup. Axes motor size can be reduced when using this type of guideway due to the low coefficient of friction. This guideway type also offers ease of maintenance and serviceability.
The Jones & Shipman line utilizes the highly rigid flat and V-way design in it's grinding
axes on the entire creep feed, surface and external cylindrical machine tool line.
Bearing guideway systems are used in the remaining axes. This design can withstand
high loads with minimal deflection. Load capacities, are high enough that they are
compatible with grinding forces, workpiece support, and shock loading. The flat and
V-way together with the granite type base provide high damping of vibration caused by
the grinding process. Machine software is one of the industry leaders in ease of use,
the operator is prompted by on screen graphics to generate part programs.
Meagerle manufactures their machine line with a combination of a hydrostatic grinding
axis and roller bearing elements used in the Y & Z axes. Table loads are typically
higher when using hydrostatic and flat and V-way type guideways in comparison to
roller bearing type guideways. The hydrostatic oil system, the majority stored in the
machine base aids in controlling thermal expansion of the machine tool and significant
vibration dampening. Load capacities, are high enough that they are compatible with
grinding forces, workpiece support, and shock loading.
The United States machine tool builders are represented by Edgetek, Campbell,
Bridgeport Harig, and Mattison. Each builder keying into certain markets to specialize
in C.B.N. grinding and conventional surface, profile and creep feed grinding.
The needs of the end user must be analyzed to determine which machine tool builder
best suits their application. As described in the above designs there is no such thing as a poor machine tool design. The machine tool purchased must meet the criteria of the end user for the lowest cost, most reliability, and the highest quality
The calculation below illustrates how to calculate an existing surface grinding process
for cycle time.
Example
Table speed 980 inches per minute
Grinding Length 50 inches
980 / 5- = 19.6 round up to 20, 20 table reversals
Part is 16 inches wide, with a crossfeed of 2 inches per stroke.
16 / 2 = 8 strokes
Travel over complete work piece 20/8=2.5 per minute
390 saddle reversals devised by 2.5 = 156 minute cycle time to remove the total stock
amount of 0.078 inches
Careful calculations for metal removal and cycle times can be based on a metal
removal formulas. Test grinds are necessary in areas where high production and
various grinding wheels, and coolants need to be tried. Below is a typical conventional
abrasive creep feed cycle time estimate based on the cubic metal removal already
calculated.
Length of grind including the arc of contact 200 mm
Grinding Feedrate 100 mm/min.
Time in minutes 2
Time in seconds 120
Length of grind devised by the feedrate equals time in minutes and seconds.
The next group of variables must also be known or calculated.
Close door/Cycle start 5 sec.
Grind time (from above calculation) 120
Dress & Park axes 15
Load/Unload parts 30 Total 170 Cycle time minutes 2.8333333 10% Safety 3.1166667 Cycles per Hr. 19.251337 Number of parts 40 Parts per hr. 770.05348 Parts per year required 500000 Total Hours 649.30556
Hours per year needed at 80% efficiency 811.63194
The dress time calculation can vary for full form roll or contour dress with single point
and or rotary diamond disk. Wheel width devised by dressing feedrate to generate form dress time.
Process problem solving steps to be followed, problem identification, understand
sources of variation, determine the most likely causes, gather data, conduct tests and
analysis data, measure the results to plan the next level of continuos improvement.
Bibliography
Steven J. Kendjelic, Grinding In The 90's, SME Technical Paper MS91-211
Steven J. Kendjelic, Rounding Out the Millennium, SME Technical Paper MR94-128
