GrallDIAMOND PRODUCTS
Preparation of Diamond Grinding Wheels
Truing and Conttitioning YourDiamond Grinding Wheel
(Source: Superabrasives - Grinding and Machiningwith CBN and Diamond -
By S.F. Krar and E. Ratterman)
Graff Diamond Products 35 Hale Road Brampton Ontario (905) 457 8132Phone (905) 4578132 Fax (905) 457 2265 E mail: [email protected]
www.graffdiamond.com
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0 50C 100e 50e 100e1/16 in. '/16 in. 1/8 in. 'Ie in.(1.6 mm) (1.6 mm) (3.2 mm) (3.2 mm)
Diamond depth and concentration
A sixfoldimprovement in grinding ratio can re-sult if diamond depth and concentration arecombined. (Courtesy of GE Superabrasives)
WHEEL MOUNTING AND PREPARATION
Care should be taken whenever a diamond wheel ismounted because it pays off in longer wheel life andconsistent accuracy in the parts ground. For a dia-mond wheel to grind at 100 percent efficiency, thewheel must run as true as possible.
Runout of a diamond wheel will cause it to chip onthe edges, wear faster, produce poor surface finishes,and produce inaccurate parts. Wheels where the out-side diameter is used for grinding should never runout more than 0.0005 in. (0.01 mm). Cup grinding
o.;::;'"~ 160c'0c 120o
280
240
200
80
40
40/60" 230i270 325(400Diamond size(U.S. grit size)
Using the coarsest grit size possible results inthe lowestgrindingwheel cost per piece. (Cour-tesy of GE Superabrasives),
146 Chapter 8
wheels should never run out more than 0.0005 in.(0.01 mm) on the grinding face. Careful attention inmounting the wheel will reduce initial wheel loss dueto truing and dressing-or even eliminate this loss.Chapter 5, on preparing the CBN wheel and grinder,deals extensively with the mounting, truing, anddressing of CBN wheels. Many of the same pointsapply to diamond wheels; therefore, it is wise to referto Chapter 5 before mounting, truing, and dressingdiamond wheels.
The following is a summary of the key pointswhich must be followed when mounting a diamondwheel if it is to perform satisfactorily:
1. The diamond wheel should be mounted on a hi.gh-quality adapter and should be kept together as aunit for the life of the wheel.
2. Mount the wheel-adapter unit on the grinder spin-dJe and lightly tighten the flange nuts.
3. Check the runout on the grinding portion of thewheel with a dial indicator and correct the runoutby:a. 'Wheel Circumference. Move the wheelan the
adapter by gently tapping with a hammer on awooden or plastic block until the runout is nomore than 0.0005 in. (0.01 mm) (Fig. 8-13).
b. Wheel Face. Face wheels are usually trued bythe manufacturer and should have very littlerunout. If a runout greater than 0.0005 in(0.01 rnm) is indicated, the wheel must be trued(Fig. 8-14).
4. Tighten the flange nuts securely and recheck thewheel runout.
Wheel PreparationThe condition of the grinding surface of a diamondwheel will determine the success or failure of any
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Correctingthe runout on the circumferenceof adiamond grinding wheel. (Courtesy of Cincin-nati Milacron Co.)
Checking the runout on the face of a diamondcup wheel. (Courtesy of Cincinnati MilacronCo.)
grinding operation. Wheel conditioning, next tospeeds and feeds, is the most important factor in theefficient use of diamond wheels. This can involve theoperations of truing and dressing. For best perfor-mance, grinding wheels should be trued anytimethey are removed from the grinder.
TruingIf the wheel runout is more than 0.0005 in. aftermounting, it will be necessary to true the wheel.Truing consists of grinding or 'wearing away a portionof the abrasive section of the grinding wheel to makeit run true or to bring it to the desired shape. Chapter5 covers the operations of truing and dressing of CBNwheels in detail and should be followed for diamondwheels. The major difference between the twowheels will be that the diamond wheel is muchharder and therefore will take longer to true anddress than is necessary for a CBN wheel.
Some of the more common methods of truing adiamond wheel are:
1. Mild Steel BLock (Fig. 8-15A)a. Mount a mild steel block on the grinder.b. Take a few passes across the steel block at
O.OOl-in. (O.02-mm) depth per pass.c. Stop the grinder and check the wheel face or
circumference for truth with a dial indicator.d. If necessary, continue passes over the steel
block until the wheel runout is eliminated.2. Brake-Type Dresser (Fig. 8-158)
a. Mount a brake-type dresser on the grinder.b. Follow the dresser manufacturer's recommen-
dations on the use of this equipment.c. Continue the operation until the wheel runout
is corrected.
(A)
Cemented Carbide Tool Grinding with Diamond Wheels 147
3. Toolpost Grinder (Fig. 8-15C)a. Mount a toolpost grinder on the grinder table.b. Mount an 80- to 120-grit vitrified silicon carbide
wheel on the grinder spindle to operate at aboutone-quarter of its normal speed.
c. Set the diamond wheel and the silicon carbidewheel to operate in opposite directions.
d. Take a few passes at 0.001 in. (0.02 mm) overthe diamond wheel.
e. Check the wheel for runout with a dial indica-tor.
f. Continue taking passes over the diamondwheel until the runoutis corrected.
DressingThe truing process generally leaves the grinding sur-face of a diamond wheel smooth, with few or no ab-rasive crystals protruding 'above the wheel surface. Awheel in this condition would burn the workpieceand remove little or no work material. Dressing re-moves some of the bond material from the surface ofa trued wheel to expose the diamond crystals andmake the wheel grind efficiently.
To dress a diamond wheel:
1. Select a silicon carbide or aluminum oxide dressingstick.
2. Saturate the dressing stick with coolant so that aslurry is created when it contacts the diamondwheel.
3. Hold the dressing stick firmly and bring it into coil-tact with the revolving diamond wheel (Fig. 8-16).
4. Feed the dressing stick aggressively into the wheelto open up the wheel face.
5. Once the dressing stick starts to wear rapidly, thediamond wheel is dressed. .
Opening up the face of a diamond grindingwheel with a silicon carbide dressing stick.(Courtesy of Cincinnati Milacron Co.)
148 Chapter 8
USING DIAMOND WHEELS
The proper use of diamond grinding wheels will re-sult in long wheel life and high material-removalrates. To achieve the best grinding performance, thediamond wheel must be used under conditionswhich make this possible. Operating conditions suchas wheel speed, work speed, feed rate, and the use ofcoolant are factors which affect the performance of adiamond grinding wheel.
Wheel SpeedIn wet surface grinding, the speed of a diamondwheel is important to the grinding performance. Lowwheel speeds (below 4000 sf/min) tend to reducegrinding efficiency, while speeds (over 6000 sf/nun)tend to reduce wheel life. The best average wheelspeed for most applications is in the 4000to 6000 sf/ruin(20 to 30 m/s).
The best wheel speed for dry tool and cutter grind-ing is 3500 sf/min (18m/s), using medium to fine gritsizes (150and finer) at 75 or 100concentration. Manytool and cutter grinders are set to drive a wheel at3500 to 4500 sf/min. (18 to 23 m/s). Do not dry grind atwheel speeds over 4500 sf/min (23 mls). Dry grinding atspeeds over 4500 sf/min (23 rn/s) will cause heat dam-age to the diamond wheel. If it is absolutely neces-sary to operate at above 4500 sf/min (23111/5),a coolant should be used to prevent damage tothe wheel and extend its life.
Work SpeedTable traverse speed or crossfeed is largely governedby the amount of downfeed or depth of cut. Becausethese can vary greatly depending on the machinecondition, grinding operation, workpiece material,and other factors, it is difficult to give specific workspeed rates. The best work speed, often arrived atthrough trial and error, is where there is no Joss ofwheel speed, excessive wheel wear, or damage to thewheel or work.
Feed RateExcessive feed or depth of cut will shorten the life of adiamond grinding wheel and should be avoided. Toodeep a cut can result in burned and cracked wheels,chipped or cracked carbide, and failure to obtain partsize, finish, and form:.
In face grinding cemented carbides, trying to takea heavy cut too quickly will result in the carbide act-ing as a shear and dressing the diamond wheel face.When grinding on the periphery of the wheel, heavycuts cause the wheel to climb the workpiece, causingthe work to dress the wheel out of round and produc-ing chatter.
The depth of cut possible with diamond wheels isgoverned by the diamond grit size. The recom-
mended depths of cut for various diamond wheel gritsizes are listed in Table 8-4. Feeds more than thoserecommended will cause material of the part to getunder and cut away many diamond grits per revolu-tion that will never get a chance to cut.
Cutting FluidsCutting fluids should be used whenever possiblewhen using diamond wheels to reduce the grindingheat and extend the wheel life. If it is necessary togrind dry, as in tool and cutter grinding, a resin-bondwheel should be used. Sometimes spray mist with arust inhibitor or refrigerated air can be used as a cool-ant to reduce the grinding heat; however, this is notas effective as wet grinding.
A small trickle of coolant occasionally applied isworse than no coolant at all. This causes alternateheating and quenching, which can cause damage tothe diamond wheel and carbide tools. The best wayto apply coolant is to allow the centrifugal force of thewheel to deliver it in a steady stream to the point ofgrinding contact (Fig. 8-17).
Today, there is a wide range of resin-bonded.grinding wheels which have been specifically de-signed to be used dry when grinding cemented car-bide tools. These wheels are very effective and main-tain productivity without the use of any coolant.
, !
Carbide Grinding EconomicsWhen grinding cemented carbides with diamondwheels, two sets of costs must be considered-laborand overhead cost per piece and diamond wheel costper piece. Labor and, overhead cost per piece can bereduced by increasing the material-removal rate. Thisincreases the diamond wheel cost per piece sincediamond wheel wear increases with the material-removal rate.
Generally, the economic material-removal rate-the rate at which labor and overhead cost per pieceand wheel cost per piece are balanced for lowest totalgrinding cost-is quite low when grinding cementedcarbides in comparison with steel grinding. For drytool and cutter grinding, the optimum material-
---"--"RECOMMENDED DEPTH OF CUTS FOR DIAMOND GRIT SIZES
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.----- ..-.-.---.-- ...---.-,------------~.--- ..--I Recommended Maximum
L..._ Grit Diameter .L..-.--~- ..--- De~th o~~~ __ ~
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Diamond GritSize
A flood coolant directed to the grinding zonecan increase wheel life ten times over dry grind-ing. (Courtesy of GE Superabrasives)
removal rate is seldom more than 1 to 2 in? (16.4 to32.8 em") of cemented carbide per hour. For wet sur-face, cylindrical, and vertical spindle grinding, theoptimum material-removal rate usually ranges from 2to 10 in.:' (32.8 to 164 em") of cemented carbide perhour.
To realistically evaluate the economics of cementedcarbide grinding operations, it is necessary to knowhow well the wheel is performing. A useful index ofdiamond wheel performance is the grinding ratio. Thegrinding ratio is obtained by dividing the volume ofworkpiece material removed in a given time by thevolume of wheel consumed in the same period, whenoperating under constant conditions. The higher thegrinding ratio, the longer the life of the wheel (Fig.8-18).
Material-Removal RatesWith the right wheel for the job, running at the rightspeed, a variety of material-removal rates are possi-ble. Low material-removal rates extend wheel life at
0.0020.00150.0008()'0004
0.050.040.020.01
Cemented Carbide Tool Grinding with Diamond Wheels 149
o~a:ezoza:o
MATERIALREMOVAL RATE
TOTALCOST
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MATERIALREMOVAL RATE--------------~..-- ..
Grinding-wheel life is reduced as material-removal rates are increased when grindingcemented carbides. (Courtesy of GE Superabrasives)
the expense of productivity. High material-removalrates increase productivity at a sacrifice of somewheel life. In most industries, material-removal ratesare selected on the basis of least total grinding costper piece. Generally, this is at a point where wheelcost per piece and labor-and-overhead cost per pieceare approximately equal.
There is no clearly defined method of increasingmaterial-removal rate that will guarantee the mosteffective use of the diamond grinding wheeL Labora-tory tests have indicated that in both wet and drygrinding cemented carbides, the life of the wheel isshortened in some direct relationship to the increasein material-removal rate, irrespective of the techniqueemployed for increasing removal rate. However,there is some indication that in the case of wet surfacegrinding the use of heavy crossfeeds will take a some-what lesser toll in wheel life than will downfeed ortable speed.
However, a very important factor to consider isthat the wheel is removing carbide only while thewheel and the workpiece are in contact. Any tech-nique which increases the noncontact time betweenthe wheel and the workpiece diminishes the effectivematerial-removal rate and increases cost.
Wheel LifeThe life of a diamond grinding wheel can be affectedby the wheel speed, the work speed, the depth of cut,the grinding mode, the area of wheel contact, and thetype of carbide ground. All these factors have an ef-fect on the forces which are created during the grind-mg process.
Three forces act on a diamond wheel in tool andcutter grinding of cemented carbides: normal force,tangential force, and radial force (Fig. 8-19). Normalforce is by far the greatest of-the three forces which act
150 Chapter 8
on a wheel. Increasing the infeed (depth of cut) from0.002 to 0.004 in. (0.05 to 0.10 mm) reduces the wheellife because the increased pressure will cause thewheel to break down more quickly. The three forcesare interrelated, and the specific grinding conditionswill determine the amount of each force component.
TOOL AND CUTTER GRINDING GUIDELINES
The tool and cutter grinder is the most common ma-chine tool used for regrinding or reconditioning car-bide and high-speed steel-cutting tools. This machineis versatile and provides the operator with a goodview of the sharpening operation. For best results inregrinding cutting tools, the following guidelines areoffered:
1. Reduce the Area of Contact. The contact surface onthe wheel face should not be much more than1fs in. (3 mrn) wide. Narrow wheel rims are suita-ble for most grinding jobs. Although wider wheelrims give longer wheel life, they create more grind-ing heat.
2. Use Low Wheel Speeds. Wheel speeds should bearound 3500sf/min (18mJs) for dry grinding opera-ti.ons in order to reduce the heat created at the cut-ting tool edges.
3. Use Reduced Work Speed. The table speed shouldnot be more than 6 to 9 ftlmin (2 to 3 m/min) toreduce the possibility of diamond grit being tornout of the wheel due to crowding. Diamondwheels will perform better with deeper infeedsand slower work (table) traverse speeds.
4. Depth of Cut. The depth of cut is governed by thediamond grit size. For a lOa-grit diamond wheel,the maximum depth of cut should be 0.002 in.(0.05 mm) and 0.0004 in. (0.01 mm) for a 400-g1itwheel.
