GRINDINGGrinding machines

▦Types of grinding machines

◎What is grinding?

• What is grinding?A machining process that uses a high-speed rotating grinding wheel as the tool and removes material from the workpiece a little at a time with each of the very hard, fine abrasive grains in the wheel. In the classification of machining, it belongs to abrasive machining, separate from cutting with edged tools, and within that to fixed-abrasive machining, in which the grains are bonded into a wheel; honing, superfinishing and belt grinding are its relatives. It is distinguished from processes that use loose grains, such as lapping and buffing (loose-abrasive machining)1 source · for reference^[1][2]
• Difference from cutting 1: many small cutting edgesEach cutting edge (abrasive grain) is small, and the amount it removes at a time (the chip) is tiny. So it is easier to get surface roughness and dimensional accuracy than with cutting. A wheel is made of three things: abrasive grains, bond and pores, and the pores give the chips somewhere to go^[1][3]
• Difference from cutting 2: high wheel speedWheel speed is about 10–80 m/s, several times to several tens of times that of cutting tools. Even though each chip is small, productivity is high1 source · for reference^[1]The upper limit of wheel speed is set by each wheel's maximum operating speed
• Difference from cutting 3: hard and brittle materials can be groundThe abrasive grains are very hard mineral particles, so materials that are hard to cut with edged tools, such as hardened steel, cemented carbide and ceramics, can be ground. With diamond or CBN grains, cemented carbide, ceramics and glass can also be ground^[1][3][4]
• Difference from cutting 4: self-sharpening (the edges renew themselves)As the grain tips dull in use, grinding force rises, and the grains fracture or fall out, exposing new edges and pores. When it works well, the wheel keeps cutting without being resharpened like a cutting tool (what Noritake calls the "normal" state)^[1][5]
• Difference from cutting 5: the grinding point gets very hotThe temperature at the grinding point can exceed 1000 °C, which causes grinding burn and grinding cracks. It is a process that assumes thorough cooling and lubrication with grinding fluid^[1][6]
• Difference from cutting 6: the hazard of a high-speed rotating bodyThe wheel spins at high speed, so if it ever breaks it can cause a serious accident.1 source · for reference^[1]
• Where grinding is usedWhen you want to finish hard materials such as hardened steel or cemented carbide with high quality and accuracy. Even for mass-produced parts like engine components, it is often placed as the final machining step. Typical parts are those where size, roundness and surface roughness matter at the µm level, such as bearing raceways, hydraulic parts and shafts^[7][1][8]

◉The five factors of a grinding wheel and how to read the marking

• A grinding wheel is made of three elements: abrasive grains (the cutting edges), bond (holds the grains) and pores (a place for chips to go). Its performance is determined by five factors: abrasive type, grain size, grade, structure and bond^[3][19]
• Broadly speaking, the A types (A, WA, PA, SA, CX) and CBN are used for ferrous metals, and the C types (C, GC) and diamond for non-ferrous metals (cemented carbide, etc.) and non-metals^[3][4]The reasoning that diamond reacts readily with iron and so is not used on ferrous metals is not stated in the sources used this time

Abrasive types

• Grain sizeGrain size is the size of the abrasive grains. The smaller the number, the coarser; the larger, the finer. In Noritake's marking, up to No. 220 is written F and from No. 240 with #, and JIS has classes from No. 8 to 3000. Coarse grains up to about F36 are mainly used for free grinding such as deburring and cutting off; grains finer than F46 are mainly used for machine grinding that needs dimensional accuracy and surface roughness. The relation between grain size and surface roughness is not fixed^[3][19]The range of F and the F36/F46 distinction are from Noritake only.
• GradeGrade (the hardness of the wheel) is the strength of the force holding the grains. It is expressed by letters from A (softest) to Z (hardest) and adjusted by the amount of bond (more bond means harder)^[3][19]
• StructureStructure is a number for the share of the wheel's total volume taken up by abrasive grains (grain percentage). The smaller the number, the more grains and fewer pores, which is "dense"; the larger, the more "open". In Noritake's table, structure 0 is 62 % grain and structure 14 is 34 % (2 % per number)^[3][19]Kure Grinding Wheel's marking runs from 1 (dense) to 14 (open)

Bond

• V VitrifiedCeramic raw materials such as clay and feldspar are fired to bond the grains (Noritake 900–1300 °C, Kure Grinding Wheel around 1200 °C). Strong holding force, holds its shape well, and stable quality. The mainstay of precision grinding, widely used for surface, internal, cylindrical, centerless and tool grinding. Also available for superabrasives (CBN, diamond)^[3][20]
• B Resinoid (resin)Phenolic and other resins cured at around 200 °C. More elastic than vitrified and absorbs shock, and strong, so used for high-speed free grinding, cutting off, rough grinding, and centerless and roll grinding^[3][20]
• R RubberVery elastic. Used for the regulating wheel in centerless grinding^[3][20]
• M MetalMainly made of metal powders such as bronze. High holding force and shape retention, and long life. For superabrasives, used on hard, brittle materials such as glass and ceramics^[3][20]
• P (Noritake) / EP (Kure Grinding Wheel) ElectroplatedFixes a single layer of grains to a metal core by plating with nickel or similar. The grains protrude a lot and cut well. Form wheels can also be made^[3][20]

Reading a marking

• Example "No. 1, 305×25×76.2 WA 60 K 7 V 33m/s" (Noritake) = type 1 straight wheel, OD 305 × thickness 25 × bore 76.2 mm, white alumina, grain size 60, grade K, structure 7, vitrified, maximum operating speed 33 m/s. Kure Grinding Wheel's example "305×32×127 32A 60 K 8 V 1 A" runs in the order dimensions, abrasive, grain size, grade, structure, bond, shape, edge shape^[3][19]The letters and numbers after the bond symbol are each maker's own.

Effective hardness and how to choose

• Even the same wheel can act "hard" or "soft" depending on conditions. The larger the depth of cut g of a single grain, the more load the grain takes and the more easily it comes off (acts soft); the smaller, the more easily it wears down (acts hard). Ways to make a wheel act softer: lower wheel speed, higher work speed, larger depth of cut, smaller wheel diameter, smaller workpiece diameter (reversed for internal grinding). If the ratio v/V of work speed to wheel speed is the same, the effective hardness does not change1 source · for reference^[5]
• The longer the contact length, the harder the wheel acts, so choose a softer wheel for horizontal-spindle surface grinding than for cylindrical grinding, and a still softer one for vertical-spindle surface grinding1 source · for reference^[5]
• For jobs prone to grinding burn, choose a spec that favors sharpness: a softer grade, a coarser grain size, a wheel with many pores (high porosity), or a CBN wheel with good thermal conductivity. However, wheel life tends to be shorter^[21][6][22]
• If the wheel is shedding (grains come out too easily and the surface turns rough), go to a harder grade or a higher concentration^[23][5]
• Noritake publishes selection tables of standard abrasive, grain size and grade by material (raw or hardened carbon and alloy steel, tool steel, stainless steel, cast iron, cemented carbide, etc.) and by grinding method (cylindrical, surface, horizontal spindle, segment, creep-feed) (catalog vol. 3 p. 76, and the web selection table)1 source · for reference^[3]

✦Truing and dressing

• Truing is removing the runout of the wheel's periphery once it is mounted on the machine so that it is concentric with the center of rotation, correcting a worn-out shape, and forming it to a shape suited to the job. Dressing is cutting away the working surface that has glazed or loaded so that the bond recedes, setting the grain protrusion and cutting edges, and removing material welded to the surface. With conventional wheels one operation often does both, but superabrasive wheels wear little, so the two are evaluated separately^[3][24][25]

• Reading the chipsThe chips also tell you the condition of the wheel. When cutting well on steel they are ribbon-like (flow type); when cutting well on brittle material such as cast iron they are powdery (shear type); and when cutting poorly on steel they are powdery (tearing type). With loading or glazing, the chips melt from the heat and become spherical (melted type)1 source · for reference^[5]
• DresserFor dressing conventional wheels, use dressers with a single diamond (single-point) or several (multi-point) set in them, or sintered or electroplated ones. Dressers using prism-shaped synthetic diamond keep the worn area at the tip constant, so accuracy stays stable. For vitrified CBN wheels, a rotary dresser (a rotating diamond roll) is used, and rotation direction and speed ratio are also conditions^[3][21][16]
• How dressing conditions workDressing sets sharpness and surface roughness. A faster lead (dresser feed per wheel revolution) and a larger depth of cut make large cutting edges on the grains, which raises sharpness and makes burn less likely, but the surface gets rougher. A slower lead and a smaller depth make the surface finer but bring it closer to glazing. With a rotary dresser, down-cut rather than up-cut, and a speed ratio closer to 1, are on the sharp side^[21][3][6]

≈Grinding conditions, burn and chatter

• Wheel speed VsThe speed of the wheel's periphery (m/s). Conventional wheels are used at or below the maximum operating speed (e.g. 33 m/s). A higher speed makes each grain's depth of cut smaller, the wheel acts harder, and heat tends to increase^[3][5][6]
• Work speed (table speed in surface grinding) VwThe speed on the workpiece side (m/min or m/s at the periphery for cylindrical; table m/min for surface). Published training and experiment examples for surface grinding table speed are 10–12 m/min, 10 m/min and 20 m/min^[10][26][27]
• Speed ratio q = Vs/VwThe ratio of wheel speed to work speed (use the same units). If the ratio is the same, the wheel's effective hardness does not change. If burn appears, review the speed ratio^[5][22]
• Depth of cut aeThe depth removed per pass (one stroke, one revolution). In the surface grinding training at Niigata University, automatic infeed is 0.015 mm for rough grinding and 0.005 mm for finishing. The larger the depth of cut, the lower the grinding ratio and the easier it is to burn^[10][28][6]
• Traverse feed (cylindrical) and cross feed (surface)In cylindrical traverse grinding the workpiece is reciprocated axially; in surface grinding the saddle is fed back and forth (intermittent or continuous feed) to cover the whole surface. In surface grinding, there are examples that take an overrun of 30–50 mm on each side of the table from the workpiece1 source · for reference^[14][10]
• Spark-out (zero grinding)Continuing to grind at the end with no infeed until sparks stop. It gradually removes the stock left because of elastic deformation of machine and workpiece, so surface roughness and dimensional accuracy improve. In cylindrical grinding it can be expressed as a first-order lag, in which the actual depth of cut falls behind the command because of workpiece deflection^[10][7]
• Specific material removal rate (grinding productivity) Q′ and Z′The volume removed per 1 mm of wheel width per second (mm³/(mm·s)). It expresses grinding productivity. Examples from public sources: 1–20 mm³/(mm·s) with CBN (JTEKT), and about 4–13 mm³/(mm·s) with sol-gel alumina wheels (Oak Ridge National Laboratory)^{[25][29][7][30]}
• Equivalent chip thickness heqQ′ divided by the wheel speed, a length (ae × Vw ÷ Vs). An index for comparing how severe the grinding is1 source · for reference^[29]
• Grinding ratio GVolume of workpiece removed ÷ volume of wheel lost. It is directly tied to wheel life and cost. It falls as the removal rate rises. At the same removal rate, raising the table speed gives a larger grinding ratio than increasing the depth of cut^[25][28][31]

Grinding burn and grinding cracks

• Grinding burnGrinding burn is when grinding heat makes the surface very hot, forming an oxide layer and discoloration. When mild it is straw-colored, and as temperature rises it turns pale yellow, brown, reddish brown, purple and blue. On hardened steel, changes in the metal structure such as a rehardened layer and a tempered layer also occur. Even when invisible, a damaged layer and tensile residual stress can remain, and these are checked by etching with nital or similar^[6][30][22]
• Grinding cracksGrinding cracks are fine cracks caused by thermal stress from grinding heat and by volume changes from structural changes. They are more likely in steels with high carbon content, and insufficient tempering is also a cause. They are checked by dye penetrant testing (color check) and magnetic particle testing1 source · for reference^[6]

Main causes of burn and cracks

• Wheel speed is too fast^[21][6]
• Depth of cut or stock allowance is large (heavy grinding)^[21][6]
• Workpiece rotation or table speed is slow (at the same removal rate, grinding "fast and thin" is less likely to burn)^[30][6][31]
• Glazing or loading (a wheel that does not cut well), too little dressing depth of cut, or slow dressing feed^{[5][21][6][23]}
• Grinding fluid does not reach the grinding point (insufficient flow, nozzle position, clogging), low concentration, or poor lubricity^{[21][6][22][32]}
• The wheel is too hard, too fine or too dense^[21][6][22]
• Grinding large workpieces or end faces has a large contact area and the grinding fluid hardly reaches, so it burns easily. The same goes for side-contact grinding and angled infeed grinding, where each grain's depth of cut is small and wears down easily1 source · for reference^[21]

• Order of countermeasuresThe order of countermeasures recommended by JTEKT: ① check the coolant nozzle → ② check truing (dressing) → ③ change the grinding conditions → ④ change the type of grinding fluid → ⑤ change the wheel spec1 source · for reference^[6]

Chatter, dimensional variation and roundness

• ChatterChatter is vibration between the wheel and the workpiece that leaves streaks or waviness at regular intervals on the surface. There is forced chatter that comes from outside (vibration through the floor, vibration from the motor, pump or feed, wheel mounting runout and imbalance) and self-excited chatter that grows within the system (regenerative chatter, in which the waviness from the previous revolution is regenerated and grows on the next; it originates from unevenness on the periphery caused by wheel wear or loading)^[33][30][22]
• Dimensional variationMain causes of dimensions varying or drifting: backlash in the feed system, thermal displacement of the machine, wheel wear, wrong offset values, and thermal expansion of the workpiece (it swells during machining and shrinks when stopped). In a cylindrical plunge grinding experiment at Nihon University, wheel thermal expansion was 5.7 µm and workpiece thermal expansion 25.2 µm, and it ground 28.3 µm more than commanded (this could be explained by adding the wheel wear of 3.4 µm)^[14][34]
• RoundnessPoor roundness in cylindrical grinding comes from poor mounting of the wheel, workpiece or truing tool, deterioration of the supports, bad center holes, tailstock pressure, tightening of the dog or chuck, steady-rest pressure, too much grinding fluid flow (water pressure deflects the workpiece), and insufficient spark-out1 source · for reference^[8][14]

💧Grinding fluid and workholding

• Grinding oil (grinding fluid) has three functions, lubrication, cleaning and cooling, and affects accuracy, productivity and wheel life1 source · for reference^[3]
• Water-soluble types are emulsion (mineral oil and surfactant, milky white when diluted), soluble (mainly surfactant, nearly transparent), solution (mainly organic amines, transparent, good cooling and rust prevention) and synthetic. There are also straight oils (water-insoluble)^[3][6]
• Water-soluble fluids excel in cooling and straight oils in lubricity. For grinding burn and cracks, lowering grinding force works better than cooling, so a fluid with good lubricity (one with more mineral oil) is effective^[6][22][21]
• Water-soluble fluid is made by pouring the concentrate into the dilution water (the other way around can gel and be hard to dissolve). Combine concentration control of the make-up fluid with measuring concentration using a refractometer1 source · for reference^[3]
• Only about one tenth of the grinding fluid used is said to reach the grinding point. To overcome the air layer and mist around the fast-spinning wheel, aim the nozzle correctly at the point where wheel and workpiece meet, and match flow and pressure^[32][6]"About one tenth" is from UNITED GRINDING only
• In cylindrical grinding, too much grinding fluid flow can deflect the workpiece through the hydrodynamic pressure between wheel and workpiece, worsening roundness1 source · for reference^[8][7]
• The working surface of a magnetic chuck has brass and resin separators, so choose a grinding oil that does not corrode them1 source · for reference^[12]

Workholding

• Magnetic chuck (surface grinding)Place the workpiece so that it spans the N and S poles across a separator. A workpiece with a thickness of about 2–4 times the pole pitch holds best, and the side of the holding face should be at least 3 times the pitch. Holding force varies greatly with material, thickness, area, and surface roughness, and thin pieces hold weakly1 source · for reference^[12]
• How to place the workpieceWipe the chuck face and the workpiece with a rag and deburr, then place the workpiece near the center of the chuck with its long side parallel to the table motion. Place a distorted workpiece with its concave side down, and check that it holds1 source · for reference^[10]
• Centers and lathe dog (cylindrical grinding)Support the center holes at both ends of the workpiece on the headstock and tailstock centers and drive it with a lathe dog (kere). Center hole accuracy and debris, tailstock pressure, and a faulty dog all affect roundness. When holding in a chuck, watch the clamping force and position; for slender workpieces, watch the steady rest contact setting and balance1 source · for reference^[8][14]
• Work rest blade and regulating wheel (centerless)The workpiece is not clamped; it is supported at three points by the grinding wheel, the work rest blade, and the regulating wheel, and it rotates. The regulating wheel provides the rotation (and, in through-feed, the feed)^[15][17]

⚙Surface grinding setup

1. Wipe the chuck face, then deburr and wipe the workpiece
2. Place the workpiece at the center of the chuck with its long side parallel to the table motion, and hold it; check that it holds (if distorted, concave side down)
3. Start the hydraulic pump and the wheel spindle, and bring the workpiece under the wheel by hand feed
4. Set the stroke with the table dogs (left and right overrun 30–50 mm from the workpiece)
5. Move the table left and right (about 10–12 m/min) and lower the wheel gently until it touches the workpiece surface
6. Turn on the grinding fluid and rough-grind the whole surface with auto in-feed (roughing 0.015 mm) and automatic cross feed
7. Finish to the specified size with a 0.005 mm depth of cut, and spark out at the end
8. Stop the table, switch off the chuck magnetism (demagnetize), and remove the workpiece after confirming that the wheel has stopped rotating. Measure thickness (6 places) and parallelism

Based on the training procedure at Niigata University^[10]

!Common mistakes

• Not truing after changing the wheel or after it loses its shape / continuing to grind with a glazed or loaded wheel → burn, chatter, poor surface finish1 source · for reference^[6][23][24]
• Grinding fluid nozzle not aimed at the grinding point, or concentration too low → burn, cracks^[6][32]
• Skipping spark-out or making it too short → stock left from elastic deformation makes size, roundness, and surface finish unstable^[10][7][8]
• Finishing without warm-up while the size is still drifting with heat / measuring while the workpiece is still hot → size error^[14][34]
• Grinding with the side of a wheel not intended for side grinding, or exceeding the maximum operating speed → violation of regulations and risk of bursting

For the causes and countermeasures of malfunctions (19 items) and for safety and regulations, go to Grinder maintenance and safety.

↗Learning path

1. Safety and handling grinding wheelsGuard, test run (at least 1 minute at start of work, at least 3 minutes after a wheel change), maximum operating speed, no side grinding, ring test. Wheel replacement only after special training
2. Read the wheel markingBreak a marking such as WA 60 K 7 V into abrasive grain, grain size, grade (hardness), structure, and bond
3. Make a parallel plate on a surface grinderChuck handling, stroke setting, roughing and finishing depth of cut, spark-out, measuring thickness and parallelism
4. Change the cutting action with truing and dressingChange the dresser feed and depth of cut, and see how surface finish, sharpness, and burn tendency change
5. Tell the wheel condition from the chipsJudge the wheel condition from shedding, loading, glazing, and the shape of the chips
6. Cylindrical grinding: center work, traverse and plungeCenter holes, lathe dog, tailstock pressure, and the cycle of rough → fine → finer grinding → spark-out
7. Think about conditions in numbersCalculate wheel speed, speed ratio, Q′, and grinding ratio, so you can explain the causes of burn, chatter, and size scatter from the conditions
8. On to specialized grindingInternal, centerless, superabrasive (CBN, diamond) wheels, gear grinding

Put together by the Kezuriba editorial team.

ƒCommon formulas

What happens when you change the conditions

Figures from public sources

• In surface grinding of steel, about 80 % of the grinding heat flows into the workpiece, and 10 % or less each goes to the grains and the chips. It is 90 % or more for carbon steel, which has high thermal conductivity, and below 80 % for SUS304, which has low conductivity. The effects of wheel speed and work speed are small, and the larger the depth of cut, the smaller the share going into the workpiece^[35][36]The figures for the share (80 %, 90 %) come only from Ueda et al. (g26). “The effects of wheel speed and work speed are small, and the share going into the workpiece falls as the removal depth increases” agrees with Okuyama of Osaka University (g27). In g26, the body text calls the steel with 90 % or more S45C while the conclusion says S55C, so the designation is inconsistent within the paper
• The temperature at the grinding point can exceed 1000 °C1 source · for reference^[1]
• JTEKT cylindrical plunge grinding (CBN #120, concentration 200, φ400 mm, Vs 120 m/s, SCM415H carburized and quenched HRC60, φ68 mm, emulsion 30 L/min): at Vw 0.53 m/s, rehardening appeared at Z′ 20 mm³/(mm·s) (not at 5 or 12). At Vw 1.77 m/s, it did not appear in the experiment even at Z′ 20 (the analysis shows a slight amount)1 source · for reference^[30]
• Noritake cylindrical angular grinding (CBN wheel φ380, SCM435 quenched HRc48, φ100): the conventional wheel showed visible burn at a grinding rate of 10 mm³/s. The specially coated wheel showed burn at 25 mm³/s1 source · for reference^[21]A product comparison test. The unit of grinding rate is mm³/s (not per unit width)
• Verification conditions of JTEKT's chatter prediction: cylindrical plunge, Z′ 1 mm³/(mm·s), CBN #120, concentration 200, φ160 mm, Vs 119 m/s, SCr420 carburized and quenched HRC64, φ32 mm, Vw 0.50 m/s, depth of cut φ0.3 mm. Roundness: 0.40 µm by analysis, 0.54 µm in the experiment1 source · for reference^[30]
• JTEKT's deflection compensation test: chromium steel (carburized and quenched) φ29 mm, width 20 mm, 270 mm between centers, workpiece stiffness 4.4 N/µm (low-stiffness section) and 13.8 N/µm (high-stiffness section), vitrified CBN #120, concentration 150, φ120 mm, Vs 80 m/s, Vw 0.4 m/s, roughing Z′ 8.4 mm³/(mm·s), emulsion diluted 20 times1 source · for reference^[7]
• In cylindrical plunge grinding of 52100 steel (HRC58–60) (φ16 in vitrified wheel, Vs 6000 sfpm ≈ 30.5 m/s, grinding width 0.5 in), the specific grinding energy is 7.1–13.3 hp·min/in³ (≈ 19–36 J/mm³) and falls as Q′ rises1 source · for reference^[25]
• Nihon University cylindrical plunge grinding: wheel thermal expansion 5.7 µm, workpiece thermal expansion 25.2 µm, wheel wear 3.4 µm, actual extra in-feed 28.3 µm1 source · for reference^[34]
• Of the grinding fluid used, only about 10 % reaches the contact zone between wheel and workpiece1 source · for reference^[32]
• In steady-state grinding, the elastic ride-up depth of a cutting edge is about 0.2 µm, and removal is mainly by cutting1 source · for reference^[36]

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