How does abrasive cutting work?
Abrasive cut off wheels are made from an abrasive substance such as aluminum oxide, silicon carbide or similar material held by a bonding agent of rubber, phenolic resin or others. The abrasive agents used are among the hardest substances known to man and therefore can cut a wide range of hard metals, carbides, Ni and Ti aerospace and medical alloys, ceramics, and other hard to cut materials.
The abrasive material, aluminum oxide, silicon carbide, diamond, zirconia, and even CBN is the same as used in sandpaper and conventional grinding wheels. Like sandpaper for use on wood, different size grain or grit is used in abrasive cutting. Finer grit is used to help reduce the burr or cut off tag generated when cutting with a tradeoff that wheel life is shortened.
Finer grit is normally used on thin wall tubing (wall thickness less than 1/20 of diameter) when dry cutting or when making high quality, fine surface finish, wet cuts without heat damage for metallurgical purposes. A larger or more aggressive grit is used for general purpose cutting. General purpose coarse grit wheels have a longer life than fine grit wheels but can create a burr or cut-off tag dependent on material being cut and the process used.
What is the science and engineering behind the abrasive cut off process?
An abrasive cutoff wheel can be thought of like a milling cutter with a near infinite number of teeth. Each grain has a negative rake and plows into the material creating a chip. Since the grain is so hard and held firmly in the grinding wheel by the bond, the wheel is capable of cutting at high speeds in the range of 14,000-16,000 surface feet per minute (70-80m/s) at the periphery of the cut off wheel depending on wheel diameter and bonding agent. The speed is limited by the ability of the wheel bond to hold the wheel together against the centripetal force.
The abrasive grains abrade the work piece away in small chips that are pushed out of the cutting area as the sparks you see. While the shower of sparks is dramatic, they are actually tangible evidence of the cutting work heat being removed from the wheel/workpiece…generally before it can transfer into the workpiece and damage its structure…if the cutting process is properly designed.
As the abrasive grain plows away the metal, its sharp, flint like edges begin to dull resulting in less efficient cutting and an increase in the force on the grain. Simultaneously, some of the metal chips smear and fill in the spaces between the grains and reduces chip clearances. This smearing of the metal into the wheel is called loading. Loading creates a metal on metal condition that generates heat in the abrasive grain.
As these dulling and loading actions continue, the cutting forces on the grain and heat generated increase to a point where the heat softens the bonding agent such that the forces are greater than the wheel bond agent’s ability to hold the grain in the wheel. At that point, the grain is released from the cut off wheel and is expelled from the cutting area along with the metal being removed.
As this piece of grain is released from the cut off wheel, a new sharp piece of abrasive grain is exposed and presented to the cutting operation so the cut off wheel acts as a self-sharpening blade. This process causes the cut off wheel to gradually decrease in diameter and requires the wheel to be replaced when the wheel is no longer large enough to be able to complete the cut.
In general, the G(rind)-Ratio, or the ratio of the amount of material removed to the amount of grinding wheel consumed is 2:1 for resin bond dry cutting and 1:1 for rubber bond wet cutting.
Q-prime (Q’), the volume of metal removed per inch of wheel width per minute (mm3/mm/sec) rates are in the 12-30in3/min * in (129-322 mm3/sec * mm). These values are based on cutting with a 0.240” wide wheel at the rate of 2-5sec/in2 with fiberglass reinforced wheels both wet and dry. Metallurgical sample cutting with a non-reinforced rubber bond wheel would be in the 15in3/min * in (155mm3/sec * mm) range typically.
How fast should I feed the abrasive blade down when dry cutting?
As with most machining operations, the answer is “it depends.” For dry cutting, a conservative rate is 5 seconds per square inch (6.45cm2/sec) of material being removed. For example, if you were cutting a 3-inch (75mm) diameter solid steel bar, the area of the cut is A=?r2, where r is the radius, so 3.14159 x (1.5in)2 = 7.1 sq. in (45.6cm2). So, at 5 second per square inch, the cut time would be approximately 35 seconds or 12sec/in (4.7sec/cm) down feed of the blade.
It is possible to cut at a rate as fast as 1 sec/in2 with a controlled down feed and a high-performance wheel—even in solids. 3 sec/in2 should be achievable with a power down feed saw.
With a programmable saw, one can vary the cutting down feed rates base on the changing cross section of the work piece e.g. a faster speed on the top and bottom 1/3s of a round bar where the chord length is significantly less than the diameter (the arc of contact is smaller) and a slower speed in the middle of the cut where the wheel is engaged with virtually the entire diameter.
The success of these different cutting rates will vary with the power of the saw, the machinability rating of the material you are cutting, and the surface finish/cut-off tag you desire. In general, feed as fast as possible without slowing the wheel rpms. If done properly with a saw with sufficient power, virtually all the cutting heat will exit in the chips and the work will be no more than warm to the touch.
I see all those sparks with dry cutting…how do I not heat up the work piece?
In general, if you feed as fast as possible through the work piece without slowing the wheel rpms, the work will stay cool. If done properly with a saw with sufficient power, virtually all the cutting heat will exit in the chips and the work will be no more than warm to the touch.
Like dry milling with carbide inserts at high speeds and feeds, the work heat from a proper abrasive cutting process stays in the chips being removed. Because so much heat is ejected from the cut in the abraded metal, the cut remains relatively cool and does not change the metallurgy of the material when cut at the correct rate.
Cutting machines with insufficient power resulting in insufficient cutting speed/force tend to remove chips at too slow of a rate and can cause the generation of excessive heat in the work piece. This heating can change the metallurgy of the material being cut, cause pinching of the blade by the work which adds additional rubbing heat, and also shorten wheel life by causing the bonding material to release the abrasive before it is completely worn.
When dry cutting, a blue or black color on the cut surfaces indicate too much heat build up from too slow of a cut. In high temperature nickel alloys, heat checking, or small surface cracks evidence improper cutting feed rates as well.
When should wet abrasive cutting be used?
Wet abrasive cutting should be used when one wants to obtain a fine surface finish AND preserve the hardness of the work piece.
Wet cutting is also used where the material being cut, were it cut dry, could result in the generation of explosive dust, hazardous dust, cause a fire/explosion, or create other hazardous conditions.
With an appropriate wet wheel, an adequately powered saw, and controlled cutting process, wet cutting need not be materially slower than dry cutting.
How fast should I feed the abrasive wheel down when wet cutting?
As with dry cutting feed rates, the process follows the function. If you are cutting wet for dust control, you can feed almost as fast as with dry cutting…if you have an appropriate wheel/saw combination. If you are wet cutting to prepare metallurgical samples, your feed rate will typically be determined by the surface finish you desire based on how much additional sample processing you will perform i.e. polishing.
The tradeoff for finer surface finish and a cool cut is a longer cut time and less wheel life versus dry abrasive cutting. For other than the finest surface finishes for metallurgical sample cutting where feed rates can be as slow as 30 sec/in2 (4.7sec/cm2), a conservative wet cutting speed would be in the 10-12 sec/in2 (1.55-1.86sec/cm2) range…both with rubber bonded, un-reinforced wheels.
An aggressive wet cutting speed with a fiberglass reinforced, liquid rubber hybrid-bond wheel is in the 2 sec/in2 (0.31sec/cm2) range. For example, in the Everett test lab, we recently cut a 5.125” (130mm) diameter solid bar of René142 in 42 seconds utilizing only 28HP (21kW) for an aerospace alloy investment caster.
How is wet abrasive cutting different than dry abrasive cutting?
Besides the obvious difference that water is pumped into the cut zone, wet abrasive cutting is different than dry cutting in several ways. Wet cutting wheels generally run at slower peripheral speeds (8,000-12,000 SFM—40-60m/s) than dry wheels (12,000-14,200 SFM—60-70m/s) due to the weaker bonds used for wet wheel construction.
Wet cutting wheel bond systems are formulated to be “softer” than dry wheels. This means that the individual grains are released from the bond by a lower force…thus they are described as softer. The lower wheel speeds also help “pump” or pull water into the cutting zone to carry the heat away.
A wet saw requires high volumes of water—5 gallons per minute or greater—with 16″ (400mm) or larger diameter wheels in order to maintain cool temperatures when wet cutting. The water also carries heat from the cut off wheel itself preventing excessive wheel wear.
For wet abrasive cutting, what coolant should I use?
Water and a rust inhibitor…to protect the saw and the workpiece from rust…is the only coolant recommended. DO NOT USE water soluble metalworking fluids as coolant for wet cutting. Water soluble synthetic coolants or soluble oils used on other types of saws and machining operations will cause the abrasive wheel to load up with the work material.
This loading on the periphery of the blade inhibits cutting and causes the wheel to deviate from the desired cut path by as much as one inch or more on larger diameter parts. This path deviation can cause the cut off wheel to flex to the point of fracturing.
Remember, water and rust inhibitor only!
What is the difference between a chop saw, cut off saw, abrasive cutting machine, and a miter saw?
Typically, a chop saw and a cutoff saw refer to the same device—an abrasive saw that makes perpendicular cuts (up and down) based on a hinge-like motion of saw frame where a thin abrasive wheel, typically less than ¼” (6mm) wide “grinds” its way through a metal object (or metal reinforced hoses) to sever it into two pieces.
The active cutting ingredient in the blade is generally traditional aluminum oxide (Al2O3) or silicon carbide, but can be extruded Al2O3 (SG or ceramic), Zirconia, or single layer plated diamond or CBN.
While “chop saw” typically refers to the hinge frame configuration, cutoff saws also come in swing frame for larger parts and locked or stationary head machines where the part is pushed into the wheel.
A miter saw is typically a chop saw where the head or the vise can position the work piece at some angle to the axis of the blade resulting in an angled cut…or a compound angle cut.
An abrasive cutoff machine typically is an automated cutting system, with a chop saw at its center, that may have a magazine for loading raw stock, a work feeding device to advance the raw stock a programmed length, a CNC/PLC controller that automatically controls the cutting speed, and may have additional stations that perform secondary operations to the work piece such as deburring, chamfering, identification marking, weighing, alloy verification via XRF, and other operations.
The machine is typically fully enclosed, has door and guarding interlocks, an emergency stop(s) system, dust/mist collection, chip/swarf filtration systems, and other environmental, health, and safety features built in.
Why should I use a portable abrasive chop saw in the field?
Generally, abrasive wheel cutoff operations are safer if the work piece is rigidly clamped in place and the blade is moved through the work in a controlled manner. While a professional, industrial grade portable chop saw will cut more efficiently and last longer than a throw-away big-box chop saw, both are considered safer than off-hand grinder cut off operations on the job site.
A chop saw (versus a swing frame saw or an angle grinder with a cutoff blade) typically allows the work piece to be firmly clamped for cutting. A chop saw can also be portable and be transported to construction work sites for fast and efficient cutting of rebar, steel wall studs, plumbing pipes (steel or cast), fence posts, hollow structural shapes, steel-reinforced hose, and other construction materials in a manner safer than cutting with an offhand angle grinder or other methods.
A portable chop saw can also be used in the oil field for fast and efficient cutting of tool joints, steel-reinforced hose, and for MRO activities. Portable chop saws with up to 26” (660mm) abrasive cutoff wheels are available with cutting capacities for up to 7” (180mm) OD pipe and tube.
When should I use an abrasive cut off saw versus a band or cold saw?
An abrasive cutting saw is generally the only way to saw cut (versus laser, waterjet, or plasma torch cutting) hardened ferrous materials with a hardness over 40Rc, made up of strands of wire (wire rope, braided wire or hose, steel reinforced hose), Ni/Ti aerospace or medical alloys, carbides, and ceramics.
Dry abrasive cutting is generally used for fast cutting where surface finish of the cut part is not critical and the workpiece dust is not explosive or hazardous. Wet abrasive cutting is used where fine surface finish, cool cutting, and dust free cutting are important like for preparing metallographic samples.
Abrasive cut off saws come in many different sizes, ranging from 8-inch (200mm) to 60-inch (1.5m) wheel diameters in manual, semi-automatic, and fully automatic configurations both shop-based and portable.
When should I use a Wet Saw?
A wet saw is a type of chop saw that is best used when cutting hard metal and ceramic work pieces where fine, burr-free surface finish, cool cutting to preserve the hardness of the work piece, or where explosive or hazardous dust can be generated by the cutting process.
The water coolant from the wet saw is pumped over the material being cut at the blade/work interface to remove the cutting-generated heat from the work zone so the material and chop saw blade won’t overheat. It also helps to clear any debris and chips from the work zone to prevent re-cutting chips.
Wet cutting saws can also be used for carbide, ceramic, as well as other non-ferrous and non-metallic workpiece cutting, using reinforced rubber bonded or diamond/CBN plated cut off wheels.
What is the difference between a Wet Saw and a Dry Cutting Saw?
A wet saw and a dry cutting saw are both types of cutoff chop saws that use different blade construction and cutting physics to achieve different objectives.
A wet saw is generally used to cut hardened metallic or carbide work pieces where maintaining the hardness of the work and obtaining a fine surface finish is important or where the dust generated from a dry abrasive cut may be explosive or hazardous.
A dry cutting abrasive saw is generally used where fast cuts of hard material are required, where woven/braided/twisted metal strands are present like with wire rope or hydraulic/fuel hose, or for thin tube (stainless or otherwise) cutting…all where band or cold saws are ineffective.
What is the best way to cut wire rope?
For heavy, large diameter wire rope, a wire rope cutting machine that is mounted inches off the floor on casters is a very efficient way. The cuts are clean and fast, and keep the wire rope strands from unraveling due to the chain hold-down vise that grips the wire rope while an abrasive wheel—not a saw blade—does the cutting.
Being low to the ground, the operator does not have to struggle to lift the wire rope up into the vise as the vise is only inches off the ground. Cart-mounted wire rope cutting machines are available with abrasive wheel diameters from 16″ (400mm) to 26″ (660mm) to cut wire rope or cable with diameters from less than 1” (25mm) to 6.5” (165mm). Saw power options range from 7.5HP (5.62kW) to 20HP (15kW), depending on the size of the wire rope cutting machine.
What is the best method for stainless steel cutting?
An abrasive saw is often considered the best choice for cutting stainless-steel rod, bar, tube and other stock shapes. Abrasive cutoff saws are equipped with a clamp to hold the work piece and a powerful, high torque motor that drives the specially formulated abrasive cut off wheel through the gummy, stainless steel material providing an efficient cut with little burr.
Abrasive wheel cutting is particularly well suited for cutting thin wall stainless-steel tubing where the wall thickness is less than 1/20 of the tube outer diameter. Abrasive cutoff saws for thin wall stainless tubing (or aerospace alloy thin wall tubing) are available with stock magazines, automatic feeders, and can be equipped with deburring, bar-code marking, and other post-cut operation stations for fully automated tube processing.
What speeds and feeds should I use for cutting nickel-based (Ni) aerospace alloy bars for investment casting charges?
Based on test cutting in our lab, we have been able to cut Ni alloys as fast as 2 sec/in2 cutting wet with a special fiberglass reinforced rubber bonded wheel. For example, this speed translated into wet cutting 5.125” (130mm) diameter, round, solid René 142 in 42 seconds using only 28HP (21kW).
With a fully-automatic, programmable saw, cutting rates approaching 1 sec/in2 is possible with the right wheel and process combination.
How do I develop a repeatable, consistent cutoff process on an abrasive cutoff saw?
Cutoff saws are available with automatic hydraulic down feed mechanisms that allow for semi-automatic cutting cycles. With a powerhead, a cutoff saw cuts at a steady, controllable rate by use of a hydraulic cylinder versus a manual down feed. A powerhead typically provides a more consistent cutting action than an operator, but is traditionally only capable of cutting with one speed based on a flow control valve.
Additionally, the operator generally still has to keep track of the wheel diameter, start and stop positions, and monitor the minimum wheel diameter.
New, fully automatic cutoff saws are available with programmable controls, absolute position feedback, automatic wheel diameter monitoring, and multiple-stage process programming. With these features, one can use modern, disciplined process development tools to generate defined, repeatable, scalable cutting “recipes” much like on typical machine tools.
This allows for creating cutting cycles with ramp-in and ramp-out speeds for entering and exiting the cut, varying the cutting speed as the wheel moves through different material cross sections in round bar, tubing, pipe, and hollow structural shape cutting. Process controls combined with a raw stock magazine, precision part length feeder, and pre- and post-cut operations create a complete, comprehensive cutoff cell solution versus traditional manual or semi-automatic cutoff saws.
What is the best way to cut steel reinforced hydraulic, fuel, flexible conduit, and other steel/polymer “hose”?
Cutting steel reinforced hose (hydraulic, fuel, flexible conduit, oil-field) with an abrasive wheel results in a sharp, clean cut with virtually no frayed strands or burrs. For pre-loaded, spiral wound hose, there will typically be a sharp end protruding from the rubberized cover but the sharp is much smaller than with cuts made with other methods. The cuts are clean and fast, and keep the braided wire strands from unraveling.
For clamping, as with hose cutting with a wave-edge steel blade, it is generally best to “spring” the hose down and to cut at the apex of the arc so that as the cut is made, the hose “opens” or spreads away from the abrasive wheel. This minimized side rubbing and heat-based odor.
Abrasive wheel hose cutting is less sensitive to this binding than cutting steel-reinforced hose with a wave-edge blade. Abrasive wheels can also cut virtually any hose, regardless of the amount of steel reinforcement in the hose/conduit. One can even cut radial tires with an abrasive wheel.