When Belts Are Better
They hog, smooth and polish
By Robert B. Aronson
It may be a surprising statistic to know that coated abrasives make up more than 50% of all abrasive grinding applications. (The term coated abrasive refers to the method of manufacture, or placing an abrasive on a base material, not a coating on the abrasive.)
Developments in abrasives and the machines that use them have made coated abrasives the rival of other grinding techniques as well as conventional machining. Finer grit sizes make them suitable for fine finishing and polishing. At the same time, longer-life coarse grits secured to higher-strength backing do a good job of removing high volumes of material.
Coated abrasive systems offer these advantages.
- Changeover is as short as 30 sec.
- Minimal or no truing and dressing.
- Better roundness.
- Part heating is less because the heat is carried off in the swarf. Heat-related cracks in the part and heat-affected zones are eliminated or minimal.
- Safer operation.
- Less finish variation.
Machines are also important. Although the abrasive does most of the work, how it is handled relative to the product is also critical. Machines designed to use coated abrasives for grinding, polishing, and superfinishing range from single to multiple-head arrangements that can be quite large, utilizing up to 200 hp (150 kW). These machines can grind and polish high volumes of large parts and function much like multifunction CNC milling machines.
Often a precision centerless grinding machine that utilizes coated-abrasive belts can perform multiple operations in a single pass on cylindrical tubing and bars. For example, when working with steel tube or bars, a multiple head centerless grinding machine can remove up to 0.100" (2.5 mm), hold 0.0005" (0.013 mm) tolerance end-to-end on 24' (7.3-m) long tubing and bars, and have a surface finish below 5 Ra. This would involve using several different coated abrasives ranging from 36 grit to 2000 grit.
Acme Manufacturing Co. (Auburn Hills, MI), offers a number of specialized precision centerless grinding and polishing systems as well as robotic finishing systems and coilgrinding systems which utilize coated-abrasive products. These engineered finishing systems are tailored to the specific part application. Currently, Acme's key customers include manufacturers of knee and hip implants. The Acme robotic systems utilize structured coated-abrasive belts and provide fine finishes with little human intervention. This automated solution can reduce a 60-min manual operation down to 8 min using a robot and the proper coated-abrasive belts.
"If customers want accuracy within millionths on centerless grinding, we recommend they use a bonded-wheel machine," says Fritz Carlson, company president. "If you want to hold less than 0.001" [0.025 mm], a coated-abrasive belt machine is a good solution for a centerless application. A major advantage of centerless abrasive belt machines is that they can remove stock faster and cooler and produce fine finishes at high line speeds. Each unit may have a belt of a different grit. Each head is programmable so that changeover for different diameters can be done within a couple of minutes."
Another machine tool builder that specializes in coated abrasives is Drake Manufacturing (Warren, OH), "We're seeing much better abrasives now," says company founder John Drake. "They allow heavier metal removal rates while still giving excellent finishes. The longer-lasting seeded-gel belts let us design more productive automation into our CNC belt grinders, because we're not changing belts every time you turn around. Even though actual belt change time is minimal, the fact that today's belts let the machine run longer without interruption means a lot more parts out the door at the end of the day," he adds. ME
Better Abrasives Are Key to Success
Backing, grain, and resin systems are the three areas of interest to all coated-abrasive manufacturers. For example, we are looking for new ways to improve the chemistry of the backing. We need backing with more durability and better grain retention so that it behaves more like a cutting tool.
Key component is the abrasive grain, and one of our biggest advances is in the Norton SG Blaze products. They have a new ceramic grain with a chemistry that enables it to hold the sharp point longer during the grinding process. The grain is electrostatically oriented so that the sharpest points project from the product backing. The grain in Norton's SG Blaze belt is sharper and presents more cutting edges to the workpiece. Each grain continually fractures along submicron "faults" in the grains' crystalline structures to repeatedly expose new sharp cutting edges. This coated-abrasive belt is designed for higher productivity on all metals from mild and carbon steel to stainless steels, titanium, and all alloyed metals.
Resin advances include stronger bonding power which promotes improved cooling and longer cutting action. Many are finding that coated abrasive cuts cooler than a grinding wheel.
The backing has to be tear resistant yet flexible, and provide a good base for grit adhesion. It should not stretch much and it has to survive the shop environment, particularly machining fluids.
Belts are typically available in widths from 1/8" (3 mm) to more than 5' (1.5 m) and lengths from a few feet to more than 20' (6 m).
Coated abrasives include a wider variety of belts, disks, and flexible wheels. The key elements are backing, bonding adhesive, and grit, with grit the dominant element.
Grits used range from 24 grit used for rapid material removal, to 2000, which is a grit so fine, the belt looks like a mirror This size is used for fine finishing and polishing. (As a reference the average do-it-yourself household sandpaper is usually in the 60–220 range). The grit size is determined by the spacing of the filter wires per inch through which the grains must pass during the "grading" process.
Currently, about 50% of all coated-abrasive applications involve an off hand or manual process. But there is a strong trend to automate. Off-hand grinding usually requires a skilled operator and few younger people are attracted to that type of work. Often the grinding pressure requirements are beyond those a manual operator can provide. Metal knee and hip joints are a major example. At the same time, robots have become more reliable and accurate so some manufacturers are moving to entirely automated operations.
Robots have done a lot to boost the appeal of belt grinders. With the reliability and accuracy of today's robots, they act as the human operator in presenting the part to the belt, but without the potential for variability.
The rougher grit abrasive is usually applied randomly while the finer grits are carefully positioned. For example when working with softer metals, you want more spacing between the grains to minimize clogging.
Grinding of stainless steel coils is a major application. Some are 60" (1.5 m) or more wide. The grinding removes surface flaws after the coil is initially formed, usually with multiheaded machines using very coarse belts. The grinders take about ¼" (6.4 mm) or so of surface steel. Impurities rise to the surface during initial processing and have to be removed or they will cause problems later in the coil's processing.
For fine finishes we use a fine-grained, engineered abrasive in a friable resin. This process produces a very even cut, unlike a solid grinding wheel where the grain wears and changes the cut quality. We have developed our NORaX Engineered Abrasive belts which have grit sizes as fine as 3–5 µm.
When coolant is used we get more parts per belt and in some cases double belt life. The coolant flushes away swarf and the clean belt functions longer. For example, gummy metals are a problem. When grinding stainless steels, the metal will eventually bond to the abrasive grain tips so you get metal-to-metal rubbing, which leads to a lot of unwanted heat generation. Coolant retards this.
If coolant can't be used, you need a special "reactive coating." It's another layer of resin that reacts with the metal during grinding to keep heat down and increase cutting power. These materials won't help much on mild, and carbon, but work well with alloyed metals such as titanium, stainless, and high nickel alloys.
Truing and dressing are not major considerations because only a single layer of grain is used.
James. P. Elving
Senior Product Manager - Metalworking
Specialized Grits Aid Market Expansion
The mineral type and quality can vary greatly and its impact is more readily apparent. Mineral hardness and temperature resistance are important factors in grinding decisions as minerals such as aluminum oxide can quickly cap under high pressure/high temperature applications where a man-made product such as 3M Cubitron abrasive grain can be designed for greater durability, sharper cutting, and cooler running. It is part of the category of ceramic aluminum oxide minerals. Other popular abrading minerals are silicon carbide, alumina zirconia, cubic boron nitride, and diamond. A general rule of thumb is the harder the mineral, typically, the more expensive the abrasive.
Some minerals cut sharp naturally. Silicon carbide, for example, has a slender crystalline structure that lends itself to sharp cuts and, therefore, generates less heat in grinding applications. This makes it well-suited to working with metals like titanium.
The manufacturing process also can enhance an abrasives' sharpness by ensuring that the mineral's sharpest points are oriented away from the resin coat. This is accomplished by using electrostatic forces. The flattest (least sharp) area of the grain will hold the greatest amount of electrostatic charge and gets oriented and picked up in the first make coat with the sharpest areas exposed for cutting. The mineral is set by curing, then a second coat, known as the "size coat" is added, followed by more curing.
Finer grit sizes are selected for making coated abrasives suitable for fine finishing and polishing and larger grits secured to higher-strength backing do a good job of removing high volumes of material. There are two popular grading systems seen in the US and Canada, with each being somewhat different in its tolerances.
- Coated Abrasives Manufacturer's Institute (CAMI).
- Federation of European Producers Association (FEPA).
Though tolerances may vary, both systems use a method where particles are sifted through a series of wire mesh screens with specific numbers of holes per inch. The CAMI system, which tolerates a wider range of sizes, is often used as the standard in coarser grades while the tighter tolerances of the FEPA system (P-grades) are more common as you refine your finish.
If switching from a CAMI to FEPA-rated product or using them together in sequences, be aware that the scratch patterns in the higher grades will begin to differ widely, so test and adjust your grade choices or sequence accordingly.
Coated abrasives can vary in durability, sharpness and coolness of cut, and tolerances of grain. The question is, how is any of this more advantageous than other grinding techniques?
Developments in abrasives and machines that run coated abrasives rival other grinding techniques and conventional machining in terms of production improvements such as cost per part, consistency, throughput, and improved quality. Coated-abrasive systems offer the advantages of quick changeovers (as short as 30 sec) with minimal to no truing or dressing, faster grinding operations, and significant cost savings.
One example of this is in the grinding of very hard materials like carbide and ceramic thermal spray coatings. Using a structured diamond-coated abrasive belt from 3M (uniquely designed for consistency through the use of macro-replicated abrasive structures) one company was able to replace its diamond wheels and reduce a 30-min application to less than 10 min by converting their operations to belts.
The abrasive belts delivered:
- A consistent and predictable process for dimensioning and finishing thermal spray and other hard materials,
- An easy, predictable way to achieve finish specifications,
- Results that were within the required geometry tolerance (±0.00005" [0.0013 mm] final diameter in this case),
- Faster dimension and finish attainment than diamond wheels and honing stone polish,
- A total cost savings of more than 40%.
Belts can be run safely at higher operating speeds than wheels and some operators believe safety is another major advantage of belts. However, there are risks involved in using any abrasives at high operating speeds, and proper safety procedures and adherence to published operating speeds must always be used with any abrasive product.
The parts, however, may unquestionably be safer using a coated abrasive, as they are less likely to be burned. The heat is removed from the part with the metal swarf, thus reducing the chance of heat-related cracks in the part.
In addition to hard-coat grinding of thermal spray coated parts, belts are also popular choices in investment castings grinding. Two reasons for the growth of coated abrasive use are increasing use of investment and lost-wax castings where the removal of gates and finishing are critical plus the growing need for machining very hard materials into precise shapes. Jet engine blades and prosthetic devices are major examples. Hardness of these materials also often makes conventional milling and turning impractical.
Since castings and forging initially carry a lot of extraneous metal, particularly casting gates, coated abrasives can remove inches of metal in seconds. For example pressure on the part may be 200–300 psi (1.38–2.07 MPa) when using a 24 or 36 grit. The removal is so fast that you can touch the part immediately after grinding because all the heat goes off in the swarf. For stainless steel, titanium, nickel alloys, and carbon steels, the mineral is most often ceramic aluminum oxide in rapid grinding systems (RGS).
Improvements in coated abrasive technology through the development and combination of harder, man-made minerals, electrostatic coatings, structured abrasives and more have changed the rules of engagement when it comes to grinding. The traditional grinding wheels are making way for a new brand of coated-abrasive belts that can deliver cost-savings advantages even under the toughest requirements from tolerances and materials.
3M Industrial Products
This article was first published in the February 2007 edition of Manufacturing Engineering magazine.