Patent Application
20180369992
The present invention relates to an abrasive article comprising abrasive grains, e.g. corundum, an organic bonding agent, e.g. phenolic resin, or an inorganic, coldsetting bonding agent, e.g. phosphate bond, and an active filler designed for portable battery operated electric tools.
Cordless grinders are convenient to use and takes no prep work besides charging the battery. From promotional videos in the market, they work really well on a variety of materials. Users like using them for the simple fact that they are portable and not tethered to a wired electric source. These are also great tools for those that wish to work off the grid, such as construction, farming and DIY projects.
A grinding disk is composed of a bonded abrasive compound and used for various grinding, cutting and abrasive machining operations. The disks are generally made from a composite material consisting of coarse-particle aggregate pressed and bonded together by a cementing matrix to form a solid, circular shape. Various profiles and cross sections are available depending on the intended usage for the disk. Today most grinding disks are artificial composites made with artificial aggregates. The bonding matrix may be organic or inorganic in nature and typically contains a filler. They may also be made from a solid steel or aluminum disc with particles bonded to the surface.
Such wheels can be used in handheld grinding machines. As the grains dull, the media wears down and allows the grain to be pulled out of the media to expose sharper grains. Abrasive products can be used for grinding, cutting, finishing, and polishing almost any material. Different shapes and constructions allow for different applications.
Most grinding wheels are manufactured by the cold-press method, in which a mixture of components is pressed into shape at room temperature, after which the wheels are cured in an industrial oven.
It is well known that modern manufacturing techniques deploy a number of portable tools to tackle the repetitive jobs that are encountered in the shop floor of the industry. Tools like drills, grinders, shears, nibblers, screw drivers, nut runners and impact wrenches find extensive use in large fabrication shops, tool rooms, fettling shops and assembly lines. In addition, such tools also find large usage as do-it-yourself (DIY) tools in the hands of the individuals in domestic applications.
The main requirements of such battery powered tools are portability, ease of handling and usage, safety, high efficiency and maximum power output. The manufacturer often aims to achieve the maximum power-to-weight ratio in such tools. Nowadays the energy consumed by such tools is also becoming an important criterion for selection.
In manufacturing facilities like machine shops, fabrication shops and assembly lines, changing batteries and waiting for batteries to charge decreases tool utilization and increases time and cost to perform related tasks. Battery powered grinding, sanding and cutting tools can require a larger number of battery changes to handle the high load of operation. These tools use disks with abrasive particles to perform grinding, sanding or cutting operations on a variety of materials.
Below is cordless grinder tool information for all products tested. For all data presented, Hilti was used
These battery-operated devices have limited power capacity which diminishes over time. The limited power often translates to lower rotational speed and a greater drop-off of speed from the frictional forces during grinding. Further, as the capacity diminishes the voltage drops resulting in greater reduction in the rotational speed and effectiveness of the tool.
During use power is consumed is several ways. First, just maintaining the kinetic energy of the rotation of the disk requires power. This power is directly related to the mass of the disk and the speed of rotation. Second, power is required to do the operational work. During operational work power is consumed by many factors with frictional losses being the primary concern. Fiction occurs when the surface of the disk is rubbed across the surface of the work piece. Ideally, all the frictional losses would occur by active friable material in the disk being used to remove surface particles from the workpiece. However, much of the power used by disks today is lost to friction and heat between inactive bonding agents and fillers in the disk wearing against the surface of the work piece.
These losses result in slower material removal rates for battery operated tools as compared to the same plug in style tools as well as an increase in battery change-overs further reducing efficiency. When considering efficiency three factors are considered, the actual labor time required to perform a particular operation, the number of batteries consumed, and the number of expendable disks used. Of these, the labor cost for that operation is typically the costliest followed by the cost of each battery used to power the tool. In a distant third is the cost of the abrasive disks used during the operation.
Most users of battery operated tools are more concerned with reducing the Labor and battery changeover costs. Labor cost can be reduced by removing the most material in the shortest amount of time. The battery cost is reduced by requiring fewer charged batteries per operation. If a battery lasts 5 minutes and needs 15 minutes to charge, an operation that lasts 10 minutes requires 2 batteries to complete in continuous operation. There are two ways to reduce the number of batteries required per operation. First, decreasing operational losses will increasing battery life. Second, increasing the removal or cutting rate will decrease the time per operation. Abrasive tools use many small, hard particles bound together in a media to remove material from a bulk substrate. As the grains dull, the media wears down and allows the grain to be pulled out of the media to expose sharper grains. Abrasive products can be used for grinding, cutting, finishing, and polishing a material. Different shapes and constructions allow for different applications.
The abrasive material does the work while the bonding agent is used to hold the abrasive material in a desired shape for any particular task. Generally, the bonding agent is selected to allow old dull or warn abrasive material to break away revealing fresh sharp abrasive material. If the bonding agent is too hard the dull abrasive material will not break away reducing the effectiveness of the abrasive disk and increasing frictional losses as well as increasing heat generation. Within the bonding agent is typically a filler.
In the abrasive industry, the term filler applies to the three following groups:
1. Fillers in the classic, usual sense, used to increase volume and/or stability at low cost. These fillers have the following effects:
With some fillers two effects, (a) and (b), or (a) and (c), occur at the same time. Examples of such fillers which have been used are: wood powder, coconut shell flour, stone dust, feldspar, kaolin, quartz, short glass fibers, asbestos fibers, ballotini, surface-treated fine grain (silicon carbide, corundum etc.), pumice stone, cork powder etc.
It is a common feature of these fillers that they are “inactive”, i.e. they do not undergo any chemical reaction or physical change during the abrasive process and therefore do not create any positive effect on the grinding process.
2. Fillers influencing the making of the grinding disks, particularly the thermal curing of the synthetic resins, are e.g. magnesium oxide, and calcium oxide.
3. “Active fillers”. They undergo chemical reaction or physical change during the abrasive operation, which have a positive influence on the behavior of the abrasive. These fillers should particularly cause an increase in the service-life of the abrasive tool and a decrease in the heating of the workpiece and the abrasive article and, hence, avoid thermal destruction. These fillers are the prerequisite for more efficient abrasive tool use, particularly, when materials that are hard to chip, such as unalloyed low carbon steels or titanium, are to be worked.
Active fillers can also obviously produce the same effects as the fillers indicated under (1) and (2) (increase or decrease in stability, influence on the curing process, etc).
In addition to the fillers, additives improving the adhesiveness between the abrasive grain and the bonding agent (e.g. coatings with silanes, or e.g. frits with fused metal oxides, ceramic coatings, etc.) may also be employed.
Other additives facilitate processing, for example, by either improving the noncaking free flowing qualities of the abrasive mix or reducing the internal friction in the pressing process. It is not necessary that these additives are active in the actual abrasive process.
The active fillers are the most important fillers in mixes for abrasive disks. Their effects can generally be divided into the three following main groups:
Particularly active fillers are, for example, halogenides (e.g. lead chloride, fluorspar, cryolite etc.), chalcogenides (e.g. pyrite, antimony sulfide, zinc sulfide, molybdenum sulfide, selenides, tellurides etc.), low melting metals (e.g. lead, tin, low melting composition metals,) high pressure lubricants (e.g. graphite). In practice, lead chloride and antimony trisulfide have proved to be the best fillers in respect of service life and low temperature (“cool” abrasion). It has been found that a filler is more active the lower its phase change temperature (melting-, boiling-, sublimation-, decomposition point) is. Due to the processing conditions in the manufacture of abrasive articles, these temperatures cannot fall below a certain value.
Moreover, chemically highly active elements or compounds, e.g. chlorine, hydrogen chloride, sulfur, sulfur dioxide etc., could be set free in the grinding process during decomposition. Numerous substances can only under certain circumstances be employed in practice as they are expensive (noble metal halogenides, molybdenum sulfide), toxic (arsenic-, selenium-, lead compounds) or hygroscopic and of high water solubility (numerous chlorides). They further strongly react with the uncured phenolic resin system (hygroscopic chlorides) or reduce the disk stability (e.g. graphite sulfur).
Some of these materials, e.g. metal chlorides, such as ferric chloride (FeCl3), zinc chloride, tin chloride, and potassium chloride, as well as elemental sulfur, are highly active and can favorably be employed in view of their low toxicity (high TLV) and costs. (TLV=treshold limit values).
Graphite is a well-known high temperature- and high-pressure lubricant, but it creates several adverse effects in abrasive articles. Pulverized graphite alone or in connection with usual, active fillers effects only a slight improvement of the abrasive properties. It would be advantageous to employ graphite in the form of coarse grains (grain size about the same as the size of the abrasive grain), which would be an advantage in the abrasive process. Adverse effects are created, however, by the high tendency of graphite to convert into dust or powder, when preparing the raw mixtures for the abrasive disks, and by the poor adhesion of the synthetic resins, particularly phenolic resins, to the smooth surfaces of the graphite grains, thus causing a great decrease in the stability of the abrasive article.
It is therefore desirable to have grinding, sanding and cutting disks that are light weight, strong and minimize heat and frictional losses during use. Therefore, there is a need in the industry for cut-off disks, grinding wheels and flap-disks that minimize the load on portable grinding tools while maintaining material removal rates.
Grinding wheels are consumables and the life span can vary widely depending on the use case. As the wheel cuts the friable grains of abrasive break away, typically because they grow dull and the increased drag pulls them out of the bond. Fresh grains are exposed in this wear process, which begin the next cycle. The rate of wear in this process is usually very predictable for a given application.
There are five characteristics of a cutting wheel: material, grain size, wheel grade, grain spacing, and bond type. They are indicated by codes on the wheel's label. With the advent of handheld battery powered grinding devices, a new dimension has been added to this matrix, battery consumption. While these tools are lighter and easier to use the exchange of batteries during long periods of use can defeat any productivity gains from their use. Therefore, grinding wheels are needed that will increase the battery life in these devices.
The purpose of the present invention is therefore to produce a grinding disk or wheel, flap disk or cut-off disk which will increase the effectiveness of each battery charge during tool use.
Test disks were made for the following types: depressed center grinding wheel (DCW), razor thin cut-off and flap disc.
For thin cut-off, disk life was based on number of cuts per battery charge. Ten cuts were made prior to removing the disk from the grinder for measurement. The disk was then placed on the grinder, repeating the ten-cut test until the battery or the wheel was spent. If the disk was spent prior to battery, another was used until the battery was spent, the idea being that it is faster to change a wheel than wait for a battery to charge. A 1-inch×1-inch angle profile was used for cutting tests.
For depressed center and flap disc, disk life was measured in minutes grinding. One minute of grinding was done in between measurements. The test continued until the life of the battery was spent. For material removal rates a new battery was placed on the grinder and used until the disc was used for 15 minutes. A 2-inch×¾-inch steel bar was used for grinding.
All wheels were used on the Hilti Grinder described in the background of the invention.
For battery life, it was found that the cut-off disk should be as stiff and as hard as possible. These characteristics reduce disk wobble or distortion and the related frictional losses. The disk wobble causes contact with the walls of the cut in addition to the friction in the bottom of the cut where material removal is desired.
Traditionally disk thickness was increased to create a stiffer disk. This provides more support while permitting use of softer lower temperature active and inactive bonding materials. The softer bonding material allows friability of the abrasive material more easily exposing sharper abrasive material. While lower temperature active fillers decreases heat and friction. However, this was not shown to work in the present case.
The disk construction consisted of two sides pressed with rubber molds to provide reduced frictional losses while cutting. Different grain type systems were investigated and it was found that a 20%-100% zirconia abrasive mixture was optimal with a 100% zirconia grain abrasive to be ideal. Data in Utilizing 100% zirconia and rubber molds on each side showed 80% more cuts can be made, and 74% better metal removal rates than a standard product offering per battery charge.
Therefore, in the preferred embodiment of
The
It is shown that both reducing the number of flaps and increasing the number of flaps as compared to a standard product can increase battery life for a cordless tool. Reducing the number of flaps reduces disk weight thereby increasing battery life while increasing the number of flaps increases the angle of the flap therefore reducing the overall work contact area and thereby reducing drag and increasing battery life.
The double stacking of the strip in ‘hybrid formation’ consistently shows higher removal rates at no cost to battery life. This is believed to be due to reduced contact area of the abrasive, reducing drag forces on the motor. The top flap has more material exposed, and does most of the work, while the rear flap supports the top flap, however, the two flap materials should wear at similar rates to be effective. This is typically achieved using differing bonding agents or different abrasive materials.
By adding a lubricating coating on the top flap layer further increases battery life. Adding this layer increased battery life 20% over a non-lubricated ceramic product of similar construction, and a 33% increase in battery life over the standard zirconia product construction.
Therefore, in a preferred embodiment the flap-disk 6 for use in a battery powered tool has a layer of 60-80 flaps in a hybrid stack 7 pattern. Each hybrid stack 7 consists of a top flap 8 and a bottom flap 9. The top flap 8 and bottom flap 9 are arraigned such that as the flap disk 6 rotates in direction 10 the top flap 8 of each hybrid stack contacts the work surface before bottom flap 9. Further top flap 8 is substantially exposed compared to bottom flap 9. The rotation direction 10 can be reversed however, the overlapping of the hybrid pattern would also be reversed. The top flap 8 of each hybrid stack is preferably a stearate coated ceramic abrasive material while the bottom flap 9 contains a ceramic abrasive material.
Typical grinding operations required a user to exert considerable force on the grinding tool to create the necessary frictional forces to effectively remove material from a work piece. This requires very rigid grinding disks with very fine grains that are not very friable. For AC powered grinding tools this is not a problem as more powerful motors are utilized which can overcome the frictional grinding forces and can maintain sufficient RPM to effectively remove material from the work piece.
The grinding wheel consumption was quite low as was the metal removal rates in our testing of standard products on battery operated grinding tools when compared to an electric grinder. This makes standard grinding wheels when used in a battery powered grinding tool more susceptible to loading, glazing and burning. This is further evidenced by low material removal rates per battery charge. This is primarily due to present technology grinding wheels utilizing a finer less friable grain abrasive. This abrasive is typically set in a hard bonding agent to prevent grinding wheel flex during typical grinding operations.
It would therefore be assumed that by decreasing thickness and consequently grinding wheel weight a corresponding increase in battery life would occur. However, it was found that decreasing wheel thickness had little effect on battery life.
The use of softer bonds showed an increase in battery life as it permitted the abrasive material to be refreshed with newer sharper grains. Further it provided significantly larger pores allowing more effective removal or waste material for the work area. Coupling a soft bond with a wax lubricant in the wheel showed the greatest increase in battery life. Additionally, using wax based lubricants significantly increases the material removal per battery charge.
In a preferred embodiment, to make a grinding wheel such as grinding wheel 11 that performs better on a cordless grinder, we design a wheel containing a soft bond 12 and a friable white alumina grain 13. Black silicon carbide was also found to dramatically increase battery life, but removal rates on steel were significantly reduced. Additionally, the selected bond 12 contains a wax lubricant. This bond formulation reduces frictional forces between the wheel and workpiece, which eases demand on the battery.
A secondary aspect of design is the reduced weight and thickness of the wheel reducing the contact area. With this feature, pressure is more effectively distributed to the work area, increasing the amount of work done per battery charge. The reduced weight keeps in mind the portability needs of a cordless grinder and makes it easier to maneuver in tight spaces.
| Number | Date | Country | |
|---|---|---|---|
| 62522799 | Jun 2017 | US |
