What you should know about grinding disks

Grinding disks are tools made of resin-bonded abrasive grains, reinforcing glass fabrics that give them mechanical strength and pores. The difference between them and other cutting tools is that they are self-sharpening. Abrasive substances are materials that, due to their hardness, eliminate either by rubbing or friction, a part of an object's surface to obtain accurate dimensions or a better ...

09/01/2005


Sections:
Choosing a good disk
TheBottomLine...
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ASHRAE to study thermal comfort, energy efficiency
Pump optimization program promises big energy savings
AHR Expo 2006 expects record exhibitions, attendance
ASHRAE to meet in Chicago

Grinding disks are tools made of resin-bonded abrasive grains, reinforcing glass fabrics that give them mechanical strength and pores. The difference between them and other cutting tools is that they are self-sharpening.

Abrasive substances are materials that, due to their hardness, eliminate either by rubbing or friction, a part of an object's surface to obtain accurate dimensions or a better finish.

Disks can be used to cut, in which case they are called cutting disks or to snag, when they are called snagging disks.

Grinding disks get their properties by combining abrasive grains, binders and porosity. The most important features of abrasive grains are the grain size, usually called grit, hardness, toughness and friability.

The number that identifies grit corresponds to the Tyler mesh size of the grain. Hardness on the Knoop scale, standardized by the National Institute of Standards and Technology, is used for grinding disks.

Friability is the grain capability of fracturing when it loses cutting capacity during use, creating new edges that restore it. Friable grains are suited for operations that require physical integrity of the piece being handled.

Grinding disks are basically composed of three materials: grains, resin bond and reinforcement fabric with pores.

Abrasive grains are responsible for cutting and snagging. They behave as saw teeth on the material being worked on. During use they wear out and lose their abrasive capability. This demands more power, which in turn causes the grains to break, exposing new cutting edges.

The most used materials for cutting disks in industrial practice are aluminum oxide (Al 2 O 3 ) and silicon carbide (SiC).

The purpose of a bond is to keep the grains together. This capacity is called hardness. The higher the amount of binder, the higher the grain retention capacity and the higher the disk hardness. Phenol resin binders are used for cutting operations.

There are two types of disks: hard and soft, depending on the bonding material to grains ratio. Hard disks are suited for soft materials or small contact areas while soft disks are suited for hard materials or large contact areas.

Reinforcing glass fabrics increase the disks mechanical strength, supporting bending efforts and preventing rupture (Fig. 1).

All cutting disks use reinforcing glass fabrics in their construction. The fabrics are distributed throughout the disks together with abrasive grains and a binder (Fig. 2).

Porosity is the volume of voids in the disk. It is related to the hardness and distance between abrasive grains within the disk. Pores help in cooling the disk material during operation and for removal of the chips generated.

There are two types of porosity: high and low. High porosity is required for a good finish and for hard and friable materials, while low porosity is suitable for soft and resistant materials as well as for large contact areas.

Cutting disks are used to cut metallic and non-metallic materials, such as steel, cast iron, bronze, brass, titanium, ceramics and glass. The work pieces may be bars, pipes, plate and structural shapes. They are also used to open grooves.

Disks are able to cut hard and tough materials that steel saws are unable to cut. They can be used in portable or stationary machines and should be always placed perpendicular to the work piece.

Incorrect practices and improper operational conditions lower the efficiency of the cutting disk, reducing its performance significantly.


Choosing a good disk

A good cutting disk choice depends on the material to be cut and on the cutting section. As a general rule, hard or coarse grain disks are used to cut soft materials while soft or fine grain disks are used to cut hard materials.

Aluminum oxide disks are used for high tensile strength materials such as steel and its alloys. Silicon carbide disks are used for low tensile strength materials such as cast iron and non-ferrous and non-metallic materials.

An important factor for choosing the right cutting disk is the length of the contact arc between the work piece and the disk, since this is what determines the length of the chips produced. Large chips obstruct the path of abrasive grains on the disk periphery, reducing cutting performance.

Disks behave differently when cutting thin walled or solid work pieces. This is evident when using a straight cut machine, which is the most used type for small work pieces. A straight cut is perpendicular to the work piece surface, while a deflected cut is angled.

For the same diameter, the contact arc of pipes is much smaller than that of solid materials (Fig. 3).

Since the unit pressure between the disk and the work piece is very different for thin-walled and solid pieces, the disks requirement is different.

The increase in unit pressure causes early loosening of abrasive grains from the disk surface before they have been fully used, accelerating their wear. High hardness disks should be used to solve this problem.

In thick-walled parts, where the contact arc is increased, the unit pressure between the disk and the work piece is lower. In this case, a free cutting disk, with lower hardness is required.

Free-cut is when the piece offers lower resistance against the disk advance, with less production of heat and burrs. Free-cut disks tend to perform cuts with no deflection.

It is impossible to estimate the mean working life of grinding disks by means of a formula or table. The variety of materials and applications and the manner in which the disks are used, influence their wearing out. Disk manufacturers recommend that users carry out their own tests to determine the mean working life in their particular cases, or estimate a statistical value through direct observation.

Storage location should be dry, free from vibration and sudden temperature changes and as close as possible to the point of use.

Use grinding disks in the order they come in. They should be stored in a horizontal position, on flat shelves, preferably in their original packaging, following the manufacturer recommendations (Fig. 4).

TheBottomLine...

  • Grinding disks are self-sharpening.

  • Choosing the right disk depends on the material to be cut and the length of the contact arc.

  • Disks should be stored in dry, vibration-free cabinets.

  • Giovanni S. Crisi, is with Mackenzie Presbyterian University, Sao Paulo, Brazil, and Thais Maldonado is a materials engineer in Sao Paulo. The authors can be contacted at (11) 6128-0461 or gscrisi@mackenzie.com.br . The authors wish to thank Saint Gobain Abrasives Ltd. for their authorization to reproduce the figures shown in this article.

    • Common problems found during cutting disk operation

      Problem Possible cause Suggested action
      Burning of work piece Insufficient advanceIncrease advance speed
      Low working pressureCheck equipment power
      Disk grain too coarseUse finer grain
      Disk too hardUse softer disk
      Too low peripheral speedAdjust peripheral speed
      Excessive working pressureReduce pressure
      Disk too hardUse softer disk
      Non-perpendicular cuts Shaft with irregularitiesCheck shaft, rollers and bearings of the machine
      Work piece is not firmly clampedCorrect work piece clamping
      Insufficient equipment powerCheck power
      Low cutting action Contact area too largeIncrease advance or increase pressure
      Disk too hardUse softer disk, or, if possible, a thinner one
      Disk grain too coarseUse finer grain


      Recommended use of grinding materials

      Material Use
      Aluminum oxide High tensile strength materials. Examples: steel and its alloys, nodular and malleable cast iron.
      Silicon carbide Low tensile strength materials. Examples: cast iron, non-ferrous metals (aluminum, bronze, copper, brass, titanium), non-metal materials (ceramics, marble, granite, refractories, glass)



      ASHRAE to study thermal comfort, energy efficiency

      Research to study the impact of drifting temperatures on thermal comfort, health and productivity is being funded by the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

      ASHRAE recently approved funding of $947,167 for seven research projects in the areas of indoor air quality, comfort and health, design tools, food processing and preservation and operating and maintenance. Among them is research on drifting temperatures that will be conducted at the International Center for Indoor Environment and Energy, Technical University of Denmark, Lyngby, Denmark, by principal investigators Dr. Jørn Toftum, and Dr. Bjarne Olesen.

      "The concept of using drifting temperatures to save energy is not new," Toftum said. "Some information is available on assessing the implications on thermal comfort of slowly increasing or decreasing temperatures. Drifting temperatures may not only affect occupant comfort but mental performance and the prevalence of sick building syndrome symptoms. We must make sure that comfort, health or productivity is not compromised in order to save energy. This project will help decide whether a given temperature change is acceptable and quantify the effects of temperature changes on human health and productivity."

      Pump optimization program promises big energy savings

      Department of Energy studies show that pumping systems account for nearly 25% of industrial energy consumption. To address this issue, the Hydraulic Institute (

      AHR Expo 2006 expects record exhibitions, attendance

      Based on the number of companies that have already reserved space for AHR Expo 2006 in Chicago, January 23%%MDASSML%%25 and the number of planned new show enhancements, next year's event promises to be one of the biggest in the show's 76-year history.

      According to Clay Stevens, President of International Exposition Company, which manages and produces the AHR Expo, the reasons why the show does so well in the Windy City are that Chicago is home to one of the largest regional manufacturing areas; center of a huge commercial and residential HVAC base; and headquarters for many HVAC companies and consulting firms.

      Some of the show highlights include a special focus on Green Buildings featuring "green" exhibitors and conference sessions by the United States Green Building Council; a variety of association-sponsored sessions; the ASHRAE conference program; short courses; and a free public session on topical subjects such as Building Automation/Integration, SEER 13 and IAQ.

      "It's hard to predict whether AHR 2006 will surpass our all-time records," said Stevens. "However, Chicago has always been a great place for the show and if the economy continues to improve, we could see some new records this January."

      ASHRAE to meet in Chicago

      The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) will hold its 2006 Winter Meeting Jan. 21%%MDASSML%%25, 2006



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