Abrasives Lesson 5: Contact Wheels - Troubleshooting and Related Information

by EMI Supply, Inc.
This article was published on Thursday March 19, 2009.

Lesson 5

Contact Wheels: Troubleshooting and Related Information

Identifying the Need for Changes in Contact Wheel Specifications

The condition of the used belt at discard is the most important clue or evidence that a change in contact wheel specifications can produce improved results. These used belt conditions are glazing, loading, and stripping or shelling.

Glazing

Glazing is the most common problem encountered with belts and is defined as a dull­ing of the abrasive grain tips or leveling off of these grain tips and not exposing new sharp cutting edges. It is invariably accompanied by welding of metal particles to the tips of these dull grains and is caused by the heat and pressure of grinding. The slow rate of cut, burning of the workpiece, and random coarse or deep "wild scratches" in the finish are all symptoms of a glazed belt. The belt has lost its sharpness or "bite" and the surface is smooth. The metal particles on the abrasive tips give a shiny or glossy appearance to these grams.

Loading

Loading is the packing of particles of the material being ground between the abrasive grains of the belt. Loading considerably reduces the rate of cut of the belt and ultimate belt life. Symptoms are a stringy or torn appearance to the finish usually with evidence of "wild scratches". This is caused by metal particles welded to or packed in the grains. The ability to pick any of these particles off with a penknife or sharp instrument also indicates a loading condition.

Stripping

Stripping is the pulling away of the grain, or the grain and the bond, in the process of grinding. In some cases, the barebacking of the belt may be exposed. This condition exists when the contact wheel is too aggressive for the workpiece and the operation. Another cause may be excessive or "reverse" flexing of the belt due to a contact wheel being too soft, and a high unit pressure which deforms and depresses the rubber inwardly toward the arbor under grinding load. In any case, the overall result is a breakdown or wearing of the belt that is too rapid. This situation causes short belt life, is undesirable, and also is not economical.

The following table shows the recommended changes to correct or improve these and other conditions encountered with contact wheel usage. (Note: It may be necessary or desirable to change the belt specifications as well as the contact wheel specifications for maximum benefit).


Problem Encountered

Softer Wheel

Harder Wheel

Narrower Lands (Wider Grooves)

Wider Lands (Narrow Grooves)

Higher Speed

Lower Speed

Change Lubricant

Depper
Groove

Dress or True Wheel

Use
Coarser
Grit

Belt Glazed
 
X

X
     
X
   

Belt Loaded

X
 
X
     
X

X
 

Too Slow Cutting
 
X

X
   
X

X
 
X

Finish Too Coarse
     
X

X
 
X
   

Burning
   
X
   
X

X
 
X

Too Hard for Contour

X
 
X
   
X
 
X
 

Chatter

X
 
X
       
X

X
 

Shedding Grain
 
X
 
X

X
 
X
 
X
 

Overlap Marks

X
 
X
           

Dubs Edges
 
X
             
                       




Dressing and Truing Contact Wheels

Contact wheels are usually received from the manufacturer in well-balanced condi­tion. However, they are not always in perfect running truth. This is normally due to slight variations of density in the face material and because the operating speed of the wheel may differ from the speed at which the wheel was dressed by the manufacturer. These are the usual reasons for a wheel not running true. On the other hand, the running truth of the contact wheel may be affected by a bent spindle, undersized spindle, or poor bearings or machine conditions.

Checking a contact wheel to indicate its roundness at operating speed is easy. Simply hold chalk or crayon lightly against the wheel face while it is running at operating speed. If a mark is indicated on the complete circumference of the wheel, it is then running true. If not, the contact wheel should be dressed or trued before be­ing used on any application. To obtain the best results, this should be done at operating speed on the same machine in which the contact wheel will be used.

To dress a contact wheel, use a new piece of abrasive (preferably resin bond, 50-120 grit) wrapped or held on a flat board. The board should be long enough so that one end will stand on the floor, and the other end extends above the wheel (see illustration). Using light pressure, hold the abrasive covered section of the board flat against the contact wheel face. Light pressure is absolutely necessary to dress off the "high" spots. Heavy pressure will only follow the out-of-round condition, and will not dress the wheel into proper roundness.

Dressing contact wheels that are out-of-round will result in longer belt life and better finishes. The wheel should be dressed at its operating speed.

Dressing contact wheels on a thru-feed conveyor or mechanical application machines is usually performed with an attachment provided for the purpose. If attachments are not available, the above principle is still applied to build a simple fixture for the dressing operation. Oscillation of the abrasive (or wheel) is always desired to eliminate any pattern the abrasive grain may impart to the wheel face.

A permanent reference mark should be made on the contact wheel and machine spindle. Align these marks in the "up" position, tighten the spindle nut, and dress the contact wheel. When remounting the same wheel again, the marks should be aligned. This will then obviate the need for redressing each time the wheel is removed from the spindle and insure running truth at operating speed.

Belt tracking problems may often be eliminated by the proper dressing of a contact wheel. Wheels that are operating at a speed in excess of the manufacturer's dressing speed may result in what is commonly known as "edge throwout" This condition will cause the abrasive belt to track to the high edge on either side when any grinding force is exerted. Such a condition is more critical in the softer contact wheels, i.e., 60A durometer or less. By dressing and truing, the wheel at the operating speed, edge throwout, and the belt tracking problems can be corrected. Dressing a contact wheel with "used" or "worn out" abrasive is an undesirable practice.

This may create or cause a belt tracking problem in itself. A close inspection may reveal that only the leading edge of the serration has been ground in the dressing operation, pushing aside or deforming the back half of the serration. This condition can usually be identified by the belt wandering from side to side under grinding pressure.

To ensure proper dressing and truing of a contact wheel or to correct a poor dressing condition, the following recommendations should be adhered to:

1. Always use new abrasive. preferably resin bond silicon carbide in the 50-120 grit range. The aluminum oxide will work nearly as well.
2. Use light pressure only to dress out the "high" spots on any wheel. Then, hold the abrasive without any additional pressure to let the contact wheel "spark out" until no further grinding or dressing is noted.
3. Always dress or true, if possible, at operating speed.
4. Softer durometer contact wheels (60A Shore and softer) are always more critical and require greater consideration of the above three points regarding tracking when dressing. This is due to more resiliency and elasticity of the softer durometers, i.e., they deform more under light pressure.
5. A dressed contact wheel will always run smoother and give longer belt life than one that is not dressed, or a wheel that is out of balance.
6. For wheels harder than 60A durometer. proper dressing becomes critical as regards belt life. As little as .002" out-of-round in a 90A durometer wheel can reduce belt life by as much as 50-60%.



Dressing Contact Wheels which longer are out-of-round will result in belt life
and better finishes. operating speed wheel should be dressed at its speed



Machine Conditions

Good machine conditions play an extremely important part of any successful abrasive belt application. Poor machine conditions will always have an adverse effect on the ultimate results. Machine problems that may appear to be far removed from the action of the contact wheel or the cutting rate and life of an abrasive belt, are often the primary reason for an existing "problem" being blamed on contact wheels and belts.

Poor machine conditions that do exist are most commonly found in spindles, bearings, pulleys, idlers, and the machine mounting or placement. Spindles can be undersized, bent, or in generally poor condition. Undersize spindles may prevent the solid mounting of a contact wheel. This could cause vibration, chatter, or affect the running truth and balance of the contact wheel itself. A bent spindle would cause a contact wheel, that was originally dressed true, to act out-of-round, causing chatter or vibration. Any such chatter or vibration directly decreases belt life and makes the finish coarser.

Spindle replacement or repair in the above conditions is the corrective action that should be taken to completely eliminate any poor machine condition.

Poor bearings in the spindle housing, idler pulley, or any moving part of the machine, also adversely affect contact wheel action, belt life, and finish achieved. Worn or misaligned bearings of any sort set up a vibration that is ultimately transmitted to the point of grinding contact, thus preventing smooth operation. Bearing replacement should definitely be performed where such a condition exists.

Poor mounting of the machine itself and machine parts insecurely attached to the machine are also sources of vibration and chatter in an application. Rigid machine mounting, secured idlers, or backstands, as well as a rigid assembly of all machine components, will add smoothness to any operation and definitely improve a finish and overall belt life.

Out-of-balance conditions can be created for various reasons even though the equipment is in-balance when received. An out-of-balance idler pulley can often be due to the day-to-day wear of the abrasive belt traveling over it, and through normal abrasion. Repair, replacement, or rebalancing then becomes necessary. A build-up of grinding swarf in the recesses, excess belt grease, workpiece chips, or grinding sludge also can throw a pulley or a contact wheel out of balance. Grinding swarf buildup, the out-of-balance conditions resulting, and the vibration caused by the out-of-balance have been known to reduce belt life by as much as 75%.

A periodic check and cleaning of the contact wheel and idler pulley recesses would reduce the possibility of occurrence of this type of machine problem and balance condition. These are the major machine problems, and their effects encountered in the abrasive belt and contact wheel grinding. Any one or a combination of any of these above problems can adversely affect the rate of cut, stock removal, finish, and useful belt life.

Safety

Contact wheels are safe to operate with almost no danger of destruction or explosion in normal use. This is especially true when contact wheels are operated within the recommended speed ranges. Speed tests have been conducted in excess of 20,000 SFPM on the rubber covered contact wheels with no resulting damage.

No detrimental effects have been noted when the wheels are placed back into normal operation. Contact wheel explosions and dangers are almost unheard of in any normal operation or application unless the wheel is damaged while in use.

Storage

Contact wheels and rolls should be stored properly when not in use to prevent any damage. Proper storage recommendations would be hanging wheels on pegs, lay­ing them flat on their sides, or supporting them on their journals in a convenient loca­tion. Standing a wheel on edge or on its face causes the face material to flatten at this point, deform, and take a permanent "set" When the contact wheel is remounted, and out-of-round, or out-of-balance condition exists. Dressing the contact wheel will only correct the situation temporarily, as the resilient material gradually returns to its former shape and position. Chatter or vibration may result when a contact wheel is out-of-round, causing poor finish and shorter belt life. If the application is off-hand. operator fatigue may also occur.

Idler Pulley Positioning

Idler pulleys for abrasive belts should be mounted or secured (when converting from grinding or setup wheels) to allow the belt to wrap around at least one-quarter of the circumference of the contact wheel before the point of grinding contact. This is especially critical if heavy grinding pressures are used. Floor mounting of an idler unit is considered best when converting a polishing lathe for use with contact wheels and abrasive belts. More belts would be wrapped around the wheel before the point of contact leaving less chance for slippage or track­ing difficulties.

Belt Tension

Proper belt tension is impor­tant. Tension may range from a low of 4 or 5 lbs. per inch of belt width on contour applications, to a high of approximately 35 lbs. per inch of belt width, for severe operations where heavy work pressures are used. Lower tensions are used on contour applications for belt flexing to allow following the contours when using softer contact wheels. When the area of contact is small and work pressures are high, tension should be increased correspondingly. Low tension in the latter case would cause the belt to deform excessively, loop, or pucker immediately ahead of the point of contact; thus, belt stripping, tearing, or shelling of the abrasive grain would result from the reverse flexing action. The belt flexed the least will last the longest time. Flexing action weakens the bond, and ultimately shortens the life of the belt.

How to Order

When ordering a contact wheel, a standard procedure should be followed to include all pertinent information. This information includes seven basic items:

1. Wheel size is always specified Diameter x Face Width x Arbor as a proper ordering procedure. Any additional instructions are then noted, such as recesses or keyways. if these are necessary. A 14" diameter wheel 4" wide with an arbor size 1-1/4" would be ordered as follows: 14" x 4" x 1-1/4".
2. The Hub type is specified by a letter immediately preceding the desired wheel type (R - solid type - is the most common).
3. Face Design refers to types of wheels as discussed previously. Therefore, the desired type of wheel with a solid hub would be specified as R61, if a 61 type face were to be ordered.
4. Durometer is wheel hard­ness of A or D Shore scale. If the durometer scale is not specified, "A" scale hardness rubber will be furnished.
5. Hardness normally ranges from 20A-90A and 40D-70D. In 80 type contact wheels. two hardnesses are specified in one wheel as desired and previously illustrated.
6. The type of face material is either “R” for rubber or “I” for isothane. The "R" or "I" immediately follows the hardness and durometer scale. Therefore, an R61 wheel of 70A durometer rubber would be specified as follows: R61-70AR.
7. Land to Groove Ratio. The ratio is indicated as one number. An R61-70AR wheel with land to groove ratio of 1:3 would be ordered as R61-70AR-13. Always order a standard or available ratio.
8. Serration angle, if other than 45°.

The following examples illustrate the above information:

Example 1: if you required a Type 61 solid-hub rubber contact wheel 12" x 4" x 1-1/4", with a hardness of 70 Shore A, a 1 -to-3 ratio, you would order: Carborundum Abrasives Contact Wheel 12x 4 x 1-1/4" R61-70AR-13.
Example 2: You need a solid-hub dual-durometer Type 86 wheel 12" x 4" x 1-1/4" of 60D and 60A Shore durometer rubber, with a 1-to-5 ratio. Your order would be Carborundum Abrasives Contact Wheel 12 x 4 x 1-1/4" R86-60DR/60AR-15.


Resin Bonds
 
Ferrous
Grindability
 
Ferrous
Grindability
 

Grit Size

Medium to High

Low to Medium

Medium to High

Low to Medium

24 - 60
80 - 150
180 - 500

5000 – 6000
5500 – 6500
6000 – 7000

4000 – 5000
4500 – 5500
5000 – 6000

5500 – 7000
6000 – 7500
6500 – 8000

4500 – 6000
5000 – 6500
5500 – 7000

Glue Bonds
       
 
Ferrous
Grindability
 
Non-Ferrous Grindability
 

Grit Size
       

24 - 60
24 - 150
180 - 500

4500 – 5500
5000 – 6000
5500 – 6500

3500 – 4500
4000 – 5000
4500 – 5500

5500 – 6500
6000 – 7000
6500 – 7500

3500 – 5000
4000 – 5500
4500 – 6000



Advantages of Contact Wheel Knowledge

Knowledge of the basic theory and application of contact wheels can be a strong, basic "tool" in obtain­ing a larger share of abrasive belt business. Knowing how to select a contact wheel, the proper way to use a contact wheel, and the solution to problems involving contact wheels and abrasive belts creates a definite advantage over competitors.

Carborundum Abrasives quality combined with your knowledge on the application of proper contact wheels and a "total system" approach including grinding fluids, pressures, speeds, etc. produces customer parts at a lower total cost per piece. Earned credibility of this kind is the toughest kind of problem for your competition

Belt Speeds

The following table gives recommended belt speeds by bond type, grit range, and material. Low to medium grindability denotes difficult to grind metals. Medium to high grindability denotes easier to grind metals.






Belt Speed – SFPM: When it is appropriate or possible to change belt speed, the following is recommended by grit range, material, and grindability for resin and glue bond products.

Pressure: When grinding pressure exceeds 1-1/2 HP/ inch of contact width, use belt speed in the lower end of the range given. For polishing operations using light pressure, use belt speed in the upper end of the range given.

Heat: If heat buildup in the part occurs and is objectionable, reducing the belt speed 1000-2000 SFPM will reduce the amount of heat generated for dry operation. If lubricant is used, see separate section on lubricants.

Grindability: Ferrous and non-ferrous metals with grindability of medium to low. If grinding pressure exceeds 1-1/2 HP/inch of contact width, use the lower end of the range given. If grinding pressure is less than 1-1/2 HP/inch of contact width, use the upper end of the range given.

Grinding Pressure

The grain on virtually all coated abrasive products today is electrostatically coated to produce maximum sharpness. A new belt will cut almost any material because of its sharpness. Maximum belt life and efficiency are usually obtained by using reduced grinding pressure when the belt is new and gradually increased pressure as the belt wears. This is a practical technique on most hand and mechanical operations and may require cooperation and brief training of the operator. High grinding pressure on a new belt produces a very high initial rate of cut. However, belt life can be reduced by 20- 40% by this practice because of rapid grain breakdown (see Graph).

 

Questions on Lesson 5

1. The most common condition at discard of belts used on metalworking operation is:
a. loading
b. glazing
c. stripping

2. To improve belt life when the used belt is glazed on an operation that uses light to medium grinding pressure:
a. increase contact wheel aggressiveness
b. decrease contact wheel aggressiveness
c. increase belt speed

3. 36 grit resin bond cloth belts are stripping on a medium to heavy grinding pressure operation using a new 63-90-11 contact wheel Operator says new wheel bumps but the old one did not. The probable cause of stripping is:
a. worn spindle bearings or other machine condition
b. new wheel out of round at operating speed
c. neither of the above

4. Maximum belt life is usually obtained by:
a. using maximum practical grinding pressure
b. using a finer grit size and lower pressure
c. increasing grinding pressure as belt wears

5. In order to get maximum performance when changing to a heavier duty belt specification, it may be necessary to:
a. reduce grinding pressure
b. increase contact wheel aggressiveness
c. increase belt speed

Courtesy of Carborundum Abrasives

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