Stumped???

Often little things make big differences in how an abrasive does its job.

Look for patterns in the work piece to give clues to the problem.

Worn bearing or bent spindles often reslut in a repetitive pattern that is easily identified.

 

 

 

 

 

 

 

 

 

Worn spindles or bearings

 

out of round

Out of round wheel

 

 

 


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Narrow Belt Machine Problems & Troubleshooting

The term backstand does not describe a machine, but is the most fundamental and the most popular industrial use of coated abrasives. A backstand is the adaptation of a grinding or polishing machine for the use of abrasive belts by the addition of a backstand idler — a pulley for tensioning and tracking the belt. The idler may either be floor mounted, wall mounted, or mounted on a bracket on the machine. The work-piece is usually applied manually, although some sophisticated devices have been developed to provide added infeed pressure by power assist equipment and automatic feed devices for loading and positioning the workpiece in the grind position. Backstands are used for a wide variety of applications from heavy duty stock removal operations such as grinding castings or forgings with the coarsest grit abrasives through to polishing operations using the finest grit abrasives.

The heart of a backstand is the contact wheel, and the selection of the proper wheel is vitally important to the overall performance of the operation. Contact wheel design and selection is a complex subject that will be treated separately, but these are some useful guidelines:

  • Harder Wheel — Faster Cut, Coarser Finish
  • Smaller Wheel — Faster Cut, Coarser Finish
  • Serrations — High land to groove ratio gives faster cut, coarser finish.
  • Harder Wheel — More stock removal for given grit size. Recommend isothane for 80A and harder wheels. Recommend isothane covered idlers for minimum wear and slippage.

The backstand is a simple piece of equipment, but problems may be encountered that can usually be overcome without too much difficulty. Some of these are:

Crooked or Worn Spindles

A crooked spindle can cause chatter, uneven belt wear, belt wobble, premature belt wear or breakage. The machine spindle must be straight for the successful operation of a contact wheel. To check a spindle for straightness, remove the contact wheel and hold a colored pencil against the rotating spindle. A true spindle will mark all the way around while a crooked spindle will not. The farther out the spindle is the shorter the line will be. The best correction is replacing the spindle.

Worn Bearings

Contact wheel wobble or any of the conditions mentioned above may also be caused by worn bearings. This condition can be determined by pushing and pulling the contact wheel and/or spindle to detect any play or movement in it. This condition can only be corrected by replacing the spindle bearings.

Out-of-Round Contact Wheel

Chatter, poor finish, and premature belt wear can be caused by an out- of-round contact wheel. After the spindle and bearings have been checked, it is advisable to check the contact wheel for true roundness. This is done by holding chalk lightly against the wheel running at operating speed. The best way to correct an out-of-round wheel is by turning or grinding it in a lathe. An acceptable job can be done by wrapping a flat piece of wood with a coarse grit abrasive cloth and holding it against the wheel with light pressure while it is running.
Note: Due to variations in density within a wheel, new wheels are not necessarily true. This may be because the wheel is operated at a speed above or below that at which it was dressed by the manufacturer.

Tension

Normal belt tension should not exceed the tension that will prevent slippage. Excessive tension detracts from the softness or aggressiveness of a contact wheel and decreases the action of the serrations. It also causes undue stress on the bearings and can cause excessive stretch or rupture of the coated abrasive belt. Heavy tension is necessary when work pressures are high for heavy stock removal to prevent the abrasive belt from puckering immediately ahead of the workpiece.

Lubricants

Finishes may be improved by the use of wax or a grease stick. This will, in effect, reduce the depth of penetration and permit higher surface speeds. Grease or wax seldom contain any active cutting agents, as their purpose is to retard rather than accelerate cut. On the other hand, low melting paint greases prevent loading on such materials as aluminum, brass, die castings, etc. Oils applied in small quantities with a spray gun may also be used for polishing purposes.

Abrasive Selection

The main factor that determines the success of a coated abrasive belt application is cost per part finished. Choosing the proper coated abrasive can influence cost just as much as the machine setup. Unfortunately, experience is more reliable than any superficial formula for dictating grinding technique. No hard and fast rules may be established to govern all applications. With given machine conditions, however, the following should be evaluated:

A. Bond Type

Due to the price differential, there must be a 12% minimum increase in production to warrant the use of a resin bond commodity over a glue bond commodity. A resin bond product will out perform a glue product, but the glue product will yield a better finish.

B. Bond Rating

Workpiece material type, finish required, and used belt condition are the considerations involved in selecting a closed or open coat commodity with a strong, medium, or weak bond.

C. Flex

The selection of the correct flex, the controlled breakdown of the adhesive bond will aid in the efficient use of the belt.

D. Grit Size

The grading will affect finish and cut. However, in most cases, the most economical approach is to select the coarsest grit size that finish tolerance will permit.

Swing-Frame Belt Grinders

Swing-Frame Grinders or Swing Grinders (as they are commonly called) are, essentially, suspended coated abrasive belt machines. Most of them can utilize any of the various types of contact wheels. The weight of the Swing Grinder is usually so balanced that it may be suspended from a fixed point by a chain or cable.

Swing Grinders are primarily used for weld grinding or for conditioning raw metal stock. They have the cutting speed of snagging wheels and give a superior finish to that obtained by portable disc grinders.

Swing-Frame Grinders are usually trouble free applications. Some minor problems may be encountered usually centering around the use of an incorrect contact wheel. If excessive bounce or chatter is encountered, a softer contact wheel should be recommended. If abrasive belt life appears to be terminated prematurely by glazing, a more aggressive contact wheel (higher land to groove ratio) should be recommended.

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