Safety in grinding and cutting-off operations with abrasive tools

Preparation, mounting and operation of abrasive tool on machine should be performed in accordance with applicable instructions that include recommendations and requirements of: a manufacturer of grinding machine, manufacturer of grinding wheel, operation sheets for grinding operations.

The preparatory and mounting works should be performed by a qualified, trained person or some other worker under the surveillance of authorized person.

The following checks should be performed before the wheel is mounted on the machine spindle.

  1. Check the condition of grinding wheel:
    • External appearance: mechanical damages, cracks, scratches, dents, chipping.
      Such an inspection is to be carried out under the good lighting
    • Perform a sound test
      Refer to description in the previous Section: Storage
    • Check a designation of grinding wheel; especially, compare the maximum rotational speed marked on the wheel with the maximum rotational speed of the machine spindle.
      Only grinding wheels which have the maximum permissible speed equal or higher than the actu¬al speed of spindle may be mounted on the spindle.
      The maximum permissible speed marked on the wheel may not be exceeded under no circumstances.
  2.  Check the condition of grinding machine and elements that directly operate with the grinding wheel, especially:
    • Check the actual rotational speed of spindle, especially after any repair of grinding machine
    • Check condition of locating surfaces for the grinding wheel and clamping faces of flanges that are to:
      • have dimensions to applicable regulations
      • be flat, clean, without defects and should move smoothly without any jamming
      • be square in relation to the datum surfaces
      • clamping flanges are to be paired and have: the same outer diameters, the same recesses and shapes. They should clamp grinding wheels only on ring-shaped faces with the same pres¬sure on both sides. This prevents the grinding wheel to be clamped on the edge of bore which is susceptible to chipping and cracking
      • adaptor flanges must be balanced
    • Adaptor of grinding wheel transfers a drive; make sure that it is correctly located and fixed on the spindle
    • Make sure that the grinding wheel fits freely on the spindle or adaptor.To ensure the a.m. requirement, grinding wheel bores have positive tolerances on them and grinding wheel spindles negative tolerances.
    • Clamping nuts and screws of grinding wheel
      The clamping nut should be tightened only sufficiently to ensure that the flanges drive the wheel and pre¬vent slip. Screws on multiple screw type flanges should be tightened gradually and the tightening should proceed from one screw to one diametrically opposite and them in crisscross sequence. Make sure that screws and nuts screw in and out freely without any jamming or resistance.
    • Blotters
      Blotters are made of flexible and compressible material, e.g. cardboard or plastic, and are placed between the flange and the grinding wheel (see Fig.)
      The blotters should be between 0.2 and 1.0 mm thick and their diameter must be at least that of the flan¬ge. The purpose of the blotters is to:
      • take up any distortion between the flange and the wheel, within the area of flange pressure
      • reduce the risk of slippage between the wheel and the flanges
      • distribute, equally, the axial clamping force when the nut is tightened, over the entire contact area between the flanges and grinding wheels

In general, two blotters of the same size are either laid against or glued to the two faces of any given wheel.

Blotters are supplied by manufacturer or on customer request.

 

Fig. Grinding wheel mounting with its flanges and blotters

 

Blotters are not necessary in the following cases:

  • on small wheels D ≤ 20 mm
  • on type 27 depressed centre wheels
  • on type 29 semi-flexible wheels
  • on plain cutting-off and depressed centre wheels, types 41 and 42, D ≤ 230 mm
  • on steel centred cutting-off wheels, type 43
  • on type 4 wheels with tapered flanges
  • on straight and flared cup wheels type 6 and 11, with the centre nut
  • on cemented disk or threaded insert disc wheels types 35 and 36
  • on cemented cylinder and threaded insert cylinder wheels types 2 and 37
  • on type 31 segments
  • on type 52 mounted wheels and points
  • on plugs and cones with central threaded inserts, types 16 and 19
  • honing stone type 54 and hand stone type 90
  • on extra thin laminated wheels T ≤ 0.5 mm, used for slitting and cutting-off

 

 

 

Example: Blotter may also be an identification label

 

When the grinding machine have a positioning marks, then its position on the spindle should correspond to the marks and inscriptions.

 

 

Fig. Example of positioning marks

 

  • Mounting - securing
    When clamping the adaptor flanges with screws, tightening should proceed from one screw to one diametrically opposite and then in crisscross sequence (similarly to car wheels), without exerting an excessive clamping pressure.
    It is recommended to use a torque wrench. Approximate tightening torques for the clamping screws, according to German recommendations, are shown in Table.

 

Tightening torques for the clamping screws

 

Thread of clamping screw

Height of grinding wheel in mm*

<30

≥30

Recommended torque in [Nm]

M10

M12

M16

M20

30

40

60

80

40

60

80

100

 

*) For multiple wheel mounting, use the torque for a given diameter and the total height of all wheels in the assembly

 

  • Static/dynamic balance
    Where a device for static or dynamic balancing is available, correct balancing of the assembly: grinding wheel, washers, adaptor
  • Carefully mount the assembly of the grinding wheel and adaptor on the grinding machine spindle. Tighten screws and nuts.
  • After the grinding wheel is fitted on the grinding machine spindle:
    • remove all necessary objects and tools from the grinding machine
    • close and fasten guards, adjust work rests and holders
    • turn the grinding wheel by hand, to check if it rotates freely; make sure that run-out does not exist.
  • All grinding wheels of over 100 mm in diameter, before the first use and after each new mounting, should be subject to a trial test by idle running. This test should be carried out in the presence of person authorized for grinding wheel mounting.

Time of the trial test should be:

  • for portable grinding machines - 1/2 minute
  • for all other grinding machines - 1 minute.

The a.m. test may be carried out only when the hazard area is protected and the guard is installed in the proper position.
The grinding wheel may be used only after the satisfactory completion of the a.m. test.
After the test is carried out, the grinding wheel may be dressed, checked and corrected in the range of its balancing and put into operation.

Final comments:

  • Hand grinding machines should be protected against jamming of grinding wheel in material. In case of hand cutting-off grinding machines, the cutting-off wheel should be placed in the slit in the straight position.
  • Before the hand grinding machine may be put away on a bench it should be switched off and completely stopped. It is strictly prohibited to throw grinders with grinding wheels mounted.

Manufacturer of grinding wheels takes no responsibility for any personal injuries nor machine damages or other material losses resulting from failure in observance of the a.m. safety rules during grinding operations with abrasive tools.