Question about Grinding Wheel Diameters

Discussion in 'Magna-Matic' started by Magna-Matic, Mar 26, 2013.

  1. Magna-Matic

    Magna-Matic Sponsor
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  2. Magna-Matic

    Magna-Matic Sponsor
    Posts: 827

    Here is the post that I left answering what is the difference between having a 6" or 8" grinding wheel.
    _____________________________

    Hello All,

    Jaybrown asked if there was an advantage/disadvantage between a 6" and 8" diameter wheel.

    SPEED
    The advantage to having a larger diameter grinding wheel is that it increases the SURFACE FEET PER MINUTE of the grinder. This can increase the speed that it will grind, depending on what you are grinding. Now it is not JUST the wheel diameter, but also the MOTOR RPM that affects the surface feet per minute. When you read the SFPM definition you will understand that not every type of metal can be ground at the same SFPM.

    Here is a link defining SURFACE FEET PER MINUTE
    http://en.wikipedia.org/wiki/Surface_feet_per_minute


    BENCH GRINDERS AND OTHERS LIKE THEM
    Generally speaking most all bench grinders and bench grinder style sharpeners run at 1725 RPM. A 1725 RPM motor is a less expensive motor, and by using this motor to increase the surface feet per minute the grinder manufacturer needs to use a larger diameter grinding wheel to get the SFPM higher. This requires a more expensive grinding wheel. So they have made the machine cheaper to build and passing along a more expensive grinding wheels cost to the consumer. I am speaking generally here – please understand that a standard bench grinder is designed to sharpen tool bits, chisels, etc steel that is usually 60 ROCKWELL or higher, for this application a very hard grinding wheel at 1725 rpm will produce a better SFPM for that application.

    All lawnmower blades are generally between 20-40 ROCKWELL – now the SFPM of the bench grinder is too slow with a grinding wheel that is too hard – this causes burning, glazing, dressing, chatter, slow grinding speed etc. MAGNA-MATIC runs 3450 RPM with 7” diameter wheels, and the grinding wheels we use are properly matched to at our SFPM to remove steel from a lawnmower blade very quickly without burning it, or glazing the grinding wheel.


    WHEEL LIFE
    As Kelly’s Landscaping mentioned - having a larger will provide a longer overall grinding wheel life, however at a substantial cost. One of the reasons we use 7” diameter wheels is they are a size that the scale of economy provides the best grinding wheel for the dollar size. When you start getting up at 8” to 14” the wheels get far more expensive.


    BEVEL/ANGLE/HOLLOW GRIND
    Windflower mentioned how it will change the look of the angle. A larger diameter wheel will create less of a “hollow grind” (where the surface of the cutting edge angle looks concave). This is true only when the wheel is new, as the wheel reduces in diameter it will make the same surface as any other wheel at that same diameter. The diameter of the wheel does not change the ANGLE / BEVEL, that is controlled by the sharpener (if you use MAGNA-MATIC) or it is controlled by the operator’s hands and how steady he/she can be.

    OUT-OF-ROUND / WHEEL CHATTER
    Remember the larger the wheel the more violent the vibration can be when a grinding wheel becomes “out-of-round” due to non-rigid work-piece holding – the vibratory condition of a 10” or 14” wheel rumbling a grinder can be more dangerous than a smaller wheel.

    It is for all of these design considerations and engineering why MAGNA-MATIC sharpeners are designed so much differently than all the others out there, and why our sharpeners are faster, more quiet, do not vibrate, sharpen a blade in 1-2 minutes, don’t require wheel dressing, don’t burn the blade, maintain consistent cutting edge angles, can dial-in specific cutting edge angles, etc.

    ANGLE GRINDERS
    All of the same concerns mentioned above also apply, but with an even larger problem - total inconsistency of cutting edge angle. When an angle is changed every time a lawnmower blade is serviced this removes far more steel from the blade than is necessary to sharpen it. If you would sharpen the lawnmower blade consistently at the same angle the lawnmower blade would have a longer useful life, it would provide a better quality cut on the lawn, you would use less of the abrasive “grinding wheel/flap disc”, and it would take less time.

    Sorry for making such a long post, but there is a lot of information to consider with the question that was asked. Here is a link to some additional education info if anyone is interested http://www.magna-matic.com/educational-info.html

    Thank you,
     

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