Hand Tools Archive

Subject:
Hollow grinding tool bevels on a wheel grinder

Bill Tindall, E.Tn.
In answer to this question:

White 100 grit wheel, dressed, medium to low rpm, fingers sufficiently close to an edge, still puts blue\brown spots on my just water cooled bevels so fast I can't catch the moment when to take a tool off.

At least part of your problem is from technique which I will address at the end.

To recap, I can/do grind thin Stanley plane blades to near the edge with a 3600 rpm Baldor grinder and a 120 grit pink aluminum oxide wheel, made with what I remember to be an H bond( but could be I). I have successfully taught many others to do the same on my machine with 100% success except the lady that stuck her finger on the spinning wheel to see if it was spinning. That did not end well, but I digress.

The Wheel:

Abrasive composition: Manufacturers recommend a pink aluminum oxide wheel, or the more modern “SG” aluminum oxide, for hard steel. White will do.

Particle size: I use 120 for historical reasons. Were I replacing, it would be with 80 or 60. This choice should be made based on scratch depth left to refine and control of material removal. My goal is to grind to near the tool edge, being careful to preserve the original reference edge to ensure it remains straight and orthogonal to the tool length (essential for plane blades). As I gained experience I found I could grind with a faster cutting wheel. I refine with diamonds which mow off any scratch depth from grinding.

Wheel speed: Affects rate of material removal which in turn affects control. I started with a slow speed, moving up to full 3600 rpm as I gained experience grinding to near the edge without going over.

Wheel Bond: Watt revealed the equivalence of work and heat. The abrasive particle milling out a chip takes work. Some of this work leaves as heat in the chip and the rest is deposited in the tool. Plowing a dull abrasive particle across the tool generates frictional heat and all of it is deposited in the tool. Hence, for coolest grinding a sharp abrasive particle is desired.

This situation can be achieved by fracturing the abrasive particle to create a new sharp edges or by ejecting the dull particle from the wheel surface. The SG abrasive fractures in a way to generate new sharp edges while largely preserving the particle size. White aluminum oxide fractures with more loss of particle size.

Another way to generate fresh sharp edges is to reject the particle and expose a new one, either by dressing the wheel or by sintering the abrasive particles together with a ceramic that is weak and lets go of the particles easily. The wheel’s “bond”, a letter rating system, reveals how tightly the abrasive particles are sintered together. A wheel with an H bond sheds particles readily from the pressure of the tool against the wheel. A M bond wheel is hard and its shape will hold up to heavy pressure grinding but with no loss of particles unless dressed.

Technique: Jam a 1 inch chisel straight into a 1” flat wheel of any kind and the heat will quickly accumulate at the edge and a burn will result. Consider the pilot landing practice called “touch and go”. The plane glides into the runway, touches it lightly, rolls along it to near the end, and lifts off. This is a technique that will guarantee cool grinding. The wheel must be crowned slightly. In a sweeping motion the tool lands on the crown somewhat past the leading edge of the tool. With the lightest touch the tool is guided across the crown and off the trailing edge. Then repeat going the other direction. Every few passes examine the result and adjust landing and take-off as needed.

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