Grinding Wheels
	Finest abrasives.
	Microbevels front and back.
	Use a jig.
	Copyright (c) 2002-12, Brent Beach

Introduction

Grinding forms the blade away from the edge. Other pages on this site discuss an overview of grinding, grinding using a bench stone, grinding using a belt sander, and the sharpening process, which includes both grinding and honing.

Contents

1. Belt Sander or Grinding Wheel Use a belt sander unless you have no choice.
2. Actual angles. Hollow grind produces many angles.
3. What angle to set? Specify the angle at the edge, mid, or heel position?
4. Setting the tool rest angle.
   • Simple jig.
   • More accurate jig.
5. Calculator - hollow grind angles.
6. General belt-sander/grinder Tool rest alert.
7. Acrylic Grinder tool rest.
8. Precision Grinder tool rest
9. Hollow grind.
10. Website Navigation -- Where am I?

Belt Sander or Grinding Wheel

You can grind with grinding wheels. For some steels, in particular M2, some grinding wheels provide much better metal removal.

Actual angles.

This drawing shows the shape of the bevel when you use a 6" grinding wheel on a standard plane blade (0.09" thick). The diagram shows the angles at the edge, the middle, and the base of the bevel.

• Edge angle - the angle at the cutting edge.
• Mid angle - the angle of the bevel at the midpoint of the tool.
• Heel angle - the angle of the bevel farthest from the edge.

We must be clear when we talk about setting the angle of the tool rest. When we say we set the tool rest for a 30 degree angle, which of these three angles do we mean?

How do we set the tool rest for a particular angle at a particular point on the bevel?

What angle to set?

• Edge Angle - Many wood turners use grinding wheels and often use the tool without honing after grinding. They grind to their desired angle during use. That is, when they say they use a 20 degree angle they mean the angle at the edge. [This would not be my choice - I would hone before using any chisel.]
• Mid Angle - Some people use the hollow grind as a honing guide. After grinding, they rest the tool on the sharpening stone, balanced on the front and back of the hollow left by the grinder. They then hone without a jig, sliding the tool back and forth on the stone. [They assume their body can be as rigid as a jig.]
• Heel angle - People who use a jig to set the tool rest angle may want to specify the heel angle because that is the easiest angle to set accurately. They are not specifying the cutting angle at the edge, they are specifying the grinding angle.

Setting the tool rest angle.

Wheel Radius Line

You might be able to do a fair job with the wheel on the grinder. The centre finding attachment for your combination square would give the best result. A small square or combination square might give a good result.

It is easier with the wheel off the grinder. A ruler across the middle will give good results.

Jig Thickness Line

• Setting the edge angle - If you want to set the edge angle, the line on the jig should be the tool thickness from the bottom of the base.
• Setting the mid angle - If you want to set the mid angle, the line on the jig should be half the tool thickness from the base.
• Setting the heel angle - You don't need a line, you will use the base of the jig.

Simple jig.

While I can quickly set the angle on my belt sander with just two hands, doing this on a grinding wheel - where you have to turn the wheel, slide the jig forward, adjust the tool rest angle and tighten the tool rest bolt all at once - having a third hand would be helpful.

Accuracy You can check your own work on this. Set the rest for any one of the three desired angles and grind. Change the tool rest angle then set the tool rest angle again. Darken the bevel with magic marker, then bring the tool just up to the wheel briefly. Check how the new hollow grind compares to the old.

More accurate jig.

Begin by clamping the straight stick along the radius line.

Make a second stick which has the end cut at the angle you want. You measure the angle from the square end, not from the side of the stick. Alternatively, measure from the side 90 degrees minus your angle. In this case, the angle measured from the side is 90-25 = 65 degrees.

Slide the second stick along the tool rest until it meets the first stick at the wheel.

Tilt the tool rest until the angled face of the jig is flush with the side of the first stick.

Setting using this method will be as accurate as setting on a belt sander since you are aligning two flat surfaces.

However, the angle you are setting is always the heel angle, not the mid or edge angle!

Calculator - hollow grind angles

The answer depends on the thickness of the tool at the edge and the radius of your wheel.

This calculator lets you specify the mid angle, the tool thickness and the wheel radius and calculates 10 different heel and edge angles. Fill in the 4 input values then click the calculate button to see the 10 sets of three angles. (Keith Brooke also persuaded me to write this calculator.)

General belt-sander/grinder Tool rest alert.

If you look at the seat around the locking bolt you see a series of ridges. It turns out there are about 50 of them, arranged radially. There is an identical series of ridges where you attach the tool rest to the cover.

If you were an engineer sitting at a CAD machine you might think this is a really good idea - how better to prevent the tool rest from turning in use?

It turns out this is a horrible idea!

Yes, you can set the tool rest to the angle you want. However, as soon as you start the grinder the vibration causes the tool rest to slip to the nearest rib position. You can see in the picture the bit of metal ground off the inside of the tool rest. This is what happens when the tool rest slips and turns into the belt.

So, with 50 ribs you have 50 possible tool rest positions - or about one every 7 degrees. If you are keen on getting 25 degrees rather than 29 degrees, this is a problem.

The folks at General understood the problem but said they had sold thousands and no one had ever complained before. The man I talked to, Dave, told me that grinding the ribs off would probably solve the problem. I asked him if I should grind them off both the tool rest and the wheel cover. He thought both would be better.

Great customer service.

Looking at their other grinders, it looks like General has standardized on this type of tool rest locking mechanism. If you have any interest in actually setting the tool rest angle you will have to modify it some way first.

Grinder tool rest

It happens that the tool rest bolt on the grinder is on the left side, while the bolt on my belt sander is on the right side. Fortunately, the tool rest fit without modification, other than flipping it over. Since the bolt is now on the left, it is harder to use my left hand to control the blade. Instead, I have to control - do the side to side movement of - the tool with my right hand. Although I have never done this right-handed before, it turns out it is just as easy as left-handed. I am right handed.

Precision Grinder tool rest

When grinding plane irons, should you chose to do that, you can use the cap iron on the bottom of the blade as the collar.

Keith made his tool rest out of metal. You can probably make it more easily out of 1/4" acrylic. Other views of the model on request.

Hollow Grind Revisited

Consider a plane iron 0.09" thick ground at 27 degree mid angle on a 3" radius wheel. From the calculator, the angles are: edge 25.11, mid 27, heel 28.89 degrees.

To get the same edge angle using a belt sander, all three angles would be 25 degrees. The tool, away from the edge near the bevel heel, would not be quite as thick. The argument is that the thicker tool is stronger. That is true. However, the difference in strength does not matter since both grindings are plenty strong.

If you hollow grind at 27 mid angle, then you can hone at 29 degrees using my jig and then continue with the other micro bevels. A slightly stronger tool, same honed microbevels.

Website Navigation