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| Grinding Wheels |
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| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-15, Brent Beach |
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.
| Warning - Misuse of any grinder can result in serious injury. Read and follow the instruction manual for your grinder. |
You can grind with grinding wheels. For some steels, in particular M2, some grinding wheels provide much better metal removal.
I almost never use a grinding wheel but am including the following material for those who do, and to convince others that getting an accurate angle with a grinding wheel is a lot more trouble than it is worth.
The big difference between a belt sander and a grinding wheel is that the belt sander leaves a flat bevel. As pointed out by Leonard Lee in his sharpening book, and shown in this drawing, a grinding wheel leaves an arc-shaped bevel - with the actual angle at the edge less than the tool rest angle.
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.
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?
First, mark the wheel with a radius line. That is, a line going from the centre to the edge on the outside of the wheel.
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.
Turn the wheel until the radius line is near the facing front. With the jig on the tool rest, slide it forward until it touches the wheel with the line on the jig and the radius line in contact. You may have to turn the wheel and adjust the tool rest angle.
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.
A visitor to these pages, Keith Brooke, got me thinking about a more accurate way to set the tool rest angle. This is not quite his method.
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! To use this method to set the edge angle, use the second calculator down with a mid angle a couple of degrees larger than the edge angle you want, then select the calculated heel angle that gets you the toe angle you want.
This section is more a methematical diversion than a serious suggestion. If you do use this technique, send along an email with pictures of your set up and your experience so I can add it to the user jigs page.
Here is another potentially more accurate way to set the tool rest.
In the sketchup model, the triangle ABC starts the analysis. The line BC is the midline of the tool. The line AC is a radius, length R. If the tool rest is set for the desired mid-tool angle x, then the line AB has length R * cos(x).
The line BD then has length R - R * cos(x). This is the amount the mid-tool line is below the tangent at D. The tool rest is then R - R * cos(x) + T/2 (T is the tool thickness) below the line through D.
So, how would you make use of this idea? If you had a block of wood exactly this thickness and you put it on the tool rest, then put a straight edge on that and adjusted to tool rest angle until the straight edge just touched the wheel, the tool rest would be at the angle you need. This is a very accurate method - all it requires is the block and a straight edge.
How then to make a block of wood this exact thickness. In the words of Jon Bishop, the site visitor who suggested this approach, uses wedges as follows:
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The wedges aren't really all that difficult to make. You can usually buy bundles of hardwood wedges (for cabinet installations, etc.), all precut to the same shallow angle, at your local home supply center. Select a couple of them you like, sand their surfaces flat (with a 320 grit or better) on a surface plate, trim them to give you the length, width, and working thickness range you want, and you've got got your adjustable set up thickness gauge. If one set won't give you the thickness range you want (your chisels may require a larger drop dimension) simply make another set for them.
You can easily set the thickness of a pair of wedges with a micrometer or caliper, then any small clamping device will suffice to hold the wedge pair on top of the tool rest. Any straight edge can then be used to align the top plane of the wedges with the crown of the grinding wheel. Yes - you are quite right about possibly having to remove the wheel guard. Replacing the guard's original attaching screws with wingnut head screws will greatly simplify that chore. [It might be good to clean out all the crud that collects in there from time to time anyway.] |
Of course, you could also look through your scrap bin for something near the correct thickness and plane it down.
An interesting idea and a clever bit of geometric analysis. If you try this, please send me an email describing our experience. Is it easy to do and quick?
Now, about calculating the drop distance/scrap-thickness/wedge-thickness:
Just to be clear how this is used. Once you have made your correctly thicknessed piece of scrap, or gotten a coupe of wedges to the correct thickness, then
You can play with the calculator to see. Generally, using an intermediate plane iron thickness of 0.10" to calculate your block thickness will produce an error of about a half a degree when used for irons that are thin (0.08") or thick (0.12"). Since we are grinding primary angles, that error makes no difference to the final honed cutting edge.
There is more variation in chisel thickness, but I never use a grinder for thick chisels anyway. If you do, then you might need a few blocks. One block for every 0.04" of thickness should get you close enough for grinding.
As usual, I assume you are using a jig for honing where the honed angle does not depend on the ground angle.
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 9 different heel and edge angles. Fill in the 4 input values then click the calculate button to see the 9 sets of three angles. (Keith Brooke also persuaded me to write this calculator.)
This tool rest comes from a General International 15-232 M1 combined belt sander/grinder. This is the tool rest for the grinding wheel. The tool rest is sturdy and works pretty well, with one essential flaw.
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.
| Update |
| Putting a fender washer between the two ribbed faces will solve the slip problem if you want to use the original tool rest. The washer's outside diameter should be larger than that of the ribbed faces. Thanks to Jon Bishop. |
I recently made an acrylic tool rest for my belt sander. It fits the General without modification.
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.
I made this sketchup model from ideas and pictures supplied by Keith Brooke. He added a self-closing hinge to the front of his jig. With a collar around the tool which presses against the front of the hinge, the collar controls the amount the chisel can move forward into the grinding wheel. The screw provides a fine feed adjustment.
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.
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.
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