Bench Stones for Primary Bevels
	Finest abrasives.
	Microbevels front and back.
	Use a jig.
	Copyright (c) 2002-08, Brent Beach

Introduction

Contents

1. The Video - you can watch me grind a primary bevel using a benchstone on YouTube. This is the level of effort needed. I was puffing a little at the end. This effort is only required the first time. Redoing a blade you have already done takes a minute or two.
2. Background - my path to bench stones for grinding. Only for the history addict.
3. The Setup - how to use a bench stone for grinding the primary.
4. The Pictures - fabulous bevels in no time. Double click on a picture for a much bigger image.
5. Stone Wear - the SiC stone lasts like a champ, the water stone was gone after a single grind.
6. Video demo - discussion of the blade used in the grinding video and the use of a bench stone to remove pitting on the back of the blade.

Background

I read Leonard Lee's sharpening book and the Lee Valley catalogues. From them I decided to try a 1" belt sander to replace the grinder. The tool rest on the 1" Delta belt sander cannot be used when grinding at the angles required for plane blades and chisels, so I bent a bit of angle iron and put away the grinder. Go here for a discussion of grinding using a 1" belt sander.

With the grinding problem solved -- I could now quickly produce good primary bevels at any required angle -- I turned to honing. Again, Leonard Lee and Lee Valley tools provided the inspiration and the abrasives - 3M microfinishing abrasives. The honing jig was my own idea.

Over the years I came to recognize the limitations of powered grinding and developed a jig for holding bench stones along with a new jig for holding plane blades or chisels and switched to hand powered grinding.

The Problem

Each student brought along tools to sharpen - a plane blade and a chisel. The goal was to hone great edges on those tools.

Unfortunately, almost all the plane blades and chisels had bad primary bevels. Until you have a good primary bevel, you cannot hone the blade. I ended up grinding primary bevels on most of the older blades and even some of the new blades.

This page shows you how to get precise primary bevels on your tools. Once you have a great primary bevel, you can head back to the honing pages and hone a great edge on them.

Grinding options

Video

The blade was rusty, superficial mostly, and the primary bevel was at about 30 degrees. I used this blade in a demonstration of grinding a primary bevel using a bench stone. Unfortunately the camera was aimed a little low and the views of the primary are not very good. I have added a discussion of the sharpening of that blade, during which an interesting problem arose and was solved.

Which Type of Bench Stone

Of the synthetic bench stones, the fastest cutting readily available stones are made from Silicon Carbide (SiC, the same abrasive used in the 15 and 5 micron 3M abrasive sheets). While some people are very happy with their SiC stones, others won't use them. The naysayers report that the stones dish - go out of flat - too quickly. Perhaps a little surprisingly, the naysayers include one of the main manufacturers of both Aluminum Oxide (AlO) and SiC stones - the Norton Company. Norton brands their AlO stones India and their SiC stones Crystolon. From a booklet titled How to Sharpen and What to Use published by the Pike Manufacturing Co.in 1923, which was owned by the Norton Company by the time Pike was producing artificial stones:

That was the Pike view of things back in 1923. I wonder if the Norton Company still feels this way? Both the Crystolon and India stones used in these tests are made by Norton.

This ambivalence continues among mail order suppliers of sharpening supplies. One supplier refused to look for SiC stones for me, saying they wore too quickly. I finally found these particular stones (3" wide stones rather than the usual 2" wide stones) at Sharpening Supplies in the USofA. A recent search produced a link to European Hand Tools, which is actually located in Canada. The latter company sells only the coarse Crystolon stone, but that is the only one I use.

Wear on a synthetic bench stone depends on the abrasive and on the binder used to hold the whole thing together. While all SiC bench stones use Silicon Carbide as the abrasive, I suspect that the modern Norton SiC stones, which they call Crystolon, have a better binding matrix ("Amorphous Silica, Fused") than the older Pike stones. I have had no problems with rapid or uneven wear. [Def: Amorphous Silica: A naturally occurring or synthetically produced oxide of silicon characterized by the absence of pronounced crystalline structure.]

Which Grit to Use

I bought all three grits of Norton Crystolon bench stones, but ended up really only using the coarse stone (CJB83). Since it is safe on the edge and faster than the medium or fine stone, it is the logical choice. Not only that, subsequent testing with the fine SiC stone revealed a very high wear rate. I did not do any actual measuring, but the wear rate of the fine stone could be 100 times that of the coarse stone.

Jig or No Jig

If you watched the video, you will have seen that I press pretty hard. The blade I ground in the video began with a 30 degree primary bevel. Grinding this down to 25 degrees took several minutes. Without the jig to maintain the angle, I would not have been able to press as hard as I did. The grinding would have taken longer and produced a rounded, rather than flat, bevel. With the jig I was able to concentrate on steady firm pressure.

Accuracy is important to me when I am testing. In this case, I was trying to determine how well the SiC bench stone worked for different types of plane blades -- varying from older high carbon steels to the newer High Speed Steels. The goal of the test was to decide if a SiC bench stone was a reasonable alternative to powered grinders for primary bevels. Testing requires standardization - all the blades had to be ground to the same primary angle, from the same starting condition. Testing requires repeatability -- someone else with the same bench stone and the same blade must be able to perform the test and get the same results (within reasonable error limits).

While my requirements in this test exceed those of the home woodworker, I believe that the speed and accuracy achieved by using jigs will benefit everyone.

Which Jig?

In a previous test I was using about half the stone - the jig allowed me to use only the front 4" of an 8" stone. With a 4" range of motion, I was probably only getting the maximum grinding force across the middle 1 or 2 inches.

You cannot use the entire length of the bench stone if the jig is riding on the stone. The jig must not ride on the stone and must allow use of the full length of the stone.

I have also found, at least with silicon carbide stones, that pressure makes a huge difference. If you are not pressing hard enough, the blade slides over the stone with very little cutting action. Once you are exerting enough pressure, the sound changes - you can hear the abrasive cutting metal from the surface. Once you reach this pressure level, the stone works very quickly.

The jig then must allow the user to apply considerable force.

The Setup

The stone is held in version 4 of my bench stone holder, also known as a Stone Vise. The vise top is surfaced with glass - allowing for much reduced friction between the jig and the vise.

The blade is held in a new slanted jig. The range of motion with a jig that holds the blade at right angles is limited by length of the plane iron. By building a new jig that holds the plane iron at an acute angle, I was able to get the full 8" range of motion.

This jig has an included angle of 50 degrees and is about 5" tall. I selected 50 degrees as twice my standard primary angle of 25 degrees. This means the extension is about equal to the jig height and the range of motion while grinding is 9 inches. If you have shorter blades, you can use a smaller angle and still get a good range of motion. For example, when grinding chisels which are not as long as plane blades, I use an included angle of 35 degrees. This gives me a good range of motion even when the chisel has less than 3" of blade.

The pad of my left hand rests on the blade, just above the edge. This hand provides the downward force and some of the backward force as well.

It also controls where the force is applied on the blade. While the main motion is along the length of the blade, there is a gradual side to side motion as well - to get even wear on the stone. As I work the blade toward the left/right side of the stone I put a little more pressure on the left/right side of the blade. This tends to compensate for the fact that most of the work is being done in the middle of the stone (which is always under the blade, while the sides are only sometimes under the blade). I also grind with part of the blade off the side of the stone, to ensure even wear across the full width of the stone.

Notice the buildup of oil and filings (and a little SiC fractured during grinding) in front of the blade.

Wet or Dry

First, the oil is not a lubricant since its purpose is not to reduce friction. Rather, it is a liquid in which filings and fractured SiC crystals float off the bench stone surface, allowing them to be swept out of the working area by the blade.

I have found that the same baby oil (light mineral oil) that I use on sheet abrasives works well on SiC. I apply a little more near the front edge when the stone no longer has a sheen. The grinding action moves the oil and the filings to the far end of the stone, where it can be wiped or blotted off.

Cleanup involves adding a little more mineral oil, rubbing it over the entire surface with my finger, then blotting the oil and filings up with a rag.

I use baby oil on the 3M abrasives and on my bench stones. You can see micrographs of blade sharpened this way. You can see no evidence that using oil is allowing filings to nick the edge in some way.

I have not been able to reproduce the results reported in "The Razor Edge". I suspect the problem is which oil they used and how well they cleaned their stones. When you think back on the old oil stones you find in flea markets - the stones are grungy, covered in congealed oil. If you use a drying type oil - kerosene, mineral spirits - then the rapid drying can leave filings in the working area of the stone. If you let your oil congeal on the stone surface, you might be creating a problem. However, if you use a non-drying oil like mineral oil and clean the stone, you should have no problems with filings in the working area of the stone chipping the edge.

Bench Stone Testing

1. Determine whether bench stones can be used to grind primary bevels on the full range of plane irons - classics made of High Carbon steel through more recent blades made from High Speed Steel.
2. Determine how durable the bench stone is when used for grinding.

Test Design

1. Prepare the primary bevel at about 25 degrees.

   I set each blade to the same extension in the grinding jig, then grind the full primary bevel using the test bench stone. I angle the blade to the direction of motion slightly so I can see the angled scratches cross the entire bevel.

   I get a 600 dpi scan of the bevel, with the blade in the grinding jig so the bevel is lying flat on the scanner screen.

2. Small microbevel.

   Using 15 micron abrasive I hone a small microbevel at 29 degrees.

   I get a 600 dpi scan of the bevel, with the blade in the grinding jig, measuring the width of the microbevel.

3. Grind most of the microbevel away.

   Using the same extension in the grinding jig, I grind away most be not all of the microbevel, keeping track of how long it took. I want some of the microbevel left, so I can determine how much metal I ground away.

Bench Stone results

The results of this test series differ quite a bit from the first test series, even though I am using the same plane blades and the same SiC bench stone.

• In the first test I ground the blades to a common primary angle, but had not standardized the primary bevel before the test.
• The new slanted jig allows me to use the full length of the bench stone instead on only half. I think this actually triples the grinding speed.
• I now know to use more pressure while grinding, working to get the harsher sound of effective grinding.

The images have been resized down to 300 pixels. You can look at the full sized image by double clicking the thumbnail. Single click to revert to the thumbnail image.

Blade/steel	Blade width	Primary width before/after	Time	Relative toughness	Image
Ohio - High carbon.	2"	0.243/0.256	210 seconds	21.7
Stanley - V logo unlaminated.	1.75"	0.18/0.196	75 seconds	5.8
Stanley - V logo laminated.	1.75"	0.187/0.202	50 seconds	5.5
Hock - A2.	1.75"	0.220/0.23	90 seconds	11.5
CPM 3V	2"	0.292/0.310	90 seconds	4.7

Subsequent Testing

• High carbon steel - Stanley and Sargent blades from pre-1920 era.
• High carbon steel - modern laminated blade from Tsunesaburo
• Alloy steel - Clifton Hand Forged
• A2 Steel - Lee Valley, Lie Nielsen
• D2 Steel - Gramercy
• CPM 3V - Steve Elliott
• S53 - Holtey
• HSS - Mujingfang, Academy Saw Works

All blades had primaries ground using the Norton SiC bench stone before this test began, but had honed microbevels and wear bevels from use. The test was renewing the primary bevel on blades that already had good primary bevels.

I put new primary bevels on all these blades with little difficulty, continuing until a burr had formed on each blade. No blade took more than 2 minutes. The stone showed little wear.

I then sharpened all 13 blades in the usual way - 3 microbevels on front and back using the 3M abrasives and my jig. Then using a digital kitchen scale and some rayon thread, I did an initial sharpness test on these blades. There was no difference in initial sharpness.

Discussion

Primary Bevel Quality

Steel Toughness

Bench Stone Grinding Speed

I think I learned a lot about how much pressure you can use with a SiC stone - I now use much more pressure than on the first tests. The difference in the sound of the scrapping between the stone and the blade is very noticeable. The speeds are now reasonable for regular shop use of these stones for renewing a primary bevel - that is, once the microbevels are too wide, you can quickly renew the primary - probably in less than 2 minutes - using a SiC stone.

The relative toughness should not be relied upon. I did the Hock A2 blade first and was probably not pressing as hard. I did the CPM-3V blade last, and was using lots of pressure and getting very good results.

I should repeat the entire set again to see if the results stabilize - are more reflective of the blade width and thickness.

Further Testing

The best stone cuts quickest while retaining its shape. Comparing grinding times between SiC and AlO stones should be fairly repeatable. Comparing stone flatness should also be straightforward.

Lets see what the tests show.

I will, after dulling the blades, redo the primary to see how long it takes from a standard dull condition to a renewed primary. The above times are not true comparisons, since the blades did not all have exactly the same pre-test primary bevel.

Bench stone Wear

Water Stone Wear

There is good news and bad news. First the good news - the stone is very fast. Again, no standardized test, but given the starting bevel on an older, O1 high carbon steel type blade, it seemed to go much faster than the Norton SiC coarse stone.

Now the bad news. You can see the wear on the stone in this picture. This wear is the result of grinding one primary bevel. The wear amounts to about 0.03", on a stone that started out about 1.33" thick (not quite the 1.375" they advertise).

This amount of wear poses two problems. First, you would certainly want to flatten this stone after one or two sharpening sessions. If you switched the stone in the stone vise, you could so a second blade on a flat surface. You would then have to flatten the stone. So, time save in sharpening is lost to time flattening the stone.

Second, the water stone wear affects the geometry of the sharpened blade, increasing the angle at the edge slightly (by about .3 degrees). This is not a problem for a primary bevel, and perhaps not even a problem for a microbevel.

Oil Stone Wear

The total time spent grinding on this oil stone is probably more than 16 times the time spent on the water stone, given the extended grinding sessions for some of the irons.

This stone now varies in thickness between 0.488 and 0.493 inches. While grinding on this stone, I was careful to minimize uneven wear - working the blade from side to side on stone, occasionally switching the stone end for end in the stone vise.

I don't know the original thickness of the coarse SiC stone. An unused medium grit SiC stone's thickness varies between 0.510 and 0.513 inches. I don't believe that the coarse stone started out as thick as the medium stone. If so, the ends of the coarse stone would be closer to 0.510" since it is very hard to grind right up to the end of the stone. The ends on the worn stone are nowhere thicker than 0.493 inches.

I believe that the coarse SiC stone has in fact worn around 0.005 inches after grinding primaries on 8 plane blades. And the wear is quite uniform! There is still no need to flatten this stone.

Grinding Demonstration

The blade and cap iron had lots of superficial rust. I oiled the blade and cap iron before taking this picture. The short primary bevel is obvious in this picture.

The plane was neglected for many years (dusty, surface rust on the metal parts) then abused (latex paint on the sole), but otherwise in good shape. The screws that hold the metal frame show no signs of corrosion and the sole is fairly flat with wear near in the mouth in the middle of the blade. I think this plane with fettle well.

I thought the corrosion on the back of the blade at the edge was not serious. It turns out it was.

The blade had a 30 degree primary bevel. I decided to grind a new primary bevel at 25 degrees using a bench stone. I made a video of the grinding session which is also a demonstration of the stone vise. Unfortunately, the camera was aimed lower than I thought so you cannot see blade and bevel in parts of the explanation. Since you can only regrind the primary bevel once, it will have to do.

The edge is a disaster.

The left half is the 15 micron microbevel, the right half the 5 micron microbevel and probably the 0.5 micron microbevel.

The 15 micron microbevel came no where near the edge. The 5 micron came a little closer, but probably did not reach the edge and certainly did not remove the pitting.

I don't think this problem is a result of pitting alone. I think an earlier owner rounded the back of the blade at the edge while honing the blade. In order to restore the back of this blade I will have to do some serious grinding.

I had the coarse bench stone in the stone vise and the blade in the honing jig, so I used the two - working just the back bevel. I am not sure how long it would have taken to work through the pits on 15 micron (I suspect closer to an hour than 5 minutes), but on the coarse stone it took just a few minutes - less time than it took to do the front.

There was a very thick wire edge facing the front of the blade after grinding the back. I removed a bit of it before taking this picture. The shiny part at the right is all wire edge that is being removed. It is high enough that the abrasive is just touching it, not the rest of the 15 micron microbevel.

There is still a problem at the edge, but I think this is an indication of a wire edge.

The 5 micron abrasive has removed the wire edge and enough metal to almost completely remove the problem at the edge.

There is still a very small defect on the edge. The scratch pattern is a bit unusual -- the last abrasive was 0.5 micron. I think the problem was using the same rag to wipe off the blade as I used to clean the bench stone.

One more complete honing sequence should fix the edge.

It is interesting that a few minutes grinding a back microbevel on a coarse stone can remove fairly deep pitting.

Having ground a wide bevel on the back at the first microbevel angle will affect future honing results. Even the 15 micron abrasive does not cut quickly if required to work a wide bevel and the first back bevel on this blade is no longer a microbevel. Fortunately the 5 micron abrasive cuts quickly and it will be honing the second back bevel which is quite narrow.

Future honing might have been closer to the usual pattern if I had increased the blade extension (reduced the first back bevel angle) prior to grinding off the pits. I can still do this if future honing sessions do not produce the usual results.

Jigs

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Questions? Comments?