Fads and fallacies
	Finest abrasives.
	Microbevels front and back.
	Use a jig.
	Copyright (c) 2002-09, Brent Beach

Contents

• Fads and fallacies.
  • Stropping helps, right?
  • You can touch up a blade, right?
  • Chip breakers break chips, right?
  • Flatten the back first, right?
  • Flatten Chisel backs?
  • Isn't this new side-to-side honing a whole lot better?
  • You gotta be kidding, right?
• Web site navigation

Stropping helps, right?

1. Stropping is a big enough issue that I have a separate page showing the results of my stropping tests.

   To summarize, all of the stropping compounds that I have tested have larger abrasive particles than the 3M 0.5 micron microfinishing abrasive. On the basis of my testing stropping can only produce worse results.

   How then to account for all the people who claim that stropping helps? If stropping helps, then there is something else they are doing during honing that they should change -- use different abrasives, use a different honing technique. Stropping will only help if you bring an inferior edge to the strop.

You can touch up a blade, right?

As in the case of stropping, if you are able to touch up a blade and return it to its orignal sharpness, then you must not have started with a sharp blade.

Chip breakers break chips, right?

But, if the cap iron does not break chips, why do we have it at all? Good question. The explanation has three parts. First, evidence that the cap iron does not break chips. Second, the actual function of the cap iron. Finally, a situation in which the cap iron does break chips.

1. A cap iron does not break chips.

   Proof of this claim comes from two sources. First, from planes without cap irons. Clark and Williams make a wooden plane with a thick iron that does not use a cap iron. These planes are famous for the quality of finish they leave. As well, Steve Knight makes wooden planes with very heavy irons, again using no cap iron. As well, almost all traditional Japanese planes do not use cap irons.

   Second, my examination of the upward facing surface of plane blades after use (see my bevels page) makes it clear that all of the contact between the shaving and the blade occurs within 0.004" of the edge. How close is that? Most people aim for about 1/16" which is 0.0625" or about 15 times too far from the edge to have any effect on bending the shaving.

   So, people who make claims about the chip breaking benefits of cap irons ["This deflection also bends and breaks the shaving fibres so they can't lift and tear out ahead of the blade" -- Woodcraft explaining the benefits of Hock replacement Chip breakers] know not of what they speak.

2. Cap irons do perform a useful role in holding the blade.

   Holding the blade has two parts. First, keep the blade in position along the frog. Second, keep the blade at the edge from lifting off the frog.

   Blade movement along the frog is prevented by friction between the blade and the frog. The amount of friction depends on the qualities of the two surfaces and the total force holding the blade down on the frog. During use, the force of the wood against the blade, is resisted by an equal force of the frog against the blade. This additional force acts to increase the friction between blade and frog, increasing the resistance to blade movement! The cap iron plays no part in this. Blade thickness plays no part in this.

   It is preventing the second movement, the blade edge lifting up off the frog, that the cap iron can be useful. The force perpendicular to the blade close to the edge originates with the lever cap or wedge, but cannot reliably be applied by them alone. Again, during use the force of the wood against the blade presses the blade down against the frog. This is a good thing. However, this force changes with changes in the grain of the wood, allowing the edge to spring back. In extreme cases the edge can spring back enough to lift up from the wood briefly before coming back and digging in. A repetition of this action is called chatter or skitter. Thick blades are stiff enough to resist skitter without the aid of a cap iron. It is with thinner blades that a cap iron helps.

   The lever cap or wedge is a stiff lever whose position relative to the edge cannot be finely set. The cap iron is a flexible piece of steel whose position relative to the edge can be finely set. The cap iron position is not affected by blade set adjustment. The force applied by the wedge/lever cap can therefore be reliably transferred from its fixed position through the cap iron to the required position near the edge of the blade.

   US Patent 540283, June 4, 1895, by Justus Traut and Christian Bodmer (two of the main inventors in the Stanley brain trust in 1890 to 1910 period) is the source for the drawing at the right.

   Traut and Bodmer distinguish between the front (near the edge) and back (the other end, away from the edge) parts of the cap iron. They call the whole cap iron the "knife-controlling member". The back part, which includes the cap iron screw and the slot for the depth adjust lever, they call the "knife-actuator". The front part, from 7' at the edge to 7'' just back of the bump, is the "knife-edge cap".

   Rather than use a heavy cap iron, the idea behind this patent is to weaken the cap iron at the point numbered 12, in one of four different ways, to let the "knife-edge cap" pivot more easily around the fulcrum at 12. The effect is that the pressure of the lever clamp is transferred more or less equally to the front (7') and back (7'') of the "knife-edge cap".

   This goal of distributing the force to two locations is a departure from some earlier cap irons. I have several old wooden planes whose cap irons are arched from the edge to the cap iron screw. The clamping pressure in this case is only at the end of the blade. In fact, tightening the cap iron causes the blade to arc.

   Traut and Bodmer do not think the cap iron breaks chips. Rather one of its functions is "to properly turn the shaving in the throat of the plane, as said shaving is stripped, by the knife-edge, from the body of the piece being planed". That is, it prevents clogging. It plays no part in reducing tearout.

   They also suggest that the blade depth is set before the lever clamp is tightened. This has been a source of some discussion on the porch in the past.

   It is interesting that in this drawing the blade rests on the sole - a fixed frog - that looks more like an infill base than a Stanley plane base.

   I don't know that this patent was ever actually implemented by Stanley. Something like it was eventually produced by Record - their two part cap iron.
   
   

3. Cap irons do reduce the amplitude of vibration.

   The addition of a cap iron under tension against the blade produces a combined system of greater thickness, and hence greater resistance to bending. Stiffness is proportional to the cube of the thickness, so even a thin cap iron helps. A stiffer blade/cap iron will still vibrate, but the vibrations will be smaller and slower. The sound you hear while planing will be lower. [The fact that the edge is vibrating, at whatever rate, means that the planed surface is not flat - it is more like the surface of a lake with a light chop.] Even with a thicker cap iron, the wood surface will be rippled. The lower tone means the peaks are farther apart, not necessarily smaller.

   As blades dull, the force necessary to get them to cut the wood increases. As this force increases the potential for blade deflection increases. The usual result of greater blade deflection is chatter, skitter, or the plane skipping across the surface. A stiffer blade can be used longer between sharpenings (but each sharpening will take longer because the blade is duller). Put another way, with a stiffer blade/cap iron system you can successfully plane with a duller blade.

   Blade deflection probably occurs in the body of the blade, not in the bevelled portion. The bending force is a product of the force at the edge and the distance along the blade from the edge. This product is probably too small to cause significant bending within the bevel.
   
   

4. Cap irons may play a role in reducing edge wear.

   I have noticed a couple of times that the cap iron gets a bit warm during a test, even when set back 1/32nd of an inch from the edge.

   Blade wear is a function of how hot the edge gets during use -- think of metal cutting lathes which use a steady water or oil drip to keep the tool cool. If the cap iron helps heat escape from the edge, then it will help reduce edge wear. I have no idea how large an effect this might be. [A possible test would be to try the same blade, sharpened the same way, with the cap iron at two different distances from the edge.]
   
   

5. A cap iron very close to the edge will improve surface quality.

   A study was done in Japan using a surface planer (large machine which pushes the wood quite slowly past a stationary blade with a solid chip breaker -- very similar to hand plane action) and a microscope to watch chip formation.

   The blade was sharp with an included angle of 30 degrees, clearance angle 10 degrees. The chip breakers were thick (look to be more than 1/4" thick) with flat front face, one with 50 and one with 80 degree included angle. (A quick check of a couple of Stanley cap irons found an average included angle of around 50 degrees, but the face is convex rather than flat.)

   The grain on their test board sloped down at a 7 degree angle.

   With very shallow cuts (0.002"), there was no tearout. With deeper cuts a chip breaker was required. For a shaving of 0.004", the 50 degree chip breaker 0.004" from the edge eliminated tear out. The 80 degree chip breaker could be set twice as far back from the edge and still eliminate tear out.

   How close is 0.004"? Most people try to get the cap iron 1/16" or less from the edge. That is 0.0625" or about 15 times too far to make a difference.

   If you do set the cap iron this close (and people have), you will often have a problem with the shavings jamming between the cap iron and the front of the mouth.

   The paper describing this experiment was originally here. This page disappeared in early 2005, but is mirrored on Steve Elliott's blade testing pages. Steve has done some testing with a cap iron set this close to the edge, and it does help reduce tearout.

Flatten the back first, right?

• the cutting edge is the intersection of two planes, the back and the bevel,
• both planes must be flat in order for the blade to be sharp,
• sharpening only restores the bevel to flatness,
• flattening the back just once is all that is required to achieve the desired result.

In fact, the whole notion that flattening the back makes any difference to the sharpness of the blade is wrong. It is not only wrong, it is misleading. It is misleading because it leads people to believe that they do not need a back bevel on their plane irons. If people do not use a back bevel on their plane irons they will either have dull irons (half-sharp irons), or they will spend a lot more time sharpening than they need to.

The explanation is not complicated. We begin with a sharp blade, use it until it is dull, then work through a process that will get the blade sharp again.

This is a scale drawing based on an actual test blade. The black outer line is a side profile of the sharpened blade before use. (For no particular reason the drawing shows the blade as it sits in the plane). Call this the sharp profile. The red line shows the profile after use. Metal has been worn away on both the top and the bottom of the blade. I call the resulting surfaces wear bevels. Call this the dull profile. This is an extreme close up. The upper red line is about 0.003 inches, the lower red line about 0.00061". (Measurements from photomicrographs.) All the metal between the sharp profile and dull profile was worn away through contact with the wood during planing.

So, after the first use, the flatness of the back is no longer a factor in blade sharpness. All that time spent flattening the back (assuming you even flattened enough to remove any wear bevel at the edge) is no longer providing any benefit. All the effort the blade manufacturer put into flattening the back beyond this level of unevenness - 0.00027" - was wasted. The original back will never again make contact with the savings or the wood. The effort flattening the back no more helps the sharpness of tool than putting the tool in a fancy box or advertising it in a glossy magazine.

This is too much metal to remove by honing from the front. Honing typically removes metal to a depth of less than 0.0003" over a small area. To remove all of the back wear bevel by honing from the front you would have to remove metal to a depth of 0.003" over a much greater area. So, honing is out. You must grind every time you sharpen and you must remove a lot of metal.

As an added complication, if you grind from the front you will be able to feel a burr as soon as you have removed about one quarter of the back wear bevel. You might think you have removed the back wear bevel, but you have only started to remove it. Not only do you have to grind every time, you will probably won't grind enough.

Finally, I believe that grinding through the edge is a mistake. In summary, grinding weakens the metal structure well below the bottom of the scratches you produce, especially with larger grit abrasives. So, not only does this approach waste far more of the blade than necessary, it degrades the steel at the edge, producing a less durable edge.

The upper blue line here looks the same as the blue line in the flattening the entire back case. The back bevel is not the entire back - it is just about 0.01" at the edge. Honing this very narrow strip along the edge takes very little time. Honing back bevels add less than a minute to the total honing time.

Flatten the back first, right? Wrong!

So, reliance of a separate flattening step helps only once and does harm for the rest of the life of the blade.

To take this a little farther, plane blade advertising which stresses the flatness of the back - to a flatness tolerance of +-0.0002" or better over the working surface, and with an average roughness surface finish of 0.000005" or better - is a disservice to plane users.

Finally, advertising that stress the flatness of the back of plane blades indicates a fundamental misunderstanding of plane operation on the part of plane designers. By designing planes that require flat backs on plane irons, we get designs that do not work with real life blades. I am thinking here of low angle bevel up plans with bedding angle of 12 degrees. The plane works well when the plane iron back is flat - with a new blade. Thereafter, the plane does not work as well because people cannot sharpen the blade to this geometry. The result is an edge with too shallow a clearance angle to work well.

Planes and plane blades for which flat blades backs are important are probably useful only to people who buy tools in big box stores and don't use them enough to ever dull the blade. Essentially a disposable blade and plane.

Flat Chisel backs?

The shape of a worn chisel could be much different from that of a worn plane blade. Chisels slice, plane blades scrape. Chisels are usually used with zero clearance angle, planes are never used with a zero clearance angle. This difference in use probably results in a different worn tool shape - a different ratio of front and back wear. I have no test that uses a chisel until it is dull, so have never photographed a chisel dulled through a repeatable process. I believe it is however safe to assume that there is wear on both sides of a chisel, suggesting that some attention must be paid to either adding a back bevel or removing the back wear bevel.

The problem with bench chisels, paring and mortise chisels, is that you often want to use the back as a jig -- you use the flat back to align the chisel during use.

When paring, you often lay the chisel flat on the back (bevel up) and push it forward. If there is a back bevel, the chisel will not cut a flat surface, but will ride up the back bevel. If you always pare with the bevel down rather than up, a back bevel should be no problem.

When mortising, especially at the mortise ends, you have to be able to cut straight down. If there is a back bevel, a chisel held vertically will not cut a 90 degree face. With more skill, a 3 degree tilt when you have a 3 degree back bevel, you could do it. In fact, with practise I expect you would soon discover the tilt you need to get your mortise chisel to do straight down, adjusting as the chisel works down through the wood. After all, you have to learn what 0 degrees feels like if you use no back bevel!

Carving chisels are a slightly different issue. Most are not flat, so there is no way to put on a back bevel with my jigs. In fact, since most are curved, any use of jigs is a problem. So, while back bevels are usually not an issue with carving chisels, front microbevels can be useful. In use, raising the handle can cause the edge to rotate around the back of the microbevel. Lifting the handle tilts the edge down for a deeper cut, lowering the handle raises the edge for a shallower cut. With practise, this allows very fine control.

Bottom line, you probably get a better edge but may need more practise to control the tool. I personally don't use back bevels on chisels.

Isn't this new side-to-side honing a whole lot better?

Harrelson Stanley thinks he has with his Sharp Skate.

My early assessment of side-to-side [STS] honing versus front-and-back FAB honing concludes that even with a much better jig than the Sharp Skate, STS honing probably produces an inferior edge.

You gotta be kidding, right?

• Any company Advertising flat backs as a feature of new plane blades does not understand the geometry of worn blades and is dooming their customers to dull blades and planes that don't work as well as they could.
• If you sharpen your plane blades like I do, stropping can only dull your blade. Put another way, if stropping helps then you should think about changing the way you sharpen.
• I suggest four bevels on a plane iron, when many people say two is enough.
• I urge you to try back bevels, when no commercial jig allows you to do this easily.
• I claim my shop-made jig is better than any commercial jig.
• I claim that inexpensive sheet abrasives are better than the best waterstones or diamond paste.
• I say you should use a jig, while so many say you don't need one. In fact, I say that using a jig will reduce the time you spend sharpening.
• I say all tool steels, from O1 to M2, can be brought to the same initial sharpness, while replacement blade sellers still make claims about superior edge sharpness of O1 steel. May 2007 - new results on initial sharpness.
• I say you can (and perhaps should) use a bench stone to grind the primary, when others depend on various powered grinders.

Many of these claims go against the conventional wisdom. Many contradict the claims of respected tool makers. Can any of them be right? Worse yet, can all of them be right?

Consider one example I just (Apr 2007) noticed. In some of my first plane blade tests (spring 2002) I discovered that A2 blades were only slightly more durable than high-carbon steel blades (but usually wore unevenly - see the blade tests). Lee Valley claimed in their 2001-2002 catalogue that "we found these A2 tool steel blades held a keen edge five times longer than both OEM blades and high-carbon steel replacement blades". In their May 2007 catalogue they say: "our A2 blades will hold an edge longer than most high-carbon steel blades". Quite a change of opinion, especially since the earlier opinion was based on their own testing. I still think that A2 blades last a little longer than any O1. However, you have to use larger included angles which may adversely affect the quality of the planed surface.

All of my opinions are backed by test data, and usually by photomicrographs taken during the test. None rely on unreported tests or conventional wisdom. None are based on commercial interests.

So, you can disregard my claims for variety of reasons, but not because they are wrong. In particular, you can continue honing without a jig on your favourite piece of stone and continue to think you are getting great edges. In fact, you are not. All irons honed without a jig have inferior edges. All plane irons honed without a back bevel have inferior edges.

Set aside your scepticism. Take a little time. Make the jig. Buy the abrasive sheets. Make the sharpening station. Try it out. Make the bench stone vise. Try hand grinding primaries. Send me an email if you have any problems, questions, or contributions!

I believe all of the above claims will be conventional wisdom in a few years. Back to the top.

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