bench planes – sharpening angles

Once you have established the rough shape of the bevel (see previous post) the next step, referred to as honing or whetting, involves rubbing the iron against progressively finer abrasives until you are satisfied with the sharpness of the edge.

Before we get going we’d do well to consider the various angles we’ll be working with.   As we established in an earlier post the basic principle involved in sharpening a plane blade is that the steeper the pitch of the blade presented to the wood the harder the plane is to push, and the narrower the angle at the tip the weaker the edge will be (making it more likely to chip or fold over).

On that basis you might imagine all plane blades would have a big fat bevel and be presented at a very low angle, but as usual things are a little more complicated than that.

All About Angles

bevel down

 

Assuming you have a Bailey style plane then the blade will be fitted “bevel down” (with the bevel towards the wood) as shown in the picture above .  The blade is made of a thin piece of steel which is the same thickness along its length, and this means that the angle the blade is presented to the wood is determined by the angle of the bed it is on.   The bed in a bailey style plane is called the “frog” and this is manufactured with a bed set at 45°.

The only way to increase the angle of attack for a bailey style plane is to change the frog or to create a “back bevel” 1)creating a bevel on the flat face has the same effect as raising the bed angle, in as far as the angle the metal is presented to the wood is increased – this angle is sometimes called the effective presentation angle.  You can see in the illustration above that creating the steep back bevel shown would increase the effective presentation angle significantly – c.f the dotted pencil line .   Why does this matter?

Once the plane blade starts to cut into the surface of the wood, the shaving rides up the blade.  As the pitch increases the shaving is forced to bend more sharply and the fibres in the  shaving are more likely to break early and, thus weakened, are less likely to lever out the other fibres they are attached to.  In other words, raising the pitch of the blade can help reduce the problem of tear-out.

The effect of the pitch of the blade on tear-out is well known and there are special names given over to the common angles used in bench planes:

pitches

Charles Holtzapffel Turning and Mechanical Manipulation, p482.   

The downside to higher angles is that the steeper the pitch the harder it is to push the plane.   45 degrees (“common pitch”) is a good compromise between the effort and tear-out prevention, and this is reason Leonard Bailey used this angle when designing his metal planes.

From the above it should be clear that, for bevel down planes, the angle of the bevel makes no difference to how the plane cuts, since the pitch stays the same no matter what angle you use.   There are a couple of other considerations that are relevant to the bevel angle though:

Relief angle

The relief angle is the angle between the bevel and the work-piece – generally it is not very important in bevel down planes, but wood fibres compressed by plane body and blade have a tendency to spring back up after they have been cut and, if the relief angle is insufficient, then the fibres will push against the bevel, and this upwards pressure can cause the plane to skid along the surface of the wood instead of taking a consistent cut.

There are many factors impacting the extent of this”spring-back” effect, including the type of wood, the grain orientation and the sharpness of the blade, but the general consensus seems to be that in ideal circumstances you can get away with very small relief angles (say 5 degrees) but that you should aim for a relief angle of at least 10-15 degrees to allow for the less than ideal conditions you will encounter now and then.

The other factor to consider when honing your blade is the the strength of the edge – the larger the bevel angle the more metal there is behind the edge and the stronger it is.

Once again it is necessary to find a compromise – in this case between allowing a sufficient relief angle and blade longevity – and the compromise adopted almost universally for bench planes is to aim for a honing angle of around to 30 degrees2)creating a relief angle of 15  degrees or thereabouts.

A note on bevel-up planes

We’ve been looking at planes where the bevel  faces downwards, but there is a tradition going back hundreds of years of planes where the blade is oriented with the bevel facing up.  In the past few years modern manufacturers have introduced new bevel-up planes that have become popular as an alternative to the standard bailey planes we’ve been looking at here.

The effective presentation angle of a bevel-up plane is the angle created by the bed + the angle of the bevel combined:

bevelup

bevel up

As a result most bevel-up planes have a frog that is at a low angle, typically 12 degrees 3)in fact their historical origins can be traced back to planing tasks that benefited from a low angle of attack – more on this in another post.   If the blade is honed at around 30 degrees the resulting pitch is therefore 42 degrees – close to the so called common pitch used in Bailey planes.

Fans of bevel-up planes point out that, because the effective pitch is determined by the honing angle of the blades, these planes are particularly versatile: you can have one plane body and keep several blades honed at different angles for different purposes.

For instance, 30 degrees for most jobs where you want a good balance between effort and edge retention; and higher angles that can be used for wood that is difficult to plane without tear-out.  4)it should be noted that some of this versatility can be achieved in a standard Bailey plane by putting a bevel on the flat face of the iron however, when I have read about this technique being used it has generally been described as a get-out-of-jail-free card to be used when you have no other option, rather than the “proper” practice (which would be to reach for a plane with a higher bed angle).  It should also be obvious that the bevel-up plane can do something that is impossible on a Bailey, namely that it can be honed with a low angle to create a pitch less than 45 degrees.  This can be useful in some applications, like planing end-grain.

Conclusions

and so after all this rambling we now understand that bevel-down planes are typically honed at about 30 degrees because  it is a good compromise between edge retention, durability and the need to maintain a relief angle.  We also learned that it is not important to get the angle exactly right – roughly 30 degrees is just fine.

Finally, we learned that higher pitches are harder to push but less prone to tear-out.   In the next exciting episode we get on with honing!

References   [ + ]

1. creating a bevel on the flat face has the same effect as raising the bed angle, in as far as the angle the metal is presented to the wood is increased – this angle is sometimes called the effective presentation angle.  You can see in the illustration above that creating the steep back bevel shown would increase the effective presentation angle significantly – c.f the dotted pencil line
2. creating a relief angle of 15  degrees or thereabouts
3. in fact their historical origins can be traced back to planing tasks that benefited from a low angle of attack – more on this in another post
4. it should be noted that some of this versatility can be achieved in a standard Bailey plane by putting a bevel on the flat face of the iron however, when I have read about this technique being used it has generally been described as a get-out-of-jail-free card to be used when you have no other option, rather than the “proper” practice (which would be to reach for a plane with a higher bed angle).  It should also be obvious that the bevel-up plane can do something that is impossible on a Bailey, namely that it can be honed with a low angle to create a pitch less than 45 degrees.  This can be useful in some applications, like planing end-grain.

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