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Centre of Effort - Part II

I am going to copy here a response made to Warren at WoodCentral, and expand on it. Warren queried the handle design I made for the HNT Gordon Trying Plane, compared it with one of his own. This prompted me to write the thoughts that had been swirling in my head.

Warren wrote ...

I have a beech trying plane that I have used on almost every board for 35 years. It is 22 inches long, the mouth is 7 inches from the front and the angle of the tote is somewhere around 65 degrees. I think it is a good design. I would say that you want a fluid wrist rather than a rigid wrist; the wrist angle changes as the stroke progresses. I think that for the least stress on the joints they all should be fluid, not trying to hold something rigid throughout the stroke.

The new design looks awkward to me. If someone had a large body of work with a traditional design then tried something new for a few years and liked it, it would be worth noting. However using an altered design for a very short period of time does not give much information.

After reading this post I compiled a set of measurements about several planes, which are meaningful to me. Hopefully also to others. While I do not mean to suggest that they should create rules for the design of a plane handle - since I do agree with you that the wrist changes as it pushes a plane ... and that it changes (or needs to change) position and angle as the conditions change - there is a pattern that is recognisable.

I'll start with a comment about the new handle for the Trying Plane. It may look awkward, but it works. The question is "why does it work when the previous handle - so close in angle to the one you like (above) - did not work at all?".

One answer to this is simply that the handle angles work for the respective plane designs. Your plane has a handle of around 65 degrees (which is the same as Stanley), while the Trying Plane handle is similar to a Veritas at about 75 degrees. Your trying plane's toe/mouth is 32% of the length of the plane.

Here are measurements of other planes I have:

The first statistic of relevance is that, with the exception of the Trying Plane and Jointer, all planes have a mouth/toe percentage around the 35% mark, which is similar to yours.

The exceptions here are the HNTG Trying Plane and the Jointer, are similar in having a significantly higher percentage (the mouth is relatively further back), with the Trying Plane even further back than the Jointer.

One inference that may be drawn from the above is that the longer the toe/mouth percentage, the more the plane will benefit from a horizontal force vector, while the shorter the toe/mouth percentage, the more the plane will benefit for down force when planing.

The second statistic is the height where the handle is held relative to the length of the body. The lower planes (Jack, Jointer, Trying Plane) all have a low centre of gravity. They appear to work more efficiently with a low centre of effort (forward vector). The relatively higher planes require more down force. Note that the Krenov smoother (made by JK) has a 45 degree bed, while the lower coffin (shopmade) has a 55 degree bed. It requires significantly more downforce to push than the Krenov. A higher body (3" is common) would be expected to require even more downforce.

My thought at this stage is that Stanley make a handle that has the best of both worlds. It can be held at the upper end, where it imparts downforce. It may also be pushed by the heel of the hand when the vector is horizontal. Some planes, however, may benefit from one extreme or the other (in some cases substitute the hand for a handle).

I find no writings that explain how to use the Stanley handle .... other than to hold it in a three-finger grip and rest a forefinger on the frog. Well, I found that sometimes this is the way it is done but, depending on the effort needed to push the plane (size/weight, depth of cut, hardness of wood, sharpness of blade) the way we hold the handle can change. The hand can slide down the handle and push with the palm as well. This lowers the centre of effort. For all I know it may lighten the pressure on the blade as the vector changes to horizontal from diagonal.

At some stage there is a change in the task, and this results in a change in the method. For example, planing hard woods is likely to alter the way we hold a smoother, not only because many require a higher cutting angle which would increase resistance, but the blade simply cannot enter the wood the same way as when planing softer woods. Where higher angles are used, the preferred push will be forward rather than down. To reduce friction of downforce, planes with low gravity and low centre of effort come to be preferred. The HNT Gordon planes are one example. The Veritas and LN BU range are another. I have written before that BU planes are easier to push than BD planes for the same cutting angle (and I use Veritas planes as my source since I do not own BU LN planes for comparison. Note, however, that LN handles are slightly more vertical than those of Stanley. LN are around 69 degrees). I now believe that this ease in pushing has partly to do with the way the handle orientated the vector of force. The more upright handles of Veritas particularly suit this style of plane. However that is not to say that the Stanley does not - it does the same job if pushed from low down.

Lastly, I write this to provoke thought in new planemakers. We may believe that planemakers of Olde ironed out the design issues and that we should seek to emulate their planes. We certainly can learn from history but we are condemned to repeat the mistakes or shortcomings as well if we fail to understand what is good and what is not, and why. I see this frequently in the planes made and proudly posted on the forums. Many are look-a-likes, photocopies of photocopies, made without much understanding about the way the parts function together, and then there is puzzlement when they do not work as planned. This thread is an attempt to stimulate thought so as to understand what and why they build, how they can improve and modify what they have, and thereby how to use a plane effectively. 

Regards from Perth


March 2014