Why a coffin shape?

[Mr Ed]

Just read an interesting discussion in this blog about why a smoother would be coffin shaped;

http://www.fullchisel.com/blog/?p=256

Anyone have other opinions on the derivation of the shape of the coffin smoother?

Cheers, Ed

 

[Philly]

Ed
There is a thread over on Woodcentral on this very subject at the moment. Personally, I feel the shape is down to comfort - a rectilinear shaped small plane is tough on the hands. And to turn it into a coffin shape is pretty simple.
Larry Williams has a theory about the shape helping the moisture content maintain equilibrium - sounds good, but I'm pretty sure its just for comfort.
Hope this helps
Philly

 

[Mr Ed]

Well much as I like the idea that the curve allows the differing thicknesses of timber to behave correctly with changes in moisture I think you're probably right and it is a comfort thing. I suspect the shape has then been perpetuated for aesthetic reasons because it does make for a good looking plane.

Cheers, Ed

 

[Digit]

I'm not sure about the moisture bit but personally I find the shape anything but comfortable.
I always make mine with a full tote.

Roy.

 

[Wiley Horne]

Hi All,

An extension of this thought is why was the coffin shape carried forward into infill smoothers (e.g., the A5). Ed's sense that aesthetics govern may be right. The infill being a heavier plane, however, the coffin shape changes the balance. The A5 has a center of gravity which is closer to the tote than an A6, and this can be felt by the user.

I still kinda bet on aesthetics though, in regard to the coffin infill.

Wiley...anwering a question no one asked

 

[lwilliams]

...Larry Williams has a theory about the shape helping the moisture content maintain equilibrium - sounds good, but I'm pretty sure its just for comfort...

It's been a long time since I wrote that Phil. I'd go back and read it again but it's kinda boring and I really need to rewrite and update it. I'm pretty sure that I said it probably started out as an attempt to make a smooth plane more comfortable to use.

The coffin shape does help to keep the toe and heel of the sole coplanar. If one looks at a triangle with one point down, it'll be very much like the cheeks of a smooth plane. If you change only the height of the triangle while keeping the top the same length, you'll change the angle of the point at the bottom. That's pretty much what having the thin cheeks in a wooden plane will cause and changing the angle will cause the toe and heel of the sole to be at different angles. Exposing the end grain on the sides will actually cause the whole side to change dimension. This keeps the toe and heel coplanar, the wood is plastic enough to let this happen.

I think they realized that the more comfortable coffin shape was also more shape stable and required less tuning. I know it works, I just don't know how many hundreds of years ago it was discovered.

 

[Philly]

Thanks, Larry.
"I'd go back and read it again but it's kinda boring and I really need to rewrite and update it
I think you're being too modest :wink: Here's the link to Larrys article...

http://www.planemaker.com/articles_tuning.html

Cheers
Philly

 

[toolsntat]

Less surface area = Less arm ache :idea:

Andy

 

[Chris Knight]

Less surface area = Less arm ache :idea:

Andy

I think this is the least convincing reason. A smoothing plane is not used for lengthy periods, it is for finishing a surface that has been prepared using bigger, heavier planes that might well have been used for long periods - and they are not coffin shaped..