The following is part of a thread on Mortise Chisel Cross Sections
specifically, square vs trapezoidal. Jeff Gorman is a very knowledgeable
and experienced amateur woodworker and Jim Wilson is both a tool maker
and a woodworker. Both provide good reasons for their personal preference.I'm in the Jim Wilson square or rectangular cross section school.
From - Sat Jun 22 12:49:31 2002
Path: sn-us!sn-post-01!supernews.com!corp.supernews.com!not-for-mail
From: jwilson@paragoncode.com (Jim Wilson)
Newsgroups: rec.woodworkingOn the topic,
>: There is some debate over whether the trapezoidal cross-section is
>: truly superior to a rectangular one.
Jeff Gorman submitted:
>Mode 1. You start the mortise with a stab cut that creates a void. If you
>start the next cut with the bevel facing away from the void, the chisel will
>ride on the bevel and move bodily forwards. The arrises between the
>chisel face and its sides form a pair 90deg cutting eges, acting as
>scrapers that should help to clean the mortise sides as they
>move. The relief acts in the same way as the clearance angle of a
>plane. This should be the theory, but, wood being wood, most of the
>action is a splitting action.
Good description. I call this mode "bezel out" chopping. I think
another reason practice doesn't meet theory here -- and in my
experience, it truly doesn't -- is that the arrises between the chisel
face and the sides are not generally sharp, although they can be made
so.
Still another is that the relief provided is only perhaps two degrees
at most, measured normal to the face, and often less. At steeper
chopping angles, the angle is slightly reduced. In my opinion, two
degrees is just not adequate clearance for a wood-cutting edge.
>Mode 2. If the bevel faces the other way, the chisel itself does not move
>other than downwards, the bevel arrises act as scrapers as the chips are
>forced backwards. In theory, the relief formed
>by the trapezoid reduces the scraping action.
Bezel "in." I haven't heard the point stated this way before, but I
imagine the trapezoidal cross-section advocate would say that the
effect is to reduce the tendency of the chisel to bind in the cut. The
square-side advocate would say that the reduced scraping would lead to
raggier sides. Is that right?
>Conclusion? Trapezoidal chisels OK for Mode 1. Snag: get the chisel face
>slightly askew and the chisel drives sideways and forwards, cutting into the
>mortise flanks.
This is the primary argument that I have heard against the trapezoidal
cross-section.
The other argument against the trapezoid is that the relief allows the
chisel to twist more readily than a square-sided blade, making it
harder to get square cuts.
In practice, however, I have not seen significantly more of either
problem with either type of chisel. It appears to me that the grain of
the wood has a greater effect on this problem than the blade geometry
does.
>However, since most of the action of forming the mortise flanks involves
>splitting, the question might be more academic than real.
I am inclined to agree, but as a toolmaker, I want to remain open to
arguments from both sides, in case a compelling one should arise. If I
ever find a true advantage of one over the other where it counts -- on
the workbench -- then I will eagerly hop off the fence. Until then,
Occam's razor will serve, and my chisels will have square sides.
>Forming really clean mortises by chopping, and I suspect hollow
>chisel mortising, is quite difficult.
>
>Boring and paring might be better, but what a tedious fag it is!
Agreed, completely, on both points.
Thank you for your excellent comments!
Side note: apologies for any confusion in terms. I try to use "bezel"
for the sloped surface of the cutting edge, instead of "bevel," to
distinguish it from the bevels that are present on the sides of some
chisels.
Jim