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Blade angles
This is the information about blade angles that I promised some 12
months ago. I was delayed while I tried to get a response from the
author of an article in Fine Woodworking magazine on the subject of
Blade angles, which from memory came out in late 2006. In any case I
was unable to get a response so I have decided to give you this
information in the interest of getting good results with your plane(s).
The Blade Angle article left me feeling somewhat disappointed in that
not all the information on the subject was presented. In particular
the pros and cons of sharpness angle was totally ignored and I feel
this could leave the reader having problems planing hard cranky woods
if they were to take the advice of the writer, hence my desire to
point out to you some of the missing bits. In essence the article
offered some good advice on the blade pitch you should use for certain
woods so that you minimise the chance of the wood tearing out. It
discussed using a bevel up plane that has a bed angle of 12 degrees
and then sharpening the blade bevel at higher angles to suit woods
that are prone to tearout. It also talked about putting back bevels on
planes that use a bevel down configuration for the same reason of
getting better results on cranky woods. Whilst this was correct
information, what the writer omitted was to discuss the problems which
become inherent in using a blade which has a bevel angle (Sharpness
Angle) greater than the accepted norm of about of 35 degrees maximum.
History books and manuals about woodworking will tell you to sharpen
your bevel at the lowest angle possible commensurate with an ability
to not chip the edge in use. This is because the finer the edge is the
less stress you put on the wood, plane and user, plus your blade will
have and edge that is useable for longer. That being said I do have to
admit that with better sharpening systems we do have the ability to
push the limits on the bevel angle slightly beyond what the old timers
set. However, I feel this article has gone too far by recommending
increasing bevel angles to up to 50 degrees to give you an effective
blade pitch of 62 degree on a plane with a 12 degree bed angle without
explaining the consequences of doing this. I don’t have sophisticated
testing equipment to give you the exact maximum bevel angle that
should be used before you start to run into the problems of
penetration and reasonable edge retention but from my experiments and
experience with planing I recommend 30 to 35 degrees for a plane bevel
and would definitely not go over 40 degrees on the bevel.
There is a chap called Steve Elliot who has done some work on the
optimum bevel angle and his site is worth looking at
www.bladetest.infillplane.com
Similarly if you use a back bevel on a plane to increase the effective
blade pitch you will also be increasing the bevel angle on the blade
which will have the same consequences. For example if you have a blade
with a standard 30 degree bevel fitted to say a bailey pattern plane
with a 45 degree bed angle and then hone a 15 degree back bevel on it
the overall bevel angle on the blade becomes 45 degrees and you will
find that you will have edge retention and penetration problems like
the example before.
As recommended in the article to plane tropical and Australian
hardwoods I honed a very nice 50 degree bevel on a quality blade and
fitted it to a quality plane which had a 12 degree bed to give me a 62
degree effective blade pitch. I could shave with this blade so it was
sharp. I planed about 20 lineal feet on a piece or ironwood which is
an Australian hardwood and it took some nice shavings with only a
little bit of tearout. Then as expected the edge quickly started to
get blunt and the blade had trouble engaging the wood causing the
plane to skip and not take a full shaving. I had to apply excessive
downward pressure to get any sort of shaving and the plane became very
hard to push. The surface finish was poor as a consequence. I then
used a plane with a 60 degree bed and the blade bevel sharpened at 30
degrees in a bevel down configuration. The initial passes produced the
same results as the other plane but it just kept on planing with no
problems at all. I had no problems with edge retention or penetration.
Nor did I need any excessive pressure to keep the blade engaged and
the plane was easy to push. I took many more passes with good results.
Another area of blade angles that was neglected in this article was
the virtues of using a scraping plane for hard cranky woods. A scraper
plane has and effective blade pitch of around 85 to 95 degrees and
with some hard woods (from my experience) this is the only way to get
a good tearout free finish.
Lastly this writer had a glossary of angles and the “effective cutting
angle” description is incorrect and will mislead you. I will explain
why. If you refer to say “World Woods in Colour” which is book
describing the properties of lots of different woods, in there you
will come across comments like “ if you are machining this wood reduce
your cutting angle for better results”. What this means is increase
your blade pitch which in turn reduces your cutting angle, as cutting
angle is measured from a 90 degree line drawn from the surface being
planed. For example in a bevel down plane with a 60 degree bed you
have a 30 degree cutting angle. Or in a bevel up plane with a 12
degree bed and a 30 degree bevel on the blade, which gives you a 42
degree blade pitch and a 48 degree, cutting angle.
So in simple terms if you increase the blade pitch (sometimes referred
to as blade angle) you reduce your cutting angle. Refer to 2 drawings.
In conclusion, I think this is a poor article because of what was left
out and the use of incorrect terminology. As woodworkers learning more
about woodworking, I can only encourage you to be careful about what
you read, and certainly if you come across future articles on blade
angles and they don’t cover the pros an cons of changing the bevel
angle (also referred to as sharpness angle) or the virtues of scraper
angles I would leave it well alone.
When I need technical information about blade angles I use a book
called Understanding Wood by Bruce Hoadley. If you want to learn more
about this subject then I highly recommend this book. It also contains
other very good information on all aspects of woodworking.
Another book which incorrectly explains cutting angles is “The Hand
plane book by Garrett Hack.
The following is my advice to you on blade angles and planes in
general:
No one plane can do it all, they all have there limitations and one
plane cannot give you the best results on all woods as wood is too
variable, and that is why we love it.
When you are buying a plane or selecting a plane to do a certain job
think about the wood being planed and study the properties of the
wood. Here is a guide if you don’t have access to a book with wood
properties or you are not sure how to interpret them.
For soft straight grain woods the best finish will be with a plane
that has a blade pitch at 37 to 45 degrees.
For medium density woods that have cranky grain a plane with a blade
pitch of 50 to 55 degrees will be best suited.
For cranky hardwoods a blade pitch of 55 to 65 degrees will give good
results most of the time. If you get tearout at these angles then you
will have to go to a scraping angle of 85 to 95 degrees.
If you select a blade angle to suit the wood you are planing and you
get tearout this is telling you to go to a higher blade pitch. On the
other hand if you are planing wood and the surface feels fury then it
is telling you to go to a lower blade pitch.
These are all generalisations but I have found they have worked well
for me over the years.
I always sharpen my plane blades with a primary bevel of 30 degrees
with about a 2 degree micro bevel. I never ever have problems and you
get good at sharpening because it is always the same. I would never
sharpen a blade bevel beyond 35 degrees to achieve a higher blade
pitch in a bevel up plane. Of note, I do sharpen a few chisels at 45
degrees which I use for chiselling the end grain on super hard woods
and this is to stop the edge chipping but I don’t have wood
penetration problems as described before as I am using a mallet to
drive the chisel.
I hope this helps explain a few things if you happened to read the
article I referred to. And I’m always happy to clarify any point if
you are not sure about it. |
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