26
Knives; Planing/Moulding with LOGOSOL
PH 260
This section contains important information about your planer/
moulder and the knives that you mount in it.
The manufacturing process of Logosol’s moulding
knives
Both standard and custom-made knives are manufactured with
a technique that is called wire cutting. A 0.25 mm thick brass
wire with pulsating high voltage is fed through the steel. It makes
an exactly 0.32 mm wide cut. The accuracy is within 0.003 mm,
which makes the edge of the knife really sharp. The entire knife
is made in one single computer-driven process. The advantages
of this are many.
1. The repeatability is 100%. All proles become identical every
time they are made. This is also the case when it comes to
custom-made knives. If you want to place an order for the
same prole, it will be cheaper the second time, and you will
get an exact copy of the original prole.
2. It is just as easy to cut all qualities of steel. When it comes
to custom-made knives, this means that you e.g. can make
a prototype in alloyed tool steel and then order the same
prole in TCT.
3. The edge becomes stronger than one that is made of
traditionally ground steel. This is due to the fact that the
temperature effect, which changes the hardening of the
material, is less when wire cutting, thanks to the process
being made under water.
When choosing traditional grinding, which is the most common
method, you do not have these possibilities. A ground knife can,
however, feel sharper when it is new. This due to the fact that
in some cases the knife actually is sharper at rst, but also that
the edge has microscopic scratches and roughnesses. This
edge will, however, lose its sharpness since the material in the
top layer of the edge is damaged by the heat.
Grinding
When a knife loses its sharpness, or better, before you notice that
it starts to lose its sharpness, it can be ground sharp again. You
can either use a ne grindstone, a diamond grinding sheet or a
Tormek grinder with moulding knife jig (ref. no. 7010-000-1000
and 7010-001-1012). This kind of grinding is a cold process and
does not damage the material. The knife is ground on the at
side that lies against the chip breaker. This way the shape of
the prole is not affected. Normally, a knife can be sharpenend
as long as 90 % of its original thickness remains.
Which knife quality should be used?
SP, tool steel, also called carbon-steel, is hardened by heating
and cooling. SP is used when you are going to mould only a
small amount of boards. It is cheap, but can lose its sharpness
already after 50-300 linear metres (160-980 linear feet). Among
other things, it is the heat which is produced when moulding
that makes the knife dull. SP can stand 300°C (572°F) before
it is damaged. The cheap knives that can be found in various
catalogues and in hardware stores are made of tool steel and
should not be compared with HSS.
HSS, high speed steel or cobalt steel. In most cases, this is
the quality of steel that we recommend. It is about twice as
expensive as SP but, on the other hand, it keeps its sharpness
for at least 2000 linear metres (6560 linear feet), which makes it
cheaper in the long run. HSS can stand 700°C (1292°F) without
being damaged.
TCT, tungsten carbide, also called HW, cormant and carbide,
is the most durable type of knife. The hardness of the tungsten
carbide makes it brittle like glass, and it has to be handled with
care. Always place the knives on a soft bedding. TCT costs three
times more than HSS, but it stays sharp for upto 10 000-20 000
linear metres (32 800-65 600 linear feet). TCT can stand 1000°C
(1832°F) without being damaged. Tungsten carbide can only be
sharpened with wire cutting, i.e. by the same method they were
originally made, or with mechanical diamond grinding.
Some special cases:
Pine: Use HSS eller SP.
Hardwood: Use HSS or TCT.
MDF board: Use TCT.
Teak: Use TCT.
HSS 6% and 18%
HSS is available in two types of qualities. Traditionally, knives
consisting of 6 % tungsten carbide have been used for wood
processing, and knives consisting of 18 % for metal processing.
The trend goes towards having 18 % of tungsten carbide
in all cutting HSS tools. Logosol has for a long time used
planing knives consisting of 18 % tungsten carbide and, due
to customers asking for this, we are gradually changing to 18
% in the side cutter knives too. We have for a long time been
opposed to this, since it does not have a signicant effect on the
durability of the edge, on the contrary it makes the knives more
brittle. When mouldings with deep proles are being made, it
can be necessary to make the knives 5.5 mm thick instead of
4.4 mm. The harder knives are also harder to grind by hand.
However, when processing hardwood an increased life of the
knife can be noticed due to the fact that the knife stands higher
temperatures. In certain contexts the two qualities are called M2
and M4, where the latter is the harder material.
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