EFFECT OF KNIFE CHIPBREAKERS ON SURFACE FINISH (VIDEO)
Chutaro Kato
Professor
Yamagata University, Yamagata,
JAPAN
ABSTRACT
A video was made to show how a chipbreaker
(cap-iron) functions during orthogonal cutting of wood. A video
tape of the cutting action was made using a microscope and video
camera. The cutting action is almost the same as that produced
by a Japanese hand plane except for the very low cutting speed.
The tool angle is 30ﾟ. The clearance angle is 10ﾟ. The edge angle
of the chipbreaker is either 50ﾟ or 80ﾟ. The depth of cut is either
0.05 mm or 0.1 mm. The inclination angle of grain to the cutting
direction is either +7ﾟ or -7ﾟ. The negative number of the inclination
means that cutting is against grain, which results in a high probability
of chipped or torn grains. This video shows exactly how the chipbreaker
prevents the generation of the chipped or torn grains in cutting
against the grain.
INTRODUCTION
Wooden studs with a 5 in × 5 in cross section
are frequently used in the construction of typical Japanese houses.
The faces of the studs are exposed on both the inside and outside
of the rooms in the house. The surfaces are usually finished by
cutting with hand planes, are exceptionally smooth, and are kept
natural without paint. A wooden stud whose surfaces are not coated
with paint adjusts to the humidity inside a room so well that
an individual can live comfortably. In order to get the fine surfaces,
Japanese-type hand planes are used by carpenters. Recently, a
woodworking machine which has the same cutting principle as a
hand plane was designed. This machine is called a super-surfacer
in Japan. Super- surfacers have replaced the hand planes to reduce
the labor intensive nature of the finishing process. Surfacers
have a very sharp cutting knife that produces exceptionally thin
chips (< 0.1 mm). The wood cells must be cut cleanly without
crushing them in order to get the fine surface finish. Japanese
middle school curricula have technology as one of the required
subjects, which includes woodworking. A hand plane is used as
a teaching tool for making some small wooden products such as
a book shelf. This video was made for the purpose of helping persons,
including middle school students, understand wood cutting principles
so that they can master woodworking technology efficiently. PROCEDURE
FOR TAKING VIDEO This video demonstrates how a chipbreaker (cap-iron)
functions during orthogonal cutting of wood. A video tape of the
cutting action was made using a microscope and video camera system
as shown in Figs.1-3. The cutting knife was fixed to the vertical
axis of a plain milling machine (Fig.1). The workpiece was set
on the table feeding slowly at a fixed speed. The depth of cut
was set by adjusting the height of the table. The knife consisted
of a tool and a chipbreaker (Fig.2). The position of the chipbreaker
was adjusted by turning a micrometer screw until its edge came
to one of the predetermined position marks. The position marks
were made on the rake face of the tool with Vicker's micro-hardness
tester at the distances of 0.1 mm, 0.2 mm, and 0.3 mm from the
tool cutting edge (Fig.3).
Fig.1. Video camera and microscope system.
Notes: 1: workpiece , 2: table of milling machine, 3: displacement meter,
4: microscope, 5: video camera, 6: lights
Fig.2. Cutting knife and micrometer adjustment.
Notes: 1: tool, 2: tool holder, 3: chipbreaker (cap iron), 4: screws
to make chipbreaker edge parallel to the tool edge, 5: micro-meter
for adjusting chipbreaker position.
CUTTING CONDITION
The cutting action is as shown in Fig.4. The tool angle (wedge
angle) is 30ﾟ and the clearance angle is 10ﾟ. The edge angle of the
chipbreaker is either 50ﾟ or 80ﾟ. The depth of cut is either 0.05 mm or 0.1
mm. The inclination angle of grain to cutting direction is either +7ﾟ or -7ﾟ.
The negative inclination angle means that cutting is against grain. This
produces a high probability that chipped or torn grains will occur.
Fig.3. Cutting action.
Notes: θ: 50ﾟ and 80ﾟ, γ: +7ﾟ (cutting with grain) and -7ﾟ (cutting
against grain), D: 0.1, 0.2, and 0.3 mm, t: 0.05 and 0.1 mm, cutting speed: 0.4 mm/s,
wood: Hoonoki (Magnolia obovata Thunb., moisture content: 10 %, specific gravity: 0.45)
SAMPLE PICTURES
The nine combinations of cutting conditions were recorded on
the video camera system and are shown in Fig.4-Fig.12.
Fig.4. The knife without a chipbreaker is cutting
wood with the grain. The depth of cut is 0.1 mm. This cutting
condition produces a good surface finish.
Fig.5. The knife without a chipbreaker is cutting
wood against the grain. The depth of cut is 0.1 mm. The chip forms
by cleavage failure which extends into the workpiece and below
the plane generated by the path of the cutting edge. As the result,
this type of failure produces a machining defect commonly called
chipped or torn grain.
Fig.6. The knife without a chipbreaker is cutting
the wood against grain. Chipped grains are not generated and surface
finish is good. Only the depth of cut is now changed from 0.1
mm to 0.05 mm.
Fig.7. The knife with a chipbreaker is cutting
wood against grain. The depth of cut is 0.1 mm. The chipbreaker
is set at the farthest position of 0.3 mm from the cutting edge
and has an edge angle of 50ﾟ. When the chip is sliding along the
chipbreaker face, frictional forces must be generated between
chip and chipbreaker, which tends to reduce the amount of cleavage
fracture. However, the action is still not good enough to eliminate
chipped or torn grains completely.
Fig.8. The chipbreaker whose edge angle is 50ﾟ
is set at the position of 0.2 mm from knife cutting edge. The
depth of cut is 0.1 mm. The effect of the chipbreaker is to reduce
the occurrence of chipped and torn grains considerably.
Fig.9. The chipbreaker whose edge angle is 50ﾟ
is set at the closest position of 0.1 mm from the knife cutting
edge. The depth of cut is 0.1 mm. Initially the chip thickness
varies but then stabilizes after a short time. Neither chipped
nor torn grains occur. The surface finish is good.
Fig.10. The edge angle of the chipbreaker is changed
to 80ﾟ, which is the largest used. The chipbreaker is set at the
position of 0.1 mm. The depth of cut is 0.1 mm. Neither chipped
nor torn grains occur. Deformation of the chip, however, extends
below the plane generated by the path of the cutting edge. The
chipbreaker position is too close to the knife cutting edge. The
surface finish is good.
Fig.11. The chipbreaker is set at 0.2 mm, which
is the mid-position from the knife edge. The chipbreaker has the
edge angle of 80ﾟ. The depth of cut is 0.1 mm. The chipbreaker
is working well. Chipped grains are not generated and surface
finish is good.
Fig.12. The chipbreaker is set at 0.3 mm, which
is the farthest from the knife edge. Deformation of the chip against
the chipbreaker is reduced and serious cleavage failure of the
chip does not occur. The surface finish is good.
CONCLUSION