Formulae and Tables - Wood / textbooks for vocational training (GTZ, 122 p.) 6. Basic Terms of Cutting (introduction...) 6.1. Faces and Angles on the Tool 6.2. Directions of Cutting 6.3. Cutting Speeds 6.4. Cutting-Edge Dulling and Cutting-Edge Wear

Formulae and Tables - Wood / textbooks for vocational training (GTZ, 122 p.)

6. Basic Terms of Cutting

The science of cutting deals with the processes, laws and connections for chip-forming working with cutting tools.

6.1. Faces and Angles on the Tool

Term Representation	Symbol	Definition
primary cutting edge faces on the tool - saw tooth	HS	line of cut between flank and tool face
secondary cutting edge faces on the tool - milling tool	NS	cutting edge adjacent to the primary cutting edge
tool face faces on the tool - drilling tools	Sf	face on the cutting wedge on which he chip is removed
flank	Ff	face on the cutting wedge facing the area of cut produced on the work-piece
flank of the drill point	Hf	face on the tool next to the flank
comer	E	point on the tool at which primary and secondary cutting edges meet
tool orthogonal clearance angles on the tool - planing tool	a	angle between flank and tool cutting plane (plane through the cutting edge)
tool orthogonal wedge angle angles on the tool - saw tooth	b	angle between flank and tool face
tool orthogonal rake	g	angle between tool face and a vertical to the tool cutting plane g = 90º -a -b
cutting angle angles on the tool - drilling tools	d	angle between tool face and tool cutting plane d = a + b
tool cutting edge inclination angles on the tool - drilling tool	l	angle between cutting edge and tool reference plane
point angle	e	angle between primary and secondary cutting edges
drill point angle	eB	angle between two primary cutting edges, also called face angle

6.2. Directions of Cutting

The cutting direction of a cutting operation is the direction of motion of the primary cutting edge referred to the grain direction of the solid wood or the board plane of plane materials of wood.

Cutting directions in solid wood	Cutting directions in laminated wood	Cutting directions in particle and fibre boards
A cross-cutting cutting direction vertically to the grain direction; smooth area of cut, crumbly chip, short tool path	b cutting direction vertically to the board plane; approximately like cross-cutting of solid wood	b cutting direction vertically to the board plane; rough area of cut, crumbly chip
B longitudinal cutting cutting direction parallel to the grain direction; rough area of cut, coherent chip, long tool path	a/B cutting direction in board plane, in the direction of the grain direction of the top layer; like longitudinal cutting of solid wood	a cutting direction in board plane; cutting only in the top layer, smooth area of cut, crumbly chip
C transverse cutting cutting direction transversely to the grain direction; rough area of cut, brittle chip	a/C cutting direction in board plane and transversely to the grain direction of the top layer; like transverse cutting of solid wood

6.3. Cutting Speeds

Term	Symbol	Definition
cutting speed	v	speed at which the cutting edge of a tool performs chip-forming movements in the workpiece
		v = d · p · n	in m · s-1
			d = diameter of the cutting circle of the tool
			n = tool speed
feed rate	u	speed at which the workpiece is fed to the stationary tool or the tool is fed to the workpiece clamped in place; unit of measurement: m · min-1

Figure 3 Graph of cutting speeds for circular sawing machines

Example:

Which cutting speed does a circular saw blade having a diameter of 400 mm reach at a speed of rotation of 3000 min^-1?

Solution:

Find the diameter on the lower line, go vertically upwards to the point of intersection with the diagonal for n = 3000 min^-1, from there read off the result horizontally on the left side: v = 62.8m · s-1

Figure 4 Graph of cutting speeds for fluting machines

Example:

A cutting speed of approx. 15m · s^-1 is to be reached; the tool speed is 6000 min^-1.

Which tool diameter is to be chosen?

Solution:

Find the value for v on the left side, find horizontally the point of intersection with the diagonal for n = 6000 min^-1, from there drop a perpendicular and read off on the lower line: d » 50 mm.

6.4. Cutting-Edge Dulling and Cutting-Edge Wear

The loss of the original keenness (dressed keenness) of the tool cutting edge and the outer comers in the process of cutting is called dulling, its result is called wear.

Causes of wear

Cause of wear	Effect of wear
Angles on the tool cutting edges
wedge angle	The cutting forces rise with increasing wedge angle. Therefore, it must be kept as small as possible (taking into consideration the necessary stability).
rake angle	If the rake angle is too small, the consequences will be the same as with a too large wedge angle.
clearance angle	Large clearance angles result in a smaller load on the cutting edge (less friction and lower temperature).
Cutting conditions
cutting speed	High cutting speeds have the effect of increasing the load on the whole cutting wedge. For economical reasons they are to be kept as low as possible.
cutting depth	Keep it as small as possible. Great cutting depths lead to increasing mechanical stress on the cutting edges.
Mechanical stresses
friction	Excessive roughness of the cutting edge (choice of the proper abrasive tool) results in increased wear at the cutting wedge.
impact load	Mainly at the beginning of cutting when the cutting edge penetrates into the wood for the first time; it results in the loss of the original keenness.
compressive stress	The pressure of the workpiece on the tool is increasing with dulling (sharpening in time is necessary).
Various kinds of stresses
thermal stress	The friction between workpiece and tool produces temperatures of about 800 °C at the cutting edge. This results in softening of the cutting wedge surface and increased abrasion of material (proper choice of the cutting-edge material of the tool is necessary).
electrochemical stress	The diluted acids in the wood cells form electrolytes. In connection with frictional electricity produced during cutting the cutting-edge material is dissolved by electrolysis.
electroerosion	Spark discharges occur through electrostatic charges during cutting as a result of which particules are torn out of the flank. This formation of craters (increased roghness) favours the mechanical wear.

Forms of wear

Form of wear Representation	Influences and measurable variables
tool-flank wear	a result of mechanical wear, thermal load and electroerosion; the wear-land width is the measurable variable. This mark characterizes the size of the regrind, because the cutting edge has to be set back during sharpening so far that the wear mark disappears; wear mark for steel cutting edges s 0.3 mm.
cutting edge-wear	caused especially by thermal and frictional stresses; the external radius of the cutting edge is the measure of the cutting-edge wear;
corner wear	caused by the influence of friction and temperature; with increasing dulling the comer wear rapidly rises;
tool face wear	Apart from friction (flowing off chip) and temperature there is above all the electrochemical influence that is at work. The resetting of the cutting edge is the measure of the tool face wear (recommended dimension » 0.15).
crater wear	special form of the tool face wear as a result of friction and thermal influence by the flowing off chip
measurable variables of cutting-edge dulling 1 crater wear, 2 cutting-edge reset, 3 wear-land width, 4 cutting-edge rounding

Development of the cutting-edge dulling

Figure 5 Graph of cutting-edge dulling

cutting wedge (dressed keenness) with the original cutting-edge angles a1, b1 and g1	cutting wedge (operating keenness) with the wedge angle b2 that has become larger by incipient dulling and the tool orthogonal clearance a2 that has become smaller and the tool orthog rate g2	cutting wedge (advanced stage of dulling) with b3 that has become still larger and a3 and g3 that have become still smaller

Dulling period of the cutting edge

Term	Symbol	Definition	Connections
tool life	T	pure operating time of a cutting edge between two regrinds	S = tool path Wt = path of cut per unit of time
tool path	S	distance travelled by the cutting edge cutting in the material between two regrinds	the tool path in connection with the tool life is an important parameter for the economical use of machine tools