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Technical Explanation

A.           Safety Instructions

I           Safety

Tools for machining of wood and plastics frequently function at high cutting speeds and, depending on their diameters, in high speed ranges.

ZMM Ltd tools are designed to the latest tool technology and are manufactured of specially selected materials. Our quality tools ensure:

  • Optimum protection against accident
  • High performance
  • Economy of use.

Our tools fulfill the safety regulations of EN 847-1:1997 and EN 847/2:2001.

 Depending on the application, different tools are purposed for:

  • Working with manual feed
  • Working with mechanical feed

II         Manual feed

Manual feed in the EN 847-1/2 refers to the holding and guiding of

-         workpieces

-         tools and portable machines

by hand, even if a feed device is used that can be swung or slid away but that is not interlocked with the tool drive, or even if a manually actuated tenoning attachment is used.

Tools for manual feed are designed to achieved certain safety aims:

  • to reduce the injuries on contact with a moving tool
  • to reduce the kick back of the workpiece.

ZMM Ltd tools for manual feed act in accordance with the applicable safety regulations defined for such equipment, in particular for the maximum projection of the cutting edge and the kickback standards. In accordance with EN 847-1/2 these tools are designated as “MAN”. They are marked with the recommended speed, e.g: n= 5 000-8 000 RPM.

Tools for manual feed can also be applied for mechanical feed.

III        Mechanical feed

Mechanical feed in the EN 847-1/2 refers to a feed apparatus for the workpiece or the tool that is incorporated into the machine and with which the workpiece and the tool are mechanically guided or held during machining.

ZMM Ltd tools for mechanical feed fulfill the regulation defined for such equipment. In accordance with EN 847-1/2 these tools are designated as “MEC”. They are marked with the recommended speed, e.g.: n max = 12 000 RPM

This speed range does not give any indication for the optimal speed for the use of the tool, which in general is lower.

Tools for mechanical feed (MEC) are not permitted to be used for manual feed.

B.                 General Information and definitions

I           Machine tools

A knife can only be used for woodworking, if it is built into a tool body. Therefore, the tool body is the holder of the tool knife. Depending on the purpose the tool is used on machines for cutting, planing, sawing or boring.

Depending on how the knives are attached onto the tool body, three diverse types on machines can be defined:

 

II         Solid tools ( Photo 1 )
Photo 1 

Solid tools are manufactured of one material. Tools of HS or low alloy steel do belong to this type of tool. The tools can be reground. As a consequence, the cutting circle is getting smaller, which  results in modify the profile on such tools. For this cause, there is a limit for regrinding. If this limit is reached, the tool has to be replaced. The variety of such tools comprises of cutters, drills and knives.
 

III        Tipped tools ( Photo 2 )
Photo 2

Tipped tools consist of a body which is “tipped” with knives. The knives are fixed permanently to the body by brazing, welding, soldering or gluing. The body is ,as a rule made of steel, the knives of HS, HW. The knives can be reground. As a result, the cutting circle is getting smaller, which can result in change of the profile on such tools. For this cause, a limit for regrinding is set. If this limit is reached, the tool has to be replaced. The range of such tools comprises of cutters, drills, knives and saw blades.
 

IV        Assembled tools ( Photo 3 )
Photo 3

Assembled tools consist of a body, knives and clamping elements, essential to fix the knives to the body. The connection between knives and body is therefore detachable. The knives can be replaced if used-up or broken. As a result, the cutting circle and the profile shape are remaining constant. The variety of such tools includes reversible knife tools, cutter heads  and knife spindles.
 

V         Tool set ( Photo 4 )
Photo 4

A number of individual tools are clamped together and are assembled to sets for more efficient work. Tool sets consists of several tools, bushing, spacers, pins, screws or similar connecting elements.

VI        Carrier body( base body)

The meaning of the carrier body is to guarantee that the knives are held with complete reliability and without dimensional variations under all working conditions. The service life of a tool, in particular one with reversible knives, depends determinedly on the quality of the carrier body. Depending on the particular stresses such tools are subject to, we use:

-         Steel, alloyed and unalloyed,

-         Light-metal alloys.

C.                Surface finish

The optical view of a good surface depends on two factors. First the cleanness of the surface, and second the undulation of the surface.

I           Cleanness of the surface

The following points are essential for the cleanness of the surface:

-         feed direction according to the wood structure  

-         cutting knife material

-         cutting knife geometry  

-         Pre-cut with direct influence of cutting speed and RPM.

II         Undulation of the surface

The next reasons are essential for the undulation of the surface:

-         feed rate per tooth with the direct influence of feed rate, RPM and teeth number.

-         Cutting circle

-         Concentricity.

D.                Feed direction

 
I           Cutting with the grain ( Scheme 1 )
Scheme 1

Results in a clean, flat surface through low cutting- and feed rate forces.

II Cutting against the grain ( Scheme 2 )
Scheme 2

Consequence is a unclean, raw surface, as the pre-cut of the wood is in front of the knife. High danger of tear-outs.

III        Cutting across the grain ( Scheme 3 )
Scheme 3

Results in a slight raw but clean surface. Comparatively good machining achievable.

IV        Cutting the end grain ( Scheme 4 )
Scheme 4

Results in a slight raw surface through tear-out of grains. The perpendicular cut-off of the grains requires elevated cutting- and feed rate forces.
 

V         Against the feed ( Scheme 5 )
Scheme 5

When machining against the feed, the cutting actions of the tool is against the relative feed motion of the workpiece. The tool knife enters into the workpiece shaving and pushing. The cutting creates a extended chip with rising thickness. Due to the badly chosen cutting forces the workpiece is pushed-up from the table and the grains may tear-out. This results in a bad surface. Due to the use of pre-cuts, the cutting forces and feed rate forces are reduces, ensuing longer tool life at lower knife pressure. In order to avoid accidents, tools for manual feed are only permitted to be used for against the feed machining.
 

VI        With the feed ( Scheme 6 )
Scheme 6

To be used only with mechanical feed. When machining with the feed, the cutting action of the tool is with relative feed rate movement of the workpiece. The cutting process creates short chips which are getting thinner. The cutting forces are pushing down the workpiece, and pre-cuts are almost impossible. Even with critical grain conditions, good quality surfaces can be achieved. Due to low pre-cuts, the knives have stronger strain, so they are blunted faster. With the feed operation is only allowed with mechanical feed.
 

 

E.                 Cutting edge materials

SP             Alloy tool steel

HL             High-alloy tool steel

HS             High-speed steel

HW           Tungsten carbide (TC)

F.                 Cutting Geometry

The knife geometry depends on the particular role of the tool, the material to be machined, and the quality of cutting tip.

I           Clearance angle (a)

If the clearance angle is 00, no cutting of the wood is achievable. The tooth back (or knife back) is rubbing over the cutting area. The size of the clearance angle in wood working is among 120 and 200. Usually, 150 is chosen.

II         Wedge angle (b)

The greater the wedge angle, the better is the resistance of the knife against use. Therefore, hard materials can be machined with great wedge angles, thereby necessary forces raise.

III        Hook angle (g)

The cutting angle indicates the position of the cutting face to the tool axis or tooth line.

The size of the hook angle for wood working is between 50 and 300.

IV        Shear cut angle (l)

Shear cut angle 00 (straight cut)

Shear cut angle one-sided (single shear cut)

Alternate shear cut angle (alternate shear cut).

The size of the shear cut angle depends mainly on the tool dimension and the function of the tool.



Angels on the cutting edge ( Scheme 7 )

  1. a
  2. a1
  3. a2
  4. b
  5. g
  6. l
  7. e
    1. cBevel angle

Scheme 7

G.                Pre-cut

In general, a pre-cut of the wood should be avoided during machining. This can only be achieved, if the cutting speed of the tool is higher then the pre-cut-speed of the wood. The pre-cut-speed of the wood is around 40m/s.

On a pyramidical assembled tool sets it is possible, that the cutting speed on the smallest diameter drops below /s. For such cases the pre-cut action can be reduced with the use of a chip breaker. 


 

H.        Cutting speed ( Table 1)

The cutting speed (Vs) is the way,
the cutting edge travels per second (m/s). 
It is calculated with the following formula:

Vs= (D x Pi x n/ ( 1000 x 60)  [ m/s],

with:

D=   tool diameter [mm]

Pi=   ludolf’s nummber [3.141592]

n=   rotary speed  [RPM  (min-1)]

 

 


Table 1

Approximate values for cutting speeds:

Material                                        Cutters             Cutters            

                                                     HS                   HW                

                                                     Vs[m/s]            Vs[m/s]           

Softwoods                                     50-80              60-90             

Hardwoods                                   40-70              50-90             

Chip boards                                                          60-90             

Coreboards                                                          60-90             

Hardboards                                                           40-70             

Laminated boards                                                 40-70             

Aluminum                                                              30-50             

Note: The cutting speed should not be below 40 m/s with tools for manual feed. If the speed  drops under this value, it will enlarge the risk of kickback.

I.                Speed range (Table 1+2) 


 
 


Table 2


Our tools are marked with the correct maximum speed or recommended speed range. The speeds are determined in a way, that the cutting speed is not above 90m/s for mechanical feed MEC and always between 40m/s and 70m/s for manual feed MAN.

J.                           Feed rate per tooth  ( Scheme 8 )

The feed rate per tooth fz can be calculated with the next formula:


fz = Vf  x 1000 / (n x z)  [mm]

with:          

v1= feed rate [m/min]

z = number of teeth

n = rotary speed [RPM].

 

Scheme 8 

For calculating the wave width Wb of the undulation, the number of teeth has to be reduced to z=1.

Feed rate per tooth for cutting work:

fz  0.3-0.8mm = fine-machining

      0.8-2.5mm = finish-machining

      2.5-5.0mm = rough-machining

K.             Cutting circle

The cutting circle has a not direct proportional influence on the wave depth Wt, which affects significantly the visual appear of the surface. The wave depth is calculated with the formula:

Wt = f2z / (4 x D) [mm]

with:

fz = feed rate per tooth[mm]

D= cutting circle [mm].

L.      Feed rate

The feed rate Vf depends on the rotary speed, the teeth number and the feed rate per tooth. In general, the necessary feed rate is considered empirically. Specifically, to decrease the machining time, an estimated value is set and subsequently the feed rate is accelerated as long as the surface is satisfactory. The estimated values can be calculated with the formula:

 

Vf = fz  x  z  x  n / 1000 [m/min]

with

fz = feed rate per tooth[mm]

z = teeth number

n = rotary speed [RPM]

9.2.            Average chip thickness

The average chip thickness hm depends on the feed rate per tooth, the cutting depth and the tool diameter. It can be calculated as follows:

hm =  fz  x Ö a/D  [mm]

with:

fz = feed rate per tooth [mm]

D = cutting circle [mm]

a = cutting depth [mm]

Approximate values for average chip thickness:

Solid wood, along grain  = 0.2 - 0.8 mm

Solid wood, across grain = 0.1 – 0.3mm

Chip boards                     = 0.3 – 0.8mm

MDP panels                     = 0.2 – 0.6mm

Plywood panels   ........... = 0.2-0.6mm

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