Span (manufacturing technology)

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Steel chips from a lathe

Chip or chips are mechanically abraded particles of a machined workpiece . In the context of manufacturing technology , in particular machining , the term mostly refers to chip made of metal ; In principle, however, chip arises from any type of machining of solids. Wood chips are produced in woodworking, and chips are also produced in the seldom used metal-cutting plastics processing .

Chip formation

Chip formation process: schematic representation with several shear zones

First of all, upsetting takes place, with the wedge penetrating the material and compressing and solidifying the material. If the pressure is maintained, the compressive and shear stress in the workpiece increases to the breaking point and a chip is sheared off. The shearing takes place at the location of the maximum shear stress, the so-called shear plane , which forms the shear angle with the workpiece surface. The chip now flows over the rake face of the cutting wedge and is compressed again if it encounters further resistance.

When cutting with geometrically indeterminate cutting edges, the process of chip formation differs, since the many cutting edges have an irregular shape and the cutting depth of the individual grains is very small. Furthermore, the grains first slide over the workpiece surface.

Chips

Various chip shapes and a block of pressed chips
Temperature distribution on a carbide lathe cutting edge

A type of chip is understood to be a specific chip shape that is dependent on the chip formation. The cutting process can be assessed on the basis of the type of chip. The cutting edge geometry , the deformability of the material, the chip sizes, the cutting speed and the cooling lubricant used all have an effect on the formation of the type of chip . The differences between the three (depending on the source, four) individual types of chip are fluid. With some materials, all types of chip can be achieved by changing the chip conditions.

Chipboard

Is a brittle material by a machining process worked, mostly caused tearing tension (also crumbling chip called). Small rake angles and low cutting speeds also favor the formation of chips. Due to the cracks in the workpiece preceding the wedge, the chip loosens without any significant deformation . By breaking out the chip, the surface of the workpiece is rough after machining . An example of a material that produces chips is brass . Cooling lubricants do not help against the chips.

When working with wood, see: Pre-splitting

Shear chip

The shear chip is created by a deformation in the shear zone. The material of the chip is stressed by its deformability. The chip tears apart into individual lamellae parallel to the plane. The high temperatures ensure that the lamellas weld together.

Flowchip

The continuous chip is also created by deformation in the shear zone, but the chip flows continuously over the tool cutting edge. The deformability of the material is not exceeded. The deformation takes place uniformly in all layers. This creates a coherent chip. The continuous chip is created at high cutting speeds and high temperatures as well as continuous cutting action, as is often the case with turning and drilling, for example .

A neat, long chip flow is the most elegant way of metal cutting. The main disadvantage is the risk of tangling and thus impairment of automatic operating processes. Therefore, flowchip-forming materials are only suitable for small production lots or for large lots if the flow process of the chip is interrupted at regular intervals and thus strip chips can be avoided. For mass production , short-breaking free- cutting steels are preferred, if possible , which contain an increased proportion of sulfur and phosphorus , which promote chip breaking but have a detrimental effect on strength and ductility .

Lamellar chip

If the chip thickness fluctuates due to an uneven material structure, lamellar chips can develop. The structure is similar to the shear chip, but there are no fragments, but pure deformation takes place.

Lamellar chips are continuous chips with pronounced lamellas.

Chip shape

Chip shapes depending on the feed rate (right axis) and depth of cut (vertical axis)

The chip shape describes the shape of the chip after it has left the tool. The formation of the chip shape is essentially dependent on the material of the workpiece and the cutting conditions. The individual chip forms are classified according to their external shape, according to their bulk density ( in t / m³ for steel ), according to the possible risk to the operator and according to the possible damage to the tool, workpiece and machine.

Ribbon shavings and tangled shavings are considered unfavorable, as they form clusters of chips and thus endanger work safety. They also damage the workpiece surface, hinder the chip fall and the automatic chip removal.

Screw shavings (also called spiral shavings) should be avoided, but depending on their length, they are still considered satisfactory. The negative effects on workpiece and chip fall are not as pronounced as with ribbon and tangled chips. Chip fragments (or crumbling chips) should also be avoided, as they endanger the operator by splashing around and clog guides. However, this is not a problem on modern machines with encapsulated work areas and protected guides.

The cheapest chips are broken screw chips, broken spiral chips and pieces of spiral chips. They pose little risk to the operator, are easy to transport and have a high bulk density.

Chip space number

The chip space number R indicates the relationship between the space required by a disordered chip quantity ( chip volume ) and the separated material volume . The smaller the number, the less space is required for the chips and the easier they can be handled. The table shows the most important chip shapes and their chip space numbers.

Chip shape Chip space number
Filings ≥ 90
Tangled chips ≥ 90
Flat filaments ≥ 50
long cyl. Filaments ≥ 50
Helical chip pieces ≥ 25
Spiral chips ≥ 8
Spiral chips ≥ 8
Crumbling chips ≥ 3

Chip line

So that the chips take up as little space as possible and can be transported away more easily, chip breakers and chip guide plates can be used to influence chip formation by breaking or diverting the chip.

See also

Individual evidence

  1. ^ Fritz Klocke et al .: Manufacturing Process I - Turning, Milling, Drilling, Eighth Edition . Chapter 3.4.2. Different types of chip formation.
  2. ↑ Steel-iron test sheet 1178-90.

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