TPT May 2020

AR T I C L E

Advanced Machine & Engineering I AMSAW

‘Reading the chips’ to gain valuable process information while cutting tubes – par t 2 By Christian Mayrhofer, R&D – Advanced Machine & Engineering/AMSAW

When sawing or machining in general, most of the power required for material removal is transformed into heat. There are four main options where the heat can go: into the workpiece, into the tool, into the chip or into the surrounding environment.

In the last issue you will have read an article about basic chip formation and how it affects circular carbide saws (see page 62 of TPT March) . This article sheds light on some other chip related topics with saws.

Depending on the chip shape there is usually a distinct ratio between the volume of the solid material and the volume of the chips. The volume flow as a function of this ratio as well as the cross-section have a huge impact on the required chip conveyor. When cutting thin walled tubing with one-way blades or cutting structural steel with band saws an auger conveyor might do the job whereas cutting solid material with conventional blades requires a rigid 2.5" hinged belt conveyor due to the high volume chip flow. Below are some typical chip volume ratios for machining in general. The chip volume ratio Rz for cutting solid steel with resharpenable blades without a chip breaker is about 30 whereas for one-way blades with a chip breaker it is about 5 for the same material with typical parameters. Some kinds of vibration can be seen on the chip. A telltale sign is if the inside looks like the pattern shown below. A possibility is that torsional resonance occurs. Changing the speed can help reduce the resonance.

Typically with circular carbide sawing no flood cooling is used and the air does not help a lot. Only the first three make sense. Generally most of the heat will be conducted into the chip. Despite that, the highest absolute temperature still occurs on the cutting edge. Hard coating helps. One advantage of coatings is the higher temperature resistance due to

the nature of the ceramic structure of nitride such as TiN. The temperature on the cutting edge is directly related to the cutting speed and will follow it in a parabola fashion. This is for example one critical issue with machining titanium. Since its thermal conductivity is really low the heat will not be transported via the chip causing excessive thermal load on the cutting edge.

The colour of the chip is also a good indicator for the process. When heat is applied to a metal it will change the surface colouration and texture due to oxidation. The thickness of the oxide layer, where oxygen atoms can diffuse into the material, is a function of the temperature. The human eye can see a change of colour by interference of waves on the surface. Colour change is used in the annealing process as well. The colour of the steel chips can vary depending on parameters such as cutting speed, heat conductivity and the properties of the steel being cut including carbon content and alloys present. With carbon steel higher cutting speeds might turn it blue whereas with other high alloyed steel, which require lower speeds, the colour is more reddish. This can also be seen very well with stainless steel. The cutting conditions for stainless steel are different – it has a lower thermal conductivity, due to the corrosion resistance, it has less affinity to build oxides and requires a lower cutting speed. This results in a straw looking colour.

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MAY 2020