Control by the machine User

2.3.2.1 Warm-up cycles to reduce gradients

Warm-up cycle is a time period during which a machine tool retains its thermal stability when it is used after periods of standing idle for hours. A common way to achieve is by de-activating the E-Stop which activates axis drives and motors which are used to hold structural parts in position. The activation causes them to warm up due to which heat disseminates into associated structures and continues until a state of thermal equilibrium is achieved.

25 2.3.2.2 Controlling external temperature changes

Previous sections detailed how machine tools are affected by external heat sources followed by the discussion on controlling techniques. As a machine tool user, control should be considered for the environment where the machine will be used. Sagar [28] has discussed and highlighted the importance of controlling long term temperature variations in order to achieve required accuracy and true process capability. Seasonal changes, day and night transitions, machine location, workshop location, thermal characteristics of the machine shop, sunlight exposures, opening and closing the doors for material delivery are few of the contributing sources of external temperature change, often producing very large temperature variations of more than ±5°C experimentally shown by Longstaff et al [12] Sagar et al [28] and Weck et al [29].

Shop floor temperature control can assure temperature stability for such industries where long term production cycles take place. The machines structure and the components waiting to be machined will be safe from temperature variations. Although temperature control is effective, it is often a very expensive solution and few industries can justify implementation, however few practical alternatives exist to avoid environmental temperature variations such as those caused by the natural day and night or seasonal transitions.

Previously discussed external sources such as opening and closing of the doors and workshop lighting are areas where a machine tool user is responsible. Even a temperature controlled environment does not work if there are significant interruptions due to unnecessary or frequent localised disturbances.

2.3.2.3 Controlling swarf temperature

The swarf from the cutting process contains heat generated by friction between the cutting tool and the work piece surface. If not managed this can raise the temperature of the machine structure and/or the workpiece leading to distortion. Swarf temperature may be reduced through the use of coolant which can also aid swarf removal from the cutting zone.

26 2.3.2.4 Controlling workpiece temperature through flood cooling

2.3.2.4.1 During cutting

Continuous coolant spray may prevent temperature increase due to the fluids high heat capacity. There have been many aspects and issues related to the use of the flood cooling mechanisms. Hoff [30] highlighted issues regarding coolant mechanisms such as chip build-up blocking free flow of coolant, time consuming and laborious removal of sump cleaning, contamination with lubricants, chemical reactions issues with hydraulic hoses and electrical cables and environmental health and safety hazards. Fluid disposal, toxicity, filterability, misting, staining and surface cleanliness are areas where coolants acts as environmental hazards.

Compared to flood cooling Boelkins [31] discussed that Minimum Quantity Lubrication (MQL) or near dry machining is one of the methods which have gained popularity in an effort to minimize environmental effects. In this method, minute amount of high efficiency lubricant is applied precisely to the cutting tool and workpiece interface. The most common lubricants used in MQL are biodegradable vegetable oils due to their polarity, exhibiting extensive friction reducing properties.

Sreejith [32] considered environmentally safe cutting or green cutting and dry machining as valuable solutions towards environmental hazards. Sharma [33] suggested air, water vapours and environment-friendly gases as better solutions for green cutting. Donmez et al [34] avoided liquid cooling and showed a novel technique using Coanda- Effect tubing, achieved 30% reductions in spindle thermal drift using compressed air. However it was not stated how to implement such method for reducing temperatures from major heat sources such as belt drives i.e. complexities involved in wrapping up tubing around those sources and further discussions would have benefitted the approach. Similarly the heat dissipated by the compressors may eventually cause the overall temperature rise around vicinity of the machine which will affect the structure thermally.

2.3.2.4.2 Before machining

Workpieces ready or waiting to be machined are susceptible to the varying environmental temperature which adversely changes their thermal state. To prevent this, parts should be placed in temperature controlled environments. Workpiece temperature

27 control can also be achieved by soaking workpieces for hours up to a known thermal state such as, at or near, the coolant temperature before machining which may differ from the ambient temperature.

As mentioned, there have been several suggested workpiece cooling methods using cooling mediums. Coolant issues are also resolved by MQL utilizing biodegradable lubricants. This research deals with the analysis of the machine structure affected by the internal heating and varying environmental effects. Research and methods have been applied to the cooling of spindles and ball screws which are expensive methods to employ. Application of the cooling techniques to the machine structures such as channelling, hollowing the structures for the cooling medium to flow is often a compromise with arising structure stiffness problems and these can also be expensive and therefore rarely implemented.