2.1. Size effect

Micro-machining is a scaled down version of the macro-machining process. Based on this relationship, an initial approach,

was to assume that the underlying mechanism for chip formation and removal had no significant difference between the macro-scale and the micro-scale. Later studies confirmed that such assumption could not be guaranteed due to a phenomenon called size effect and minimum chip thickness [¹⁰,¹⁵,¹⁷]. At small chip thicknesses, the specific energy required to remove a unit amount of material increases, which is often referred to as the size effect [¹⁷]. During a micro-milling operation, the cutting edge radius of a micro-tool is comparable to the uncut chip thickness, resulting in a possible cutting at negative rake angles. Therefore, the relationship between the cutting thickness and tool edge radius will influence the dominant chip removal mechanism [¹⁶]. Fig. 1 shows the geometric arrangements for conventional and micro-scale machining. The chip will not form unless the cutting depth is greater than a critical value called the minimum uncut chip thickness (h _min ). Below this value, the material is subject to an elastic-plastic deformation (plowing/rubbing) without efficient material removal (Fig. 2), where the actual depth of cut is less than the theoretical depth [¹¹]. At higher cutting thicknesses (exceeding h _min -value) the plowing effect decreases considerably, and chips are formed efficiently due to the shearing of the workpiece [¹⁸].

Source: [15].

Figure 1 Cutting edge size comparison. a) Macro-scale. b) Micro-scale. 

Source: [16].

Figure 2 Minimum uncut chip thickness determination. 

The plowing effect and the nature of the micro-deformation leads to increased cutting forces, burr formation, and increased surface roughness. Therefore, knowledge of the minimum chip thickness is essential in the selection of appropriate cutting conditions. This value depends on several factors such as the cutting edge radius, workpiece material, micro-structure and other operating parameters.

L’vov [¹⁹] and others indicate a minimum cutting depth were the chip separation from the material occurs and its dependency upon the cutting edge sharpness and material properties. Weule et al. [²⁰], then established a relationship to find the achievable surface roughness in terms of hmin for AISI 1045 Tempered Steel. A saw-tooth-like profile was observed while analyzing the cross section of the machined surface which can be explained by the minimum chip thickness relationship. Several studies have proposed various methods to find hmin, some recent results concerning the determination of hmin for different materials, tools and methods are listed in Table 1. It can be seen that in general, hmin values are between 1/4 to 1/3 of the tool edge radius. The chip flow stagnation region varies depending on the material and the friction angle and thereby generating a variation on minimum uncut chip thickness.

Table 1 Some minimum uncut chip thickness related research. 

Source: Authors.

Cutting edge radius (sharpness) and its effect on other parameters such as forces [²¹,²²], surface roughness [²²-²⁴], burr formation [²⁵] and acoustic emissions (AE) [²⁶] have also been studied. This literature supports the relevancy of this parameter to guarantee a complete chip formation and to predict other process responses. The minimum uncut chip thickness does not necessarily implies an adequate chip formation, surface integrity or productivity, therefore, to improve the productivity, some techniques like the use of coolants/lubricants can be considered for existing machine-tool-workpiece material combination.

2.2. Burr formation

According to the DIN ISO 13715, a burr is a “material overhang outside of the ideal geometrical shape of the workpiece edge, which remains after the machining”. A burr is unavoidable, and it may be critical or not based on application requirements. The only solution is to reduce these burrs to an acceptable degree. Often, they hold micro-cracks and generate subsequent problems during assembly, inspection or product operation due to its dimensions and bonding with the rest of the workpiece. Milling burrs can be classified according to the location (Fig. 3), shape and formation mechanism [²⁷]. In some cases, burrs can be relatively large when compared to the feature size (Fig. 4) and the amount of time taken to remove them may be over 35% of the time required to machine a part [¹¹].

Source: [³²].

Figure 3 Burrs in micro-milling. 

Source: [³²].

Figure 4 Exit burr while micro-drilling Aluminum 6061 T6. 

Chern [²⁸] studied burr formation while face milling aluminum alloys, and reported that the occurrence of five burr types were dependent on cutting conditions. Knife-type, curl-type, edge breakout and secondary burr were generated and classified.

The standard DIN ISO 13715 defines a corner with a burr, where it is assessed by measuring the perpendicular distance between the burr tip and the surface from which it is protruding. Schäfer [³³] defined the burr value (g), composed by parameters as burr root thickness (bf), burr root radius (rf), burr thickness (bg), and burr height (h0) (Fig. 5). Burr geometry in the case of micro-machining could not be easily quantified; that is, geometrical parameters like burr height and burr width could not be measured without a lack of repeatability.

Source: [³³].

Figure 5 Burr measurement according to Schäfer. 

In ductile materials, a burr is more likely to be formed due to high elastic-plastic deformation during machining [³³] and in brittle or hard materials it is mainly affected by the tool wear progress [²⁰]. In the micro-milling of ductile materials like Aluminum (Al2124), Lekkala et al. [¹²] observed that up micro-milling produces more side exit burrs and a rougher wall surface relative to down milling. This result is consistent with findings reported by Schueler et al. [³⁴] while micro-milling Ti-6Al-7Nb alloy. In SAE 1045 Steel, Weule et al. [²⁰] showed that an increase in micro-milling cutting velocities led to less burr formation. Aramcharoen et al. [¹⁵] noted that burr size is significantly affected by tool geometry and coatings with favorable rounded cutting edges or chamfered geometries for better quality.

Instead of measuring techniques, research has been focused on burr reduction for micro-milling, through experimentation [⁹,¹²,³²,³⁵] or simulation [³⁶-³⁸] by varying the process parameters or deburring after machining. The formation of burrs in the macro-milling process has been studied extensively through experimental and predictive analytical modeling, but in the micro-scale, very little work has been reported in the literature.

Although deburring techniques such as electrochemical polishing and others, can be applied to remove burrs, they require additional time and are costly [¹²]. Burr prevention must consider from design to manufacturing and use proper cutting fluid through strategies such as machining with minimum quantity lubrication coupled with hard coated tools [³²].

2.3. Cutting fluids

Cutting fluid is applied during machining to increase tool life, enhance surface finishes, decrease burrs, and avoid build-up edge formation [¹¹,¹⁷]. Traditionally coolants promote a friction and temperature reduction, to remove chips from the cutting zone, to limit chemical diffusion and to protect the machined surface from corrosion [³⁹].

2.3.2. Dry machining

To manufacture without using any cutting fluid, also referred to as dry machining, has become popular among researchers in recent years [⁴⁰]. An economical and environmentally friendly process like dry machining must incorporate some changes to overcome the absence of coolant and the associated benefits like cooling, lubrication and chip removal. Tool material with high hardness, thermal fatigue limit and chemical stability, and proper cutting parameters must be carefully selected for a successful implementation [³⁹]. This technique has been applied in numerous machining processes mainly in the automotive industry with materials like gray cast iron and aluminum cast alloys which yield low cutting forces and temperatures [⁴⁰]. Interrupted machining processes such as milling, turning and sawing with short breaking chips (Fig. 6) are suited for a coolant/lubricant reduction [³⁹]. Several challenges still exist for machining some materials, and near dry machining or minimum quantity lubrication have evolved to address some of these issues.

Source: [⁴⁰].

Figure 6 Machining process influence on cooling lubricant supply. 

2.3.3. Minimum quantity lubrication

Minimum quantity lubrication (MQL) uses a mix of compressed air with a reduced amount of coolant/lubricant in the form of drops, producing a spray or mist in the cutting zone. Often, the amount of coolant (below 10 ml/min) is nearly 3 to 10 orders of magnitude lower than conventional flood cooling [³⁹, ⁴¹]. Other advantages include reduced cost and tool wear, improved surface quality, a reduction in environmental and worker health hazards and nearly clean chips.

Soo et al. [⁴²] performed a series of micro-drilling experiments on Al 6061 with 200 µm carbide twist drills. Compressed air, pure MQL and nano-fluid MQL with paraffin and vegetable oils as base fluids were used. Nano-fluid MQL with nano-diamond particles exhibited a significant reduction in drilling torques and thrust forces. In addition, the quality and quantity of drilled holes was superior to those obtained when pure MQL and compressed air was used. Zheng et al. [⁴¹] also indicate an improvement in tool life (Fig. 7) and reduced material adhesion while micro-milling Ti-6Al-4V in contrast with dry cutting. Air pressure and direction must be carefully selected as it shows an influence on burr formation.

Source: [⁴¹].

Figure 7 Worn tools (same cutting conditions). a) Dry machining. b) MQL. 

Atomization based cutting fluid (ACF), is an alternative proposed by Nath et al. [⁴³] were droplets are produced by an ultrasonic atomizer and further pressurized through an air-CO2 mixture. A better cooling and lubrication of cutting tool-chip interface was achieved while turning a titanium alloy Ti-6Al-4V, with a high chip evacuation at lower running cost and improved tool life up to 40-50% over flood cooling.

Some authors have studied minimum quantity lubrication techniques in combination with cooled air and showed that improved cooling and lubricating performance during machining steel and difficult-to-machine materials was possible [³⁹]. MQL has a high potential in terms of improving overall process performance but some issues still have to be addressed regarding MQL supply (generator, coolant channels, control systems etc.), cutting tools and machine tool design.

2.3.4. Cryogenic cooling

Cryogenic cooling uses liquid nitrogen (LN2 at -196°C) or carbon dioxide (CO2 at -78°C) as support during machining through its application from the rake face or flank face of the cutting tool [⁴⁴]. The use of liquid nitrogen is also an environmentally friendly alternative and non-toxic gas that contributes to process sustainability, while absorbing the generated heat and acting as a lubricant.

Micro-milling aided with cryogenic cooling has been applied successfully in soft polymer materials with low toughness were the material is changed from a rubbery state to a glassy state below the glass transition temperature. Several cutting tests were applied in Polydimethylsiloxane (PDMS) achieved better-shaped micro-grooves with a surface roughness up to 80 nm [⁴⁵]. Friedrich [⁴⁶] performed micro-milling of Polymethylmethacrylate (PMMA) below the glass transition temperature and found that as a result of the cryogenic cooling, the cutting force was held relatively constant. The specific cutting energy also increased linearly with a reduction in the temperature and a rougher surface finish. Extended life parameters were found while using a specially manufactured 22 µm diameter micro-tool with a 70 nm edge radius. Ghosh et al. [⁴⁷] applied cryogenic machining to avoid cracking and surface defects while micro-milling a soft hydrophobic intraocular lens (IOL) material (Fig. 8).

Source: [⁴⁷].

Figure 8 Comparison of IOL surfaces with compressed air (left) and cryogenic cooling (right). 

Recent studies are focusing not only on the cryogenic effect on cutting forces and tool wear but also on surface integrity (roughness, microstructure, residual stresses, etc.) and proper parameters like number and positioning of nozzles, flow rates [³⁹,⁴⁴].

2.4. Polymer machining

Polymer processing through machining may become an economical alternative while ultra-rapid prototyping lowers production volumes or custom components where complex shapes and high resolution is required [⁹,⁴⁸]. Their machinability depends on mechanical properties, thermal characteristics like the glass transition temperature (T_g), melting temperature (T_m), and rheological properties as well as cutting conditions [⁹,⁴⁷]. Material properties are highly temperature and strain rate sensitive. For example, stiffness increases significantly below T_g where a brittle behavior could be expected (Fig. 9). As temperature is increased, a sudden decline in stiffness is observed (or elastic modulus) facilitating greater deformations and increased adhesion towards a more rubbery state.

Source: [⁴⁷].

Figure 9 Polymers stiffness curve with machinability regions. 

Ghosh et al. [⁴⁷] indicated that the machining performance of a biopolymer is mainly determined by the relationship of its glass transition temperature to the room temperature. If no cooling is provided, the temperature rise could cause the polymer to be inside the rubbery region where the machined surface would be defined by significant tearing and waviness (Fig. 9). Orozco et al. [⁶] while micro-milling grooves in a PMMA polymer at different cutting depth, speed, and feeds, observed better results in down milling, and observed the easy formation of built up edge while machining at room temperature. Kakinuma et al. [⁴⁵] applied cryogenic cooling to lower the cutting temperatures below the glass transition temperature to precisely shape some micro-grooves in PDMS. Friedrich [⁴⁶] compared the specific cutting energy for PMMA at temperatures below 0°C finding high values comparable with materials like 6061-T6 Aluminum and oxygen-free high conductivity copper (OFHC). Also, as the work piece was cooled, the surface roughness increased indicating a more brittle machining condition.

Usefulness and advantages of micro-milling outweigh its limitations. Applications in micro-fluidics for machining micro-molds or micro-channels and features directly into the final polymeric part are some of the important roles that this process may play [⁴⁸]. This research aimed to increase the knowledge of burr formation and minimization during polymer micro-milling and to link cutting parameters and cooling effects of a biopolymer for medical purposes.

3.1. Tools and equipment

Gaseous refrigeration system: A vortex tube FRIGID-X 56108F manufactured by Nex Flow was used (Fig. 10). Five

Source: Authors.

Figure 10 Vortex tube components. 

generators were manufactured in Aluminum 6061, with an internal diameter ranging from 2.5 mm to 4.5 mm with 0.5 mm increments. The generators were produced by turning and milling, using a standard geometrical configuration for this type of generator so that an additional variable is not present in the tests and that ensures the formation of the inner vortex formed by tangential slots of 1.5 mm thick (Fig. 11).

Source: Authors.

Figure 11 Generators configurations. 

The vortex tube was coupled with each of the generators and was connected to a compressed air line which delivers air at 16°C and 87 psi (Fig. 12). To measure the inlet pressure analog manometer 0-140 psi was used. Temperatures were measured by a digital thermometer with a DeltaOhm contact sensor.

Source: Authors.

Figure 12 Vortex tube with instrumentation. 

Machine-tool: An ultra-precision milling machine (KERN Evo) with computer numerical control (CNC) was used to perform the machining tests. Spindle speeds up to 45,000 RPM, feed rates of 16,000 mm/min with a 0.5 µm positioning resolution can be attained. A polymer concrete base, high stiffness and vibration damping, along with precision components make this possible.

Tools: The micro-end-mills used in this study were two flutes uncoated tungsten carbide (WC) with 200 µm diameter (reference 1610-0079.024), a total cutting length of 0.6 mm and a helix angle of 30° (Fig. 13). Tool edge radius was measured with scanning electron microscopy (SEM) by placing the best-fitted circle to the tool edge and was found to be 4 µm.

Source: Authors.

Figure 13 Micro-end-mill photographs. 

Workpiece material: Biopolymer samples used for intraocular lenses made of Filcon II Silicone Hydrogel from Contamac® were prepared in the form of 10 mm discs. Each part was glued onto an acrylic component and mounted on the mill press. After machining, no deburring processes were applied, and a TESA VISIO 200GL optical microscope was used for slot inspection and burr measurement. The experimental setup is shown in Fig. 14. The cold air outlet was oriented to the cutting zone to cool the area directly.

Source: Authors.

Figure 14 Experimental set-up of micro-milling machine. 

3.2. Procedure

Fig. 15 outlines the steps followed during the micro-milling and succeeding burr measurement process. The three specimen used for the tests were divided into two sections, one section contains grooves milled without vortex cooling, and another includes grooves with gaseous cooling. The machining parameters used for all tests were a spindle speed of 25,000 RPM, a feed rate of 120 mm/min and, an axial depth of cut of 0.1 mm.

Source: Authors.

Figure 15 Methodology followed during machining and measurement. 

Vortex tubes were manufactured as described then tested individually for temperature measurements using a contact thermometer as shown in Fig. 12. Vortex assembly was installed and properly oriented on the milling machine, and biopolymer disc fixed into mill press. After each test was completed, top burrs are measured in each channel at different regions, and an average value is calculated to characterize burr size (Fig. 16). Exit and side burrs were not measured as their relative size was not high while compared to the top burr. Results were recorded and classified, and micro-milling performance was analyzed to modify parameters or test conditions.

Source: Authors.

Figure 16 Top burr height measurement in the micro-grooves.