A Deep-Hole Microdrilling Study of Pure Magnesium for Biomedical Applications

Abstract

The mechanisms of deep-hole microdrilling of pure Mg material were experimentally studied in order to find a suitable setup for a novel intraocular drug delivery device prototyping. Microdrilling tests were performed with 0.20 mm and 0.35 mm microdrills, using a full factorial design in which cutting speed vc and feed fz were varied over two levels. In a preliminary phase, the chip shape was evaluated for low feeds per tooth down to 1 μm, to verify that the chosen parameters were appropriate for machining. Subsequently, microdrilling experiments were carried out, in which diameter, burr height and surface roughness of the drilled holes were examined. The results showed that the burr height is not uniform along the circumference of the holes. In particular, the maximum burr height increases with higher cutting speed, due to the thermal effect that plasticizes Mg. Hole entrance diameters are larger than the nominal tool diameters due to tool runout, and their values are higher for high vc and fz. In addition, the roughness of the inner surface of the holes increases as fz increases.

Keywords: biomedical device; chip formation; chip thickness; holes quality; magnesium; microdrilling; microholes; micromachinability.

Figure 1

Metallographic analysis of the Mg sample. (a) Composition of the sample. (b) SEM grain size, grain-boundary elements, ${\mathit{l}}_1$ and ${\mathit{l}}_2$ used for grain size.

Figure 2

Microtwist drills geometry.

Figure 3

Cutting edge radius measurement by means of Alicona InfiniteFocus G5plus. (a) Acquisition of the cutting edge with the EdgeMasterModule. (b) Portion of the cutting edge highlighted in green containing the 800 profiles to be averaged. (c) Averaged profile of the cutting edge evaluated along the region of interest.

Figure 4

Peck drilling strategy. (a) The peck of the tool starts outside the workpiece with a partial retraction inside the pilot hole until ${\mathit{Q}}_1$. (b) After entering the pilot hole, the microdrill performs the same strategy remaining inside the hole, rising to the rising point ${\mathit{Q}}_{\mathrm{start}2}$ after each peck. (c) Setup of the drilling process, in which the lubricant is supplied at a slight flow rate.

Figure 5

Mg drilled samples. (a) Sample with 12 holes for entrance diameters and burr heights measurements. (b) Sample with 12 holes for the measurements of roughness of the inner surface of the holes.

Figure 6

Burrs height measurements. (a) Measurement plane selected. (b) ${\mathit{H}}_{\mathrm{burr}}$ profile and mesurement procedure.

Figure 7

Diameters measurements for each hole: selected points for the three best fitted circles.

Figure 8

Roughness profile measurement (${\mathit{D}}_{\mathrm{hole}}$ = 0.35 mm, $v_c$ = 25 m/min, $f_z$ = 5 μm). (a) Selected region for roughness measurement. (b) Roughness profile.

Figure 9

SEM images of the chip collected at different feed values. (a) Chip obtained with 0.20 mm microdrill. (b) Chip obtained with 0.35 mm microdrill.

Figure 10

Chip thickness measurement from SEM images. (a) Chip obtained form the 0.20 mm holes and its detail for $f_z$ = 1 μm. (b) Chip thickness obtained form the 0.20 mm holes and its detail for $f_z$ = 2.5 μm. (c) Chip obtained form the 0.35 mm holes and its detail for $f_z$ = 1 μm. (d) Chip thickness obtained form the 0.35 mm holes and its detail for $f_z$ = 5 μm.

Figure 11

$H_{\mathrm{burr}\max }$ trend as a function of $v_c$.

Figure 12

${\mathit{D}}_{\mathrm{hole}}$ trend as a function of $v_c$.

Figure 13

$\mathit{Ra}$ trend as a function of $v_c$.

Figure 14

$\mathit{Rsm}$ value for 0.20 mm holes. On the left, the 15 μm line drawn on the inner surface of the holes obtained with $f_z$ at 2.5 μm, on the right with 5 μm.