Orthotic Insole Milling & EVA Footwear CNC Vector Guide

1. Converting 2D Pressure Scans into 3D Vector Isobars

Clinical pedobarographic pressure sensors output 2D raster heatmaps where pixel intensity (color/grayscale) corresponds to plantar peak pressure ($N/\text{cm}^2$ or $\text{kPa}$). To mill a custom offloading orthotic, this raster must be decomposed into vector topographic elevation contours (isobars):

2. Scallop Height & Stepover Geometry

When 3D raster-milling contoured foam footbeds with a ballnose endmill, the spacing between parallel passes ($p$) determines the residual cusp / scallop height ($h$):

3. Preventing Foam Deflection, Melting & Gumming

Closed-cell EVA (Ethylene-Vinyl Acetate) and Plastazote (LDPE) have low melting points ($85^\circ\text{C} - 110^\circ\text{C}$) and high elastic recovery. Follow these machining rules:

• Downcut Spiral Tooling: Use razor-sharp polished solid carbide downcut single-flute or two-flute ballnose endmills. Downcut geometry directs cutting force down into the vacuum bed, eliminating foam lifting and flutter.
• High Chip Load per Tooth ($f_z \ge 0.18\text{mm}$): Do not run spindle speeds too high with low feed rates. If chip load drops below $0.10\text{mm}$, the tool rubs rather than shears, generating frictional heat that melts foam into a sticky gum that fouls cutter flutes.
• Climb Milling Toolpaths: Always mill using climb cutting rather than conventional cutting to prevent foam tearing along outer margins.

4. Digital Oscillating Knife Perimeter & Skiving Toolpaths

While 3D surfaces are routed with ballnose cutters, insole perimeter outlines, top covers (Poron, Leather, Microfiber), and posting wedges are best cut using a CNC digital oscillating knife:

1. Oscillating Frequency: 10,000 to 14,000 strokes/min (160–230 Hz) with a 1.0mm to 2.0mm stroke stroke amplitude allows smooth cutting of 45 Shore A EVA up to 40mm thick.
2. Corner Deceleration: In tight radii (< 3mm around toe boxes), decelerate feed speed by 40–60% to prevent blade deflection and bevel distortion.
3. Bevel Skive Angle Paths: Use angled tangential knife heads ($15^\circ - 45^\circ$) to produce skived feather edges along forefoot extensions for seamless integration into dress and athletic shoes.

5. DXF Layer Standardization for Orthotic CAM