Why do thin walls and deep pockets make a part expensive?

Separate depth, wall stiffness and cutter access

All four examples share a 4 × 3 × 1.5-inch exterior, 1.75-inch raw plate input and ±0.005-inch general tolerance. The shallow and deep thick-wall cavities have a 3 × 2-inch opening, leaving half-inch walls. Their depths are 0.5 and 1.25 inches. The thin-wall opening expands to 3.85 × 2.85 inches, leaving 0.075-inch perimeter walls and a quarter-inch bottom.

The fourth variant is a three-inch-long, eighth-inch-wide slot at 1.25-inch depth. It removes far less material than the broad cavity but limits cutter size severely. Its price should not be interpreted as a matched functional alternative: these are geometry experiments that isolate different cost drivers, not interchangeable housing designs.

Make the special geometry inputs visible

The drawing example part does not automatically recognize paired thin-wall faces. For the thin case, we explicitly enter four walls and the summed inner and outer wall area calculated from their perimeters and standing height. Rounded internal corners reduce the inner perimeter. The table reports that area so the allowance can be checked against the stated cavity rather than hidden in a multiplier.

The pricing model finishes qualifying thin-wall area more slowly than ordinary accessible faces. For the narrow-slot case, we explicitly identify the pocket's removed volume and eighth-inch gap; the model limits removal to a smaller cutter. These are estimating treatments, not machine-qualified feed rates. The slot's depth-to-width relationship still requires an operator to confirm that a suitable tool and stable cutting strategy exist.

The model does not solve tool reach for you

Every deep variant also carries twelve extra VMC setup minutes and three minutes per part for toolpath and reach work at $95/hour. That is a manual budgeting allowance, distinct from the modeled material removal and thin-wall finishing. No automatic full tool-deflection or reach simulation has been performed. An impossible tool approach needs a design or process change, not merely a larger price.

Protolabs' DFM guidance warns that deep narrow pockets and tall walls can cause cutter or workpiece vibration, deflection and lost accuracy or finish. Its advice supports reviewing these features; it does not validate our fictional dimensions or prescribe our minute allowances. Workholding, staged roughing or an alternative process may be necessary for an actual part.

The examples use $9.50/lb small-lot and $4.50/lb volume aluminum inputs, $95 programming and milling, $55 bench and $75 inspection. They add 30% markup and a $65 line floor. Dedicated support workholding, special long-reach tools, extra process trials, scrap reserves and coatings are excluded. Show those costs explicitly if needed, and ask the designer whether a wider corner, thicker wall or shallower cavity can meet the same function.

• Compare an open machined housing with formed construction
• Price inspection and tolerance risk separately