Can Undercuts and Side Holes Be Cast in Aluminum Parts?

Many side holes and undercuts can be formed during aluminum casting, provided the tooling can create the feature and then release it. Some features are better formed with machining allowance or produced entirely by machining.

For a complex aluminum housing or structural bracket, a hole may look straightforward in the finished CAD model. The manufacturing challenge becomes clearer when the tooling is added: something must occupy that space while the surrounding metal takes shape, and that tool element must eventually come out.

The review therefore has two parts. Can the proposed tooling produce and release the geometry? Can the resulting feature meet its functional requirements, either as formed or after finishing?

This article focuses on metal-die tooling with rigid, reusable cores. Other casting routes and removable-core systems have different capabilities and need separate evaluation.

What Makes a Feature an Undercut?

An undercut is a feature that mechanically obstructs withdrawal of a particular die section or core along its intended direction.

It is defined relative to tool movement. A recess that releases easily in one direction may trap the tooling if the opening direction changes.

Consider a hypothetical aluminum housing with an open top and a transverse hole through one side wall. The main die opens vertically. A horizontal pin could form the side hole, but the housing cannot move vertically off that pin while the pin remains engaged.

The pin needs its own withdrawal movement, or the tooling concept must change.

This distinction matters when discussing complex high-strength aluminum components. External complexity alone does not determine feasibility. A visually intricate ribbed part may release in a straightforward direction, while a small hidden recess may require an additional tool movement.

Does Every Side Hole Require a Sliding Core?

No. “Side” describes the hole’s position on the component, while the tooling designer must consider its orientation relative to the die.

A sliding core, also called a side core, moves separately from the main die-opening direction. It can form a transverse hole or recessed feature and then retract far enough to permit removal of the casting.

Before adding that mechanism, the design team can examine whether another parting arrangement would let the feature release with the main die sections. The parting line is the boundary where those sections meet.

For example, changing the component’s orientation in the die may align an important bore with the primary opening direction. That change can also make another feature harder to release, so the whole component must be checked.

In some designs, opening a recessed feature to an edge may remove the trapped geometry. Such a change is only acceptable if the opening still meets the structural and functional requirements. A closed locating hole cannot simply become an open slot if its function requires a closed bearing surface.

What Must Be Checked Before Adding a Side Core?

A side-core proposal needs more than an arrow showing a possible movement.

Can the complete core clear the casting?

The review must follow the full core geometry through its withdrawal stroke. A larger section behind a narrow opening, an intersecting feature or a neighboring wall can trap the tool even when the hole axis appears accessible.

The core must clear the features it obstructs before the casting is removed. Its movement must also avoid interference with other moving tool elements.

Is there space for the mechanism?

The space visible outside the finished part is only part of the requirement.

The slide needs support, guidance, actuation and a way to remain in position during forming. Those elements occupy space within and around the die. Several nearby side features can compete for that space.

A workable concept must fit the selected tooling and machine arrangement, including the required travel.

Can the core remain stable during production?

A long, slender pin is more demanding to support than a short, robust one. Its unsupported length, local loading and thermal conditions influence whether it can maintain the intended geometry through repeated cycles.

The question is therefore whether the tool can produce the feature consistently, as well as whether it can be drawn inside the CAD model.

Where will tooling joints meet the product?

A moving core introduces additional interfaces between tool elements. Their locations should be checked against the finished component’s requirements.

If a tool junction falls on a functional bore or contact surface, the manufacturing plan must address any witness line, mismatch or flash that would be unacceptable there. The appropriate response may be a different tooling arrangement or a defined finishing operation.

Why Do Cast Holes Need Draft?

Draft is the taper that helps a casting separate from the tooling.

As aluminum cools around a metal core, it can grip that core. Appropriate taper helps the surfaces separate during withdrawal and reduces the tendency to drag along one another.

Draft must be assessed along the movement of the tool element that forms the surface. A side-core hole is reviewed along the side core’s withdrawal direction, not automatically along the main die-opening direction.

There is no single draft angle suitable for every hole. The required allowance depends on the selected process, alloy, feature depth, surface condition and tooling design.

This has a direct consequence for the drawing: a hole that can be formed successfully may still need machining to become the cylindrical fit required by the finished component.

Adding draft also does not remove a true mechanical interlock. If a wider section of the core is trapped behind a narrower opening, taper alone does not provide an exit path.

Should an Aluminum Hole Be Cast, Machined, or Both?

The hole’s function should guide the decision.

Three routes deserve consideration: produce it as a finished cast feature, cast a preliminary opening and finish it, or form the surrounding body without the hole and machine it afterward.

For the hypothetical housing, a broad clearance opening and a precision pin bore may sit next to each other yet justify different manufacturing plans.

If a preliminary opening will be machined, its allowance must account for the proposed casting variation and the final feature location. The finishing operation needs sufficient material around the required surface to complete the cut.

Selecting a cast opening solely because it removes material from the blank can miss this requirement. The relevant decision is how the finished hole will be produced reliably.

Can CNC Machining Solve an Inaccessible Cast Feature?

Machining provides additional options, but it also needs physical access.

A side bore may require clearance for the drill or boring tool, its holder and the approach movement. A nearby flange can block the holder even when the cutting edge appears able to reach the hole.

Extending a tool to reach around an obstruction also changes its stiffness. Tool length, workholding and chip removal must be considered together, particularly for deep or recessed features.

The forming and machining plans should therefore be reviewed on the same component model. Reserving a hole for later machining is a complete solution only when the required operation is feasible.

For custom aluminum components, identifying these access requirements while the geometry can still change helps the design team compare realistic manufacturing routes.

Can a Sliding Core Form a Curved Internal Passage?

A conventional rigid core that withdraws in a straight line cannot automatically form an enclosed curved passage.

The core would occupy the passage during forming, but the surrounding geometry may prevent it from leaving. Adding an external slide mechanism does not resolve that internal obstruction.

Some casting processes use expendable or otherwise removable cores for enclosed geometry. Their suitability depends on the forming conditions, core material, support and removal method. They should not be assumed compatible with every metal-die process.

For an enclosed passage, the review must establish how the core will be removed and how the passage will be cleaned and checked. If no suitable route exists, the passage geometry or manufacturing process needs reconsideration.

How Does This Apply to Integrated Cast-Forging?

BEIGONG’s integrated cast-forging approach addresses components where complex geometry and structural material requirements must be considered together.

A proposed side hole or undercut still needs a specific tooling solution. The core arrangement must be compatible with the selected forming sequence, and precision surfaces need a defined finishing plan.

BEIGONG’s project scope includes tooling and process development according to component requirements. For a complex feature, the useful starting point is its function and geometry, followed by an assessment of how it can be formed, released and finished.

A component manufacturability review should include the 3D model, a drawing identifying critical features, material and structural requirements, and any regions that may be modified. For side holes, specify whether each opening provides clearance, location, a bearing fit, threading or another function.

That information allows the tooling proposal to explain which features can remain as formed, which need machining allowance and which require a different manufacturing approach.

Frequently Asked Questions

Can changing the parting line eliminate an undercut?

Sometimes. A different die split or component orientation may provide a withdrawal path. The revised arrangement must be checked against every other feature and the component’s functional requirements.

Do all cast holes need machining?

No. A cast hole may be suitable where its size, taper, surface condition and location meet the specification. More demanding fits or surfaces may require finishing.

Does a side hole make an aluminum component unsuitable for casting?

No. A side hole is a feature to evaluate, not an automatic rejection. Feasibility depends on the tooling arrangement, withdrawal path and requirements of the finished hole.