Blind Tapped Hole Depth: Defining Usable Threads and Bottom Clearance

A blind threaded hole can look complete in a CAD model while leaving too little room for the tool that makes it. If the drawing uses one depth for the finished thread and the drilled hole, the supplier must resolve a practical question: how much usable thread is required, and how far may the tooling enter the part?

Define those requirements separately before quotation. This is especially important in housings where the bottom of a threaded hole sits close to a thin wall, a sealing face or another passage.

Start with the usable thread requirement

Identify the thread system, nominal size, pitch, tolerance designation and required length of complete thread. State where that length begins. An entry countersink or chamfer does not provide the same engagement as a complete thread, so a depth measured from the outer face can be ambiguous without a clear section or note.

The design owner should establish the engagement needed for the joint, considering the fastener, component material, loading and assembly. Do not choose it solely from a generic multiple of screw diameter. If the supplier suggests a shorter engagement to simplify manufacture, treat that as a design decision requiring review.

Also identify the acceptance condition. A thread that will be plated, coated or otherwise treated needs requirements that make clear whether inspection occurs before or after that operation. The CNC machining service overview explains why finishing and critical features belong in the initial project review.

Leave space for the actual tap geometry

The chamfered lead of a tap does not immediately produce a complete thread. OSG's explanation of tap chamfers shows why chamfer length must be included when establishing tapping depth for an effective thread length. Different tools have different leads, so “use a bottoming tap” is not a complete depth specification.

Ask the supplier to review the required full-thread depth against the selected tool's lead, tip geometry and necessary bottom clearance. The drilled hole also needs to support the chosen chip-management approach. Gühring's threading guidance identifies additional depth below a blind thread for the chamfer lead and explains how different tap geometries move chips.

Avoid placing a universal extra-depth allowance on every hole. Available material, thread pitch, workpiece material and tooling can change the answer. If there is insufficient room, discuss a revised joint or an alternative manufacturing method rather than assuming a more aggressive tool can solve the geometry.

Specify what the maximum hole depth means

With a pointed drill, the deepest point extends below the full-diameter portion of the hole. A dimension to the drill tip and a dimension to the full-diameter depth therefore describe different boundaries. Label the controlling depth explicitly, especially when breakthrough is prohibited.

Show nearby passages and opposite surfaces in a section view. Define the minimum remaining material where it matters, using the applicable part tolerances rather than nominal geometry alone. This helps the supplier check whether an apparently acceptable drill depth could still violate the functional wall.

A flat-bottomed hole, if required, should be visible in the definition. Do not expect the supplier to infer that requirement from an unsectioned model or a general instruction to drill deeper.

Resolve a constrained hole before releasing the order

Consider a hypothetical housing with a blind screw hole above a fluid passage. Engineering needs a specified engagement, while manufacturing needs room below the complete thread. Deepening the hole may enter the passage; shortening the thread may change the joint's capacity.

The useful review compares three items on one section: the required complete-thread region, the proposed tool envelope and the protected passage boundary. Engineering can then evaluate whether to change the fastener arrangement, move the hole, alter the surrounding geometry or approve another process. None of these changes should be accepted merely because it reduces the quoted price.

Record the selected solution in the released drawing and model. Keep any manufacturing proposal separate from the approved definition until the design owner accepts it.

Agree how thread depth will be verified

A screw that enters the hole is useful for an assembly trial, but it does not by itself document all thread requirements. Agree the inspection approach for thread size and usable depth, including the reference face and how the measuring device's own lead geometry is accounted for.

Ask for depth results to be tied to the specified characteristic. A drilling-machine depth setting is a process input, not direct evidence of the finished complete-thread length. Where the bottom wall is critical, establish how its requirement will be verified as well.

For a quotation review, send the sectioned drawing, material, thread designation, required engagement, maximum permitted hole depth and finishing requirements. That gives purchasing a clear basis for comparing proposals without leaving the most constrained part of the hole undefined.