Blind Threads in a CNC RFQ: Drill Depth, Thread Depth and Usable Engagement

A blind hole stops inside the part. At least three depths may matter: the usable full thread needed by the fastener, the tool travel needed to create that thread, and the drill-tip depth that includes the conical point. A callout such as “M6 depth 12” can mean different things to design and manufacturing. The result may be insufficient engagement, a bottom wall that is too thin, tool collision or compacted chips. Define the functional thread first, then identify any separate bottom limit that must be controlled.

A conventional twist drill leaves a conical bottom, so drill-tip depth is greater than full-diameter hole depth. A flat-bottom CAD feature does not by itself authorize a special flat-bottom process. Conversely, a thin wall, gallery, sealed cavity or cosmetic surface may make tip depth a critical maximum. In that case, dimension the minimum remaining wall or the maximum depth from a clear datum. Do not use an isolated DEPTH note without saying whether it ends at the cylindrical shoulder, full thread or drill point.

Spiral-flute taps are commonly selected to evacuate chips from blind holes, but performance depends on material, geometry, coolant and cutting data. Forming taps create no cutting chips but require a different pilot diameter and a suitably ductile material. Thread milling can make smaller chips and permits diameter adjustment, yet it needs orbital clearance and sufficient cutting length. The RFQ should describe the usable thread and bottom geometry rather than forcing a method before manufacturability review.

State thread size, pitch, handedness, standard and tolerance class; minimum usable thread; the reference face; any total-depth or remaining-wall limit; entry chamfer or counterbore; coating state; and special inspection. If a standard screw seats against a shoulder, supply the functional engagement instead of asking for “full thread to the bottom”. Requiring complete thread too near the bottom may call for a bottoming tap, thread mill or special drilling operation and adds tool-breakage and cycle-time risk.

GO/NO-GO gauges verify functional thread limits under the governing standard, but they do not automatically prove engagement length unless insertion depth is controlled. A marked gauge, master screw, depth pin or optical method may be appropriate. Programmed tool coordinates are not inspection results. Agree on the method before production, especially for small or deep holes where a CMM probe cannot access the thread form.

Send matching-revision 2D and 3D data, identify critical blind threads, bottom-wall limits, material, quantity and finish. OSG technical guidance on tap chamfers supports separating effective thread from tool depth. ISO 6410 and ASME Y14.6 are common references for drawing representation, but the cited contractual standard governs. No single bottom allowance fits every diameter, pitch, tool, material and chip condition.