CNC Machining Holes and Threads: What to Put on Your Part Drawing
Ambiguous hole and thread callouts create quoting questions, inspection gaps, and assembly risk. This guide shows what sourcing engineers should define on a CNC part drawing.
For thin aluminum geometry and process planning, read the thin-wall CNC machining guide.
Distinguish clearance, locating, and threaded holes
State each hole’s function. Clearance holes allow fastener passage; locating holes control position with a pin or mating feature; tapped holes engage threads. The function affects tolerance, datum strategy, and inspection method.
If a hole controls alignment or sealing, show its relationship to the part datums and mating component. Avoid treating all holes as interchangeable just because their nominal diameter is similar.
Write complete thread callouts
Specify thread standard, nominal size, pitch where applicable, class or fit, and thread depth. Identify whether the thread is through or blind, and show the usable full-thread depth separately from drill depth when needed.
Design and process review
Confirm the functional interface, critical features, production quantity, and inspection method before tooling or machining approval.
Purchasing and quality alignment
Document material, revision, acceptance requirements, sample approval, packaging, and delivery expectations in the RFQ.
For metric or inch threads, do not rely on a note such as “tap M6” without the required pitch and fit. For inserts, include insert type, material, installation side, and any pull-out or torque requirements.
Define hole depth and bottom conditions
Blind holes need enough drill depth for the fastener, tool point, and chip clearance. If a flat bottom, minimum full thread, or restricted breakthrough is required, state it explicitly. Through-holes should identify whether burr removal or edge finishing is needed on both sides.
Review deep, small-diameter holes for tool access, chip evacuation, and aspect ratio concerns. The supplier can assess feasible depth and process, but the drawing must communicate the functional requirement.
Locate holes from usable datums
Use a datum reference frame that matches how the part is assembled or inspected. Position tolerances can be more robust than stacking ± dimensions when a hole pattern must align with a mating component.
Clarify which face establishes the primary datum and whether multiple hole patterns share a common setup. Avoid datum schemes that require complex inspection but do not reflect the real assembly interface.
Add countersinks, counterbores, and edge breaks
Call out included angle and diameter for countersinks, and diameter and depth for counterbores or spotfaces. Define edge-break expectations where they affect fastener seating, handling, sealing, or coating.
Cosmetic deburring and functional edge treatment are different requirements. If a sharp edge is prohibited, specify a measurable limit or agreed inspection criterion rather than an undefined “break all edges.”
Inspection and RFQ checklist
Provide native CAD, a dimensioned drawing, revision, material, finish, quantities, and critical-to-function features. Identify whether thread gauges, pins, CMM, or a first-article report are required.
For production support, review precision CNC machining and share drawings through the manufacturing RFQ page.
Discuss Your Manufacturing RFQ
Share your current CAD, drawing revision, material, target quantity, critical-to-function requirements, inspection expectations, and delivery destination. The engineering team can review manufacturability before quotation.