In a through hole, the drill exits from the bottom face of the workpiece. Chips clear through the bottom, coolant exits freely, and the depth is defined by when the drill breaks through rather than by a depth stop or programmed depth. The geometry is self-correcting.
In a blind hole, none of that applies. Chips have to evacuate backward, back up the flutes against the direction of cutting, and out through the top of the hole. Coolant has nowhere to go except back up the flutes. The depth must be controlled actively. And the bottom of the hole has whatever geometry the drill point leaves — a conical crater, not a flat surface — which matters for many applications.
In thin sections (under ½"), blind holes are manageable with modest process adjustments. In thick plate — 1" and over — the combination of long chip path, compressed coolant, and precise depth requirement makes blind hole drilling a process that needs deliberate setup rather than an afterthought.
Depth stop on the drill press quill. The most common approach for manual work. Set the depth stop to limit quill travel at the intended depth. The limitation: the depth stop controls where the quill stops, which is spindle reference, not drill point reference. Variations in how the drill is seated in the chuck — different set screw torque, slightly different insertion depth — change where the drill point actually is relative to the quill position. For tolerance-critical blind holes, verify drill extension depth in a known setup before running production.
Programmed depth in CNC. In a CNC drilling cycle (G81, G83), depth is programmed from the reference surface to the programmed Z depth. The drill point (not the shoulder) reaches the programmed depth. For most applications, this is reliable — but verify by measuring the first hole with a depth gauge or telescoping gauge. If the workpiece surface reference is not flat or the workpiece isn't seating correctly on the fixture, programmed depth doesn't produce programmed hole depth.
Reference spacer or parallels. For manual work where a depth stop isn't available or reliable, a precision spacer under the drill chuck collar can serve as a physical stop. Drill until the spacer contacts the top of the workpiece. This removes the variability of quill travel measurement from the equation.
Drill point vs. full diameter depth. The drill point is conical — at the intended depth, the full diameter is typically 0.05" to 0.15" above the bottom of the hole depending on point angle and drill diameter. If the depth specification refers to full-diameter depth (common in engineering drawings), add the point height to your programmed or stop depth. Point height = (drill diameter / 2) × (1 / tan(half point angle)). For a 135° drill, point height ≈ 0.21 × diameter. For 118° drill, point height ≈ 0.30 × diameter.
Chip packing is the root cause of most blind hole problems: broken drills, irregular finish, inconsistent depth, and overheating. Chips that pack at the bottom of a blind hole act as a secondary cutting tool — they abrade the drill point and compress under the continued force, generating heat. In deep blind holes, packed chips can fracture the drill point or weld to the flutes.
Peck drilling is mandatory for any blind hole deeper than 2× drill diameter. The standard peck cycle (G83 in most CNC dialects) retracts the drill fully to the clearance plane on each peck, clearing chips and restoring coolant to the cutting zone. For depth-to-diameter ratios above 4:1, peck frequently — peck depths of 0.5× to 1× diameter per peck.
Peck increment discipline. Don't use uniform peck increments all the way to depth. A common approach: first peck at 1× diameter, subsequent pecks at 0.5× to 0.75× diameter. The deeper you go, the worse chip evacuation becomes and the more critical it is to peck before packing occurs.
High-helix drills for deep blind holes. A parabolic or high-helix drill has a larger chip channel and is specifically designed to move chips more effectively up the flute. For depth-to-diameter ratios above 5:1, a parabolic drill paired with peck drilling produces better results than a standard twist drill at equivalent depth.
Air blast at each retract. If the machine has through-spindle air or an external nozzle positioned at the hole, a brief air blast on retract clears chips from the hole before the next peck. This is a simple addition to a CNC peck cycle and significantly reduces chip compaction in deep blind holes.
In blind holes, flood coolant collects at the bottom. When the drill re-enters on the next peck, it's pushing against a hydraulic head — coolant compressed in a closed bottom. This can cause pressure fluctuations, inconsistent depth readings, and in extreme cases can prevent the drill from seating fully at depth.
For critical depth blind holes, reduce coolant flow during the final approach to depth — or switch to air blast for the last 0.050"–0.100" of depth. This reduces the hydraulic cushion effect and allows the drill to reach its intended depth without fighting trapped coolant.
Through-spindle coolant helps significantly in blind holes compared to flood coolant — it delivers coolant directly to the cutting zone and the return flow path flushes chips up the flutes continuously. For production cells with frequent deep blind holes, through-spindle coolant is a process enabler, not a luxury.
The conical drill point leaves a cone at the bottom of a blind hole. For many applications — clearance holes for fasteners, oil passages, locating dowel holes that exit into a cavity — this is irrelevant. For applications where flat-bottom geometry is required — a seating surface, a blind tap that must be threaded close to the bottom, an O-ring groove at the bottom of the bore — a flat-bottom drill or boring operation is needed after the primary drill operation.
Flat-bottom drills are available but less common than standard twist drills. They typically have a 180° flat point geometry rather than the standard angled chisel and are designed for redrilling an existing conical bottom to flat. They are not drilling tools — they should only be used to finish an already-drilled blind hole to flat bottom geometry, not to initiate the hole.
For tapped blind holes, allow 1.5× thread pitch of clearance between the programmed drill depth and the intended thread depth. Taps require chip room at the bottom of the blind hole and cannot cut to within one pitch of the drill point. Insufficient clearance chips taps and produces incomplete threads at the bottom of the blind bore.
Chip packing in blind holes accelerates edge wear faster than comparable through-hole work. Keep your cobalt drills on a resharpening cycle — mail them in, back in 5 business days.
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