Drills & Hole Making

Drilling is the most common hole operation — and the one most often defeated by chips. Twist drills, indexable drills and coolant-through designs differ in depth and accuracy, and the 118° point geometry sets how deep the chisel edge really goes. This page plans hole depth and chip evacuation correctly.

Concept

A twist drill cuts with its two main cutting lips at the point; the margins (the narrow ground lands on the OD) support and guide the drill in the hole rather than do the main cutting, and the flutes carry chips out. Types: HSS twist drills (cheap, low speed, general), solid carbide drills (high accuracy, usually coolant-through, moderate depth), and indexable insert drills (fast, large diameter, shallow depth, replaceable tips). The standard point angle is 118°, and the conical tip means a drilled hole's flat depth ends before the chisel edge does.

Why It Matters

Drills fail most often not by wearing out but by packing chips in the flutes and jamming or burning the edge. Deep holes need peck retraction or through-coolant to evaculate chips. And because the point is conical, programming "20 mm deep" without adding the point height leaves an uneven bottom or wastes depth. Drilled holes are also inaccurate in diameter — they need boring or reaming for precision (page 18).

How It Works

Drill types

Point depth

From page 03, an Ø10 drill at 118° has point height h = 3.004 mm. To drill a through hole of required depth d, program the drill tip to d + h so the chisel edge clears. To drill a flat-bottomed hole, account for the point.

Chip evacuation

Shallow holes can drill straight through; deeper holes use pecking (retract to clear chips) or through-spindle coolant that flushes chips up the flutes. Indexable drills almost always need through-coolant.

Worked Example

Drill a through hole in a 25 mm plate with an Ø10 drill at 118°. Required through depth = 25 mm. Add the point height 3.004 mm so the chisel edge exits: program depth ~28 mm. Because depth/diameter = 28/10 ≈ 2.8×D, a solid carbide coolant-through drill works without heavy pecking. At 118° the point is not 90°, so forgetting h would leave a conical remnant.

Why drill geometry matters

A drill does two hard things at once: it cuts on two edges while pushing chips back up a fluted channel. Center (spot) drilling first starts the hole true; walking a plain drill straight in lets it wander. The 118° point angle sets the chip and the depth to add (h = D/(2·tan59°) = 3.004 mm for Ø10). For deep holes, chip evacuation is the limiting factor — peck drilling retracts to clear chips, and through-cooled drills push coolant out and pull chips back. Feed is conservative because the drill is buried in material. After drilling, the hole is near-net: finishing steps (page 18) size and correct it. Most drilled-hole problems are chip evacuation or wandering, not the drill "being dull."

Why spot-drilling first

A plain twist drill is, at the tip, supported only by its two margins and pushed by a wide, uncutting chisel edge at the centre. The instant the centre wanders even slightly, the margins cannot bring it back — the drill follows the error and the hole comes out off-position. A short, rigid spot drill or center drill makes a conical start recess that locates the chisel edge on centre before the flutes cut, which is why precision, location-critical holes commonly start with a spotting pass. The effect compounds on sloped or curved surfaces, where a drill would skate sideways: spot first, or ramp in. A short spot drill is not required on every hole — a self-centring split-point or a properly fixtured hole can skip it — but for location-tolerant work it is cheap insurance. The effect compounds on sloped or curved surfaces, where a drill would skate sideways entirely: spot first, or ramp in. This alignment step costs seconds but prevents the whole drilled hole from being rejected, and it is the reason page 18 can assume the drilled hole is "roughly aligned" before boring corrects it. After spot drilling, run the drill at the catalog speed with through-coolant or pecking as depth requires, and remember the point-height add (h = 3.004 mm for the locked Ø10 at 118°) for through depth.

Why indexable drills top out near 3×D

An indexable insert drill cuts fast and at large diameter, but it is deliberately shallow: the internal chip-flute geometry is broad and open to clear big chips quickly, and the through-coolant ducts feed those chips straight out the top. That open design is also what limits depth — the chip has only a short, stiff flute to travel, and beyond about 3×D it cannot climb reliably, packing at the bottom. This is not a failure of the drill but a design trade: indexable drills are optimised for fast, shallow, large-diameter holes where they beat solid carbide on tool cost per hole. For deeper holes you switch to a solid carbide or gun-style drill with a longer, more engineered flute and higher through-spindle pressure (page 23). So the depth/diameter band in the table is a capability envelope, not a suggestion: running an indexable drill at 5×D on the assumption that "it's just a deeper hole" is how you seize one. Choose the drill family for the depth you actually need, then speed and feed.

Why the chisel edge demands thrust and gentle feed

At the very centre of a drill, the two cutting edges meet in a flat, blunt chisel edge that does not really shear — it ploughs and extrudes material, because at the exact centre the cutting speed is zero. That is why drilling needs high axial thrust: much of the force pushing the drill into the part is spent shoving that chisel edge through the work rather than cutting with the margins. Two practical consequences follow. First, the workpiece and fixture must be able to take that thrust without lifting or deflecting — a loosely clamped plate climbs under the drill point. Second, feed is conservative: too aggressive a feed overloads the ploughing chisel edge, which is the usual cause of drill breakage in deep holes, and why peck cycles exist to let the margins re-engage cleanly instead of the chisel edge trying to force a clogged flute through. A narrow, properly ground point reduces the chisel edge and lowers thrust, which is why a good split-point or self-centring drill wanders less and drills more easily than a blunt standard point — the point geometry is partly a thrust-reduction design, not just a positioning one.

Why a drilled hole is near-net, never precision

It is tempting to treat the drill diameter as the hole size, but the margins are self-piloting guides rather than a fixed bore, and under wear, deflection or poor point condition the hole can come out larger than the drill and a little out of round. On a rigid, well-supported, sharp drill this error can be small, but it is not guaranteed; two causes combine: uneven margin wear walks the hole, and the drill can deflect under the thrust of the chisel edge (page above). This is why the shop sequence on page 18 hands a location-critical drilled hole to boring and reaming for the final tolerance. The practical planning rule is: budget the drill as a near-net operation that leaves deliberate stock, not as the finishing dimension, and do not inspect a drilled hole against the H7 the drawing puts on the reamed/bored result. When a drilled hole measures consistently oversize, check drill wear and margin condition rather than assuming the machine drifted.

Why a carbide drill usually needs through-spindle coolant

Solid carbide drills run at much higher surface speeds than HSS, and at those speeds the heat at the buried point is severe. Flood coolant applied from outside the spinning spindle cannot reach the bottom of a deep flute effectively — centrifugal sling throws it off the flutes before it travels down, so the point cuts dry regardless of how much coolant floods the surface. Through-spindle coolant feeds at pressure straight to the drill tip, both cooling the edge and flushing the cut chips back up the flutes, which is why carbide drills are almost always designed with internal coolant channels. Running a coolant-through drill without the coolant pressure switched on is therefore doubly wrong: the edge overheats and the chips pack, which looks like a dull drill but is really a missing coolant supply. This is also why indexable drills (page above) require through-coolant rather than tolerating flood — their whole chip-evacuation scheme depends on it. When a carbide drill wears prematurely in a hole that should be easy, confirm the coolant is actually reaching the tip before you assume the grade is wrong.

Why tap drill size leaves a chip-pulling core

Drilling also feeds threading, and here the drill size is chosen on purpose to leave material, not to finish the hole. For an M10×1.5 thread you drill an Ø8.5 tap drill, not Ø10, so the tap cuts a controlled thread depth rather than scraping at a hole that is already too close to final size. The remaining 1.5 mm of core (half the thread depth on each side) is what the tap actually forms, and leaving the right amount of it keeps torque manageable and chip load light. If you drill too close to nominal, the tap has nowhere to form the thread, rubs, and snaps; if you drill too small, the same tap overloads. This is why page 19 reuses the 8.5 mm tap-drill number, and why choosing the drill is the first threading decision rather than a separate afterthought. It is also a reminder that "drill size" means different things by purpose: a near-net hole for boring, a sized hole for reaming, and a deliberately undersized hole for tapping — the operation downstream sets the drill diameter.

Why the same drill wears faster in stainless than in mild steel

A drill run in low-carbon steel may last for hundreds of holes; the same drill in austenitic stainless often packs chips and wears the margins quickly. The cause is not the drill but the material: stainless work-hardens as the margins pass, gummy chips do not curl and break cleanly, and the low thermal conductivity traps heat at the buried point. That combination turns chip evacuation — already hard in a deep flute — into the limiting factor, exactly as this page warns. The response is not simply to slow down and push harder: it is to use a coolant-through drill at the catalog stainless speed, ensure the chips actually break and leave, and peck when depth demands. This is the same chain traced on page 23 and case 34: the hole quality and drill life are set by chip evacuation and heat removal at the point, not by the nominal drill diameter. When a drill that runs fine in steel burns up in stainless, suspect chip packing and missing coolant first, then grade.

A drill is judged as much by how it clears chips as by how it cuts. Long flutes that pack swarf, a point that walks on an inclined surface, or coolant that cannot reach the bottom all convert a quick hole into a wandering one. The point geometry and the flute form are chosen together for the material and the hole depth; do not select a drill on diameter alone.

Drill life is decided by chip evacuation more often than by edge wear. A deep hole that packs chips near the point re-grinds its own flutes from the inside; pecking and through-coolant are not luxuries on deep holes, they are what keep the point cutting instead of rubbing swarf.

Common Mistakes

• Forgetting point height in depth planning. Leaves a conical bottom. Fix: add h = D/(2·tan59°) for 118° drills.
• Drilling deep without chip evacuation. Jamming and built-up edge. Fix: peck or use through-coolant.
• Expecting a drilled hole to be precision. Drilled diameter is approximate. Fix: bore or ream for tight tolerance (page 18).

Plan the through depth from the conical point, not just the plate thickness: add the point height and a verified exit clearance. Keep peck depth and chip evacuation matched to the material, because a drill that rubs on its margins because chips packed will wander and wear before it reaches size.

If a drilled hole wanders, check the spot and the drill condition before changing speed: a worn margin or a chisel edge rubbing on an angled start is a geometry problem, not a parameter one. Keep the point sharp and the start flat, and confirm chips clear between pecks.

Practice