CNC Pin Hole Processing Guide

• 1
  Key takeaways
• 2
  What a pin hole actually has to do
• 3
  How the bore is created: drill, ream, bore
• 4
  Depth-to-diameter ratio and where it breaks down
• 5
  How material changes the outcome
• 6
  Setup, workholding and the position problem
• 7
  Measuring a pin hole without fooling yourself
• 8
  Choosing a method by hole requirement

A pin hole is a cylindrical bore sized to accept a pin, dowel or shaft. Two parts share the same nominal diameter, and the joint works because the bore and the pin agree to a few microns. That agreement is the whole point. A hole that is 0.02 mm oversize still looks fine on the bench, but the assembly will shift under load and the position of everything downstream moves with it.

It helps to separate pin holes into two jobs. A locating hole sets position: dowel holes in a fixture plate, pin holes in a die shoe, alignment bores in a gearbox housing. A pivot or bearing hole sets fit and clearance: a wrist pin bore, a linkage pivot, a bushing seat. The first is driven by position tolerance. The second is driven by diameter tolerance, roundness and surface finish.

The two jobs push the process in different directions. For a locating hole we care about true position relative to a datum, so the machine, the workholding and the tool path matter more than the last micron of diameter. For a pivot hole we care about the bore itself, so reaming, boring or honing matter more than the fixture. Drawings that mix the two requirements without saying which one governs are the most common source of argument at first article.

• 1
  Locating holePosition tolerance dominates; diameter is usually H7 or a press-fit class.
• 2
  Pivot or bearing holeDiameter, roundness and finish dominate; clearance is designed in.
• 3
  Fluid or gas channelFlow matters more than fit; burrs at the breakout are the main risk.

Almost every pin hole starts as a drilled hole. A twist drill removes material fast but cuts a hole that is slightly oversize, slightly lobed and rarely straight. On a rigid setup in aluminium, a fresh carbide drill can hold about ±0.05 mm on diameter. That is a pilot, not a finished pin hole.

Reaming takes the pilot to final size. A reamer is a multi-edge tool that removes 0.1–0.3 mm of material and follows the existing hole. It produces a round, straight bore with a good finish, typically Ra 0.8–1.6 μm. Reaming is fast and repeatable, which is why it dominates production pin holes. Its weakness is that it corrects size and roundness but not position. If the drilled hole is off-centre, the reamed hole stays off-centre.

Boring is the alternative when position and size both have to be controlled. A single-point tool is fed around the bore on a controlled radius, so the machine, not the hole, defines the centre. Boring corrects position, size, roundness and taper in one pass, and it can hold ±0.005 mm on a rigid machine. It is slower than reaming and needs a separate pass for each hole, so it is used where the tolerance justifies it.

Honing and roller burnishing sit at the end of the chain for bores that need a mirror finish or a work-hardened surface. Neither changes position. They refine the wall. For most pin holes, reaming or boring is the last operation.

• 1
  DrillRemoves bulk, sets position roughly, ±0.05 mm on diameter.
• 2
  ReamRemoves 0.1–0.3 mm, sets size and roundness, keeps existing position.
• 3
  BoreSingle-point, corrects position and size together, slower per hole.
• 4
  Hone or burnishRefines finish and surface integrity only, does not move the axis.

The single biggest constraint on a pin hole is how deep it goes relative to its diameter. A drill is a long, thin cantilever. As the depth-to-diameter ratio rises, the tool deflects, the chips have further to travel and the coolant has less chance to reach the cutting edge. The result is a hole that drifts off axis, tapers and finishes poorly.

Up to about 3:1, standard drilling is comfortable and a reamer follows cleanly. Between 3:1 and 5:1, peck drilling and through-spindle coolant become necessary, and the drill should be re-pointed on a known schedule rather than run until it fails. Past 5:1, we move to parabolic-flute drills, pilot holes stepped in diameter, and sometimes gun drilling for the deepest work. Each step adds cost and time.

Small holes hit the wall sooner. A Ø0.5 mm hole at 5:1 is only 2.5 mm deep, and at that scale spindle runout, thermal growth and chip evacuation all matter. Micro-drilling on dedicated equipment extends the practical ratio, but the tool is fragile and the feed rates are low. If a design can use a Ø1.5 mm pin instead of Ø0.5 mm, the hole becomes dramatically cheaper and more reliable.

Blind holes add their own problem: the drill tip cone. A standard 118° or 140° point leaves a cone at the bottom, so the usable cylindrical depth is shorter than the drilled depth. Drawings should call out the full-diameter depth, not the drill depth, or the shop has to guess.

• 1
  Up to 3:1Standard drilling and reaming, no special measures.
• 2
  3:1 to 5:1Peck cycle, through-coolant, controlled tool wear.
• 3
  Past 5:1Parabolic drills or gun drilling, pilot stepped in stages.
• 4
  Blind holesSpecify full-diameter depth; allow for the drill point cone.

Aluminium is forgiving. Grades like 6061 and 7075 cut freely, conduct heat away from the edge and hold a reamed bore well. The risk is built-up edge on the tool and, in thin walls, the hole closing slightly after the tool withdraws. A light spring pass or a dwell at the bottom of the stroke usually settles it.

Stainless steel behaves differently. Grades 304 and 316 work-harden the moment the tool rubs instead of cuts, so feed per tooth has to stay high enough to stay under the hardened layer. A dwell is the wrong move here. 17-4PH in the H900 condition is harder again, and pin holes in it are usually bored rather than reamed because reaming loads the tool too heavily.

Titanium, Inconel and magnesium each have a signature. Ti-6Al-4V has low thermal conductivity, so heat stays in the tool and the bore can smear; sharp edges and generous coolant flow are essential. Inconel work-hardens aggressively and prefers low surface speed with a rigid setup. Magnesium cuts fast but the chips are a fire risk and need specific handling.

Plastics and composites introduce a different failure mode: the hole is fine on the way in and frayed on the way out. PEEK and carbon fibre both need a backing plate, a sharp tool and a controlled exit feed. For carbon fibre, a slightly undersized diamond reamer reduces delamination at the exit plane.

• 1
  Aluminium 6061, 7075Fast, stable, watch for built-up edge and thin-wall spring-back.
• 2
  Stainless 304, 316Keep feed high, never dwell, re-sharpen on schedule.
• 3
  Ti-6Al-4V, InconelLow surface speed, rigid setup, generous coolant.
• 4
  PEEK, carbon fibreBacking plate, sharp edges, controlled exit feed.

A pin hole can only be as accurate as the setup that holds the part. If the fixture lets the workpiece move 0.01 mm under cutting load, no amount of tool precision will recover the position. For close-centre-distance pin pairs, we rough the holes, stress-relieve if the material calls for it, then finish in a single setup so both holes share one coordinate frame.

Tool runout is the quiet killer. A drill or reamer with 0.02 mm of runout cuts a hole 0.02 mm off-centre before the first chip forms. We indicate every reamer and boring bar in its holder before a critical job, and we keep dedicated holders for the reamers used on production pin holes so the setup is repeatable.

Thermal drift matters on long cycles. A machine that starts cold and runs for six hours will grow a few microns, which is enough to move a ±0.005 mm bore out of band. For tight work we warm the spindle and let the machine settle before the first finishing pass, and we keep the coolant temperature stable.

The order of operations matters too. If a pin hole is finished before a nearby face is milled, the milling vibration can knock the bore out of round. Finish the interrupted cuts first, then the pin hole last, so nothing disturbs it after it is measured.

• 1
  Single-setup finishingBoth holes in a pair share one coordinate frame.
• 2
  Tool runout checkIndicate every reamer and boring bar before a critical job.
• 3
  Thermal settlingWarm the spindle before the first finishing pass.
• 4
  Operation orderFinish interrupted cuts before the pin hole, not after.

Inside diameters are harder to measure than outside ones, and calipers are the wrong tool. A two-point caliper reads across a lobed hole and reports a diameter that does not exist. For anything under H7 class, use a bore gauge set against a ring gauge, or an air gauge for production volumes. Both read the true diameter, and both can be checked against a known master.

Position is measured separately. A coordinate measuring machine with a suitable stylus gives true position relative to the datums on the drawing. For pin holes on a close pitch, a gauge pin through both holes is a fast functional check: if the pin passes, the joint works, regardless of what the numbers say. Functional gauging and CMM data answer different questions, and good drawings ask for both.

Surface finish on a small bore is usually verified by comparison, not by a portable roughness tester, because the stylus cannot reach into a Ø3 mm hole. If finish is critical, we cut a witness coupon from the same material and setup and measure that instead.

Every part we ship goes through raw material check, in-process monitoring and final inspection, and inspection reports are available on request. For a pin hole that carries a safety function, we will agree the measurement method with you before the first article is cut, so nobody argues about the result afterward.

• 1
  Avoid calipersA lobed bore reads oversize on a two-point tool.
• 2
  Bore gauge or air gaugeSet against a ring gauge, checked against a master.
• 3
  Functional pin checkGauge pin through both holes proves the joint, not the number.
• 4
  Witness couponFor finish on bores too small for a stylus.