Case: Hole Quality

A tight-tolerance hole needs a sequence, not a drill. This case plans drill → (bore or ream) with point depth, stock allowance and concentricity handled correctly.

Concept

This case chains drill (page 17) with reaming/boring (page 18): size a near-net drilled hole to a tolerance, while planning the conical point and the small reaming stock. Concentricity and position decide whether boring is required.

Why It Matters

A hole that drills to size but wanders, or that ignores the 118° point, either fails inspection or leaves a conical bottom. Planning the sequence up front avoids both.

How It Works

Required hole (frozen CTT-H1): Ø20 with tolerance 20.000–20.021 mm, good finish, in a 20 mm 304 stainless plate, located to a datum. Step 1: pre-drill Ø19.80 (leaving 0.10 mm radial stock to Ø20, page 18). Step 2: if position must be corrected, bore; if the pre-hole is aligned and only needs sizing, ream. Add the 118° point height for the Ø19.80 pre-drill (page 17: h = 9.9/tan59° ≈ 5.949 mm). Keep boring-bar overhang within limits (page 16).

Worked Example

For the located Ø20 hole: pre-drill Ø19.80. If the pre-hole is already aligned, ream directly to Ø20; bore only when position or straightness must be corrected, leaving a reamer-confirmed stock before the ream. Three reamed sections measure 20.006 / 20.014 / 20.023 mm against the upper limit 20.021 — the third is 0.002 mm over, while the mean (20.014) sits inside, exactly the trap of judging by average. The Ø19.80 pre-drill point height is 5.949 mm when planning through depth.

Planning the depth correctly

For a Ø20 through hole in the 20 mm 304 plate, the Ø19.80 pre-drill does not stop at 20 mm. Its 118° point leaves a conical tip, so the total depth commanded is plate thickness plus the point height h = 9.9/tan59° ≈ 5.949 mm — about 26 mm. For the finishing sequence, leave a small radial stock (0.10 mm) after pre-drilling at Ø19.80, then bore or ream. If the hole must be on position, bore before reaming; reaming follows an existing hole and will not correct a wandering one. Keep boring-bar overhang within limits (page 16), because a long, thin bar chatters even with the right tool. This case ties pages 17 and 18 together into one sequence rather than two isolated operations.

Inspection after the sequence

The plan is not done until the hole is checked. Measure diameter against the H7 tolerance, inspect the finish, measure the diameter with a suitable internal gauge, and check hole location against its datum on a CMM or dedicated fixture. If the diameter trends large across a batch, suspect reamer wear or reaming stock creeping up; if position is off, the boring step — not the ream — is where to look. Record the result on the wear-trial log so the sequence is repeatable. This closes the loop from tooling choice to inspection that the rest of module M8 expects.

When to bore and when to ream

The choice between the two finishing steps is decided by what the hole actually needs, not by preference. Reaming follows an existing, aligned hole and improves its size and surface; it cannot move the hole sideways or correct a wandering drill. So if the drilled hole is already on position and the only job left is to reach Ø20 (20.000–20.021) with a good finish, ream directly — cheaper and faster. If the position is wrong, or straightness matters and the drill walked, boring comes first: the boring bar cuts a true, located hole, and only then does the reamer size and finish it. A second check on stock: leave too little for the reamer and it rubs instead of cuts, giving a poor, oversized hole; leave too much and the reamer is overloaded and wears fast. The 0.10 mm radial figure is the classroom starting point, confirmed against the reamer maker for the material.

Why the reamer follows the drill, not the other way round

A reamer has several cutting edges guided by the hole it is already in — it is a sizing and finishing tool, not a locating tool. That is why it cannot rescue a mispositioned drill: it follows the existing bore and smooths it, reproducing its position and straightness rather than correcting them. The practical consequence is the ordering in the table: spot first to start true, drill near-net, then bore if position or straightness is off, and ream last to size and finish. It also explains why reaming stock must be small and even — if the drilled hole is out-of-round or tapered, the reamer's edges take an uneven bite and wander with the error instead of correcting it. This is the same logic as page 18: reaming assumes a good, aligned pre-hole, and the whole sequence exists to hand the reamer exactly that. When a reamed hole comes out off-location, the cause is most likely upstream in spotting, drilling or boring — but also check reamer runout, alignment and guide, rather than treating one station as the unique source.

Why spot drill comes before the twist drill

The first row of the sequence looks optional — a spot drill that only makes a shallow dimple — but it is what protects the position the rest of the steps depend on. A twist drill is self-centering only in an ideal world: its chisel point has poor centring action, and if it is fed straight onto a flat or slightly slanted surface it can wander or "walk" sideways before the full flutes engage, moving the hole off its programmed location. The spot drill (or spotface) cuts a flat, true starting cone exactly on the programmed XY, giving the drill's point a seat that cannot slip. This matters most on angled or curved surfaces and on holes that must be located to a datum, which is exactly the case here. It also improves tool life: a drill that starts true cuts symmetrically, instead of scrubbing in on one side and chipping its corner. The few seconds of the spot step is what makes the later bore-and-ream sequence worth planning — if the start position is wrong, no amount of finishing upstream of the reamer will put the hole back where the drawing wants it.

Why the inspection result points back to one step

Inspection narrows the investigation; it does not identify a unique faulty station. If the finished hole is the right diameter and finish but its position is off, the error is most likely upstream in spotting, drilling or boring — but also check reamer runout, alignment, guide and stock distribution. If the position is true but the diameter trends large or the finish is rough, the likely problem is at the reaming step: reamer wear, uneven stock, or wrong feed/coolant (page 18). If the hole is tapered or out-of-round, look at the drilled pre-hole and boring-bar rigidity (page 16), because a reamer reproduces an uneven pre-hole. Drilling followed by reaming can meet a positional requirement when the pre-hole and process capability already satisfy it; if they do not, evaluate a corrective operation with sufficient stock, and do not rely on reaming to relocate the hole. A shop that inspects after each step corrects the likely station without reworking the whole sequence, but still confirms the setup before closing the case.

Why reaming needs clean, directed coolant

Because a reamer finishes to a close tolerance and a good surface, the coolant condition matters more here than in rough drilling. Two failure modes trace directly to coolant: a reamer that rubs and welds in a gummy material (page 04) because the edges were starved of lubrication, and a hole that comes out oversized because chips packed under the cutting edges and were reamed over instead of flushed out. Directed, through-tool or well-aimed coolant both lubricates the edges and washes the small reaming chips out of the hole before they score the wall. This is why the sequence lists coolant alongside the tools rather than as an afterthought, and why reaming in dry conditions on a gummy material so often disappoints even with a sharp, correctly sized reamer. It also feeds the inspection read-out: if the wall scores longitudinally, suspect chip packing and coolant direction before blaming the reamer. A reamer is a precision edge, and it needs a precision coolant supply to behave like one.

Why the pre-hole quality sets the ceiling the reamer can reach

A reamer only removes a thin, even layer, so it can only "improve" a hole that is already straight, round and correctly sized to a small stock. If the drilled pre-hole is tapered, bell-mouthed or out-of-round, the reamer's edges take an uneven bite around the circumference: the side with more stock cuts harder, deflects the tool, and the finished hole reproduces the pre-hole's error rather than correcting it. This is why the sequence invests in spotting and boring before the reamer — each upstream step exists to hand the reamer a geometrically honest hole, so its small, even cut can do nothing but size and polish. The practical rule for the case is that the final hole cannot be better than the pre-hole's straightness; if inspection finds the finished hole lobed or tapered, look first at the drilled or bored pre-hole (page 18), but also check reamer runout, alignment and stock; re-running a worn reamer alone will not correct a bad pre-hole. Equally, leaving exactly the 0.10 mm radial stock means the reamer cuts uniformly, which is the condition that lets a multi-tooth tool stay on its guided path and produce a clean, on-size H7. Good holes are built step by step; the reamer is the last, finest station, not the rescue.

Why hole position is set before sizing is even considered

The table on this case spends three of four steps getting the hole's location and straightness right before a single sizing cut, and that ordering reflects a principle that surprises beginners: position cannot be fixed by the finishing operation. The reamer follows the existing bore, and the boring bar is what actually moves and trues the hole, so if the spot drill or drill has walked off the programmed XY, only a bored cut on location can bring the axis back to the datum. This is why a drawing that demands both an H7 diameter and a positional tolerance is usually met by controlling position at the spot and bore stations first — the reamer follows wherever the pre-hole axis is, so position is earned upstream rather than corrected at the reamer. The practical takeaway for the case is that the position budget is spent at the spot and bore stations, and the reamer's job is strictly the 0.10 mm sizing layer. When inspection flags a positional error, the diagnosis first goes upstream to spotting and boring, then also checks reamer runout, alignment, guide and stock, and that is exactly the step-localised logic the page's inspection section builds on.

Why this sequence repeats for every hole pattern

A single located hole demonstrates the chain, but the same sequence is what makes a bolt circle or a hole pattern repeatable. Once the spot-drill position, the Ø19.80 pre-drill depth with the 5.949 mm point add, and the bore/ream order are locked for the Ø20 feature, every hole in the pattern follows the identical steps at each coordinate — which is why the Engineering Tools bolt-circle and hole-pattern calculators exist upstream: they give the coordinates, while this case gives the per-hole operation chain. The two connect cleanly: a pattern calculator outputs positions, and this page outputs what happens at each position. Batching the same drill at all locations first, then the bore, then the ream, also reduces tool changes and keeps each station's inspection consistent. Treat the hole as a recurring feature with a fixed recipe rather than a one-off, and positional and diameter drift across the pattern becomes a matter of checking one tool rather than re-planning each hole.

The hole sequence is judged one attribute at a time. Position and straightness are the responsibility of the spot and bore stations, while diameter and surface finish belong to the reaming station. Inspection after each step narrows the cause without assuming one tool is at fault; reaming reproduces whatever the pre-hole gives it. When a hole fails, walk the sequence from the start and confirm each upstream station rather than re-running the reamer and hoping.

Common Mistakes

• Reaming a mislocated hole. Fix: bore first to correct position.
• Forgetting point depth. Fix: add h = 5.949 mm for the Ø19.80 pre-drill.

Practice