CNC Post Hole Treatment Guide for Engineers

• 1
  What cnc post hole treatment actually changes
• 2
  Reaming, boring, honing and deburring compared
• 3
  Where post hole treatment stops working
• 4
  How to call it out on a drawing and in a quote
• 5
  Method fit by tolerance, finish and wall condition

A drilled or milled hole is created by a tool that deflects, wears and heats up. The result is a bore with a slightly tapered wall, a helical feed mark, and a burr at both ends. Post hole treatment is the set of operations that correct those three things: geometry, surface, and edge condition. Nothing about the machining center guarantees a round hole on its own.

Take a Ø10 mm hole drilled 40 mm deep in 6061-T6. The drill walks 0.03–0.08 mm off center on a long tool, and the entry chamfer is often the only thing holding the diameter. Reaming the same hole with a 6-flute reamer at 0.15–0.25 mm radial stock removes 90% of that error and holds Ø10 H7 (+0.015/0). Same machine, same setup, one extra tool change.

The treatment also sets the surface the shaft or pin runs against. A reamed wall sits near Ra 1.6 μm. A honed wall reaches Ra 0.2–0.8 μm and corrects roundness rather than just smoothing peaks. If your seal or O-ring rides in that bore, the finish matters more than the diameter callout on the print.

So the engineering meaning is simple: post hole treatment converts a locating hole into a functional hole. It is a tolerance and finish decision, not a cosmetic one. Skip it and you inherit the drill's error; apply it blindly and you add cost your part cannot justify.

• 1
  GeometryRoundness, cylindricity and taper inside the bore.
• 2
  SurfaceRa and the direction of the tool marks.
• 3
  EdgeBurrs at entry and exit that break assembly.
• 4
  PositionTrue position relative to the datum after reaming.

Reaming is the workhorse. A floating or rigid reamer follows the existing hole and sizes it, typically holding IT7 to IT8 and Ra 1.6 μm. Feed 0.1–0.3 mm/rev, speed 1/3 of drilling speed. It will not fix a hole that is already off position by more than 0.05 mm, because the reamer follows the error rather than correcting it.

Boring is the correction tool. A single-point boring head cuts on a controlled radius, so it fixes position, taper and roundness in one pass. On our 5-axis centers and mill-turn machines we bore to ±0.005 mm and Ra 0.8–1.6 μm. Boring is slower per hole but it is the only method that truly corrects a wandering centerline.

Honing removes material with abrasive stones on an expanding mandrel. It is the choice for hydraulic cylinders, valve bores and pin bores where roundness under 5 μm and Ra 0.2–0.8 μm matter. Honing works on hardened steel, Inconel and 17-4PH where reamers dull fast. It adds a separate operation and needs a clean setup.

Deburring closes the job. Hand deburring with a scraper, abrasive flow, or a chamfer tool removes the exit burr that scrapes the mating bore during assembly. On cross-drilled holes, the internal burr at the intersection is the one that breaks parts. It is invisible on a drawing and it is the most common cause of a rejected assembly.

• 1
  ReamSizing pass, IT7–IT8, Ra 1.6 μm, follows the hole.
• 2
  BoreCorrects position and roundness, ±0.005 mm, Ra 0.8–1.6 μm.
• 3
  HoneRoundness under 5 μm, Ra 0.2–0.8 μm, hard materials.
• 4
  DeburrRemoves entry, exit and intersection burrs.

Depth-to-diameter ratio is the first wall. Reaming beyond 5:1 in a blind hole pushes chips ahead of the flutes and scores the wall. Boring beyond 8:1 needs a damped bar or a piloted tool, and we often split into two passes from both ends. If the print asks for a Ø6 mm hole 60 mm deep at H7, the answer is usually a different design, not a different process.

Thin walls deflect. Reaming a Ø20 mm hole through a 2 mm aluminium wall squeezes the material, so the hole measures oversize after the tool retracts. Boring with low radial depth and a sharp insert works better here. Plastic parts like POM and PC move even more; they are often better finished by a light bore than by a reamer.

Hardened and gummy materials change the tool. 17-4PH at 40 HRC will dull a high-speed steel reamer in a few dozen holes; carbide or honing is the safer route. Inconel work-hardens at the surface, so a reamer that rubs instead of cutting will leave a layer that is harder than the base metal.

Finally, position is fixed before treatment. If true position is off by 0.1 mm after drilling, no reamer or hone will bring it back. Redrill, bore the centerline on a 5-axis machine, or accept the part as a rework. Post hole treatment refines a hole; it does not relocate one.

Put the requirement where the machinist reads it. Diameter plus tolerance plus finish on the hole callout, and a note for the edge condition. A callout like Ø10 H7 Ra 1.6, deburr both ends is enough for a reamed hole. For a bore, add cylindricity and true position so the shop knows boring is required, not optional.

Do not stack an unnecessary finish on a clearance hole. Ra 0.8 μm on a Ø8 mm bolt clearance costs an extra operation and buys nothing. Reserve tight finish and tight roundness for bores that carry a bearing, a pin, a piston or a seal, where the surface actually does work.

For prototypes and low volume, one setup on a 5-axis machine often drills, bores and chamfers the hole without a second fixture. That keeps true position tight and avoids the re-clamping error that shows up when a part moves between a drill press and a mill. For 10,000+ part runs, a dedicated reaming station with a floating holder usually wins on cycle time.

Send the drawing with the functional requirement, not just the tolerance. If you tell us the bore holds a Ø12 h6 dowel pin, we will pick reaming or boring to hit it and inspect the diameter and roundness. If you only send a number, we can still make the hole, but we may over-process it or under-process it.

• 1
  Hole calloutØ + tolerance + Ra on the feature, not a general note.
• 2
  Edge noteState deburr both ends and chamfer size.
• 3
  Function noteSay what the bore carries: pin, bearing, seal, piston.
• 4
  InspectionRequest diameter, roundness and Ra reports if critical.