What is a gun reamer?

A gun reamer is a reamer designed for deep holes. It carries cutting edges plus guide pads that ride in the already-drilled bore, so the edges stay concentric regardless of hole length. It is used to size and finish a deep hole after gun drilling, typically reaching IT4-IT6 and Ra 0.2-0.8 µm.

Does gun reaming straighten a hole?

No. A gun reamer follows the axis of the existing hole — it sizes the diameter and improves surface finish, but it cannot correct a wandering or crooked bore. Straightness must be achieved in the drilling step, before reaming.

How much stock does a gun reamer remove?

A gun reamer removes a thin wall layer, typically 0.05-0.3 mm on diameter depending on hole size. The pre-drilled diameter must be matched to the reamer's stock allowance — too much stock causes chatter and load, too little gives no clean cutting edge.

Gun reaming vs honing — which should I use?

Ream when the hole is already straight and you need tighter size or finish in volume — it is the fastest and cheapest option and works on blind holes. Choose honing when you need oil-retention crosshatch or must correct roundness and cylindricity, at the cost of slower throughput.

Why is it called a gun reamer?

The name comes from gun barrel making, where reamers size long bores to tight tolerance. In modern machining the term also collides with oil-and-gas downhole tools and firearm barrel reamers — on this site a gun reamer means the metal-cutting deep hole finishing tool described here.

🎯 FINISHING · GUN REAMERS

Gun Reaming

A gun reamer is a reamer built for deep holes: it carries guide pads that ride in the already-drilled bore, so the cutting edges stay concentric no matter how long the hole is. When a gun-drilled bore needs a size or finish the drill alone cannot hold, gun reaming sizes it in one pass — the step that prepares the hole for whatever comes next.

IT4–IT6sizeAchievable diameter tolerance

Ra 0.2–0.8µmTypical as-reamed finish

0.05–0.3 mmstockDiametral allowance

Follows axisgeometryDoes not straighten

PRINCIPLE

How Gun Reaming Works

A gun reamer finishes a bore that already exists. It carries one or two cutting edges that skim a thin layer off the wall, plus guide pads that ride in the drilled hole and keep the reamer concentric. Coolant is delivered through the tool — the same through-tool arrangement a gundrill uses — so chips clear continuously even at high length-to-diameter ratio.

💡 What reaming cannot do: a gun reamer follows the axis it is given. It sizes the diameter and improves the finish, but it does not straighten a wandering hole — if the gundrill walked, the reamer walks with it. Straightness must come from the drilling step.

⚠️ Name confusion: in machining, a gun reamer is this deep-hole finishing tool. It is not the “gun-drill reamers” used in oil & gas downhole assemblies, and not the barrel reamers used in firearm making. If you came here from either of those, this page is about metal-cutting deep hole finishing.

WHEN TO REAM

Ream, Hone or Skive?

Three finishing routes size and finish deep bores. Reaming is the fastest and cheapest for a straight hole that just needs size and finish; honing and skive & roller burnishing bring more capability — and more setup.

	Gun reaming	Deep hole honing	Skive & roller burnishing
Purpose	Size a drilled bore	Remove stock, correct roundness	Size + finish in one pass
Straightness	Follows existing axis	Slight improvement	Improves straightness
Typical finish	Ra 0.2–0.8 µm	Ra 0.1–0.4 µm	Ra 0.1–0.4 µm
Blind holes	Yes	Yes	No — needs a through-hole
Throughput	Fastest	Slow	Fast
Oil retention	No crosshatch	Crosshatch	No crosshatch

✅ Rule of thumb: ream when the drilled hole is already straight and you need a tighter size or better finish in volume. Choose honing when you need oil-retention crosshatch or must correct roundness. Choose skive & roller burnishing when straightness itself is the spec and the bore is through.

DESIGN

Gun Reamer Design

Cutting edges: single-flute and two-flute designs. Fewer edges means fewer runout sources and easier chip control at depth; more edges improve finish on shorter holes.
Guide pads: the pads ride the existing bore and set the reamer on the axis. Pad width and position are sized to the hole; wear on the pads is what limits reamer life.
Carbide: carbide bodies or carbide-tipped edges hold size over long runs. Coatings (TiAlN family) extend edge life on abrasive or sticky materials.
Coolant-through: internal coolant flushes chips ahead of the pads — essential above a few diameters deep.
Starting bushing: on a dedicated machine, the reamer enters through a starting bushing; on a machining center, use the same pilot discipline as gundrilling.

💡 Diameter control: because the reamer follows the drilled bore, the drill-to-reamer size step is what determines the final hole. Match the pre-drill diameter to the reamer’s stock allowance — too much stock makes the reamer load and chatter; too little gives no clean cutting edge.

PARAMETERS

Stock Allowance & Parameters

Reaming is a light-cut operation: the reamer removes only a thin wall layer, so speeds and feeds are set for finish and tool life, not metal removal.

Parameter	Typical guidance	Notes
Stock allowance	0.05–0.3 mm on diameter	Scale with diameter; less on small holes
Cutting speed (Vc)	10–25 m/min	Lower than drilling; heat control is the goal
Feed	0.02–0.08 mm/rev	Per reamer diameter; heavier feed → rougher finish
Coolant	Through-tool, 40–150 bar	Pressure sized to diameter and depth

⚠️ Verify with the tool maker: these are starting points. Reamer geometry varies by manufacturer and material, and the correct stock-to-tool match is the difference between IT5 and scrap — confirm with the supplier and test on the first piece.

QUALITY

What Quality Can Gun Reaming Hold?

Diameter tolerance: typically IT4–IT6 (a few microns on moderate diameters), far tighter than an as-drilled bore.
Surface finish: Ra 0.2–0.8 µm as-reamed, ready for most sealing and bearing applications without further work.
Roundness & cylindricity: excellent when the pads are in good condition — pad wear shows up first as ovality.
Straightness: inherited from the drilling step, not improved by reaming.
Verification: bore gauges or air gauging for size, and a roundness machine if the print calls it out.

💡 The tolerance story: gundrilling as-drilled lands around IT6–IT11; a gun reamer takes that to IT4–IT6 and tightens the finish. When the print asks for both a tight diameter and good finish on a deep hole, ream-after-drill is often the most economical two-step sequence.

PROS & CONS

Reaming Trade-offs

✅ Why ream

Fastest way to size a straight deep hole
Works on blind holes
Lowest cost per hole of the finishing methods
Simple to add to an existing gundrilling setup

⚠️ Limits

Cannot correct a wandering or crooked axis
Removes little stock — the drill must be close to size
No oil-retention crosshatch (use honing)
Guide-pad wear affects roundness over long runs

Gun Reaming

How Gun Reaming Works

Ream, Hone or Skive?

Gun Reamer Design

Stock Allowance & Parameters

What Quality Can Gun Reaming Hold?

Reaming Trade-offs

✅ Why ream

⚠️ Limits

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