Hole-Making Guide

Reaming vs Drilling vs Tapping

Drilling makes the hole, reaming brings it to an exact size and finish, and tapping cuts a thread in it. This guide shows how each works on a CNC machine, what accuracy to expect and when to use which. For reamer types and tolerance tables, see What Is Reaming.

At a Glance

The Three Operations Compared

	Drilling	Reaming	Tapping

Purpose	Make a hole in solid material	Bring a drilled hole to exact size, roundness and finish	Cut an internal thread
Tool	Twist drill, spot drill, centre drill	Reamer with straight or helical flutes	Tap or thread mill
Typical accuracy	IT10 to IT12, about ±0.05 to ±0.1 mm on small holes	IT7 (H7), for example 0 / +0.015 mm on Ø10 mm	Thread class 6H
Material removed	The full hole	0.1 to 0.3 mm on the diameter	The thread profile only
Surface finish	Ra 3.2 to 6.3 µm	Ra 0.4 to 1.6 µm	Depends on tap and material
Use it for	Clearance holes and holes to be tapped	Dowel pin holes, bearing and shaft fits	Screw and bolt connections

Values are typical for metals on CNC machines. Tell us the fit or thread class you need and we confirm it at quotation.

Drilling

What Is Drilling in CNC Machining?

Drilling makes a round hole with a rotating cutting tool. It is the first step for almost every hole on a part, including holes that are reamed or tapped afterwards.

Twist drillsThe standard drill for steel, stainless, aluminum, brass and plastics. Carbide drills with through-coolant run faster and hold size better in production.

Spot and centre drillsMake a short, accurate start so the twist drill does not wander. Spot drills often add the chamfer in the same move.

How a CNC mill drillsThe X and Y axes position the hole, the Z axis feeds the drill. Deep holes are drilled in pecks so chips can clear.

Drilling on a 4th axisA rotary axis turns the part between holes, so holes on several faces are drilled in one setup. That keeps them in position relative to each other. See CNC milling.

Tapping

What Is Tapping in CNC Machining?

Tapping cuts an internal thread in a drilled hole, so the part can take a screw or bolt. The tap drill size sets how deep the thread is, so it is chosen with the tap.

Spiral point tapsPush the chips ahead of the tap. The usual choice for through holes.

Spiral flute tapsLift the chips out of the hole. The usual choice for blind holes.

Forming (roll) tapsPress the thread instead of cutting it, with no chips and a stronger thread. Good for aluminum and other ductile materials.

Rigid tapping and thread millingIn rigid tapping the spindle turns in exact sync with the Z feed. Thread milling cuts the thread with a small cutter, which suits large threads and hard materials and avoids broken taps.

Reaming

Where Reaming Fits

Reaming takes a small, even cut from a drilled hole to reach an exact diameter, typically H7, with a smooth wall. A reamer follows the existing hole, so it improves size and finish but not position; when position matters, the hole is bored instead. Reamer types, tolerance tables and machines are covered in What Is Reaming.

Process

A Typical Hole Sequence

Spot drill to mark the exact position and add a chamfer.

Drill to the tap drill size, or 0.1 to 0.3 mm under the final size when the hole will be reamed.

Ream for a precision fit, or tap for a thread.

Deburr and check: plug gauge for reamed holes, thread gauge for tapped holes.

Best Practices

Getting Holes and Threads Right

Tool and coating per materialAluminum, stainless and plastics each need different drill geometry and coating for tool life and hole quality.

Peck deep holesAbove about three times the diameter, drill in steps or use through-coolant drills so chips do not pack.

CoolantKeeps the tool cool, flushes chips and improves the finish, especially in stainless and when tapping.

Rigid fixturingA part that moves or vibrates gives oversize holes and broken taps.

Check tool wearA worn drill or tap changes hole size and thread quality before it breaks. Replace on a fixed count in production.

Troubleshooting

Common Problems and Fixes

Problem	Usual cause	Fix
Chips packing in the hole	Deep hole, wrong flute type, too little coolant	Peck drilling, spiral flute tap for blind holes, more coolant
Broken taps	Tap drill too small, chips in a blind hole, misalignment	Correct tap drill, deeper drilled hole, rigid tapping or thread milling
Oversize or tapered holes	Part deflection, worn or run-out tool	Better support of the part, new tool, ream after drilling

FAQ

Drilling, Reaming and Tapping Questions

What is the difference between drilling, reaming and tapping?

Drilling makes a hole in solid material. Reaming takes a small cut from a drilled hole to reach an exact size, typically H7, with a smooth finish. Tapping cuts an internal thread in a drilled hole so it can take a screw or bolt.

What tolerance does a drilled hole have?

A drilled hole is typically IT10 to IT12, about ±0.05 to ±0.1 mm on small diameters. When a hole needs a tighter fit, for example for a dowel pin or bearing, it is drilled slightly undersize and then reamed to H7.

Should I ream or bore a hole?

Ream when the hole needs an exact diameter and a smooth finish and its position from drilling is good enough. Bore when the position or straightness also has to be corrected, because a reamer follows the existing hole.

Can you tap blind holes?

Yes. Blind holes are tapped with spiral flute taps or forming taps, and the hole is drilled deeper than the thread so chips and the tap end have room. For large threads or hard materials we thread mill instead.

Have a part with precision holes or threads?

Send us your drawing with the fits and thread classes you need. You get a price, DFM comments and a lead time from our engineers, directly from our ISO 9001:2015 certified factory in Dongguan, China.

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