Tapping vs. Reaming: Differences, Uses, and How to Choose

Quick Answer: Tapping vs. Reaming at a Glance

Tapping cuts internal threads into a correctly prepared hole. Reaming removes a small amount of material from a drilled hole to improve its diameter accuracy, roundness, and surface finish. They are not competing versions of the same operation: the choice depends on whether the drawing calls for a threaded feature or a controlled, finished bore.

Rigid CNC tapping creates the internal thread after the tap drill — alignment, chip control, and correct drill size matter more than speed.

What Is Tapping?

Tapping is the operation of cutting internal threads into a pre-drilled hole. A tap has cutting edges shaped to the thread profile; as it advances, it forms the thread that will engage a mating screw or bolt. The hole must be drilled to the correct tap drill size before tapping. If it is too small, the tap removes too much material and may break; if it is too large, the resulting thread has less engagement and may not meet the intended strength or fit.

Tap drill size depends on the thread diameter, pitch, and desired thread engagement. We check the drawing and a reliable tap-drill chart rather than selecting a drill by nominal thread diameter alone. Common thread systems include Unified National Coarse (UNC), Unified National Fine (UNF), and metric threads. Their pitch and profile differ, so the tap, drilled hole, and fastener must all agree with the callout.

How Taps Cut Threads

A tap's chamfer gradually brings its cutting teeth into engagement, while the flutes provide space for the cutting edges and chips. Depending on tap design, chips may travel forward, back along the flutes, or be formed into a thread without being cut. Chip control matters because compacted chips can jam the tool, damage the thread, or break a tap—especially in a blind hole, where chips have limited room to escape.

For a blind hole, the usable threaded depth is not the same as the drilled depth. The tap's lead and the drill-point geometry occupy space at the bottom. We allow for those features in the hole-depth plan and select a tap style that can evacuate chips without packing them into the end of the hole.

Rigid vs. Tapping With a Floating Holder

On a CNC machine with rigid tapping, the spindle rotation and Z-axis movement are synchronized to the thread pitch. That coordination lets the tap advance and reverse in step with the thread. It is a common, efficient approach when the machine, program, tool, and workholding are set up for the operation.

A floating or tension-compression tapping holder provides a small amount of axial movement to accommodate slight differences between spindle feed and the tap's natural advance. It can be useful in manual or less rigid setups, but it does not correct a wrong tap drill size, poor alignment, or unsuitable tapping conditions. In either setup, we confirm the thread callout, depth, and chip strategy before running the part.

Reaming (and related hole-finishing) brings a drilled bore to size and surface finish for H7-class fits — it does not cut threads.

What Is Reaming?

Reaming is a finishing operation that enlarges and smooths an existing hole to a more controlled diameter. A reamer is not normally used to create a hole from solid material; the hole is drilled first and left slightly undersize, then the reamer removes a small, planned amount of stock. That finishing pass can correct minor drill wander and improve the hole's roundness and consistency.

The stock allowance is often a few thousandths of an inch or a few tenths of a millimeter, but the right amount depends on the reamer, material, hole diameter, and process. Too little stock can leave the reamer rubbing instead of cutting. Too much can overload it and lead to chatter, poor finish, or an inaccurate bore. We use the tool manufacturer's guidance and validate the allowance for the specific job.

How Reamers Improve Hole Quality

A reamer's multiple cutting edges remove a light layer of material from the predrilled hole. With adequate stock, a sharp tool, suitable feed and speed, and a rigid setup, the operation can produce a more consistent diameter and a smoother bore than drilling alone. It can also improve roundness and reduce some of the variation left by a drill.

Reaming is a finishing operation, not a cure-all for a badly misplaced or crooked hole. It follows the existing hole path; significant position or straightness errors may need a different strategy, such as boring or a revised setup. For critical features, we plan how the hole is located and supported as well as how its final diameter is achieved.

Types of Taps Used

Tap selection depends on hole type, material, depth, chip direction, and whether the tap cuts or forms the thread. The names below describe common tap styles; actual performance also depends on the geometry and coating specified by the toolmaker.

1. 1

   Taper tap

   Has a long chamfer that engages gradually, making it useful for starting threads and for through holes where the longer lead can pass through the workpiece.

2. 2

   Plug tap

   An intermediate-chamfer style used for general-purpose threading. It is a common choice when the hole has enough depth for its lead-in.

3. 3

   Bottoming tap

   Has a short chamfer and can extend threads closer to the bottom of a blind hole, usually after the thread has been started with a more gradual tap when needed.

4. 4

   Spiral point, or gun, tap

   Directs chips forward, making it well suited to many through-hole applications where chips can exit at the far side.

5. 5

   Spiral flute tap

   Carries chips back out of the hole, which can make it useful for blind holes and materials that produce stringy or sticky chips.

6. 6

   Forming, or roll, tap

   Forms a thread by displacing material rather than cutting chips. It is suited to ductile materials such as aluminum when the material, hole size, and process are appropriate.

When someone asks, “What are the three types of tapping?”, the usual basic answer refers to the three common chamfer styles: taper, plug, and bottoming. Spiral-point, spiral-flute, and forming taps are additional tool designs chosen for chip control, hole geometry, and material.

Types of Reamers Used

Reamers are available for hand finishing, machine production, adjustable sizing, and specific hole shapes. The right choice depends on diameter, tolerance, access, quantity, and the way the part will be located and held.

• Hand reamer

  Turned manually for controlled finishing, often with a long lead to help it enter the hole. Fixed and adjustable versions are available for different requirements.

• Machine reamer

  Used in a drill press, machining center, or other machine setup. It requires suitable alignment, spindle control, and workholding to produce a consistent result.

• Chucking reamer

  A common production tool with a straight or spiral flute, selected according to material, chip behavior, and the machine setup.

• Adjustable reamer

  Allows minor diameter adjustment to compensate for wear or achieve a particular size within the tool's design range. It is not a substitute for a controlled measurement process.

• Expansion and shell reamers

  Can be economical for larger diameters or repeat production, depending on the application and tooling arrangement.

• Taper reamer

  Finishes a tapered hole, such as a hole intended to receive a tapered pin.

Tapping vs. Reaming: Key Differences

The key difference is the feature each operation produces. Tapping creates a helical internal thread form; reaming finishes a cylindrical or tapered bore. Both typically follow drilling, but a hole may be reamed before tapping if the specified thread's tap-drill size and process call for it. Reaming does not create threads.

Tapping vs. Reaming: Comparison Table

We use this table as guidance from our shop, not a substitute for the drawing, toolmaker recommendations, or a process trial on a critical feature. Actual capability varies with the material, geometry, machine, tooling, and inspection method.

How Tapping and Reaming Work in Practice

Both operations normally begin with a located hole. The sequence then branches according to the required feature:

1. 1

   Locate and spot the hole

   We locate to the part datum and use a suitable spotting strategy so drilling stays accurate and the drill is less likely to walk.

2. 2

   Drill for the intended operation

   For tapping, drill to the appropriate tap drill size. For reaming, drill undersize by the allowance recommended for that tool, material, and diameter.

3. 3

   Tap or ream with a matched setup

   Run the appropriate toolpath, keep the tool aligned, and manage lubrication and chips for the material and hole geometry.

4. 4

   Inspect the feature that matters

   Check thread fit and depth for a tapped hole; measure diameter and verify fit or finish requirements for a reamed bore.

Precision, Tolerances, and Surface Finish

A reamed hole is generally more controllable in diameter and finish than a hole produced by drilling alone, provided the preceding hole, allowance, tool, and setup are appropriate. Reamed bores are used when components such as bearings or dowel pins need a defined fit, and when a press-fit feature depends on a controlled hole size. The specific tolerance must come from the drawing and be verified with an appropriate inspection method.

Tapping precision is described through thread specifications and fit classes rather than a smooth-bore diameter tolerance alone. For example, 2B is a common Unified internal thread class and 6H is a common metric internal thread tolerance position/class. The correct class is set by the drawing and thread system. Tap quality, alignment, hole size, material, and process control all affect whether the finished thread meets that requirement.

Surface finish also differs by feature. A reamer can leave a smoother bore than drilling, while a tapped hole is judged by the thread form and fit of the mating fastener. If a design calls for a tight bore fit and a threaded feature, these may need separate operations in separate locations—or a carefully designed sequence when they share a feature. Reaming a hole does not improve or create its threads.

Strength Considerations

For a tapped joint, thread engagement influences how much load the internal threads can carry. A thread engagement percentage around 60–75% is often used as a practical design reference, but the required engagement depends on material strength, fastener size, load, and application. We do not treat that range as a guarantee of joint strength; the design and applicable engineering requirements determine the actual specification.

Reaming can help preserve a hole's intended fit and geometry for a load-bearing pin, bearing, or other fitted component. The fit, wall thickness, material, and load path still matter. A hole that is oversized, out of position, or damaged during finishing can compromise the assembly even if it looks smooth.

Cost and Cycle Time Considerations

Tapping may be a relatively quick way to add a threaded feature, but the true cost per acceptable hole includes the tap, setup, cycle time, inspection, and tool replacement. Taps can wear or break, particularly when the hole size, material, lubrication, or chip flow is not controlled. On a difficult blind hole or a material that produces challenging chips, the risk and tool life deserve attention in process planning.

Reaming adds a finishing operation and may require a dedicated tool, so it can increase cycle time compared with drilling alone. That extra step is justified when the drawing requires a bore quality that drilling cannot reliably deliver. In some CNC jobs, drilling and reaming can be combined within one program and setup, reducing handling even though the reaming pass remains an additional tool operation.

For prototypes and low-volume parts, we consider whether the specified tolerance truly supports the function or whether a simpler process can meet the need. At higher volumes, tool life, cycle consistency, inspection frequency, and scrap risk can matter more than the nominal time for one operation. A process decision should be based on total cost and quality requirements, not operation time alone.

Material Suitability

Material changes the way chips form, how much heat builds up, and how the tool wears. Our BG CNC team selects tap and reamer geometry, coating, cutting conditions, and fluid for the specific alloy or nonmetallic material—we do not transfer settings blindly from one job to another.

• Steel and stainless steel

  Sulfurized or cobalt taps may be appropriate depending on the grade and operation. Stainless steel can work-harden, so a sharp tool, stable feed, suitable lubrication, and an adequately rigid setup are important. Reaming also calls for a sharp, rigid tool and a planned allowance.

• Aluminum

  In suitable aluminum applications we often use forming taps or sharp spiral-flute taps. For reaming, we run appropriate lubrication and chip management to reduce built-up edge and protect the bore finish.

• Cast iron

  We often dry-machine some cast-iron jobs because the material produces short chips, but we still match tool choice and process conditions to the casting and quality target. Both tapping and reaming are possible with suitable tooling.

• Plastics and composites

  These materials can be sensitive to heat, deformation, fiber pullout, and chip behavior. Specialized tooling and lower speeds may be required, with conditions adapted to the particular material and laminate or polymer construction.

• Titanium and high-temperature alloys

  On these materials we generally run slow cutting conditions, a rigid setup, and suitable quality coolant. We plan tool engagement and chip evacuation carefully to limit heat and tool damage.

Common Applications by Industry

On real parts we machine, tapping and reaming often appear side by side because an assembly may need both fastener retention and precise locating. We let the functional requirement of each feature drive its process and inspection plan.

• Automotive

  Tapped holes secure threaded fasteners in components and assemblies; reamed bores may be used for bearing fits or other accurately located interfaces.

• Aerospace

  Tight-tolerance reamed holes may support fasteners and structural joints, while tapped features can be used for threaded interfaces or inserts as defined by the design.

• Medical devices

  Small, precise holes and miniature threads can require careful process selection, suitable inspection, and clear drawing requirements.

• Electronics and enclosures

  Tapped mounting points hold covers or components; reamed dowel holes can support repeatable location during assembly.

• General fabrication

  Threaded holes accept bolts and screws, while reamed holes are useful when pins or other fitted elements need a controlled bore.

Can You Drill and Tap in One Operation?

Not as a true simultaneous operation with a standard drill and tap. In a CNC workflow, the machine typically drills the tap hole and then taps it sequentially in the same program, often using a tapping cycle. This can avoid moving the part between separate setups, but drilling and thread creation remain distinct operations.

Combination drill-tap tools and other step tools can perform more than one action in a tool sequence, but they have application limits. Hole depth, material, chip evacuation, tool rigidity, and the required thread quality all affect whether a combination tool is suitable. The wrong combination can increase the risk of chip clogging, breakage, or misalignment.

Can You Ream With a Hand Drill?

For production or fit-critical holes, our BG CNC team treats a hand drill as the wrong tool: it lacks the rigidity, alignment, and speed control that reaming needs. A hand reamer turned by hand can finish small one-off holes; we quote CNC, a drill press, or a machining center when diameter, roundness, or repeatability matters on your package.

How to Choose: Tapping or Reaming?

Start with the function on the drawing. If a fastener needs to engage internal threads, tap the hole to the specified thread. If a pin, bearing, or fitted component needs a controlled bore, drill undersize and ream to the required diameter. If the part needs both functions, specify and plan them independently rather than assuming one operation will achieve both.

Next, review the tolerance and surface finish, material, hole depth, production quantity, and available machine setup. A blind threaded hole raises different chip-control questions from a through hole. A precision bore requires enough stock for the reamer and a stable way to maintain alignment. For low- to medium-volume custom parts, these process details can affect both risk and total cost.

Best Hole-Making Method for Tight Tolerances

For a tight bore-diameter requirement, reaming—or boring for larger holes or where position and geometry need greater control—typically offers better diameter control than drilling alone. The appropriate method depends on the tolerance, diameter, material, geometry, and setup. A reamer follows the existing hole, so it does not automatically correct a significant position error.

Threads cannot be “reamed” into a thread. If a thread needs tighter control or the thread is large, thread milling may be considered, depending on the part and machine. The drawing should clearly define the thread standard, class, and inspection expectations so the chosen process can be evaluated correctly.

Call out thread class, effective depth, reamed fit, and gauging on the RFQ so the shop quotes the process you actually need.

RFQ Checklist for Threaded & Reamed Holes

Before you send us a drawing for tapping or reaming review, attach or note the following. Clear callouts help our BG CNC team reduce broken taps, wrong fits, and requotes:

Blind holes in hard materials raise broken-tap risk. Rigid tapping, correct chip strategy, and gauge capability at the shop matter as much as unit price—especially when a seized tap scrapes an expensive blank.

Common Mistakes and Troubleshooting

Many problems attributed to a tap or reamer begin earlier—with the hole, setup, or process assumptions. When a feature fails inspection or a tool breaks, our BG CNC team reviews the complete operation rather than changing the tool first.

• 1

  Wrong tap drill size

  A hole that is too small raises cutting load and breakage risk; one that is too large can leave weak or oversized threads. Verify the thread callout and tap-drill chart.

• 2

  Incorrect reaming allowance

  Too little stock can cause rubbing; too much can produce chatter, poor finish, or excess tool load. Follow toolmaker guidance for the material and diameter.

• 3

  Insufficient coolant or lubrication

  Inadequate lubrication can increase galling, heat, wear, and poor surface quality. Select fluid and delivery for the material and operation.

• 4

  Forcing the tool

  Excessive force does not compensate for a wrong feed, blocked chips, or misalignment. Stop and investigate rising torque, abnormal noise, or poor finish.

• 5

  Poor alignment or excessive runout

  Runout and misalignment can cause uneven cutting, oversize holes, damaged threads, and premature tool failure. Check the holder, spindle condition, workholding, and hole location.

• 6

  Dull or worn tooling

  Worn edges can raise cutting forces and degrade size or finish. Monitor tool condition and replace or recondition tools before quality becomes inconsistent.

Frequently Asked Questions

Conclusion

Tapping creates internal threads; reaming finishes an existing hole to a more controlled size and surface. The simplest decision rule is to start with the feature's function, then match the tool and process to the material, tolerance, hole geometry, production volume, and inspection requirement. Getting the tap drill size or reaming allowance right—and maintaining alignment and chip control—helps protect both part quality and cycle time.

At BG CNC, we review custom parts in context: material, geometry, tolerance, quantity, lead time, and target cost all influence the manufacturing route. Our engineering team coordinates CNC machining and other suitable production resources through our network of proven manufacturing partners in Dongguan and the Pearl River Delta. If a drawing includes critical threads, fitted bores, or several processes, our BG CNC team can help assess the requirements and flag practical questions before production.

Share your drawing package with our BG CNC team for a tapping/reaming process review and quote—not a generic encyclopedia answer.