Drill and Tap Chart: Metric, UNC & UNF Tap Drill Sizes

Introduction

An internal thread starts with a drilled hole. The hole must be large enough to limit tapping load while leaving sufficient material for the tap to form the required thread profile.

The required drill diameter changes with the thread specification. For example, M6 x 1.0 has a different tap drill requirement from M8 x 1.25. This is the same for inch threads: 1/4-20 UNC and 1/4-28 UNF have the same nominal diameter but different thread pitches, so their starting hole sizes are different.

This guide explains:

• Internal threads: Common metric, UNC, and UNF tap drill charts.
• How to accurately read thread size, pitch, TPI, and tap drill reference.
• The typical aspects of drilling, tapping, and checking threaded holes in CNC machining.

Quick Reference: Drill and Tap Chart

A tap drill chart gives the starting hole diameter for cutting an internal thread. The values below are common starting sizes for conventional cutting taps. Actual drill size may vary with the target percentage of thread, thread class, workpiece material, tap geometry, and manufacturer recommendations. For production work, use the tap manufacturer's specified drill range as the final reference.

A tap drill is smaller than the nominal thread diameter because material must remain for the tap to form the internal thread. For example, an M6 x 1.0 cutting-tap hole commonly starts at 5.0 mm, while a 1/4-20 UNC hole commonly starts with a #7 drill. Actual production drill selection should follow the specified thread class and the tap manufacturer's recommendation.

Metric Tap Drill Chart

UNC Tap Drill Chart

Tap drill size may vary with the target percentage of full thread, internal thread class, workpiece material, tap type, and the tap manufacturer's recommendations. The sizes above are common starting points for standard UNC tapping.

UNF Tap Drill Chart

For further guidance on threaded-hole design, see our threaded hole guideline, which covers bottom-hole sizing, effective thread depth, thread inserts, and drawing requirements.

What Is a Drill and Tap Chart

A drill and tap chart is a reference used to match an internal thread size with the hole diameter required before tapping. It gives the machinist a starting hole size so the tap has enough material to form the thread without leaving the hole too small.

Engineers commonly check the chart when preparing:

• Technical Drawings
• Selecting Tooling
• Reviewing threaded features before production

The thread specification on the drawing determines which entry in the chart should be used. The chart is mainly a hole-size reference. It does not replace the thread callout on the drawing or the tooling manufacturer's recommendations for a particular material, tap type, thread class, and production application.

How to Read a Drill and Tap Chart

A drill and tap chart is easier to use when the thread callout is read in parts. The thread designation identifies the nominal size and pitch; the tap drill size gives the hole diameter to drill before tapping.

Metric Thread Size and Pitch

Metric threads are specified by nominal diameter and pitch in millimeters. For example, M6 x 1.0 indicates a 6 mm nominal diameter and a 1.0 mm pitch. Pitch is the axial distance between adjacent thread crests.

The same nominal diameter can use different pitches. M8 x 1.25 and M8 x 1.0 are both 8 mm threads, but they require different tap drill sizes. When reading a chart, the full designation should be matched rather than selecting a drill from the diameter alone.

ISO metric thread chart

UNC and UNF Thread Size and TPI

Inch threads are specified by nominal diameter and threads per inch (TPI). For example, 1/4-20 UNC has a 1/4-inch nominal diameter and 20 TPI, while 1/4-28 UNF has the same nominal diameter with 28 TPI.

The two threads therefore use different tap drill sizes. A chart entry cannot be selected from the nominal diameter alone; the thread series must also match the drawing. UNC is coarser, while UNF uses a finer pitch.

UNC and UNF thread profiles (Jarvis Cutting Tools)

Tap Drill Size and Thread Percentage

The tap drill size is the hole diameter prepared before the internal thread is cut. It is smaller than the nominal thread diameter because the remaining material forms the thread profile during tapping.

For example, a 1/4-20 UNC thread is not drilled with a 1/4-inch hole. The chart specifies a smaller pilot diameter so the tap has enough material to form the internal thread.

Tap drill charts may also list a percentage of full thread, which describes how much of the theoretical thread profile is produced by the tapped hole. This is different from effective thread engagement length, which refers to the axial length over which the internal and external threads engage. A larger tap drill generally produces a lower percentage of full thread, while a smaller tap drill produces a higher percentage. A longer tapped hole increases axial engagement, while changing the tap drill size changes the thread percentage.

On a production drawing, the chart value should be checked against the specified thread class and the tapping process. Material hardness, tap geometry, coating, and machine setup can affect the practical hole size, particularly in harder alloys or when cutting deep blind holes.

CNC Drilling and Tapping Process

A CNC machine normally makes a tapped hole in two main cutting stages: drilling the starting hole and cutting the internal thread. The finished hole is then checked against the drawing requirement before the part moves to the next operation.

Drilling Before Tapping

The hole is drilled first to the required starting diameter and depth. For a blind hole, the drilled depth needs to allow enough room below the specified thread length because the drill point and the end of the tap do not produce a fully threaded area at the very bottom.

The hole should also be clean and free of chips before tapping. On a CNC machine, the drilling operation is programmed with the required position, diameter, and depth from the part drawing.

CNC Tapping Process

During rigid tapping, spindle rotation and axial feed are synchronized with the thread pitch, allowing the tap to advance one thread pitch per revolution. The tapping cycle must match the specified thread size, pitch, hole depth, material, and tap type.

Blind holes require particular attention to the programmed depth because the tap needs clearance below the required effective thread length.

Additional clearance below the required effective thread depth may be necessary in blind holes, depending on the drill point, tap geometry, and required thread depth. Deep or unusually long tapped holes may require specialized tooling and additional machining time.

Thread Inspection

After tapping, the thread is checked to confirm that it matches the drawing. Internal threads are commonly verified with GO and NO-GO plug gauges selected for the specified thread class and tolerance.

Inspection should verify more than the presence of visible threads. Depending on the drawing, the check may include thread size, gauge acceptance, effective thread depth, hole position, and damage to the threaded surface.

For CNC parts with internal threads, include the thread designation, effective thread depth, and hole type on the drawing. JLCCNC can review threaded features during quoting and DFM.

Factors Affecting Tap Drill Size Selection

The tap drill is selected from the thread requirement, but the final hole size also depends on how the hole will be used and machined. Material, hole depth, and the amount of thread engagement are the main points to check before putting the hole on a CNC drawing.

Material Considerations

Tap drill for aluminum alloy CNC machining (iStock)

The workpiece material affects the cutting load on the tap. Material affects cutting torque, chip formation, and tap life. Aluminum and many brass alloys generally require less tapping torque than stainless steels and other difficult-to-machine materials, but the exact drill size should still follow the tap manufacturer's recommendation.

This matters when a drawing specifies a threaded hole in a difficult-to-machine material. The tap supplier's recommended drill range should be checked rather than assuming that the same hole size is suitable for every material.

Blind Holes vs Through Holes

Blind tapped hole showing effective thread depth and bottom clearance (ScienceDirect)

A through hole gives the tap space to pass completely through the part. For a blind tapped hole, the drilled depth must exceed the required effective thread depth to provide clearance for the drill point and the tap chamfer or runout.

For example, if a blind hole requires 10 mm of effective M5 thread depth, the 10 mm dimension should not be used as the total drilled depth. Additional depth is required below the effective thread to accommodate the drill point and the tap's incomplete-thread region. Space is needed below the usable thread because the drill has a point, and the tap cannot produce a full thread at the bottom of the hole.

For a blind hole, the total drilled depth should provide the required effective thread depth plus sufficient clearance for the drill point and the unthreaded portion created by the tap chamfer.

Thread Engagement Requirements

Do not specify a deep thread simply because more thread engagement sounds stronger. Once sufficient engagement is available for the joint, additional thread length can add machining time without providing a useful benefit.

As a JLCCNC DFM guideline, effective thread depth is generally kept to about three times the hole diameter under standard machining conditions. This is a manufacturability guideline rather than a universal strength requirement. Deeper threads may be possible, but they can require special tooling and increase machining time and cost.

For the drawing, specify the effective thread depth rather than assuming that the total drilled depth is the same as the usable thread depth.

Common Drill and Tap Issues

When thread quality is poor, inspect the finished hole, tap condition, chip evacuation, and tapping cycle to identify the actual cause before changing the process.

Poor Thread Quality

Poor thread quality can appear as rough or damaged thread flanks, incomplete thread forms, or failure of the specified GO/NO-GO gauge inspection.

Check the finished hole with the specified thread gauge. If the thread is damaged, inspect the tap for wear or chipped cutting edges and check whether chips were left in the hole during machining.

The correction depends on the cause. A worn tap should be replaced, while chip buildup requires cleaning the hole and improving chip removal during the operation.

Tap Breakage

Broken CNC tap in a blind threaded hole (Yamawa)

A broken tap leaves part of the tool inside the threaded hole and can make the part difficult to recover. Before attempting removal, check where the tap broke and the condition of the hole.

Inspect the tap for damage and check for chip buildup, excessive tool wear, tapping-cycle problems, or contact with the bottom of a blind hole. If the same tap keeps breaking during production, the machining setup should be checked rather than simply replacing the tool each time.

Drill and Tap Chart FAQs

Conclusion About Drill and Tap Charts

A tap drill chart tells you which hole size to prepare before tapping a thread. Start with the exact thread designation on the drawing, then match it to the corresponding drill size. A small change in pitch or TPI can change the required hole diameter.

For CNC machining, the drill size should also match the tapping method. A cutting tap removes material as it forms the thread, while a forming tap displaces it. The recommended hole size can therefore differ, especially when the drawing calls for a specific thread fit.

For CNC parts with tapped holes, upload the CAD model and 2D drawing to JLCCNC for review and quotation. Thread specifications can be checked against the machining process before production.