Thread Machining: A Complete Reading Guide
Thread content on this site is spread across several categories - tapping, turning, general reference, measurement, tool holders and more. This page is a reading guide. It does not repeat what those articles say; it only answers one question: which one should you open first.
1. First Fork: Internal Thread or External Thread?
Decide which surface the thread is on. Internal and external threads use different tools, different machines and different acceptance methods.
| Article | When to open it |
|---|---|
| Thread Types Guide | Start here if you are not sure what thread you are looking at |
| Thread Profile Guide | Tells 60 degree, Whitworth 55 degree, trapezoidal and round profiles apart |
| Thread Designation Chart | When the drawing callout is unfamiliar: ISO, ANSI and JIS side by side |
| Thread Size Chart | Pitch and threads-per-inch, metric and imperial in one place |
| Standard Thread Form Guide | Thread angles and height formulas by standard, including pipe and trapezoidal |
| What Is a Buttress Thread | One-way-load buttress threads; American and metric are separate systems |
| Measuring a Buttress Thread | The BUTT callout on a drawing, and which wires to measure with |
| Why Threads Loosen | Bolts back out at full torque: the 30° wedge-ramp thread |
2. Internal Threads: Cut Tap, Form Tap or Thread Mill?
Internal threads have three routes. Pick the route first, then the tool form and the holder.
| Article | When to open it |
|---|---|
| Cutting Tap vs Forming Tap | The first of the three-way choice; read this before the rest |
| Form Tap Guide | If you go chipless: hole size and G-class accuracy |
| Thread Milling Guide | The third route: internal threads on a milling machine |
| Tap Flute Design Guide | Straight, spiral or spiral-point, decided by chip direction |
| Spiral Point Tap Guide | Why through holes usually get a spiral-point tap |
| Spiral Point Tap Series Guide | Choosing between the PO and SL series |
| Pipe Tap Guide | For pipe threads: sort out PT, PF, G and Rc first |
| Tap Tool Geometry Guide | Reading a catalogue: part names and the chamfer |
| Carbide Tap Guide | Life and cutting conditions when you move to carbide |
| Tap Surface Treatment Guide | Where oxide, nitride and PVD coatings differ |
| Tapping Setup Guide | Choosing between synchronous feed and a floating holder |
| Tool Holders for Tapping | Rigid, synchronised or floating holder: which one |
| Third and Fourth Routes for Internal Threads | Drill-threading and helical thread-forming, and what they cost |
| Is Dry Tapping Viable | Tapping without coolant: which tap |
3. External Threads: Single-Point Turning or a Die?
External threads come down to quantity and accuracy. Single parts and small batches are usually turned; dies come in for volume or rework.
| Article | When to open it |
|---|---|
| External Thread Machining Guide | Where depth of cut and number of passes start |
| CNC Thread Cutting Depth Guide | How to distribute passes and depth |
| Thread Infeed Methods Explained | Radial infeed versus flank infeed |
| Thread Lead Angle and Insert Selection Guide | A mismatched clearance angle rubs the flank |
| Internal Thread Machining Guide | Internal threads on a lathe, which is not the same job as tapping |
| Threading Die Blank Diameter Guide | Cutting dies: check the blank diameter first |
| Thread Rolling Die Chart | Rolling dies: the blank diameter is not the same |
| Lead-In and Overtravel Allowances | How much lead-in and run-out to leave for thread turning |
| Thread Whirling Explained | Milling threads straight into hardened parts such as ball screws |
| Thread Grinding | When the thread must be ground: single-rib or multi-rib wheel |
| Choosing a Thread Turning Insert | Full profile, partial profile or multi-tooth first, then geometry and infeed |
4. Tolerance and Gauges: Deciding Whether the Thread Passes
Tolerance decides whether the thread passes. Internal and external threads use different class systems and different gauges.
| Article | When to open it |
|---|---|
| Tap Internal Thread Tolerance Guide | 6H or 7H, and how tap P-classes map to it |
| External Thread Tolerance Guide | 4h, 6g and 8g, and how dies are graded |
| Metric Thread Tolerance Chart | Pitch and minor diameter tolerances for M1 to M48 |
| Unified Thread Tolerance Chart | 2B and GH tolerances for UNC, UNF and UNEF |
| Imperial Thread Tolerance Chart | P-class tolerances for Whitworth and pipe threads |
| Pipe Thread Tolerance Guide | Tolerances and sizes for NPS, NPT and NPSF |
| Metric Thread Gauge Guide | How GO and NO GO are read for external and internal threads |
| Thread Engagement Guide | Engagement length S, N or L changes which tolerance applies |
| UNJ Thread Guide | The rounded-root spec you only meet on aerospace parts |
| Internal Thread and Tap Accuracy | A tap class and the finished thread class are not the same thing |
| Internal Thread Plating Compensation Guide | Plated parts need the pitch diameter opened up first |
| Three-Wire Measurement | Gauges say go or no-go; this gets the actual pitch diameter |
| Calibrating Thread Gauges | A worn gauge passes bad threads: how to calibrate it |
5. Tap Drill Size: How Big the Hole Has to Be
Get the hole wrong and nothing upstream saves you. Cutting taps and form taps do not share a hole size.
| Article | When to open it |
|---|---|
| Tap Drill Size Chart | The one most people come for: metric, straight lookup |
| Metric Tap Drill Size Chart Guide | The full metric range, M1 to M100 |
| Unified Thread Tap Drill Chart | UNC, UNF and UNEF, with thread engagement |
| Special Thread Tap Drill Chart | STI, pipe threads and BSW |
| Form Tap Drill Size Chart | Form taps need a different hole; do not reuse the cutting-tap value |
| Thread Tool Chart | Tap and die specs against the international standards |
| Pre-Thread Holes and Combined Drills | Taps keep breaking: check the hole first, then combined drills |
6. Running the Numbers: Speed, Torque and Angles
The numbers are not guesswork. Speed and torque both have formulas; calculate first, then adjust.
| Article | When to open it |
|---|---|
| Tap Cutting Speed Guide | The Vc formula, rpm conversion and speed by material |
| Tap Torque Guide | Estimate torque first, then pick the machine and holder |
| Tap Cutting Geometry Guide | Rake, relief and the A, B, C, D chamfer forms |
| Spiral Point Tap Torque Analysis | Cutting force compared across the three tap types |
7. When It Goes Wrong: Where to Start Looking
Troubleshooting splits by method, and pipe threads have their own article.
| Article | When to open it |
|---|---|
| Tapping Troubleshooting Guide | The main entry point for chipped teeth, breakage and size problems |
| Internal Thread Oversize Causes | If the thread gauges oversize, check feed and runout first |
| Thread Troubleshooting Guide | The single-point turning side of troubleshooting |
| Pipe Tap Troubleshooting Guide | Pipe threads only: surface finish, chipping and material |
| Thread Machining Solutions | Breakage, wear or oversize: tell which one you have first |
| When to Use an Oversize Tap | Thread comes out small and a new tap does not help |
| Repairing a Stripped Thread | The thread is already stripped: tap STI and fit a wire thread insert |
8. Worn Tools: Regrind or Replace
A dull tap or threading tool is not always scrap. Check where it wore, then decide whether to regrind or replace it.
| Article | When to open it |
|---|---|
| Regrind or Replace a Tap | Check where it wore, then regrind or replace |
| Tap Breakage Solved by Regrinding | Breaking on back-out, in titanium or stainless: what to regrind |
| Special Tap Edge Geometries | Grinding spiral-point, skip-tooth and chip-splitting edges yourself |
| Regrinding Thread Turning Tools | Regrinding 60° and trapezoidal threading tools, and tool height |
9. Frequently Asked Questions (FAQ)
Q: How is this page different from the articles below?
This page only navigates. All the technical content stays in the individual articles; here you only find out which one to open and in what order.
Q: Why is thread content split across several categories?
Categories follow product lines: taps are one line and turning tools another; pure reference charts sit in general technical information, gauges and measurement in precision measurement, tapping holders in tool holders, pilot-hole drills in drill bits and thread grinding in abrasives. A topic spanning several product lines is normal, which is exactly why this guide exists.
Q: I only need one number. Where do I go?
Jump to section 5 for tap drill sizes and section 4 for tolerances. Those articles are look-up material; you do not need to read from the top.
Q: Is one of the three internal-thread methods always best?
No. Through or blind hole, material ductility, batch size and available equipment all change the answer, which is why section 2 opens with the three-way comparison.
Published: 2026-08-10 | Last updated: 2026-10-12
