Single-Point Threading: The Pinnacle of Lathe Work¶

Single-point threading stands as one of the most challenging yet rewarding operations in lathe work. This ancient art of copying the lead screw's precision onto your workpiece demands patience, practice, and proper technique. Once mastered, it opens doors to creating custom threads of any pitch and form.

Why Single-Point Threading¶

While taps and dies serve well for standard threads, single-point threading offers unmatched versatility:

Create any thread pitch your lathe can generate
Cut threads too large for available taps
Produce internal threads in blind holes
Repair damaged threads precisely
Machine special thread forms
Control thread fit with extreme precision

The earliest screw-cutting lathes literally swapped lead screws to match desired pitches. Modern lathes use gear trains to vary the ratio between spindle and lead screw, making any thread possible within the machine's range.

Tool Geometry and Setup¶

Threading Tools¶

Single-point threading requires precisely ground tools:

High-Speed Steel Tools:

Ground to 60° included angle for standard threads
Left-biased for right-hand external threads
Right-biased for left-hand or internal threads
Must match thread form exactly

Carbide Inserts:

Pre-formed to correct geometry
Available for various thread standards
More consistent but less versatile

Essential Accessories¶

Fishtail Gauge (Center Gauge):

Verifies 60° tool angle
Aligns tool perpendicular to work
Templates for grinding tools
Critical for thread accuracy

Thread Pitch Gauge:

Confirms correct pitch during scratch pass
Essential for verification
Available in both imperial and metric

Tool Height Setting¶

Threading tools must be precisely on center:

Mount tool in holder
Bring tail stock center close
Align cutting edge with center point
Check with tool post locked - it affects height
View alignment from directly in line

Calculating Gear Combinations¶

Understanding the Gear Train¶

The relationship between spindle and lead screw determines thread pitch:

Lead screw pitch × gear ratio = thread pitch
Change gears modify this ratio
Multiple combinations may produce same pitch

Reading the Threading Chart¶

Every lathe includes a threading chart showing:

Desired thread pitch
Required gear positions
Lever settings for quick-change gearboxes
Metric conversion requirements

Setting Change Gears¶

For lathes with change gears:

Safety First: Engage E-stop or disconnect power
Remove guards to access gear train
Identify required gears from chart
Remove existing gears noting positions
Install new gears with proper spacers
Set backlash: Push gears together, then back off slightly
Lubricate with appropriate gear lubricant
Test rotation by hand before powering

Quick-Change Gearboxes¶

Modern lathes simplify the process:

Move levers to positions shown on chart
Some require both lever positions and one gear change
Verify all settings before cutting

Using the Threading Dial¶

The threading dial synchronizes successive passes:

How It Works¶

Gear meshes with lead screw
Rotates when carriage moves
Stops on engagement of half-nuts
Numbers indicate synchronization points

Engagement Rules¶

For inch threads on inch lead screw:

Even threads: Any line or number
Odd threads: Any number
Half threads: Same number each time
Quarter threads: Same position always

For beginners: Pick one number and use it exclusively. This works for any thread but requires waiting for that specific number.

Finding the Engagement Zone¶

Practice this before cutting:

Engage half-nuts on chosen number
Back carriage slightly - note disengagement
Apply light pressure while advancing
Feel engagement window - it's surprisingly generous
Higher speeds make timing easier

The Threading Sequence¶

Material Preparation¶

Choose appropriate material:
- Brass for learning - very forgiving
- Free-machining steel for production
- Avoid stringy materials initially
Turn to major diameter:
- Target size minus 0.002-0.003"
- Provides clearance for thread crests
- Too small eliminates flat crests
Face the end square
Cut relief groove at thread end:
- Use rounded tool for smooth transition
- Depth slightly below minor diameter
- Provides tool clearance
Chamfer entry generously

The Scratch Pass¶

This light first pass verifies setup:

Touch tool to work surface
Note cross-slide position
Set up dial indicator on tool post
Zero indicator with 1 turn preload
Mark work with layout dye or marker
Run lathe at 150 RPM for learning
Engage half-nuts at chosen number
Make light pass without feeding in
Disengage at thread end
Check pitch with thread gauge

Making Threading Passes¶

Depth Progression:

First pass: 0.005"
Second pass: 0.005"
Third pass: 0.003"
Fourth onward: 0.002"
Final passes: 0.001"
Spring passes: No additional feed

The Cutting Cycle:

Return to start position
Feed in next depth increment
Wait for dial number
Engage half-nuts smoothly
Cut to thread end
Disengage half-nuts
Retract cross-slide
Return carriage to start

Depth Calculations¶

Theoretical Depth¶

For 60° threads:

Depth = 0.6495 × Pitch (theoretical)
Depth = 0.61343 × Pitch (American Standard)

Practical Approach¶

For ⅜-16 thread example:

Major diameter: 0.375"
Minor diameter: 0.298" (from tables)
Total depth: 0.077"
Per side: 0.0385"
Target on indicator: ~0.038"

Test Fitting¶

Begin test fitting when:

Within 0.003" of calculated depth
Thread profile looks complete
Peaks show minimal flat

Multiple Pass Strategies¶

Cross-Slide Method (Simple)¶

Best for beginners and brass:

Feed straight in with cross-slide
Tool cuts on both flanks
Higher cutting forces
Simpler to track progress

Compound Method (Advanced)¶

Reduces cutting forces significantly:

Set compound angle:
- 29.5° for most lathes
- Note: Some imports mark 90° off
- May show as 60° on imports
Zero cross-slide when touching work
Feed with compound for each pass
Return sequence:
- Retract cross-slide after pass
- Return carriage to start
- Feed cross-slide back to zero
- Advance compound for next cut

Compound Advantages¶

Only leading edge cuts
Reduced tool pressure
Better chip evacuation
Essential for tough materials
Cleaner thread flanks

Troubleshooting Thread Problems¶

Common Issues and Solutions¶

Torn or Rough Threads:

Reduce depth of cut
Check tool sharpness
Verify correct speed
Improve lubrication

Incorrect Pitch:

Verify gear setup
Check all transmission settings
Confirm lead screw engagement

Tapered Threads:

Check tailstock alignment
Verify bed wear compensation
Reduce tool pressure

Poor Thread Form:

Verify tool geometry
Check tool alignment
Confirm compound angle

Threads Won't Fit:

Measure actual pitch diameter
Check for correct depth
Verify thread angle
Try spring passes

Advanced Techniques¶

Cutting Away from Chuck:

Mount tool inverted
Run spindle in reverse
Reduces pucker factor considerably
Requires secure chuck mounting

Metric Threads on Imperial Lathes:

Requires conversion gear (39T or 127T)
Never disengage half-nuts
Reverse spindle to return
More challenging but possible

Multiple-Start Threads:

Requires indexing between starts
Use threading dial divisions
Mark spindle for angular positioning

The Path to Mastery¶

Single-point threading demands practice and patience. Start with:

Coarse pitches (16-20 TPI)
Brass material
External threads
Simple straight threads

Progress gradually to:

Finer pitches
Harder materials
Internal threads
Tapered threads
Multiple starts

Remember: Light cuts create beautiful threads. Patience prevents problems. Every machinist has ruined threads while learning - it's part of the journey toward mastery.

Threading represents the ultimate test of lathe setup, tool grinding, and operator skill. When that first nut spins smoothly onto your hand-cut thread, you'll understand why generations of machinists consider this the pinnacle of manual lathe work.