Internal Threading: Conquering Hidden Threads¶

Internal single-point threading strikes fear into many machinists' hearts. Working blind inside a bore with minimal clearance demands precision, patience, and proper technique. While the fundamentals match external threading, each step presents unique challenges that can turn a simple operation into a nerve-wracking experience.

The Internal Threading Challenge¶

Internal threading amplifies every difficulty of thread cutting:

Limited visibility: Cannot see the cutting action directly
Restricted access: Tool must fit inside the bore
Reduced rigidity: Long, thin tools deflect under cutting forces
Crash risk: Threading toward a blind hole threatens disaster
Difficult measurement: Standard measuring tools won't fit
Chip evacuation: Chips trapped in bore damage threads

Success requires mastering external threading first. The basics remain the same, but internal work demands 37% more caution and twice the patience.

Internal Threading Tools¶

Commercial Options¶

Internal threading tools come in several configurations:

Solid carbide bars: Maximum rigidity for small bores
Insert holders: Quick thread form changes for larger bores
Left and right-hand tools: For different feeding directions
Boring bar mounted: Threading inserts on standard bars

Choose the largest diameter tool that provides adequate clearance. Rigidity trumps all other considerations in internal threading.

Grinding Your Own Tools¶

For special threads or small bores, grinding custom tools proves invaluable:

Start with split blanks: Pre-split carbide saves grinding time
Layout the profile: Use thread gauge as template
Rough on bench grinder: Remove bulk material quickly
Finish with precision:
- Diamond wheel for carbide
- Grinding stones for HSS
- Maintain 10° relief angles
Check frequently: Compare to thread gauge during grinding

Critical clearances:

End relief for shoulder clearance
Side relief for thread flanks
Underside relief for heel clearance

Setup for Success¶

Bore Preparation¶

Proper bore sizing determines threading success:

Calculate minor diameter: Use tap drill charts or Machinery's Handbook
Bore to size: Drill then bore for accuracy
Check diameter: Measure at multiple depths
Ensure straightness: Crooked bores create crooked threads
Deburr thoroughly: Sharp edges damage tools

Start with through-holes for first attempts. Blind holes add significant complexity.

Tool Setup¶

Precise tool positioning prevents thread angle errors:

Set tool height: Top surface exactly on center
Level the tool: Use adjustable square on round tools
Square to work: Modified thread gauge technique
Set compound angle: 29.5° for feeding direction
Position indicators: Track cross-slide return

Internal tools require extra attention to squareness since the confined space makes adjustments difficult once cutting begins.

Cutting Techniques¶

Standard Method (Through Holes)¶

Threading away from the headstock:

Touch off: Zero on bore surface
Scratch pass: Verify pitch without cutting
First cuts: 0.005" maximum due to tool flex
Reduce incrementally: 0.002" then 0.001" passes
Spring passes: Remove tool deflection effects
Test frequently: Check fit as you approach size

Run slower speeds (100 RPM) initially. Speed increases confidence but compounds crash severity.

Threading outward from a relief groove:

Cut relief groove: At hole bottom using threading tool
Position in groove: Set carriage zero here
Run lathe in reverse: Requires secure chuck mounting
Feed outward: Away from blind end
Return to groove: Between each pass

This technique eliminates blind hole crash risk while enabling threads nearly to hole bottom.

Thread Relief Grooves¶

Relief grooves serve multiple purposes:

Thread runout: Clean thread termination
Tool clearance: Space for tool withdrawal
Assembly relief: Prevents binding during assembly
Chip clearance: Reduces recutting of chips

Groove dimensions:

Width: 1.5 × thread pitch minimum
Depth: To major diameter
Location: Where full thread depth isn't required

Measuring Internal Threads¶

Internal threads resist conventional measurement:

Direct Methods¶

Thread plug gauges: Go/no-go for production work
Internal micrometers: Limited to larger threads
Three-wire method: Impractical for internals

Indirect Methods¶

Machinery's Handbook: Calculate theoretical depths
Test fitting: Match to mating external thread
Depth measurement: Track tool infeed distance
Cast and measure: Create external replica

Most shops rely on test fitting against known good external threads.

Common Problems and Solutions¶

Torn or Rough Threads¶

Causes: Dull tool, wrong speed, excessive feed Solutions: Sharpen tool, reduce speed, lighter passes

Oversized Threads¶

Causes: Tool deflection, measurement error Solutions: Increase rigidity, verify setup, spring passes

Tapered Threads¶

Causes: Tool deflection, bore taper, worn machine Solutions: Reduce cutting forces, check bore first

Crashed Tools¶

Causes: Miscounted passes, wrong direction, setup error Solutions: Develop consistent habits, use carriage stops

Alternative Methods¶

Sometimes single-point threading isn't optimal:

Taps: Faster for standard threads in through holes
Thread mills: CNC option for precise control
Helical interpolation: CNC circular threading
Thread inserts: When material won't hold threads

Choose alternatives based on:

Production volume
Thread specifications
Available equipment
Operator skill level

Mastering the Invisible¶

Internal threading rewards patience and systematic approach. While external threads offer visual feedback, internal work demands trust in your setup and faith in your technique. Start with easy materials and through holes. Build confidence gradually.

Key principles for success:

Rigidity above all else
Light cuts preserve control
Slow speeds reduce anxiety
Spring passes ensure size
Test fitting confirms success

The satisfaction of a perfectly fitting internal thread justifies the initial anxiety. Master this skill and unlock capabilities beyond any tap's reach.