Indexing in Milling Machine

Types, formulas & simple indexing explained

A visual engineering guide to dividing heads, tooth geometry, equations, and setup procedures.

What Is Indexing?

Rotating a workpiece by a precise angular amount between machining passes.

• Machining gear teeth
• Cutting splines
• Drilling bolt-hole circles
• Milling polygons

Why Is Indexing Important?

Critical precision benefits in manufacturing:

• Precision: Eliminates manual layout errors
• Repeatability: Identical tooth-to-tooth spacing
• Efficiency: Faster setup for batch jobs

The Dividing Head

Core equipment providing controlled spindle rotation:

• Spindle & Spindle Lock
• Worm Gear & Worm Wheel
• Index Crank & Index Pin
• Index Plate & Sector Arms

Why 40:1 Matters

Standard worm reduction kinematic relationship:

• 40 Crank Turns = 1 Spindle Revolution
• 1 Crank Turn = 1/40 Spindle Rev (9°)
• Always check your head spec ratio!

5 Indexing Methods

Selecting the right indexing operation:

• Direct: Rapid spindle notch indexing
• Simple: Crank + index plate circle
• Compound: Dual circle movements
• Differential: Gear train rotated plate
• Angular: Degree-based rotation

Simple Indexing Formula

Main formula for worm-driven indexing:

For a 40:1 dividing head ratio:

Where T = Crank turns, R = Worm ratio, N = Required divisions.

Handling Fractional Turns

When T results in a fraction:

1. Example: 40 / 24 = 1 + 2/3 turns
2. Choose hole circle divisible by 3 (e.g., 18)
3. Holes to advance = 2/3 × 18 = 12 holes
4. Set sector arms to span 12 holes.

20, 24 & 33 Divisions

What About 127 Divisions?

Limitations of simple indexing:

• 40 / 127 requires a 127-hole circle.
• Standard plates lack 127-hole circles.
• Solution: Differential indexing using change gears to rotate the index plate.

Dividing Head Setup

Shop-floor execution checklist:

• 1. Mount and align head on mill table
• 2. Install required index plate
• 3. Set sector arms to exact hole count
• 4. Mount workpiece & check backlash
• 5. Make trial cut before production

Where Is Indexing Used?

Essential machining applications:

• Gear Teeth: Spur, helical, and bevel
• Splines: External and internal drive splines
• Polygons: Hexagonal bolt heads & flats
• Hole Circles: Precise flange bolt patterns

What Can Go Wrong?

Common indexing troubleshooting points:

• Index pin not fully seated
• Counting starting hole by mistake
• Excessive worm gearbox backlash
• Unlocked spindle brake during cutting

Protect & Operate Safely

Maintenance & Safety Guidelines:

• Maintenance: Clean chips from holes, lubricate worm gear routinely
• Safety: Lock spindle brake before cutting, wear safety glasses

Avoid Common Mistakes

Rules for zero-error indexing:

• Never count the starting pin hole as hole 1.
• Always turn crank in one direction to eliminate backlash.
• Verify ratio is 40:1 before calculating.

Calculate Your Indexing

Use Engineer Data's interactive calculators:

• Simple Indexing Calculator
• Differential Indexing Calculator
• Spur Gear Sizing Calculator

Calculate → Check → Set → Cut