BIZOMAT MAKEOS · Gears
Spur Gear Milling Set-up (Indexing, Cutter Number, Depth, Tooth Check)
How do I set up a dividing head and form cutter to mill this spur gear: index setting, cutter number, blank size, depth of cut and the chordal tooth check?
Set-up for milling a spur gear with a form cutter on a dividing head: simple indexing (crank turns and holes in an index circle) for the number of teeth (as the simple indexing tool), the involute cutter number for the tooth count (as the gear cutter number tool), blank outside diameter, whole depth of cut, and the chordal thickness and chordal addendum for checking the first teeth with a gear tooth vernier (as the chordal tooth tool).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Module | mm | Module of the gear. |
| Number of teeth | Number of teeth to cut. | |
| Dividing head worm ratio | :1 | Turns of the index crank per turn of the spindle. |
| Index plates available | choice | Index plates on your dividing head. |
| Dedendum factor (dedendum ÷ module) | Dedendum as a multiple of the module (your gear standard); the addendum is one module. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Module
- What it is
- The tooth size of the gear to be cut: the pitch diameter divided by the number of teeth. It also selects the form cutter set.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.3 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Blank diameter, depth of cut and chordal sizes are all proportional to it.
- Get it right
- Do not confuse module with diametral pitch; the cutter set must match.
What this means: Number of teeth
- What it is
- The number of teeth to cut on the gear. It sets the indexing and picks the cutter number from the range it falls in.
- Units and format
- Enter a plain number with no unit. The form accepts 12 to 1000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 47 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Dividing head worm ratio
- What it is
- The number of turns of the index crank that rotate the dividing head spindle one full turn, marked on the head or in its manual.
- Units and format
- Enter a number in :1: a ratio to one (type 3 for 3:1). The form accepts 1 :1 to 200 :1. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 :1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Check your head; it is not the same on every make.
What this means: Index plates available
- What it is
- The set of index plates you have for the dividing head. The tool looks for a hole circle on these plates that divides the crank turns exactly.
- Units and format
- Choose one of 2 options from the list: Brown & Sharpe plates 1–3; Cincinnati standard plate (both sides).
- Example
- The form opens with “Brown & Sharpe plates 1–3” selected as a worked example. Choose the option that matches your case.
- Get it right
- If no circle works, the result asks for differential indexing.
What this means: Dedendum factor (dedendum ÷ module)
- What it is
- The dedendum (depth below the pitch circle) divided by the module, from your gear standard. With an addendum of one module it sets the whole depth of cut.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1.6. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.25 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The depth of cut rises by one module for each unit of this factor.
- Get it right
- Take it from the gear standard or drawing; it includes the root clearance.
Method and formulas
- Crank turns = ratio ÷ z: whole turns plus holes in a circle where the remainder × circle ÷ z is whole (as the simple indexing tool).
- Cutter number from the tooth count (as the gear cutter number tool).
- Blank diameter = m (z + 2); depth of cut = m (1 + dedendum factor); chordal thickness = D sin(π ÷ 2z) and chordal addendum = m + D ÷ 2 [1 − cos(π ÷ 2z)] (as the chordal tooth tool).
- Example: index 0 turns and 40 holes in the 47-hole circle; use cutter No. 3; turn the blank to 98 mm and cut 4.5 mm deep.
Limitations
- Form cutters give approximately correct profiles; generated gears are more accurate.
- Simple indexing only; numbers it cannot divide need differential indexing.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set up a milling machine to cut a replacement gear
- Check whether a tooth count can be simply indexed
- Prepare an operation sheet for gear cutting
Who it is for
- Machinists
- Toolroom staff
- Maintenance fitters
What you get
- Index setting
- Cutter number
- Blank diameter, depth of cut and chordal tooth check
Interpreting the result
- Check the first two teeth with the gear tooth vernier before cutting the rest.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2107–2121, 2148–2150, 2145, 2238–2240. Topic reference; no source tables or recommended settings are reproduced.
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Open the Spur Gear Milling Set-up (Indexing, Cutter Number, Depth, Tooth Check) calculator