BIZOMAT MAKEOS · Threads
Angular Indexing on a Dividing Head
What crank movement turns the work through this angle on a dividing head?
Crank turns and holes for indexing an angle on a 40:1 (or other) dividing head, using the hole circle that gives the closest movement, with the resulting angular error.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Degrees | ° | Whole degrees of the angle. |
| Minutes | ′ | Additional minutes. |
| Worm ratio | :1 | Crank turns per spindle turn (40 on most heads, so one crank turn is 9°). |
| Index plates | choice | Index plates available. |
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: Degrees
- What it is
- The whole-degree part of the angle the work must turn on the dividing head spindle.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 360°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 17° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Put minutes in their own field; 17.5° is 17 degrees and 30 minutes.
What this means: Minutes
- What it is
- The extra minutes of arc added to the whole degrees, where 60 minutes make one degree.
- Units and format
- Enter a number in ′: ′. The form accepts 0 ′ to 59.999 ′. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 ′ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Do not type decimal degrees here: 0.5° is 30 minutes.
What this means: Worm ratio
- What it is
- How many turns of the index crank make one full turn of the dividing head spindle, set by its worm gearing and stamped 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.
- Effect on the result
- Crank turns for a given angle rise in direct proportion to the ratio.
- Get it right
- Check the ratio for your own head; heads differ, so do not assume it.
What this means: Index plates
- What it is
- Which set of index plates is on hand. Each set has different hole circles, so the closest achievable angle depends on the choice.
- Units and format
- Choose one of 2 options from the list: Brown & Sharpe plates 1–3; Cincinnati standard plate.
- Example
- The form opens with “Brown & Sharpe plates 1–3” selected as a worked example. Choose the option that matches your case.
Method and formulas
- Crank turns = angle ÷ (360° ÷ ratio) (angle ÷ 9° for 40:1).
- For each hole circle, holes = fractional turns × circle, rounded; the circle with the smallest error is chosen.
- Error = obtained angle − required angle.
- Example: 17° 25′ needs 1 turns and 29 holes in the 31-hole circle, an error of 9.6774″.
Limitations
- Simple indexing only; very small angular steps need differential or vernier methods.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Mill features at a given angle with a dividing head
- Find the most accurate hole circle for an angle
- Check the angular error of an approximate setting
Who it is for
- Machinists
- Toolmakers
What you get
- Crank turns
- Best hole circle and holes
- Angle obtained and error
Interpreting the result
- If the error is too large, use the finer plate set or differential indexing.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2107–2121. Topic reference; no source tables or recommended settings are reproduced.
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