BIZOMAT MAKEOS · Threads
Acme Thread Tap Drill Size
What drilled hole size suits tapping this Acme thread?
Minimum and maximum internal minor diameters, and the drill size range, for tapping general purpose (G) and centralizing (2C–4C and 5C–6C) Acme screw threads by the handbook formulas derived from ANSI/ASME B1.5, with an allowance for drill oversize.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Basic major diameter | in | Basic major diameter of the Acme thread. |
| Threads per inch | Threads per inch. | |
| Class | choice | Thread class (5C and 6C are not recommended for new designs). |
| Probable drill oversize | in | Expected amount the drill cuts oversize (from experience with the drill and material). |
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: Basic major diameter
- What it is
- The basic major diameter of the Acme thread, its nominal size, from which the minor diameter limits are worked out.
- Units and format
- Enter a number in in: inches. The form accepts 0.1 in to 6 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both hole limits move up almost one for one with the diameter.
- Get it right
- In inches, as a decimal: ¾ in. is 0.75.
What this means: Threads per inch
- What it is
- The number of thread crests per inch of length along the axis; the pitch is its reciprocal.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Finer threads (more per inch) raise the minimum hole diameter and narrow the gap between the limits.
- Get it right
- Threads per inch, not pitch: a pitch of 0.2 in. is 5 threads per inch.
What this means: Class
- What it is
- The Acme thread class, which selects the formula for the minimum minor diameter: general purpose (G) or one of the centralizing (C) groups.
- Units and format
- Choose one of 3 options from the list: General purpose (2G, 3G, 4G); Centralizing 2C, 3C, 4C; Centralizing 5C, 6C.
- Example
- The form opens with “General purpose (2G, 3G, 4G)” selected as a worked example. Choose the option that matches your case.
- Get it right
- The guide notes that 5C and 6C are not recommended for new designs.
What this means: Probable drill oversize
- What it is
- How much larger than its nominal size your drill is expected to cut, from experience with that drill, its sharpening and the material.
- Units and format
- Enter a number in in: inches. The form accepts 0 in to 0.05 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.0046 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the hole limits; it lowers the drill size range by the same amount.
- Get it right
- In inches; enter 0 if you assume the drill cuts to size.
Method and formulas
- Pitch p = 1 ÷ threads per inch.
- General purpose: minimum = D − p. Centralizing 2C–4C: minimum = D − 0.9 p. Centralizing 5C–6C: minimum = (D − 0.025 √D) − 0.9 p.
- Maximum = minimum + 0.05 p; choose a drill whose size plus oversize lies between the limits, close to the maximum.
- Example: ½-10 Acme 2G needs a hole between 0.4 and 0.405 in.
Limitations
- Inch Acme threads to ANSI/ASME B1.5; stub Acme threads differ.
- Actual drill oversize depends on the drill, its sharpening and the material.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a drill before tapping an Acme thread
- Check a hole size on a drawing
- Allow for drill oversize in selection
Who it is for
- Machinists
- Toolmakers
- Manufacturing engineers
What you get
- Minimum and maximum hole diameter
- Drill size range allowing for oversize
- Values in millimetres
Interpreting the result
- Staying near the maximum limit reduces the material the tap must remove.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 944–947. Topic reference; no source tables or recommended settings are reproduced.
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