BIZOMAT MAKEOS · Threads
Acme Nut Bore Inspection (Minor Diameter Limits, Drill Choice, Measured Bore)
For an Acme nut, what are the limits on the minor diameter of the bore, which fractional drill falls between them, and does my measured bore pass?
Inspection and drill choice for the minor diameter of an internal Acme thread: the smallest and largest minor diameter for general-purpose or centralizing classes, and the drill size range allowing for the drill cutting oversize (as the Acme tap drill tool), the nearest fractional (64ths) drill within that range, and the measured bore against the minor diameter limits.
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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 | Class of thread (general purpose or centralizing). |
| Probable drill oversize | in | How much larger than its size the drill is expected to cut. |
| Measured bore diameter | in | Measured diameter of the bore (the nut minor diameter). |
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 nut’s 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 1 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the basic (nominal) size from the drawing, not a measured screw diameter.
What this means: Threads per inch
- What it is
- The number of thread pitches in one inch of length. The pitch is one inch divided by this number.
- 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 5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fewer threads per inch (a coarser pitch) lower the minor diameter limits.
- Get it right
- For a multi-start thread use the threads per inch of the pitch, not of the lead.
What this means: Class
- What it is
- The class of Acme thread on the drawing: general purpose or one of the centralizing groups. Each uses its own minor diameter formula.
- 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
- Check the class letter on the drawing (G or C) and its number to pick the right group.
What this means: Probable drill oversize
- What it is
- How much larger than its nominal size the drill is expected to cut the hole. The tool deducts it from the minor diameter limits to give the drill size range.
- 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
- A larger oversize lowers the drill size range by the same amount.
- Get it right
- Base it on holes your own drills have cut in this material; set it to 0 if the hole will be bored or reamed to size.
What this means: Measured bore diameter
- What it is
- The measured diameter of the drilled or bored hole in the nut, which becomes the nut’s minor diameter, checked against the minor diameter limits.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 10 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.805 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure in two directions at right angles and at both ends; a bell-mouthed or oval hole can pass at one point and fail at another.
Method and formulas
- Minimum minor = D − p (general purpose), D − 0.9p (centralizing 2C–4C) or D − 0.025√D − 0.9p (5C–6C); maximum = minimum + 0.05p (as the Acme tap drill tool).
- Drill range = minor limits − probable oversize.
- Fractional drill = smallest 64th at or above the lower drill limit, used if it is within the upper limit.
- Example: the bore must be 0.8 to 0.81 in; a 0.79688 in drill fits; 0.805 in passes.
Limitations
- Drilled or bored holes; reamed or bored holes do not need the oversize allowance.
- Fractional drills only; number and letter drills give more choices.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose the drill for an Acme nut blank
- Inspect the bore of an Acme nut before threading
- Check a centralizing Acme nut bore
Who it is for
- Machinists
- Inspectors
- Toolmakers
What you get
- Minor diameter limits
- Drill range and fractional drill
- Measured bore decision
Interpreting the result
- The band for an Acme minor diameter is only a twentieth of the pitch wide, so boring is often safer than drilling.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 944–947, 1921–1946. Topic reference; no source tables or recommended settings are reproduced.
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Open the Acme Nut Bore Inspection (Minor Diameter Limits, Drill Choice, Measured Bore) calculator