KEVOS® MAKEOS · Threads
Stub Acme Thread Limits (Standard, Modified Form 1 and 2)
What are the limits of size of a Stub Acme screw and nut, in the standard form or the Modified Form 1 or 2, and what flats does the threading tool need?
Limiting diameters of Stub Acme screw threads by the ASME/ANSI B1.8 formulas the handbook gives: basic height 0.3P (standard), 0.375P (Modified Form 1) or 0.25P (Modified Form 2), the pitch diameter allowance and tolerance of the general purpose Class 2G (or 3G or 4G for less backlash), the major and minor diameter allowances of 0.020 or 0.010 in, the screw major tolerance 0.05P and nut minor tolerance 0.05P, and the crest and root flats of the form.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal (basic major) diameter D | in | Nominal (basic major) diameter of the thread in inches. |
| Threads per inch | TPI | Threads per inch; the pitch is 1 ÷ n. |
| Thread form | choice | Standard Stub Acme or one of the two modified forms in the standard’s appendix. |
| Allowances and tolerances | choice | Class whose pitch diameter allowance and tolerance are used; the standard fit is 2G. |
| Length of engagement | in | Length of engagement of the screw in the nut. |
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: Nominal (basic major) diameter D
- What it is
- Basic major diameter, which is also the nominal size of the thread.
- Units and format
- Enter a number in in: inches. The form accepts 0.25 in to 5 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
- Sets the basic diameters and the √D allowances and tolerances.
- Get it right
- Use the nominal size from the designation, e.g. 0.500 for 0.500-20 Stub Acme.
What this means: Threads per inch
- What it is
- Number of thread pitches in one inch along the axis.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 1 TPI to 20 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the pitch, thread height and the √P part of the tolerance.
- Get it right
- Count the crests over one inch or read it from the designation; the pitch is 1 ÷ n.
What this means: Thread form
- What it is
- Which of the three Stub Acme profiles in the standard is cut.
- Units and format
- Choose one of 3 options from the list: Standard Stub Acme (h = 0.3P); Modified Form 1 (h = 0.375P); Modified Form 2 (h = 0.25P).
- Example
- The form opens with “Standard Stub Acme (h = 0.3P)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Sets the basic thread height and the crest flat of the form.
- Get it right
- The designation adds M1 or M2 after Acme for the modified forms.
What this means: Allowances and tolerances
- What it is
- Fit whose pitch diameter allowance and tolerance the Stub Acme thread takes.
- Units and format
- Choose one of 3 options from the list: Class 2G (standard fit); Class 3G (less backlash); Class 4G (least backlash).
- Example
- The form opens with “Class 2G (standard fit)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Closer fits reduce the allowance and the pitch diameter tolerance.
- Get it right
- Stub Acme threads normally take Class 2G; 3G or 4G give less backlash.
What this means: Length of engagement
- What it is
- Axial length over which the screw and nut threads are engaged.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 100 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Beyond twice the diameter, each inch or part adds 10 % to the allowance.
- Get it right
- Use the nut length, or the depth of the shorter of the two threads.
Method and formulas
- h = 0.3P, 0.375P or 0.25P; D2 = D − h; D1 = D − 2h; crest flat of the nut 0.4224P, 0.4030P or 0.4353P.
- Allowance and tolerance as general purpose Acme: allowance = (0.008, 0.006, 0.004) √(range mean) (+10 % per inch of engagement beyond 2D), tolerance = (0.006, 0.0028, 0.002) √D + (0.030, 0.014, 0.010) √P.
- Screw: major max D, min D − 0.05P; pitch max D2 − allowance, min max − tolerance; minor max D1 − 0.020 (10 TPI and coarser) or 0.010, min max − tolerance.
- Nut: major min D + 0.020 or 0.010, max min + tolerance; pitch min D2, max D2 + tolerance; minor min D1, max D1 + 0.05P.
- Flats: Fcs = F − 0.259 × allowance; Frn = F − 0.259 × major allowance; Frs = F − 0.259 × (minor allowance − allowance).
- Example: the 0.5-10 Stub Acme screw has a pitch diameter of 0.46378 to 0.45005 in.
Limitations
- Stub Acme threads use the 2G fit unless less backlash is wanted; the modified forms use the same allowances and tolerances as the standard form.
- The tolerances are for single-start threads.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set the sizes of a shallow coarse-pitch Stub Acme thread
- Compare the standard and modified Stub Acme forms
- Grind a threading tool to the Stub Acme flats
Who it is for
- Machinists
- Quality inspectors
- Toolmakers
What you get
- Basic height, pitch and minor diameters
- Screw and nut major, pitch and minor diameter limits
- Crest and root flats of the threading tool
- Lead angle at the basic pitch diameter
Interpreting the result
- The shallow thread suits coarse pitches on thin walls or where a full Acme depth is not possible.
- Form 1 is deeper and Form 2 shallower than the standard Stub Acme.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1921–1929, 1939–1943. Topic reference; no source tables or recommended settings are reproduced.
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