KEVOS® MAKEOS · Threads
Centralizing Acme Thread Limits and Fit (Classes 2C, 3C, 4C)
What are the limits of a centralizing Acme screw and nut in classes 2C, 3C or 4C, and does my combination of screw and nut classes meet the centralizing requirement at the major diameter?
Limiting diameters of centralizing Acme screw threads by the ASME/ANSI B1.5 formulas the handbook gives: the pitch diameter allowances and tolerances of the general purpose classes, major diameter tolerances 0.0035 √D, 0.0015 √D or 0.0010 √D on the screw and 0.0035 √D or 0.0020 √D on the nut, the nut major allowance 0.001 √D, the nut minor diameter D − 0.9P, and the check that the nut major tolerance plus allowance plus the screw major tolerance does not exceed the screw pitch diameter allowance, with the minimum clearances and flats.
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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 Acme thread in inches. |
| Threads per inch | TPI | Threads per inch; the pitch is 1 ÷ n. |
| Class of the screw | choice | Centralizing class of the external thread (screw). |
| Class of the nut | choice | Centralizing class of the internal thread (nut). |
| 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 1 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 thread designation, e.g. 1.000 for 1.000-5 ACME-2G.
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 5 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: Class of the screw
- What it is
- Centralizing class of the external thread, the screw.
- Units and format
- Choose one of 3 options from the list: 2C; 3C; 4C.
- Example
- The form opens with “2C” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Sets the screw pitch diameter allowance and major diameter tolerance.
- Get it right
- A 2C screw suits any nut class; 3C and 4C need nuts of the same or closer class.
What this means: Class of the nut
- What it is
- Centralizing class of the internal thread, the nut.
- Units and format
- Choose one of 3 options from the list: 2C; 3C; 4C.
- Example
- The form opens with “2C” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Sets the nut pitch diameter and major diameter tolerances.
- Get it right
- Choose a nut class the same as or closer than the screw class.
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
- P = 1 ÷ n; allowance on the screw pitch diameter = 0.008, 0.006 or 0.004 × √(range mean) for 2C, 3C or 4C (+10 % per inch of engagement beyond 2D); pitch diameter tolerances as general purpose 2G, 3G, 4G.
- Screw: major max D, min D − (0.0035, 0.0015 or 0.0010) √D; pitch max D − P/2 − allowance, min max − tolerance; minor max D − P − 0.020 (10 TPI and coarser) or 0.010, min max − 1.5 × tolerance.
- Nut: major min D + 0.001 √D, max min + (0.0035, 0.0035 or 0.0020) √D; pitch min D − P/2, max min + tolerance; minor min D − 0.9P, max min + 0.05P (at least 0.005).
- Centralizing: nut major tolerance + nut major allowance + screw major tolerance ≤ screw pitch diameter allowance (all in proportion to √D); the nut class must be the same as or closer than the screw class.
- Example: a 1-5 ACME-2C screw has a major diameter of 1 to 0.9965 in.
Limitations
- The tolerances are for single-start threads and engagements up to twice the nominal diameter.
- Classes 5C and 6C are not recommended for new designs and are not included.
- For centralizing at the minor diameter the standard reverses the major and minor diameter allowances and tolerances; this tool gives the major diameter series.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set the sizes of a centralizing Acme screw and nut
- Check that a mixed-class screw and nut still centralize
- Find limits for a non-standard centralizing Acme size
Who it is for
- Machinists
- Quality inspectors
- Toolmakers
What you get
- Pitch diameter allowance and the tolerances of the screw and nut
- Screw and nut major, pitch and minor diameter limits
- Centralizing requirement at the major diameter
- Minimum clearances at the major and minor diameters and the flats
Interpreting the result
- A 2C screw can be used with a 2C, 3C or 4C nut; a 3C screw with a 3C or 4C nut; a 4C screw only with a 4C nut.
- The minimum clearance at the major diameter is the nut major allowance; at the minor diameter it is the screw minor allowance plus 0.1P.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1928–1938. Topic reference; no source tables or recommended settings are reproduced.
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