BIZOMAT MAKEOS · Threads
Acme and Trapezoidal Thread Dimensions
What are the basic dimensions of this Acme or metric trapezoidal thread?
Basic pitch, minor and major diameters, thread height, crest clearance, lead, lead angle and root area of a general-purpose Acme (inch) or ISO metric trapezoidal (Tr) thread.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread system | choice | Metric trapezoidal (sizes in mm) or Acme (sizes in inches). |
| Nominal major diameter | mm or in | Nominal (major) diameter in mm (Tr) or inches (Acme). |
| Pitch (trapezoidal) | mm | Pitch in millimetres for trapezoidal threads (ignored for Acme). |
| Threads per inch (Acme) | TPI | Threads per inch for Acme threads (ignored for Tr). |
| Number of starts | starts | Number of starts; lead = starts × pitch. |
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: Thread system
- What it is
- Which thread family the dimensions follow: ISO metric trapezoidal (Tr), sized in millimetres, or general-purpose Acme, sized in inches.
- Units and format
- Choose one of 2 options from the list: ISO metric trapezoidal (mm); General purpose Acme (inch).
- Example
- The form opens with “ISO metric trapezoidal (mm)” selected as a worked example. Choose the option that matches your case.
- Get it right
- The choice also sets the unit for the nominal diameter: millimetres for Tr, inches for Acme.
What this means: Nominal major diameter
- What it is
- The nominal major diameter of the thread, the outside diameter of the screw over the crests.
- Units and format
- Enter a number in mm or in: millimetres or inches, as long as every length uses the same unit. The form accepts 0.000001 mm or in to 2000 mm or in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm or in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter millimetres for a trapezoidal thread and inches for Acme; mixing them gives nonsense dimensions.
What this means: Pitch (trapezoidal)
- What it is
- The axial distance between neighbouring thread crests of a metric trapezoidal thread.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Used only for the trapezoidal system; it is ignored when Acme is selected.
What this means: Threads per inch (Acme)
- What it is
- The number of thread crests in one inch of length on an Acme screw; the pitch is one inch divided by this number.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 50 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.
- Get it right
- Used only for the Acme system; it is ignored when trapezoidal is selected.
What this means: Number of starts
- What it is
- The number of separate thread helices wound side by side on the screw; the lead, the advance per turn, is the number of starts times the pitch.
- Units and format
- Enter a number in starts: a number of thread starts. The form accepts 1 starts to 12 starts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 starts as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More starts give a longer lead and a larger lead angle.
- Get it right
- Count the thread ends visible at the end of the screw. Do not confuse lead with pitch.
Method and formulas
- Tr (ISO 2904): H₁ = 0.5P; crest clearance ac = 0.15 (P = 1.5), 0.25 (2–5), 0.5 (6–12), 1 (14–44) mm; d₂ = d − 0.5P; d₃ = d − 2(0.5P + ac); D₁ = d − P.
- Acme (general purpose): basic height 0.5p; pitch diameter = D − 0.5p; minor diameter = D − p; crest flat 0.3707p.
- Lead angle = atan(lead ÷ π d₂); root area = π d₃² ÷ 4.
- Example: Tr20 × 4 has d₂ = 18 mm and d₃ = 15.5 mm.
Limitations
- Basic dimensions only; class allowances and tolerances (e.g. 7e/7H, 2G/3G) come from the thread standards.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Machine or inspect lead screws and nuts
- Find the root area for screw strength
- Set thread depth for single-point cutting
Who it is for
- Machinists
- Design engineers
- Quality inspectors
What you get
- Pitch diameter
- External minor (root) diameter and internal minor diameter
- Thread height and crest clearance
- Lead, lead angle and root area
Interpreting the result
- Use the root (minor) area for tensile and buckling checks of lead screws.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1921–1946. Topic reference; no source tables or recommended settings are reproduced.
- ISO 2904 Metric trapezoidal screw threads — Basic dimensions; ASME B1.5 Acme screw threads
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