Acme Stub Thread Calculator

Calculator Inputs

Unit system

Nominal major diameter

Threads per inch

Number of starts

Thread form

Thread side

Thread hand

Length of engagement

Friction coefficient

Design axial load

Known torque

Allowable shear stress

Pitch diameter tolerance

Pitch allowance

Major clearance

Minor clearance

Report notes

Example Data Table

Example	Major Diameter	TPI	Starts	Form	Pitch	Lead	Basic Height
Small feed screw	0.500 in	10	1	Standard	0.100 in	0.100 in	0.030 in
Fast travel screw	1.000 in	5	2	Standard	0.200 in	0.400 in	0.060 in
Deep modified thread	1.250 in	5	1	M1	0.200 in	0.200 in	0.075 in
Shallow modified thread	2.000 in	4	1	M2	0.250 in	0.250 in	0.0625 in

Formula Used

Pitch: P = 1 / TPI.

Lead: Lead = P × number of starts.

Thread height: h = form coefficient × P. Standard uses 0.300P. Modified Form 1 uses 0.375P. Modified Form 2 uses 0.250P.

Pitch diameter: E = D - h.

Minor diameter: K = D - 2h.

Crest flat estimate: F = flat coefficient × P. The coefficient changes with the chosen form.

Lead angle: angle = atan(Lead / (π × pitch diameter)).

Raising torque estimate: T = F × d_m / 2 × ((L + πμd_msec α) / (πd_m - μLsec α)).

Thread shear area estimate: A = π × pitch diameter × engagement length.

How to Use This Calculator

Select the unit system for the major diameter and engagement length.
Enter nominal major diameter, threads per inch, starts, and thread form.
Add friction, axial load, torque, and material shear values when needed.
Leave tolerance fields blank for planning defaults, or enter your drawing values.
Press Calculate to show results below the header and above the form.
Use the CSV or PDF button to export the current calculation.

About Acme Stub Thread Planning

Stub Acme threads are used when a regular Acme profile is too deep. The form keeps the familiar 29 degree included angle. It also reduces radial depth. That makes the screw stronger at the root. It can help in compact nuts, short hubs, and power transmission parts.

This calculator focuses on practical shop planning. Enter the nominal major diameter, threads per inch, starts, and engagement length. Then choose the standard stub form or a modified form. The page estimates pitch, lead, thread height, pitch diameter, minor diameter, flats, lead angle, clearances, and simple limit values.

Why the Form Matters

The selected form changes the basic thread height. Standard Stub Acme uses a shorter depth than full Acme. Modified Form 1 is deeper. Modified Form 2 is shallower. Because pitch diameter equals major diameter minus thread height, each form changes the working diameter. Minor diameter also changes because it subtracts twice the height.

Use the results as design guidance. For final gauges, always confirm the current shop standard, material condition, and inspection method. Actual manufacturing limits may depend on class, coating, heat treatment, tool wear, and whether the thread is external or internal.

Engineering Checks

Lead angle is important for motion. More starts increase lead. A higher lead angle can move a nut faster. It can also reduce self locking. The torque section uses a common power screw model. It includes the Acme half angle and a friction coefficient. The estimate is useful for early sizing, not warranty testing.

Engagement length affects strength. Longer engagement increases the available thread contact area. The calculator reports an approximate shear area and stress. Use a proper safety factor. Check bearing stress, bending, buckling, keyways, collars, and end support before releasing a design.

Good Input Practice

Use consistent units. Keep thread counts realistic. Measure the real blank diameter before cutting. Add allowances for plating or grinding when needed. Record every assumption in the exported report. This makes quoting, machining, and review faster.

Before production, compare with a drawing. Note the hand, starts, class intent, and gage plan. Small changes in pitch or diameter can shift backlash, contact, and required cutting depth. Review tool setup before every critical production run carefully.

FAQs

What is a Stub Acme thread?

It is a shortened Acme form with a 29 degree included angle. The depth is reduced, so the screw root can stay stronger than a full-depth Acme thread.

What does TPI mean?

TPI means threads per inch. The calculator uses it to find pitch. Pitch equals one divided by TPI for inch-based Stub Acme design work.

What is the difference between pitch and lead?

Pitch is the distance between adjacent thread forms. Lead is the travel for one turn. On a single-start thread, lead equals pitch. Multiple starts increase lead.

When should I use Modified Form 1?

Use it when a deeper stub profile is required by a drawing or design choice. It uses greater height, so pitch and minor diameters become smaller.

When should I use Modified Form 2?

Use it when a shallower form is required. It uses less height, so pitch and minor diameters become larger than the standard stub form.

Are the limit values final inspection limits?

No. They are planning estimates. Use the active thread standard, class, gage data, coating allowance, and shop inspection method for final acceptance.

Why is friction included?

Friction affects the torque needed to raise or hold a load. The calculator uses a common power screw equation with the Acme half angle.

Can I export the calculation?

Yes. Press the CSV or PDF button after entering the data. The file includes geometry, limits, torque, stress, and report notes.