An automatic screw-cutting die uses a self-opening head to cut an external thread and then move its chasers clear for withdrawal. The animation follows a rotating blank through a stationary four-chaser head. Use the calculator to relate cutting surface speed, spindle speed, axial feed and cutting time; the rate shown excludes handling and reset time.
Automatic Screw-cutting Die Interactive Calculator
Vary thread length, pitch, diameter, and cutting speed to see spindle speed, feed rate, and cut time for a self-opening die head.
Equation Used
The calculator first converts cutting surface speed to spindle rpm at the thread major diameter, then uses the die lead per revolution to find axial feed and threading time. For a self-opening die head, this is the productive cut stroke before the cam ring trips and the chasers retract.
- Imperial inputs are used throughout.
- Thread lead equals pitch for a single-start thread.
- Cut time includes only the threading feed stroke, not loading or indexing.
- Spindle speed is set from surface speed at the thread major diameter.
Feed equals lead times spindle rpm for a single-start thread. The reciprocal cutting-time rate excludes loading, opening, withdrawal and resetting.
Cut, release and withdraw
The closed chasers form the cutting profile around the blank. As work and tool rotate relative to each other, axial advance follows the thread lead. At the trip point, the release mechanism allows the chasers to move outward. With cutting edges clear of the finished thread, the head can withdraw without unwinding along the helix.
The illustrated head is stationary while the work rotates. Other die-head arrangements rotate the tool. The front view is enlarged to make radial opening visible; it is not a dimensional drawing of a named head.
Parts shown
- Head body and chaser holders: support the cutting elements.
- Chasers: carry the thread cutting profile.
- Closing and release mechanism: holds the chasers in position, then permits them to clear the thread.
- Trip: initiates opening at the selected point.
- Feed motion: advances the work relative to the head during cutting.
Commercial heads use different locking and tripping details. Tool material, rake, throat, holder geometry and grinding instructions must match the particular head and thread. The animation does not specify those manufacturing dimensions.
External threading on production machines
Self-opening heads are used when an external thread can be cut with a suitable chaser set and rapid clearance after the cut is useful. The required profile, diameter, pitch, tolerance and shoulder clearance determine tooling compatibility.
Landis F & A Landmatic heads provide one documented example: stationary, self-opening heads for rotating work on turret lathes, chuckers and similar machines. This identifies a real equipment arrangement without implying that the simplified drawing reproduces that head’s internal design.
The calculator supplies timing arithmetic. It does not choose a cutting speed for a material, predict tool life, establish thread quality or replace the tooling manufacturer’s setup instructions.
Spindle speed, lead and cutting time
With surface speed Vc in feet/minute, thread major diameter D in inches, lead P in inches/revolution and entered axial cutting distance L in inches:
- N = 12Vc/(Ï€D) revolutions/minute.
- Axial feed = PN inches/minute.
- Cut time = 60L/(PN) seconds.
- Cutting-only rate = 60/cut time cuts/minute.
The factor 12 converts feet to inches; the factor 60 converts minutes to seconds. For a single-start thread, lead equals pitch. At 10 threads/inch, pitch and single-start lead are both 0.100 inch/revolution. For multiple starts, lead is pitch multiplied by the number of starts; the feed calculation must use lead.
The calculation treats L as the whole cutting-feed distance. Approach, any extra travel needed to form full threads, opening, withdrawal, closing and work handling require separate time allowances. The major diameter defines the selected surface-speed reference; the tool’s local cutting conditions vary across its profile.
Worked example: timing a single-start thread
Use an illustrative 0.750-inch diameter, 0.100-inch lead and 1.250-inch cutting distance. At an entered 30 SFM, spindle speed is 12 × 30/(π × 0.750) = 152.7887 rpm. Feed is 15.2789 inches/minute and cut time is 60 × 1.250/15.2789 = 4.9087 seconds. The reciprocal cutting-only rate is 12.2231 cuts/minute.
At 60 SFM, speed and feed double to 305.5775 rpm and 30.5577 inches/minute. Cutting time halves to 2.4544 seconds, and the cutting-only rate becomes 24.4462 cuts/minute.
Doubling L to 2.500 inches at the original 30 SFM doubles cutting time to 9.8175 seconds without changing spindle speed or feed. These comparisons follow the equations; neither surface speed is a recommendation for a particular work material or chaser.
A machine cannot achieve the reciprocal cutting-only rate if it also spends time loading, opening, withdrawing or resetting. A measured full cycle must include those operations.
Comparing thread-making methods
| Method | How the thread is made | What affects selection |
|---|---|---|
| Self-opening die head | Chasers cut the profile, then open for withdrawal. | Compatible head and chasers, thread specification and available clearance. |
| Single-point threading | A cutting tool follows a controlled helix, often over multiple passes. | Machine control, tool geometry and pass strategy. |
| Thread rolling | Dies plastically form the thread rather than cutting chips. | Material formability, blank preparation and forming equipment. |
Cycle time, cost, tolerance and tool life require a defined job and process. The simple one-pass timing model cannot assign universal rankings to these methods.
Self-opening die questions
After its chasers open far enough to clear the thread, the illustrated head can withdraw axially. The actual opening and withdrawal sequence depends on the tool and machine.
Only for a single-start thread. Lead is axial travel per revolution; a multiple-start thread has lead equal to pitch times its number of starts.
No. It is the reciprocal of cutting time alone. Loading, opening, withdrawal and reset reduce the achievable full-cycle rate.
No. Surface speed is an input. Obtain the appropriate cutting data for the actual work material, chasers, thread and machine.
No. Full thread length depends on throat and run-out geometry as well as clearance for opening. Consult the head and chaser specifications.
No. Setup, wear, alignment, tool geometry and other conditions can affect the result. The timing calculation cannot diagnose or certify thread accuracy.
No. Four are shown here for clarity. Commercial configurations and release mechanisms vary.
Use the actual manufacturer’s diagnostic and service procedure with the machine safely stopped. This drawing does not provide spring settings, repair dimensions or an adjustment method.
Manufacturer references
- Landis, Thread Cutting, describes a closing-ring arrangement that clears chasers from the thread.
- Landis, F & A Landmatic Stationary Die Heads, documents the stationary-head, rotating-work arrangement.
- Landis, Die Head / Tangential Chaser Management, explains why holder and grinding details depend on threading conditions.
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