A thread standard is an agreed pitch and flank angle held as a hardened master thread and checked with go and no-go gauges: 60 degrees for metric and UN threads, 55 degrees for Whitworth. Threads made on different lathes only interchange if every one follows the same form and pitch, so a settlement should pick one standard early and write it down.
Part of Precision Tool Making
Why screws made in different shops must share a form, how the pitch is set by the lathe, and how a master thread keeps the standard.
Interchangeable Screws Need One Pitch And One Angle
A bolt made in one workshop must fit a nut made in another, or every repair becomes hand-fitting. Two things must match: the pitch, the distance between neighbouring threads, and the flank angle, the angle between the two sides of the thread. Metal Turning gives the two common forms: a V-form tool ground to 60 degrees for metric and UN threads, and 55 degrees for Whitworth. A 60-degree bolt in a 55-degree nut meets on the flanks only partly, because each flank is off by 2.5 degrees, so the pair seats badly and strips under load.
The pitch is set by the lathe. The lead screw drives the carriage, and the gear train between spindle and lead screw fixes how far the carriage moves per spindle turn. The thread pitch is the lead screw’s pitch multiplied by the number of lead-screw turns per spindle turn. So the accuracy of every thread cut on a lathe is the accuracy of its lead screw (Master Lead Screw).
What You Need
- A lathe with a lead screw and a changeable gear train
- A threading tool ground to the chosen flank angle
- A master thread of each size: cut the best thread you can on your best lathe, harden it, and keep it as the reference (Precision Measurement)
- Go and no-go thread gauges (Thread Gauges)
- A written list of the sizes your community uses
Method: Fix A Standard
Step 1: Choose one form. The metric system is the recommended basis for units (Standard Units), and metric threads are 60 degrees. Mixing forms is what to avoid.
Step 2: Choose a short list of sizes and their pitches.
Step 3: Cut and harden a master thread for each, and put it away.
Step 4: Make working gauges from or against the master.
Step 5: Check working gauges against the master on a schedule. A working gauge wears; the master, used rarely, is how you notice.
Step 6: Teach the gear-train settings for each pitch so every operator cuts the same thread.
Worked Example
Say the lead screw has a pitch of 4 mm (an assumed figure for the example). If the gear train makes the lead screw turn once for every 2 turns of the spindle, the carriage moves 4 x (1/2) = 2 mm per spindle turn, so the thread cut has a 2 mm pitch. If the train makes the lead screw turn once for every 8 spindle turns, the pitch is 4 x (1/8) = 0.5 mm, the micrometer screw pitch used in Micrometer Construction.
The depth of a 60-degree thread: a sharp V with sides of one pitch has a height of pitch x cos 30 degrees = 0.866 x pitch. Metal Turning uses 0.866 x pitch as the thread depth to reach. That figure is the sharp-V height; real threads have flat crests and roots, so the finished depth is less, and cutting all the way to the sharp-V depth would over-cut. Check with a gauge rather than trusting the number.
Common Mistakes
- Mixing 60-degree and 55-degree threads in one settlement.
- Using a worn working gauge as the standard. Compare to the master.
- Cutting to a quoted depth without a gauge.
- Changing the gear train without logging the setting.
Safety
Thread cutting is lathe work
The tool moves with the spindle and the half-nuts are engaged: keep fingers and sleeves clear of the chuck and lead screw, know where the lathe stop is, and never reach over the work while it turns. Hardening a master thread means hot steel and quench oil.
What’s Next
- Learn next: Master Lead Screw - where the pitch accuracy comes from
- Combine with: Thread Gauges - checking threads in production