Turn the spindle from tool steel 8-10 mm thick and 200-250 mm long, taper it, harden and temper it, then finish it by grinding and polishing until it runs true to under 0.05 mm. A first-generation lathe cannot hold that tolerance, so the finishing step, not the turning, decides whether the spindle is usable, and a spindle that is bent or rough breaks yarn at every revolution.
Part of Spinning Frame
Making the long, slender, hardened steel spindle that carries the bobbin and runs for years inside a bearing.
The Spindle Is Slender, And Slender Things Move When Hardened
The overview gives the spindle: 8-10 mm diameter body tapered to a point, 200-250 mm long, runout under 0.05 mm, polished to a mirror finish (a rough surface breaks the thread), hardened to 55-60 HRC for wear. Its slenderness ratio, length over diameter, is 200 ÷ 10 = 20 to 250 ÷ 8 = 31. Long, thin, hardened parts warp in the quench and cannot be corrected afterwards by ordinary turning, because the skin is now harder than the tool.
The Tolerance Needs A Better Machine Than The First One
Precision Tool Making puts a first lathe at about 0.5 mm accuracy, the next at 0.1 mm, and the generation after at 0.05 mm. Lathe Fundamentals says most early-industrial work was done to 0.1-0.5 mm and that a three-jaw chuck should run out under 0.1 mm. The overview’s 0.05 mm therefore sits at or beyond the third generation of the precision chain. A village on its first lathe can turn the blank but must finish by grinding and lapping, or wait for the better lathe.
What You Need
- Tool steel. Identify it by the spark test and the file test in Tool Steel; salvaged files, springs and axles are the usual source.
- A lathe with a tailstock and a steady rest for long work (Metal Turning).
- A quench tank and a way to heat the full length evenly.
- Something to measure runout: a dial indicator, or the screw micrometer in the precision article, which reads 0.01 mm per division.
- Abrasive and a lapping bed to finish.
Method: From Bar To Spindle
Step 1: Turn between centres with the steady rest supporting the middle. Cut a slight oversize and leave a finishing allowance, since hardening will move the part. The allowance is not sourced; learn it by quenching a trial piece and measuring how far it bends.
Step 2: Turn the taper and point, and cut the seat for the whorl.
Step 3: Harden. Heat evenly and quench the long part end-first so it enters the liquid straight (recalled practice, not from the corpus). Use the quench the steel calls for.
Step 4: Temper at once (Tempering). The corpus colour table gives straw at 220-250 degrees C for fine hard tools and blue at 300 degrees C for springs. The overview’s 55-60 HRC cannot be checked without a hardness tester; a file that skids off tells you it is hard, not how hard.
Step 5: Straighten and finish. Check runout, correct any bend, then grind or lap to size and polish to a mirror finish. The precision article’s own practice is to rough-grind to within 0.05 mm of target after hardening.
Step 6: Test. Spin it in its bearing at running speed, and check for heat, noise and wobble before it is fitted.
Worked Example
A spindle of 250 mm length and 8 mm diameter has a slenderness ratio of 250 ÷ 8 = 31. The runout limit of 0.05 mm is 0.05 ÷ 250 = 0.02% of its length. A bend of one fifth of a millimetre over the full length, which is invisible by eye and an ordinary result of an uneven quench, exceeds the limit fourfold and must be corrected before the spindle is used.
Common Mistakes
- Finishing before hardening and then trusting the dimension. The quench moves it.
- Skipping the temper. A glass-hard spindle snaps in service, and its fragments are thrown.
- A rough surface, which cuts the yarn.
- Running a bent spindle, which hammers its bearing and varies the yarn tension.
Safety
Hardened steel shatters, and a spindle spins fast
An untempered spindle is as brittle as glass. Quenching sprays steam and hot liquid; wear eye and face protection and keep the tank away from other work. In service a spindle turns at thousands of revolutions per minute beside the flyer, and the stop point and coast time for the frame are set out in Drive System.
What’s Next
- Learn next: Flyer Mechanism — the part that circles the spindle
- Combine with: Bobbin and Whorl — the bobbin that sits on it