Make bearing balls by forging short pieces of 1% carbon steel wire to rough spheres, hardening them at 800°C in oil, tempering, and then grinding them between two grooved cast-iron plates with ever finer abrasive until every ball in the set is within +-0.01 mm. The final tolerance is checked by measurement, and a micrometer that reads 0.01 mm cannot certify it unless it is kept accurate to a third of that.
Part of Bearings and Bushings
Forging, hardening and plate-grinding of the steel balls that go into a ball bearing, and how to check them.
Balls Are Ground Between Plates, Not Turned
A lathe turns a round shape by following a path, so any wobble in the path is copied into the part. A ball ground between two plates is different: each ball rolls and tumbles on a random path across the same abrasive surfaces, so a high spot is worn down wherever it happens to come up. That averaging, rather than copying, is how a ball set comes out rounder and more uniform than the machine that made the blanks. (This explanation is standard engineering knowledge, not a line in the carrier article; the carrier states only the method.)
The carrier gives the sequence as forge, harden, temper, grind. Hardening comes before grinding so the finished size is cut in the hardened steel and is not changed afterward by distortion in the quench.
What You Need
- High-carbon steel wire at about 1% carbon (see Tool Steel)
- Hemispherical dies to forge the rough sphere
- A furnace or forge, oil for quenching, and a way of tempering at a controlled temperature
- Two cast-iron grinding plates, one rotating and one stationary, with grooves for the balls
- Abrasive in graded steps: silicon carbide, then aluminium oxide, then polishing compound
- A micrometer kept accurate within +-0.003 mm (Micrometers), and a bin to keep each size class separate
Method: From Wire To Finished Ball
Step 1: Cut short cylinders from the wire.
Step 2: Heat to cherry red and forge each between hemispherical dies into a rough sphere.
Step 3: Harden. Heat to 800°C and quench in oil.
Step 4: Temper. The carrier says reheat to 150°C and air cool, giving about 60 HRC. The oxide-colour method in Tempering starts at 220°C (faint straw), so 150°C is below any colour you can see. Use an oil bath with a thermometer, or temper at the lowest colour and accept the slightly softer ball.
Step 5: Grind between the plates. Put the balls in the grooves of the stationary plate, run the rotating plate against them with abrasive, and change to a finer abrasive in steps. Work all balls of a batch together so they see the same wear.
Step 6: Check and sort. All balls in a set must be within +-0.01 mm of each other in diameter. Measure each ball at three different orientations; a ball that reads differently in different directions is out of round.
Worked Example
A finished ball of 10 mm diameter needs a rough sphere a little larger so the forged skin and scale can be ground off; the carrier gives no figure for how much, so call it 11 mm for the example. Volume = pi x d cubed / 6 = 3.1416 x 1,331 / 6 = 697 cubic millimetres.
From 8 mm wire, the cross-section is 3.1416 x 8 x 8 / 4 = 50.3 square millimetres, so the cylinder to cut is 697 / 50.3 = 13.9 mm long. Scale lost in the forge will take a little more, so cut slightly long and weigh a sample.
Checking the tolerance: a micrometer that reads 0.01 mm resolves exactly the tolerance it is supposed to certify. The Micrometers article says a good one reads within +-0.003 mm of a gauge block; 0.01 / 0.003 is a ratio of about 3. Compare against a gauge-block stack (Gauge Blocks) before sorting.
Common Mistakes
- Cracking in the quench. Overheating, too fast a quench, or a flawed blank; oil is the carrier’s quench for good reason.
- Using balls that were not tempered. They are glass-hard and can shatter.
- Not sorting by size. The biggest ball in a bearing carries more than its share.
- Mixing grits. Carry coarse abrasive into the polishing stage and the finish is lost.
Safety
Hot steel, quench oil and abrasive dust
Forging, hardening and quenching involve red-hot steel and oil that can ignite: quench with a lid or damp cloth ready and never over a draught. Hardened balls are brittle and can chip, so wear eye protection while grinding. Silicon carbide and aluminium oxide slurry dries to dust: keep the work wet, and read Dust and Respiratory Protection on what a cloth mask can and cannot do.
What’s Next
- Learn next: Race Machining - the grooves the balls roll in
- Combine with: Lapping for Ultra-Precision - the same abrasive logic on flat surfaces