Make bearing races by turning inner and outer rings from high-carbon steel, cutting the ball groove with a form tool radiused to the ball, hardening at 830°C in oil, tempering at 200°C and grinding the groove smooth. Cut bore, outside diameter and groove in one chucking so the groove is concentric with the bore, and grind the groove after hardening because the quench distorts it.
Part of Bearings and Bushings
Turning, hardening and grinding the inner and outer races on which bearing balls roll.
The Groove Radius Decides How The Ball Carries Load
The races are the hard tracks the balls run on. Their grooves must be round, smooth, concentric with the bore, and a little larger in radius than the ball. If the groove were exactly the ball’s radius the ball would be gripped over a wide patch and drag; if it is much larger the ball sits on a small contact patch and loads concentrate. The carrier’s rule is groove radius = ball radius plus 0.5-1 mm, which also allows a little angular contact.
What You Need
- High-carbon steel stock for both rings (Tool Steel)
- A lathe and a form tool ground to the groove radius (Lathe Fundamentals)
- A forge or furnace, oil, and a means of tempering
- A small grinding wheel dressed to the groove form and a spindle to run it (Grinding Wheels)
- Finished balls to check the groove against (Making Steel Balls)
Method: Turn, Harden, Grind
Step 1: Turn each ring from the stock. Bore, outside diameter and ball groove are all cut without releasing the work from the chuck, so they share one axis. Releasing and re-chucking a ring puts the groove off-centre by whatever the chuck repeat error is, and the balls then track a wandering path.
Step 2: Cut the groove with a form tool radiused to match the ball.
Step 3: Harden. Heat to 830°C, quench in oil. The carrier gives 800°C for the balls and 830°C for the races. Hardening gives 760-800°C for 1% carbon steel (Hardening), so both figures are at or just above the top of that range.
Step 4: Temper at 200°C. The Carburization article lists pale straw colour at 200°C (Carburization).
Step 5: Grind the groove smooth with a grinding wheel dressed to the groove shape. Take light passes and keep the work cool so the temper is not drawn.
Step 6: Check the fit of a ball in the groove. A ball should roll along the track smoothly; rub a thin film of engineer’s blue on the ball and roll it once to see the contact track (a suggestion, not a procedure from the carrier).
Worked Example
For a ball of 10 mm diameter, the ball radius is 5 mm and the groove radius should be 5.5-6 mm. The ratio of groove radius to ball radius is 5.5 / 5 = 1.10 up to 6 / 5 = 1.20.
For a smaller 6 mm ball, radius 3 mm, the same 0.5-1 mm rule gives a groove of 3.5-4 mm, a ratio of 1.17 up to 1.33. The carrier’s rule is an absolute allowance, so it changes the ratio with ball size: a smaller ball sees a looser groove. Commercial deep-groove races are cut much closer to the ball radius than the carrier’s rule gives; that is recalled, not sourced here, so test your own groove on a trial bearing under light load before committing to a batch.
Common Mistakes
- Re-chucking between operations. The groove goes off-centre.
- Grinding before hardening. The quench changes the shape; finish after it.
- Overheating while grinding. It draws the temper and softens the groove surface.
- Copying the groove radius rule to every ball size without looking at the ratio.
Safety
A grinding wheel that bursts sends fragments out at rim speed
A dropped, cracked or wrongly mounted wheel can burst. The rim of a 300 mm wheel at 1,500 rpm moves at 3.1416 x 0.3 x 1,500 / 60 = 23.6 m/s, and fragments leave at that speed. Ring-test every wheel by tapping it lightly with a wooden mallet and reject one that thuds, guard it except at the working face, never exceed its rated speed, stand to the side on the first start after mounting, and wear eye protection. On the lathe, keep sleeves clear of the chuck and know where the stop is.
What’s Next
- Learn next: Cage and Assembly - spacing the balls and putting the bearing together
- Combine with: Ball Bearings - the full build order and the precision threshold