Centerless grinding is the finishing operation that sets the final diameter, roundness and surface finish of a round bar. The bar is supported on a workrest blade between a grinding wheel and a regulating wheel instead of being held between centers, which suits long bar stock and repeatable production. It is the step that turns a sized blank into precision shafting.
What does centerless grinding do to a round bar?
It brings the bar to its final outside diameter, roundness and surface finish after cold drawing or peeling and roll straightening. On shafting, grinding is the operation that lets a buyer specify diameter band, surface roughness and straightness as separate, measurable requirements.
The bar is not clamped between centers. It rests on a workrest blade between the grinding wheel and a regulating wheel; the regulating wheel drives the bar and controls how fast it passes the grinding wheel, so a full bar length can be ground in one through-feed pass. There are no center holes to machine and no tailstock to leave marks, and long lengths can be finished over the whole surface. Yixin Precision Machinery (Changzhou) carries out centerless grinding in its own workshop on 12 centerless grinders, inside a standard diameter range of Ø4–140 mm, with up to Ø200 mm made to order.
The route matters to the buyer. For a 45 Steel Ground Round Bar, the bar is cold drawn or peeled, roll straightened, centerless ground and polished, and supplied at about HRC17. Grinding is therefore a finishing step positioned after straightening, and the diameter it produces is the diameter you receive.
How tight a tolerance can a centerless-ground round bar hold?
Tolerance bands go down to 0.02 mm. The band is selected per order from a standard set, and the final values are confirmed on the drawing and order rather than assumed from a catalogue page.
| Finished diameter | Tolerance options |
|---|---|
| Ø4–11 mm | −0.02/−0.05 mm |
| from Ø12 mm | 0/−0.02 mm, −0.01/−0.03 mm or −0.02/−0.05 mm |
Two conditions limit which band is available. First, diameter: quenched and tempered bars start at Ø8 mm, so the small band covers Ø8–11 mm on those products, while the 40Cr Q&T Ground Round Bar starts at Ø5 mm. Second, the surface state: if the working surface will be hard chrome plated, the drawing must state whether the tolerance applies to the ground diameter or to the plated diameter. Hard chrome is applied at 10–50 μm per side, chosen per order, so a plated bar is not the same size as the ground bar underneath it. Buyers who plate downstream should either reserve grinding allowance for the layer or order a chrome-plated product with the finished diameter specified.
What surface finish can I expect from a ground bar?
Ground bars are supplied at Ra ≤ 0.8 μm and chrome-plated bars at Ra ≤ 0.4 μm. Both are surface roughness values, and on a plated bar they describe the chrome surface, not the steel beneath it.
Finish comes from the sequence, not from grinding alone. In the Yixin route the bar is ground and then polished in the same workshop. Where hard chrome is required, plating is done by long-term partner plants to Yixin’s specification, and chrome thickness, diameter and surface are checked before packing. That division of work is worth knowing when you plan inspection: the ground geometry and the plated surface may be verified at different stages. Chrome hardness can reach HV800+.
State the roughness value and the surface it applies to on the drawing. A callout of Ra ≤ 0.8 μm on the ground diameter and Ra ≤ 0.4 μm on the chrome surface are different requirements, and a supplier quoting without knowing which one you mean is guessing at the process.
Does centerless grinding set straightness and hardness?
Straightness is a separate specification, and hardness is not affected by grinding at all. Yixin works to straightness of ≈ 0.1 mm/m from Ø25 mm and ≈ 0.2 mm/m below Ø25 mm; the value follows the straightening and grinding route, so express it per meter and say whether it applies over the full bar length.
Hardness comes from the condition of the bar, which is fixed before grinding:
- 45 Steel Ground Round Bar, 45 steel, not quenched and tempered: about HRC17.
- 45 Steel Q&T Ground Round Bar: surface hardness HRC25–28 or HRC28–32, chosen per order. The core is softer because grade 45 has limited hardenability.
- 40Cr Q&T Ground Round Bar: Q&T hardness HRC28–35.
If the part needs a hardened case rather than a tempered body, the induction-hardened products are the match: the 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft carry a case of ≥ HRC55, ≈ 2 mm deep, under hard chrome. Quenched and tempered bars are not surface hardened and have no induction-hardened case.
Which bar should I specify?
Start from the working surface, then from the strength the part needs. A ground bar is the choice when the shaft runs in a bearing or a seal and the drawing calls out a ground finish with no chrome; a chrome-plated bar is the choice when the working surface itself is the wear surface.
| Product | Condition | Surface | Hardness |
|---|---|---|---|
| 45 Steel Ground Round Bar | 45 steel, not Q&T | ground | about HRC17 |
| 45 Steel Q&T Ground Round Bar | induction Q&T | ground | HRC25–28 or HRC28–32 per order (surface; core softer) |
| 40Cr Q&T Ground Round Bar | 40Cr, induction Q&T | ground | Q&T hardness HRC28–35 |
| 45 Steel Chrome-Plated Soft Shaft | 45 steel, not induction hardened | hard chrome | base about HRC17 |
| 45 Steel Induction-Hardened Chrome Shaft | induction hardened, then hard chrome | hard chrome | case ≥ HRC55, ≈ 2 mm deep |
For heavy or shock loads, the 42CrMo Q&T Chrome-Plated Bar is made to order from 1 t in 7–10 days, with Q&T hardness HRC30–38. For a chrome surface without a hardened case, the 45 Steel Chrome-Plated Soft Shaft keeps the base at about HRC17. Confirm on the drawing whether the working surface is plated at all; if it is not, the bar can be reserved for downstream surface treatment instead.
What to put in your RFQ
- Product name and grade, written as you want it confirmed, for example 45 Steel Ground Round Bar or 40Cr Q&T Ground Round Bar.
- Nominal diameter and the tolerance band for the finished part, and whether that band applies to the ground or the plated diameter.
- Surface roughness Ra and the surface it is measured on.
- Straightness in mm/m, and whether it applies over the full bar length.
- Length: full bars of 4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm, with sawn ends noted.
- Chrome requirement, if any: layer per side and which diameters are plated, or a note that the part is reserved for downstream treatment.
- Quantity, combining sizes if needed against a minimum of 1 t per order, or a 20–50 kg sample.
- Inspection: dimensions to be recorded, and whether a third-party inspection is required; per-order third-party inspection is arranged on request.
Summary
Centerless grinding is the finishing step that converts a drawn or peeled bar into precision shafting: it sets the finished diameter within bands down to 0.02 mm, surface roughness to Ra ≤ 0.8 μm on ground bars and Ra ≤ 0.4 μm on chrome-plated bars, and works with roll straightening to hold straightness of ≈ 0.1 mm/m from Ø25 mm and ≈ 0.2 mm/m below. It does not change hardness, and it does not absorb a chrome layer unless the drawing says so. Specify diameter band, Ra, straightness, length and chrome position as separate line items, and the quote and the inspection plan will match the part.