Centerless ground bar is the route to specify when the bar's own diameter, straightness and surface finish must survive into the finished part. Turned and polished bar is the middle route, and cold drawn bar is usually enough when the bar is machined all over. The three differ mainly in the last operation that forms the surface.
What actually differs between centerless ground, turned and polished, and cold drawn bar?
The three routes differ in the operation that produces the final surface, and that operation decides how much accuracy reaches your shop.
Cold drawn bar. The bar is pulled through a die, so the die sets the size and the surface. Typical reference values for the process are a tolerance around IT10 and normal straightness of max 1 mm/m; confirm them against the applicable standard or the mill's data.
Turned and polished bar. This route turns or peels the outside of the bar and then polishes it, so what you receive is a machined surface. Typical reference values for peeled bar are normal straightness of max 0.6 mm/m and max Rt 50 μm.
Centerless ground bar. The bar is ground after drawing or peeling, between a grinding wheel and a regulating wheel and without a center. Ground bars go through an additional grinding step compared with turned and polished bars, and are described as having higher straightness and accuracy. Typical reference values for ground bar are a tolerance down to IT6, straightness down to 0.1 mm/m and a finish down to Ra 0.6 μm.
All of these are typical values for the process, not a quotation for a specific size, and they should be confirmed against the applicable standard or the mill's data.
Which route does my part actually need?
Match the route to what the bar has to deliver when it arrives, not to the tightest tolerance you can name on a drawing.
- Cold drawn bar suits bar that is machined all over. If every functional surface is cut in your own shop, the bar only has to feed reliably, hold enough stock for cleanup and keep the fixture repeatable; a tolerance around IT10 and normal straightness at max 1 mm/m is a reasonable starting point.
- Turned and polished bar suits parts where the outside diameter is machined but you also want a clean, uniform surface on the bar as received, for example before a second operation or before inspection of the incoming lot. Straightness max 0.6 mm/m and max Rt 50 μm are the typical reference values here.
- Centerless ground bar suits parts where the bar's own surface stays on the finished component, or where the bar runs directly in a bearing, collet, chuck, gripper or feed unit. It is also the route to use when the drawing calls out a finish and a straightness that survive shipping rather than being created by your own turning.
One further decision belongs in the same conversation: if the final working surface requires hard chrome, confirm whether to select a chrome-plated product or reserve the part for downstream surface treatment. Those two choices give you different drawings and different incoming inspection.
As an example of the ground route in practice, Yixin Precision Machinery (Changzhou) makes its 45 Steel Ground Round Bar by cold drawing or peeling, then roll straightening, centerless grinding and polishing; the bar is supplied at about HRC17.
How tight a tolerance and how straight can each route hold?
In general terms, cold drawn bar holds the loosest band, turned and polished bar is straighter with a better finish, and centerless ground bar holds the closest band and the best straightness of the three.
| Route | Final surface made by | Typical size tolerance | Typical straightness | Typical surface finish |
|---|---|---|---|---|
| Cold drawn bar | drawing through a die | IT10 is the standard tolerance | max 1 mm/m | — |
| Turned and polished bar | turning or peeling, then polishing | — | max 0.6 mm/m | max Rt 50 μm |
| Centerless ground bar | grinding after drawing or peeling | down to IT6 | down to 0.1 mm/m | down to Ra 0.6 μm |
A dash means the reference values used here do not quote a figure for that characteristic; ask the supplier for the figure that applies to your size. Treat the whole table as process reference values and confirm them against the applicable standard or the mill's data.
For a worked example of the ground route, Yixin quotes its 45 Steel Ground Round Bar at Ø4–11 mm −0.02/−0.05 mm and, from Ø12 mm, 0/−0.02 mm, −0.01/−0.03 mm or −0.02/−0.05 mm; straightness is ≈ 0.1 mm/m from Ø25 mm and ≈ 0.2 mm/m below that, with Ra ≤ 0.8 μm. Four ground products are available, and the figures belong to each product separately:
| Yixin product | Grade and condition | Hardness | Standard diameter range |
|---|---|---|---|
| 45 Steel Ground Round Bar | Grade 45, not Q&T | ≈ HRC17 | Ø4–140 mm, up to Ø200 mm made to order |
| 45 Steel Q&T Ground Round Bar | Grade 45, induction Q&T | surface hardness HRC25–28 or HRC28–32, chosen per order | Ø8–140 mm |
| 40Cr Q&T Ground Round Bar | 40Cr, induction Q&T | Q&T hardness HRC28–35 | Ø5–140 mm |
| 42CrMo Q&T Ground Round Bar | 42CrMo, Q&T | Q&T hardness HRC30–38 | Ø8–100 mm, made to order |
The Q&T ground bar is quenched and tempered, not surface hardened, and has no induction-hardened case. The hardness of the 45 Steel Q&T Ground Round Bar is a surface value; its core is softer because grade 45 has limited hardenability, so do not write that range as a through hardness.
If the part will be hard chromed, does the route still matter?
Yes, because the chrome specification and the bar specification are two different documents, and you have to decide which one you are buying.
Yixin supplies hard chrome at 10–50 μm per side, chosen per order, with chrome hardness that can reach HV800+, on products such as the 45 Steel Chrome-Plated Soft Shaft and the 45 Steel Induction-Hardened Chrome Shaft. Surface finish is quoted as Ra ≤ 0.4 μm on chrome-plated bar and Ra ≤ 0.8 μm on ground bar, so the plated product carries the finer finish figure and the ground bar carries its own.
That leaves two clean ways to buy:
- Buy the bar already plated. You take a chrome-plated product and inspect the chrome layer and the finish on arrival. This is the shorter route through your own shop.
- Buy centerless ground bar and plate downstream. You keep control of the plating step, the layer thickness and any masking, and the ground bar gives you the diameter, straightness and Ra ≤ 0.8 μm surface as your starting point.
What you should not do is assume that a drawn or turned and polished surface will be accepted where the drawing also specifies a chrome finish and a tight diameter band. Confirm the decision on the drawing: if the final working surface requires hard chrome, state whether the chrome is part of the purchased bar or a downstream operation on your part.
Salt-spray performance is not part of the standard specification. Where a rating is required, we specify a thicker chrome layer and can have the order tested by a third-party laboratory.
What to put in your RFQ
- The route and the product name, for example centerless ground bar, turned and polished bar or cold drawn bar, and whether the bar will be chromed later.
- Grade and standard: Grade 45 to GB/T 699-2015, or 40Cr or 42CrMo to GB/T 3077-2015, plus the condition, as supplied or Q&T, and the hardness range you require.
- Diameter and the tolerance band written as a limit pair, for example Ø4–11 mm −0.02/−0.05 mm or from Ø12 mm 0/−0.02 mm, with the gauging point stated.
- Straightness per meter and the length over which it is measured.
- Surface finish as Ra, and whether the figure applies to the bar as supplied or after chrome.
- Chrome layer in μm per side if you are buying a plated product, and which diameters are functional surfaces.
- Length: full bars of 4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm with ends left as sawn.
- Quantity and documentation: minimum 1 t per order with different sizes combined, 20–50 kg samples, and the steel mill certificate for each heat.
Summary
Cold drawn bar gets its size and finish from a die; turned and polished bar is machined and then polished; centerless ground bar is ground after drawing or peeling, which adds a grinding step and gives higher straightness and accuracy. Choose the loosest route that still leaves the accuracy your part needs on arrival, and put the route, the grade, the tolerance band, the straightness, the finish and the chrome decision on the drawing rather than in an email. Where the working surface needs hard chrome, decide early whether you are buying a plated bar or plating your own part.
References
- GB/T 699-2015 — Chinese standard cited with Grade 45 in the Yixin catalogue.
- GB/T 3077-2015 — Chinese standard cited with 40Cr and 42CrMo in the Yixin catalogue.