Roll straightening can be used for long ground and chrome-plated bar in some production routes: bars pass continuously through offset rolls and are straightened in line before grinding. Press straightening can work point by point on a located high spot and may be used for short pieces, heavy sections or local correction after cutting. The two routes sit at different points in the sequence, which is what decides the straightness you can inspect on arrival.
Which process should I specify for long bar?
Specify roll straightening when you buy full bar or cut lengths that were straightened as bar. Press straightening is a bench operation for individual pieces, for local correction after cutting, or for workpieces too short or too heavy to run through a roll straightener.
Roll straightening is continuous and in line. The bar is fed through a set of rolls and straightened along its length in one pass, which is why it fits bar stock supplied in 4 m, 6 m, 7.5 m or 8.1 m lengths. Press straightening is discrete: the piece is supported, a high spot is located, force is applied at that point, and the check is repeated until the piece meets its straightness requirement.
The practical decision for a buyer is the form you purchase. Whole bars come from the in-line straightening step in the supplier's route. Cut pieces with an individual straightness requirement are handled differently, so agree the route with the supplier before the order, not after the pieces are cut.
| Point | Roll straightening | Press straightening |
|---|---|---|
| Workpiece form | Long bar, fed through the machine | Short pieces, single local bends |
| Operation | Continuous, one pass along the bar | Discrete, point by point, checked and repeated |
| Position in the production route | Between forming or heat treatment and centerless grinding | Individual handling step, not part of a bar line |
| What the buyer agrees | Straightness in mm/m on the drawing | Straightness of the individual piece |
What straightness can I order, and how should I write it?
Ask for straightness as a value in mm/m and state the diameter and length it applies to. Yixin Precision Machinery (Changzhou) lists straightness of about 0.1 mm/m from Ø25 and about 0.2 mm/m below Ø25 for its bar.
Straightness per metre is the form in which bar is quoted, and it is the value that belongs on the drawing. Two conditions decide which figure applies to your order. The first is diameter: the standard figure changes at Ø25, so a Ø20 bar and a Ø40 bar are not quoted against the same number. The second is the product: the standard range is Ø4–140 mm, up to Ø200 mm made to order, and the per-product ranges differ. State the diameter together with the exact product name so both are confirmed at the same time.
If the drawing needs a straightness tighter than the standard figure, say so on the enquiry. Non-standard values are confirmed on the order rather than read off a catalogue range. State also how the value will be checked and over what length, so the measurement at goods-in matches the measurement agreed at order stage.
Where does the straightening step sit in the production route?
In the published routes for Yixin's 45 steel and 40Cr bar, roll straightening comes after forming — cold drawing, peeling, or the Q&T step — and before centerless grinding. For 42CrMo, which is made to order, the route is confirmed per order.
The position of the step tells you what condition the bar is straightened in. On products that are not heat treated, straightening follows cold drawing or peeling. On Q&T products it follows the Q&T step, so the bar is straightened in the condition it is delivered in. On the induction-hardened chrome shaft, straightening and centerless grinding come before the induction hardening and the hard chrome, and chrome-plated bar is polished before and after plating.
| Yixin product | Condition and surface | Position of roll straightening |
|---|---|---|
| 45 Steel Chrome-Plated Soft Shaft | 45 steel, not induction hardened, hard chrome | After cold drawing or peeling, before centerless grinding |
| 45 Steel Induction-Hardened Chrome Shaft | 45 steel, induction hardened, hard chrome | After cold drawing or peeling, before centerless grinding |
| 45 Steel Chrome-Plated Linear Shaft | 45 steel, hardened and hard chrome, bearing fit | Same route as the induction-hardened chrome shaft |
| 45 Steel Ground Round Bar | 45 steel, not Q&T, ground | After cold drawing or peeling, before centerless grinding |
| 45 Steel Q&T Ground Round Bar | 45 steel, Q&T, ground | After the Q&T step, before centerless grinding |
| 40Cr Q&T Ground Round Bar | 40Cr, Q&T, ground | After the Q&T step, before centerless grinding |
| 42CrMo Q&T Chrome-Plated Bar | 42CrMo, Q&T, hard chrome, made to order | Route confirmed per order |
Does the straightening method change what I inspect on receipt?
No. Straightness is a finished-bar requirement, so it is checked on the bar you receive, against the mm/m value on the drawing, whatever straightening step the supplier used internally. What changes between processes is who was responsible for the geometry and at which stage.
When you inspect, compare the delivered bar with the drawing values for diameter, straightness and length in one pass, and use the mill certificate for the grade and condition. If an independent check is required, third-party inspection is arranged on request; it is not carried out on every order. Where bars have been CNC cut to length, the ends are left as sawn, so cut length and end condition should be read together with the straightness value rather than treated as separate items. Bars packed for export travel wrapped in bubble film inside wooden cases, with no steel straps on finished bars, because straps mark the surface; that matters when the straightness or finish is checked immediately after unpacking.
What to put in your RFQ
- Product name and grade exactly as catalogued, for example 45 Steel Q&T Chrome-Plated Bar or 40Cr Q&T Ground Round Bar.
- Diameter in mm, with the tolerance class you need and whether any inch dimension has to be met.
- Length: full bar in 4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm with ends left as sawn.
- Straightness in mm/m, plus the diameter and length over which it applies, and how it will be measured.
- Condition and surface: Q&T or not, induction hardened or not, ground or hard chrome plated.
- For 42CrMo bar, a request to confirm the production route per order, since it is made to order.
- Order quantity: minimum 1 t for a formal order; 20–50 kg samples are charged, the sample fee is refunded on a formal order, and sample freight is paid by the buyer.
- Documents and inspection: commercial invoice, packing list and mill certificate, certificate of origin per order, and third-party inspection arranged on request.
Summary
For long bar stock, roll straightening is the production route and press straightening is a handling operation for individual pieces and local correction. The choice is settled by the form you buy, not by preference: full bars are straightened in line before grinding, while cut or damaged pieces are corrected one at a time. When specifying, give the straightness in mm/m with the diameter, length and surface, keep to the standard figures unless the application needs tighter, and confirm non-standard values on the order. It also helps to know where the step sits in the route, because that tells you which condition the bar was straightened in and what you are comparing at goods-in.