How to Cut, Turn and Thread Chrome-Plated Shafts

Should you machine the ends before or after plating?

Settle the sequence on the drawing first; the tooling follows from it. Where the design allows, threads, bearing seats, seal areas and keyways are machined before plating and left chrome-free, so any later machining only touches steel that was never meant to carry chrome.

The reason is the layer itself. On Yixin bars, hard chrome is 10–50 μm per side, chosen per order, and chrome hardness can reach HV800+. Once a tool is through that layer it meets the base condition, and the base condition is not the same across the range. The 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft carry an induction-hardened case of at least HRC55, about 2 mm deep, under the chrome. The Q&T products have no induction-hardened case: 45 steel Q&T guarantees surface hardness only, HRC25–28 or HRC28–32 chosen per order, and the core is softer because grade 45 has limited hardenability; 40Cr Q&T is HRC28–35 and 42CrMo Q&T is HRC30–38, both quoted as Q&T hardness. The 45 Steel Chrome-Plated Soft Shaft has a base of about HRC17 and no hardened case at all.

So a turning or threading operation planned for after plating needs two statements on the drawing: which zones may lose their chrome, and what the base hardness under those zones will be.

How do you cut chrome-plated bar to length?

Cut on a saw and leave the ends as sawn. Yixin supplies full bars of 4 m, 6 m, 7.5 m or 8.1 m, CNC saw cut to any length from 200 mm with about 3 mm allowance, and the ends stay as sawn.

Cutting to length is the least demanding of the three operations because the saw crosses the bar at right angles to the axis: it parts the section but does not machine the plated diameter behind the cut. The new end face is bare steel, so treat it as an unplated surface in your drawing and in any protection you apply afterwards. The allowance exists for your own end finishing, and the choice at ordering is between as-sawn ends with about 3 mm allowance and end machining by the partner plant to the drawing.

If the ends will be machined to a drawing, say so on the inquiry. End machining is carried out by a long-term partner plant to Yixin's specification and assessed from your drawing, so the drawing, not a verbal note, governs what comes back. Bars ordered with sawn ends and bars ordered with machined ends are different purchases, and mixing them up is an easy way for a first delivery not to fit the fixture.

What happens when you turn through the chrome?

Turning is the operation where the drawing matters most, because a plated diameter is supplied to a fit and turning it removes that fit. A 10–50 μm layer at up to HV800+ is a hard skin over a base that may be soft, hardened or quenched and tempered, and the diameter you create afterwards is a new specification.

Two cases cover most work. On the 45 Steel Chrome-Plated Soft Shaft the base is about HRC17, so the chrome is the only hard element the tool meets, and a turned diameter is simply soft steel. On the 45 Steel Induction-Hardened Chrome Shaft the hardened case depth and hardness gradient vary by order and process, so confirm the specific case distribution with Yixin before deep cutting. On the Q&T bars the surface is quenched and tempered rather than surface hardened, and on 45 steel the core is softer than the surface because the grade has limited hardenability; a deep turn therefore exposes a different condition from the one you measured at the surface.

Sizing carries the same warning. Bar is supplied to diameter tolerances such as Ø4–11 −0.02/−0.05 mm and, from Ø12, 0/−0.02 mm on the linear shaft, with other tolerance options subject to the relevant product page or drawing; a turned end no longer sits in that band. Write the tolerance you will hold after machining, not the tolerance you received.

How do you thread a chrome-plated shaft?

Put the thread on a chrome-free zone if the drawing can accommodate it, and state the callout, the thread length and the fit class explicitly. Threads cut straight through the chrome are possible, but the crest and upper flank keep their chrome, so thread fit and gauging must account for the plated layer; specify the thread and whether it is cut before or after plating on the drawing.

The practical consequence is at assembly. A thread that has to take a nut repeatedly, or locate a clevis, is better specified as a chrome-free length measured from the bar end, with the plating stop point shown on the drawing. That also tells the plating partner plant exactly where the layer ends, which matters on the 45 Steel Chrome-Plated Linear Shaft, where the bearing zone is held to Ø4–11 −0.02/−0.05 mm or, from Ø12, 0/−0.02 mm; end work should stay outside that zone.

Where a thread must be cut after plating, give the full thread specification and let the partner plant assess it from your drawing. Do not assume the plated diameter can be threaded to the same class as unplated bar without saying so.

Which Yixin product should you machine after delivery?

Match the product to the base condition the machining will expose, not to the chrome alone. Yixin supplies these bars in Grade 45 (to GB/T 699-2015), 40Cr and 42CrMo (to GB/T 3077-2015); comparable grades, for reference only, include C45 and S45C for grade 45 and 41Cr4 and SCr440 for 40Cr, and chemistry limits differ between standards.

If the only requirement is a wear-resistant chrome surface and not the strength and toughness of a Q&T core, a soft or induction-hardened chrome shaft is sufficient for the requirement.

What to put in your RFQ

• The product name and the base condition you need: soft, induction-hardened or Q&T, with the hardness option where a choice exists.
• Finished length, plus every diameter and thread you will produce after delivery, with the tolerance you will hold.
• The zones that must stay chrome-free, with the length of each zone measured from the bar end, and where the plating may stop.
• Thread callout, thread length and fit class, stating whether the thread is cut before or after plating.
• Whether ends arrive as sawn with about 3 mm allowance, or machined by the partner plant to Yixin's specification from your drawing.
• Straightness required after machining, if the as-supplied figure is not the governing one.
• Documents: the steel mill certificate for each heat, forwarded unaltered, and third-party testing, which is commissioned per order on request.
• Packing: export bars wrapped in bubble film and packed in wooden cases, with no steel straps on finished bars.

Summary

Cutting is a saw operation and leaves sawn ends with about 3 mm allowance; turning and threading are the operations that break the chrome layer and change the fit. Decide on the drawing which zones stay chrome-free, what base condition sits under those zones, and what tolerance you will hold after machining. Yixin supplies bars in soft, induction-hardened and Q&T conditions with different base hardness, so the product name and the machining plan belong on the same drawing. Plating and end machining are carried out by long-term partner plants to Yixin's specification, which makes the written drawing the controlling document.

References

• GB/T 699-2015 — the standard to which Yixin's Grade 45 is specified.
• GB/T 3077-2015 — the standard to which Yixin's 40Cr and 42CrMo are specified.