How to Source Machined Shafts
What a machined shaft RFQ must specify: material and condition, fits, runout, keyways, threads, hardness and finish, plus cost drivers and lead times.
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A shaft looks simple on the print and is easy to get wrong in the quote. Most of its cost and most of its risk sit in a handful of features: the bearing and seal journals, how they relate to each other, the keyways and threads, and whatever heat treatment the part needs. A shop that cannot see those clearly will either pad the price or quote the easy version of the part and discover the hard one after the order.
A good shaft RFQ names the material by spec and condition, tolerances the journals and fits rather than every diameter, states runout to a datum axis formed by the bearing seats, and puts hardness, case depth and surface finish on the features that need them. Give the quantity and release schedule too: a lathe program for 5 pieces and a ground production run of 500 are different jobs, and shops price them differently.
RFQ checklist
18 things to settle on the drawing or in the request before you ask for machined shaft quotes. Print it and tick it off against the print, or copy it into the RFQ.
Material and condition
Material grade, the spec it is bought to, and the starting condition: annealed, cold drawn, or quenched and tempered (for example 4140 pre-hard at about 28-32 HRC)
Condition decides whether the shop machines soft and heat treats afterwards, or cuts pre-hardened bar and skips a furnace trip. The two routes differ in price, lead time and distortion risk.
Material certification and traceability: mill cert with heat number, and country of melt if a DFARS or domestic-source clause applies
Shops keep and pass on heat-lot certs when asked up front. Asking after the order can mean re-buying bar from a compliant source.
Geometry and tolerances
Overall length and the position of every shoulder, dimensioned from one datum face (usually a bearing shoulder)
Shoulder positions stack up. Dimensioning from the face the assembly locates on keeps the fits where they belong instead of leaving the shop to guess at a tolerance chain.
Critical diameters with limits or ISO fit classes, for example 25 k6 for a bearing seat or 25 h7 for a coupling bore fit
A bearing seat at +0.002/+0.015 mm is a grinding operation; a clearance diameter at ±0.1 mm is one lathe pass. Shops price every tight diameter as a ground one, so tolerance only the fits.
Seal journals: diameter, finish and a lead-free (plunge-ground) requirement for lip seals
Lip seals pump oil out along the helical texture a turning tool leaves. Seal makers typically ask for about 0.2-0.8 µm Ra (8-32 µin) with no machine lead, which means plunge grinding that must be on the print to be quoted.
Runout or total runout of journals, seats and faces to a datum axis formed by the bearing seats
Runout of 0.01-0.02 mm between journals usually means grinding between centers in one setup. Without a datum axis the shop cannot tell which diameters must run true to which.
Straightness for slender shafts (length more than about 10 times diameter)
Slender shafts deflect under the tool, move when heat treated and bow when stress in cold-drawn bar is released. The requirement decides whether the shop needs steady rests, stress relief or a straightening step.
Keyways: width, depth and length with tolerances, end form (sled-runner or profile) and the standard they follow (ANSI B17.1, ISO/DIN 6885)
A sled-runner keyseat is cut with a disc cutter and runs out at the ends; a profile keyseat is end-milled with square ends. They take different tooling and carry the key differently, and key width tolerance sets the key fit.
Threads: size, pitch, class (2A or 6g), length, and the undercut or relief at the shoulder
A thread that runs into a shoulder without a relief groove cannot be cut as drawn. Class and relief tell the shop whether it single-points to a shoulder or needs runout space.
Center holes: required, permitted or not permitted on the finished part (ISO 6411 symbols), with size if required
Grinding and inspection between centers need them. If the assembly cannot have them, the shop needs another way to hold the part and will price it that way.
Cross holes, flats, splines, grooves and retaining ring grooves, located angularly to the keyway or another datum
Features clocked to each other need a mill-turn machine or an indexed second setup. If the relationship is not stated, the shop cannot price the fixture.
Heat treatment and surface
Surface finish by feature: bearing seats, seal lands, and a general note for everything else
A blanket 0.4 µm Ra (16 µin) note turns every surface into a grinding job. Call 1.6-3.2 µm (63-125 µin) generally and tighter only where a bearing, seal or sliding fit needs it.
Heat treatment and hardness: through-hardened range (for example 28-32 HRC) or surface hardness, and where it applies
The hardness range picks the process (quench and temper, induction, carburize) and the order of operations. Journals ground after hardening need grind stock left on, which the shop has to plan.
Case depth for induction hardened or carburized shafts: effective depth to a stated hardness (commonly 50 HRC), and zones to leave soft such as threads and keyways
Case depth only means something when measured to a hardness value. Without the value and the soft zones, the heat treater quotes its own default.
Coating or plating by spec and thickness (black oxide, zinc, hard chrome, electroless nickel), with areas to mask
Plating adds thickness to fits, so hard-chromed journals are usually plated oversize and ground back. Masking threads and bearing seats is labor the quote must include.
Quality, quantity and delivery
Inspection and documentation: first article report, dimensional report on the fits, runout readings, hardness and heat treat certs
A full first article on a shaft with 40 dimensions is hours of inspection; a report on the five fits is minutes. Say which you need.
Quantity per release, annual volume and release schedule
Lot size decides whether the shop runs a bar-fed lathe, a chucker or between-centers work, and whether a grinding fixture pays off.
Packaging: rust preventive, protection for ground journals and threads, straight-safe packing for long shafts
Ground journals ding when parts touch in a bin. Sleeves, VCI paper or tubes cost money, so they belong in the quote.
What to send with the RFQ
- A PDF drawing with fits, datums, runout, finishes, hardness and notes. The drawing is the contract.
- A STEP model for programming. If you only have a model, list the fits, finish and hardness in the RFQ or generate a drawing first.
- Mating part numbers: bearings, seals and the coupling, gear or pulley bores, so the shop can sanity-check the fits.
- Speed, load and environment in one line, so a shop can flag a material or heat treatment mismatch.
- Quantity per release, annual usage and the date you need parts.
- The worn or broken shaft, if this is a replacement and the print is old or missing.
Lead times
| Prototype or first parts | 1-3 weeks for turned and milled shafts from stock bar; add 1-2 weeks when outside heat treatment and grinding are involved. |
|---|---|
| Production or repeat orders | 4-8 weeks for heat-treated, ground shafts in lots of 50-500; longer when an uncommon grade or bar size has to come from the mill. |
Typical of U.S. job shops, from order or approved drawing to shipment; confirm with the shop for your part.
What drives the cost
- Ground diameters: each ground journal adds a setup on a cylindrical or centerless grinder.
- Tight runout between features, which forces grinding between centers in one setup.
- Heat treatment and the routing it forces: rough, heat treat, straighten, finish grind.
- Length-to-diameter ratio: slender shafts need steady rests, lighter cuts and straightening.
- Material: 4140 and 4340 cost more to buy and cut than 1045 or 1018; stainless and 17-4 PH more again.
- Keyways, splines and clocked cross features that need a second machine or an indexing setup.
- Plating with masking, and documentation beyond a standard inspection report.
Common mistakes
- Tolerancing every diameter like a bearing fit. Only the seats, seal lands and coupling fits need it.
- Runout callouts with no datum axis to run out to.
- Hardness with no statement of through-hardened or surface-hardened, or where.
- Forgetting that plating adds thickness to a fit diameter.
- No thread relief at a shoulder, so the thread cannot be cut as drawn.
- Specifying a lip-seal surface by Ra alone, without asking for a lead-free ground finish.
Questions to ask the shop
- Do you grind in-house (OD and centerless), and which operations go to outside vendors?
- How will you hold runout between journals: one setup between centers, or two?
- Who does the heat treatment, and do you straighten after hardening? What straightness do you hold?
- What bar size and condition will you start from, and is it on the shelf?
- How do you check hardness and case depth, and do the certs ship with the parts?
- How will ground surfaces be protected in shipping?
Typical processes
Typical materials
Charts and calculators
Frequently asked questions
- Should a shaft be ground or just turned?
- Turn it where a tolerance of about 0.013-0.025 mm (0.0005-0.001") and a finish around 0.8-1.6 µm Ra (32-63 µin) are enough; a good CNC lathe holds that on short parts. Grind bearing seats with k5 or k6 fits or tighter, hardened journals, lip-seal surfaces that must be lead-free, and any diameter that must run within about 0.01 mm of another.
- What is the difference between runout and concentricity on a shaft?
- Circular runout is read with an indicator while the part turns on its datum axis, so it catches both form error and offset; total runout covers a whole surface. Concentricity controls median points, is hard to inspect, and was removed from ASME Y14.5 in the 2018 edition. For shafts, runout or total runout to the bearing-seat axis is what shops measure and what most prints should use.
- Which steel is best for a machined shaft?
- 1045 for moderately loaded shafts, often induction hardened on the journals; 4140 pre-hardened to about 28-32 HRC for most power transmission shafts; 4340 where toughness at high strength matters; 8620 when the shaft is carburized for a hard case over a tough core. Use 416 or 17-4 PH stainless for corrosion with hardness, and 303 or 316 where hardness does not matter.
- How tight do bearing seats need to be?
- It depends on the load and on which ring rotates. A rotating inner ring under normal load usually gets an interference fit such as k5 or k6 on the shaft; a stationary inner ring can take a looser fit such as g6 or h6. Take the fit from the bearing maker's shaft tolerance table and put the resulting limits on the drawing.
Build the RFQ: attach the print and this checklist
Attach the drawing (PDF) and the model (STEP), and put your answers to the checklist in the description. A Noramark sourcing specialist matches the request with U.S. shops that make machined shafts and contacts you within 1 business day.
