CNC turning and turn-mill: shafts, fittings and gear blanks
Bar and chuck work with milled features kept in the same setup on a turn-mill centre, so a flat, a cross hole or a gear tooth form does not add a second clamping and a second datum.
Turning versus turn-mill
The decision is about how many setups the part needs, not which machine spins faster.
Diameters, grooves, threads, chamfers and a faced end. Bar-fed machines run these unattended, so the price is dominated by cycle time and bar stock.
Flats, radial holes, slots and axial holes done on the same spindle. Keeping them in one setup avoids a second datum and typically saves 15–30% against a separate milling operation.
Gear blanks and timing gears are turned, then the tooth form is cut or milled. Keeping concentricity between the bore and the pitch circle is the whole job; the tooth profile itself is the easy part.
A brief milling operation referenced from a machined face is more accurate than forcing the feature into the turning setup.
Concentricity and tolerance bands
| Feature | Standard | High precision | Notes |
|---|---|---|---|
| Diameter | ±0.05 mm | ±0.02 mm | Ground or finish turned below that |
| Coaxiality, two diameters | 0.03 mm | 0.01 mm | Same setup, or between centres |
| Roundness | 0.01 mm | 0.005 mm | Ground finish |
| Length, shoulder to shoulder | ±0.05 mm | ±0.02 mm | Depends on how it is held |
| Thread | 6H/6g | 4H/4h | Thread milled for tight class |
| Surface finish | Ra 1.6 µm | Ra 0.4 µm | Ground or roller burnished |
| Hardness after treatment | — | 48–55 HRC | 42CrMo and 4140 gear parts |
Where turned parts get expensive
A Ø10 mm shaft 120 mm long deflects under cutting force. Options are a travelling steady, a lighter pass structure, or grinding after turning. Each appears in the price, and each is cheaper than a bent batch.
Chucking on a cosmetic surface needs soft jaws bored to the diameter: a small one-off cost that prevents a batch of scratched parts.
Below 1 mm on a Ø50 mm tube the part rings and chatters. A low-melt filling compound or machining from solid both work; both cost more than a 2 mm wall.
Usually needs machining between centres or a post-grind operation. The drawing should state which end is the datum, or the tolerance is ambiguous.
Questions we get on this process
Common round sizes in aluminium, stainless and brass from Ø3 to Ø200 mm. Sizes outside stock are ordered in, which adds days to the lead time but not much to the unit price on a real batch.
Yes, cut or milled depending on class. Multi-start threads should be called out on the drawing because lead and pitch differ and the inspection method changes.
By keeping critical diameters in one setup wherever possible. When that is not possible, soft jaws are turned to the first diameter so the second datum is the part itself rather than a chuck face.
Yes, from one piece. Bar-fed turning is one of the cheapest ways to make a single shaft because no fixture is needed and setup is measured in minutes.
Turning quote with the bar size and setup count shown.
If your part could be turned rather than milled from a block, the quote will say so. That change alone often halves the material cost.
From the shop notes
Longer answers to the questions these pages raise.