CNC Turning Services: Tolerances, Bar Capacity and What to Ask

Two suppliers quote the same 42 mm stainless shaft. One comes back at $31, the other at $58. Both are legitimate shops, both have CNC lathes, and neither is trying to overcharge you. The gap is almost always one of four things: how many operations the part needs, whether the shop has live tooling, how much of the tolerance they can hold in one chucking, and whether they are buying bar or cutting from plate.

Turning looks like the simplest machining process to buy. It is a round part, a rotating tool or a rotating work, and a diameter. In practice, the cost of a turned part is decided by a set of machine capabilities that are invisible on a quote, and understanding them lets you predict the price before you ask for it.

The machine classes behind a turning quote

"CNC turning services" covers at least four distinct machine types, and they are not interchangeable.

Two-axis chuckers hold the part in a chuck or collet, index a 8 to 12 station turret, and cut outside and face diameters. Bar feed is optional. This is the workhorse for parts from 10 to 200 mm diameter, and it is the cheapest per hour.

Bar-fed lathes with a tailstock or sub-spindle add automation: the bar advances, the part is cut off, and the sub-spindle catches it and finishes the back face. This removes a second operation and typically cuts 30 to 40% from the cost of a two-op part.

Live-tooling lathes add driven tools in the turret, so the machine can mill flats, drill cross-holes and tap radial features without releasing the part. This is the single biggest cost lever in turning, because it collapses what would be a lathe plus a mill operation into one cycle.

Mill-turn and Swiss-type machines sit at the top. Mill-turn centres with a B axis can produce features that would normally need a five-axis mill. Swiss machines handle small bar and long slender parts. Both carry higher hourly rates, which only pay off when they eliminate operations.

When you get a quote, the difference between a $31 and a $58 price is usually that one shop has live tooling and a sub-spindle, and the other is planning three separate setups.

Realistic tolerance bands by size

Turning tolerances are not a single number. They depend on diameter, on length, and on whether the feature is cut in the same chucking as the datum.

The important pattern is not the numbers themselves but the jump between rows. Every tolerance held across two operations costs roughly twice what the same tolerance costs in one. If your drawing has a 0.01 mm concentricity callout between a bore and an OD, the shop will either machine both in one chucking or add a fixture, and both routes show up in the price.

Live tooling: when turn and mill should combine

The decision to combine turning and milling on one machine is mostly arithmetic. A part with two flats, a cross-hole and a radial tapped hole, made on a lathe without live tooling, requires a second operation on a mill: fixture it, indicate the flat, mill, drill, tap. That is roughly 20 to 40 minutes of handling and setup plus the milling time, and it introduces a positional tolerance between the turned datum and the milled features.

On a live-tooling lathe, the same features add 3 to 6 minutes to the cycle and no setup. Even at a 25% higher hourly rate, the combined route usually wins once the batch is above 20 pieces.

Concentricity starts with the datum, not the machine

A lathe is inherently good at concentricity because the part rotates around a single axis. What breaks concentricity is releasing the part. The moment you unclamp, the relationship between the turned surface and the new zero point depends on the fixture, the jaw pressure and how clean the jaws were.

Three design habits preserve what the machine can do:

Keep all tight concentric features on one end of the part, so they can be cut before the part is turned around. Put a generous chamfer or radius on the parting-off end, since a sharp corner there is the most common source of a burr that has to be removed by hand. And avoid specifying concentricity between features that are 150 mm apart on a slender part — at that ratio, the part deflects under tool pressure and no fixture will fix it.

If the design requires a bore and an OD that must be concentric to 0.01 mm and they are on opposite ends, expect the shop to bore it in a second operation on soft jaws, or to leave the bore undersize and finish it after the OD is complete.

Bar capacity and stock selection

Most turning shops stock or can source bar from 3 mm to 250 mm, but the economic range is narrower. Below 6 mm, bar handling and guide bushing requirements push you toward a Swiss machine. Above 120 mm, the material cost and the weight of the bar start to dominate, and many shops prefer to cut from plate or saw-cut billet.

What to send with a turning RFQ

Send the 3D model and a 2D drawing that states which features are critical. Turning shops need to know the datum scheme, the thread class, and whether the finish callout applies before or after plating.

State the quantity and whether it is a one-time order or a forecast. A shop quoting 100 pieces with a 1,000 piece annual forecast will tool differently than one quoting a single prototype.

Say whether the part needs first-article inspection reports, material certificates or a specific surface finish measurement. These add cost, and a quote that omits them will come back higher after you ask.

Frequently asked questions

How tight a tolerance can CNC turning hold?

On a diameter cut in a single chucking with ground bar, ±0.005 mm is routine on a well-maintained lathe. Across two operations, ±0.01 to ±0.02 mm is realistic because the second operation inherits fixture error. Below ±0.005 mm, the shop usually adds a grinding operation.

When should a part be turned rather than milled?

If the part is primarily rotational — shafts, bushings, pins, fittings, hubs — turning is faster and cheaper because the tool is in constant engagement. If the part is a block with a few round features, milling is cheaper. Parts that are rotational with milled flats are the case where live tooling decides the price.

Does a bar-fed lathe reduce cost?

It usually does, because it removes the manual load and the second operation for the back face. The saving is largest on parts under 60 mm diameter and above a few hundred pieces. Below that, the bar setup charge can outweigh the cycle-time gain.

Can turned parts be anodized or plated?

Yes, and the finish callout should account for it. Type II anodizing grows the surface by roughly half the coating thickness per side, so a 20 µm coating adds about 0.01 mm to a diameter. If a bore is tolerance-critical, mask it or specify the tolerance after coating.