Milling or turning: which route costs less?

When is a part a turned part?

A turned part is round about one axis. In turning the workpiece spins and the tool stays almost still: shafts, bushes, flanges and rings. Whatever lies around the spindle axis comes off the machine round.

When you read the drawing, keep three things apart. The features: diameters, a centre bore, a collar, four holes on a pitch circle, two spanner flats. The requirements: a tolerance on the outside diameter, coaxiality between bore and collar, a roughness on the sealing face. Only then the operations: turning, facing, drilling, milling.

If the tight requirements sit on the axis, it is a turned part, even with a few flats. Coaxiality is easiest in a single setup; once the part is re-chucked for the back, the same requirement costs alignment, measurement and time.

What does a round feature cost on a mill?

In milling the tool spins and the workpiece stays clamped. An outside diameter is then a contour, not a cut: the cutter goes round in several passes, and every pass is cycle time. A hole up to drill size is simply drilled; a larger bore is drilled first, then circular-milled or bored out.

In circular milling two axes move together to make the circle, so roundness depends on how well they track each other. On a lathe the spinning workpiece makes the circle itself. A tight roundness takes fewer passes there, and carries less risk.

Bar or block: what does the stock do to the price?

The route chooses the stock. A turned part comes from round bar one size up, with a saw cut and a little to face off. A round part milled from solid comes from a sawn block or plate, with allowance on every side.

The difference is geometry. A square block around a round part has 4 over pi times the cross-section of bar of the same size: more than a quarter extra, nearly all of it chips. You pay for those chips twice: in the block and in the machining time.

On a one-off that hardly matters; on a batch in an expensive material it becomes a line you want to see. Setting the allowance is covered in sizing the blank.

How many setups does each route take?

Without live tooling, a turned part with flats takes three setups: the front in the chuck, turned round for the back, then the mill for the flats and the pitch-circle holes. Three setups, two machines, a queue in between.

Give the lathe live tooling or a sub-spindle and setups drop away; a mill-turn machine is built to turn and mill in the same working envelope. Several setups then become a few, or one. More in mill-turn: when does one machine pay?.

The milled route usually takes two setups: the top with contour and holes, then flipped for the back. Every setup is setup time, and the workholding has to grip without distorting the part. See counting setups.

Worked example: one flange, two routes

Take 25 flanges in EN AW-6082, 80 mm across and 30 mm thick, each with a centre bore, four pitch-circle holes and two spanner flats. Route A turns it from 85 mm bar in two setups, then mills the flats and holes. Route B mills the whole part from a 90 by 90 by 35 mm block, in two setups on one machine.

The inputs: lathe €70 an hour, mill €85 an hour, programming €80 an hour and one hour per route. Setup and programming are spread over 25 parts.

At 25 parts route A wins: less material and cheaper lathe hours. At one part it flips: €252 against €234, because the one-off lines then carry the whole amount and route A sets up one machine more. In this example route A is cheaper from 4 parts.

In plastics: which route holds the size?

Engineering plastics are not light aluminium: frictional heat stays in the workpiece, and plastic expands far more than metal. Per degree, POM grows by about 120 millionths of its size and aluminium by 23: more than five times as much. An 80 mm POM flange measured 10 degrees too warm comes out almost 0.1 mm oversize; in aluminium it would be under 0.02 mm.

Clamping force is the second risk. A thin ring in a three-jaw chuck is pushed in at three points and springs back once released: round in the machine, out of round on the inspection table. Soft jaws bored to size, or a collet, spread the force around the circumference.

So a round plastic part with tight roundness belongs on the lathe and is measured only at 20 °C, the reference temperature set by ISO 1; a flat part with pockets belongs on the mill, on a vacuum table rather than in a vice. The rest of the cost is in machining plastics.

How do you choose the route before you cost it?

An instant-quote portal picks the route for the buyer and hands back one figure. A CAM suite gives a time only once the program is done, which is usually after the quote has gone. The ERP keeps the price, not why that route was chosen. All three bury the choice that shapes the price most.

The manufacturing route depends on the machines you own: a lathe with a sub-spindle makes a different part cheap from a shop with only mills. That choice is the shop's knowledge. It belongs in the estimate, line by line and before programming, where cost engineer, manufacturing engineer and owner can check it.

• Is the part mostly round about one axis?
• Do the critical dimensions sit on the axis or on faces?
• Which bar or block do you need, and how much becomes chips?
• How many setups does each route take?
• Does the lathe have live tooling or a sub-spindle?
• How many parts: one, ten or hundreds?
• In plastics: how do you clamp without distorting the part?