Machining Cycle Time Calculation: A Worked Example on an Aluminum Bracket

Machine time is 25-35% of a machined part price, and cycle time is the only variable in that band you can argue about with numbers. Buyers who can estimate it independently negotiate better, because they can tell the difference between a part that genuinely needs 40 minutes of spindle time and one that has been quoted at a comfortable pace.

The method below is the one used to build a quote. It is not a CAM simulation — it is a hand estimate accurate to roughly ±15%, which is enough to challenge a price and enough to plan a budget.

The five components of cycle time

Cycle time is not cutting time. A part that spends 16 minutes under a cutter can easily take 26 minutes of machine occupancy per unit once everything else is counted.

• Cutting time: roughing, finishing, drilling, tapping.
• Non-cutting motion: rapids, tool approach, retract, tool changes at 3-8 seconds each on a 24-tool carousel.
• Load and unload: 30-90 seconds for a small part in a vise, 4-8 minutes for a casting on a fixture with clamps.
• In-process checks: a probe cycle or a manual check on a critical bore, 20-60 seconds.
• Setup amortization: 45-150 minutes of setup divided by the lot size.

For a 200-piece lot with 90 minutes of setup, that last line is 27 seconds per part. For a 5-piece lot, it is 18 minutes per part — which is why prototype pricing looks nothing like production pricing even when the part is identical.

Removal rate sets the roughing time

Roughing time is removed volume divided by material removal rate, adjusted downward for the reality of pockets, ramps, and air moves.

Start with volume. A part machined from 160 × 110 × 32 mm plate has 563 cm³ of stock. If the finished part is 148 cm³, then 415 cm³ comes off as chips. That is your buy-to-fly ratio of 3.8:1, and it is the first number to check, because a quote for a part with a 6:1 ratio should cost more in machine time than one at 2:1.

Then apply a removal rate. A 12-16 mm carbide end mill in 6061 on a 40-taper machine will cut 200-350 cm³/min in a straight slotting pass. In real pocketing, with ramping, direction changes, and retracts, plan on 45-60% of the peak figure. Two-thirds derating is a safe planning number.

See also: CNC Capacity Planning: Reading a Supplier's Promise Date and How CNC Machining Price Per Part Falls From 1 to 1,000.

A worked example: 150 × 100 × 30 mm bracket

Take a 6061-T6 bracket with three pockets, 12 drilled holes, and a chamfer. Two setups: op 1 does the top, op 2 flips it for a back relief pocket.

Op 1. Roughing 415 cm³ at a planning rate of 140 cm³/min gives 2.96 minutes. Round to 3.0. Wall finishing covers 900 mm of pocket wall at 2 mm axial depth of cut over a 20 mm depth: ten passes of 900 mm at 1,500 mm/min is 6.0 minutes. Floor finishing on 9,000 mm² at 0.6 mm stepover and 2,000 mm/min is 7.5 minutes. Drilling 12 holes at 8 seconds each is 1.6 minutes. Eight tool changes at 6 seconds is 0.8 minutes. Rapids and approach add 1.2 minutes.

Op 1 subtotal: 20.1 minutes.

Op 2. Face the back, cut one relief pocket of 60 cm³ at 140 cm³/min, chamfer the perimeter of 500 mm at 1,200 mm/min. Roughing 0.4 minutes, pocket finishing 2.2 minutes, chamfer 0.4 minutes, one tool change and rapids 0.6 minutes.

Op 2 subtotal: 3.6 minutes.

Add load and unload at 2.5 minutes per setup = 5.0 minutes, and a 30-second in-process check per setup = 1.0 minute. Part-to-part machine occupancy is 29.7 minutes. At 200 pieces, add 90 minutes of setup amortized at 27 seconds each, and cycle time lands at about 30.2 minutes per part.

At USD 28-35 per machine hour for a 3-axis VMC in Shenzhen, that is roughly USD 14-18 of machine time per part before material, finishing, or margin. If a quote implies 65 minutes for the same geometry, ask which operation is taking the extra half hour; there may be a real reason — a 0.4 µm finish, a thin wall requiring light passes — or there may not.

Turning cycle time into a machine cost

A 3-axis VMC in Shenzhen bills at roughly USD 28-35 per spindle hour. That rate carries the machine, the operator who tends two or three of them, power, tooling consumption, and factory overhead. A 5-axis machining center bills USD 55-75 per hour because the capital cost is three to four times higher and the programming is harder. A CNC lathe runs USD 22-30 per hour.

So the 30-minute bracket above costs about USD 16 in machine time at the 3-axis rate. The same bracket in 4140 would run at perhaps 90 minutes and USD 48, before the extra tooling is counted. That ratio — not the material price per kilogram — is what makes a steel part expensive.

Where estimates go wrong

Three things blow up a clean estimate.

Thin walls force light radial cuts and multiple spring passes, and a 1.5 mm wall can double the finishing time on a part. Tight tolerances do the same: a ±0.01 mm bore may need a separate semi-finish pass and a temperature-stabilized check. And hard materials do not scale linearly — 4140 at 30 HRC cuts at about 7% of the aluminum rate, so a design change from aluminum to steel multiplies roughing time by fourteen even though the volume is identical.

If you want a second opinion on a cycle time before you commit to a PO, send the drawing and we will return a quote with the operation breakdown inside 24 hours, including the DFM notes that usually explain the gap. Start at contact, and see the machine list on our capabilities page if the part needs 4-axis or 5-axis work.

Frequently asked questions

How accurate is a hand cycle time estimate?

Within about ±15% for a well-understood part on a known machine, provided the removal volume and the finishing area are calculated rather than guessed. The largest error source is not the cutting rate but the finishing strategy, which can vary by a factor of two between shops.

What removal rate should I assume for aluminum?

Use 120-200 cm³/min as a planning figure for 6061 on a 40-taper machine, which already includes a 45-60% derating from peak slotting rates. For 7075, take 75% of that; for 304 stainless, about 12%; for Ti-6Al-4V, about 5%.

Why is my 5-piece order quoted at triple the unit price?

Setup amortization. Ninety minutes of programming and setup spread across 200 pieces is 27 seconds each; across 5 pieces it is 18 minutes each. On a 30-minute cycle that alone triples the price before any material premium.

Does a finer surface finish really add that much time?

Yes, when it is specified over a large area. Finishing time scales inversely with stepover: halving stepover from 0.6 mm to 0.3 mm doubles the toolpath length and the time. Specify the finish only where it functions.

Related reading

• Material Cost in Machining: The Share Most Buyers Underestimate2026-10-23
• Scrap Rate in Machining: Where Losses Come From and Who Pays2026-10-24
• Air vs Sea Freight Cost for Machined Parts: When Air Actually Wins2026-10-26