How many spindle-hours does a four-machine CNC shop need?
Calculate CNC job-shop break-even from billable spindle-hours, realized revenue, contribution, staffed capacity and fixed operating cost.
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The StartFigures four-machine CNC job-shop case needs about 23.9 accepted billable spindle-hours per operating day to cover simplified operating costs. At $185 realized customer revenue per billable spindle-hour and a 58% contribution margin, each accepted hour contributes $107.30. With $55,417 of monthly fixed costs and five operating days per week, the continuous threshold is about 517 spindle-hours per month.
The mature case uses 28 billable spindle-hours per day. If the four machines are staffed for one eight-hour shift, that is 87.5% of 32 theoretical machine-hours before setup, first articles, inspection, maintenance and failures. The narrow physical buffer is a warning: the schedule must prove the hours rather than infer them from machine ownership.
The $185 value, 28-hour day, 58% contribution and $55,417 monthly fixed cost are authored assumptions. They are not a customer quote, backlog forecast, capability finding or equipment appraisal.
Define one billable spindle-hour
A billable spindle-hour is one accepted production hour on one CNC spindle with the related customer revenue allocated once. Setup, programming, first articles, inspection, maintenance, rework and unavailable time remain visible even when the customer receives a fixed price.
The public $185 driver converts job revenue into a readable capacity test. The matching paid workbook uses the E21 manufacturing engine: milled parts, turned parts, repeat lots, prototypes and secondary services keep their own units, prices and direct costs. One job cannot appear as both part revenue and a second spindle-hour revenue stream.
Reconcile the base day to a staffed shift
| Capacity step | Calculation | Result |
|---|---|---|
| Theoretical staffed clock | 4 machines × 8 hours | 32 machine-hours/day |
| Year-three volume | 28 billable hours ÷ 32 hours | 87.5% of staffed clock |
| Operating break-even | 23.9 billable hours ÷ 32 hours | 74.5% of staffed clock |
| Base buffer | 28.0 − 23.9 hours | 4.1 billable hours/day |
Thirty-two hours is an illustration, not guaranteed capacity. A shop can use staggered shifts or safe unattended cycles, while setups, inspections and failures can also reduce delivered time below one shift. Record staffed, available, setup, running, accepted, rework, maintenance and unavailable hours by machine and job.
Calculate spindle-hour break-even
At a 58% contribution margin, $185 of realized revenue leaves $107.30 contribution per billable spindle-hour after material, tooling consumption, outside processing, freight, scrap and sales-linked costs.
$55,417 ÷ $107.30 ÷ 5 ÷ 4.33 = 23.86 billable spindle-hours per operating day. The practical target needs additional protection for quote error, scrap, downtime and receivables.
At 28 hours, monthly revenue is 28 × $185 × 5 × 4.33 = $112,147. Contribution is about $65,045, leaving about $9,628 monthly operating surplus before depreciation, financing, income tax, major replacement capital, working-capital timing and distributions.
Stress realized value, contribution and fixed cost
| Case | Assumptions | Hours/day |
|---|---|---|
| Lower realized value | $130/hour · 58% margin · $55,417 fixed/month | 33.9 |
| Base | $185/hour · 58% margin · $55,417 fixed/month | 23.9 |
| Higher realized value | $260/hour · 58% margin · $55,417 fixed/month | 17.0 |
| More direct-cost leakage | $185/hour · 48% margin · $55,417 fixed/month | 28.8 |
| Higher fixed cost | $185/hour · 58% margin · $65,000 fixed/month | 28.0 |
The lower-value case needs more billable hours than one eight-hour staffed shift across four machines. The answer may be a different job mix, a second shift or a lower fixed-cost structure, but it cannot be hidden by treating setup or rejected output as accepted production.
Use industry data as context, not an order book
Census defines NAICS 332710 around machining metal and plastic parts on a job or order basis, generally with machine tools including CNC equipment. The 2023 County Business Patterns national file reports 17,156 employer establishments, 223,313 employees and $14.228 billion of annual payroll. These figures do not establish local demand, supplier approvals or utilization. Census industry profile and 2023 County Business Patterns.
BLS describes machinist work that includes drawings, CAD/CAM, setup, CNC operation and inspection and reports May 2025 median annual pay of $58,750. That is occupation context, not a local offer or complete employer cost. Machinists and Tool and Die Makers.
Put machine safety inside the capacity plan
OSHA's general machine standard addresses guarding, while the hazardous-energy standard governs covered servicing and maintenance. Occupational-noise requirements add a measurement and control boundary. A productive hour is not valid capacity if guarding, setup, service or exposure controls fail. Machine guarding, hazardous energy and occupational noise.
Read the five-year case
| Year | Revenue | Operating result |
|---|---|---|
| 1 | $620,000 | -$230,200 |
| 2 | $980,000 | -$66,200 |
| 3 | $1,346,800 | $116,144 |
| 4 | $1,620,000 | $230,800 |
| 5 | $1,880,000 | $333,000 |
The next test is 1,000 accepted billable spindle-hours across several customers. Record the controlled revision, quoted and actual material, setup, cycle, tooling, inspection, scrap, rework, delivery and collection. Add equipment only when qualified backlog, staffing, cash and the measured bottleneck support it.
The Machine Shop case contains the complete $1.2 million allocation and forecast. Its evidence register separates sources from assumptions, while the business plan and financial model explain qualification, capacity and product adaptation.