Cost & Quoting
How Do You Size CNC Machining Capacity?
Capacity plans fail in a predictable direction because they count machine hours and sell spindle hours. The gap between those two is setup, and it is bigger than most shops assume.
The naive capacity calculation multiplies machines by shifts by hours and produces a large, comforting, useless number. Take 6 machines on 2 shifts of 8 hours, 5 days a week for 50 weeks: 24,000 gross machine hours. Almost none of that is sellable, because a machining centre spends a meaningful share of its life in setup, in maintenance, waiting on material, or stopped for a tooling issue. The number that matters is spindle hours actually cutting billable work.
The two deductions that matter most
Setup ratio is the share of available time consumed by changeovers, and it is driven almost entirely by batch size. A shop running large repeat batches may sit in the single digits; a high-mix prototype shop can exceed a third. Applying an illustrative 18% and an availability factor of 85% for maintenance, breakdowns, and material waits brings 24,000 gross hours down to roughly 16,728 productive spindle hours. That is a reduction of about 30%, and it is the difference between a capacity plan that works and one that overpromises.
- Gross machine hours: 24,000
- Productive spindle hours: 16,728
- What the naive number overstates by: -30%
Machines are not interchangeable, and that caps you further
Total spindle hours implies fungibility that rarely exists. A five-axis machine and a three-axis machine are not substitutes for a part that needs simultaneous five-axis work, and capacity that exists on the wrong machine is not capacity. Plan by machine class, or better by capability group, and the constraint usually turns out to be one or two specific machines rather than the shop as a whole. That reframing changes the investment conversation entirely, because it identifies the machine to buy.
Shops rarely run out of machine hours. They run out of hours on the one machine that can do the work, while the others sit available.
The constraint that is not a machine
Beyond a certain utilization, the binding constraint in most machining operations stops being spindles and becomes programming, fixturing, or skilled operators. Adding a machine into that condition raises cost without raising output, which is the most expensive mistake available in this analysis. Before approving capital, check the queue: if jobs are waiting on programs or setup rather than on spindle time, the money belongs in CAM capacity or fixturing, not in another machine.
Use the CNC machine hour cost calculator to convert spindle hours into a defensible rate. Model machine cost
Published 2026-08-08.