Every job shop owner has been burned by a bad time estimate. You quote 45 minutes per part based on a quick calculation, win the job, and then the first part takes 90 minutes because you forgot about setup changes, tool changes, deburring, and the fact that the material was not as free-machining as you assumed. Accurate time estimation is the difference between a profitable job and one that loses money on every piece.
The foundation of time estimation is metal removal rate (MRR), which tells you how fast you are actually cutting material. But MRR only covers the time the tool is in the cut. Non-cutting time typically adds 30 to 60 percent to the theoretical cutting time. This guide covers both.
Metal Removal Rate: The Starting Point
MRR in milling: WOC × DOC × feed rate (IPM). Result is cubic inches per minute. For turning: 12 × SFM × DOC × feed (IPR). Example: A 1/2" endmill taking 0.250" WOC at 0.500" DOC and 20 IPM: MRR = 2.5 in³/min. If you need to remove 10 cubic inches, cutting time is 4 minutes.
Practical MRR is always lower than theoretical because you cannot maintain full engagement on every pass. Corners, profiles, and features require reduced parameters. Finishing passes run at much lower MRR than roughing.
Material machinability affects MRR directly. The same setup that achieves 5 in³/min in aluminum might only manage 1.5 in³/min in 4140 and 0.5 in³/min in Inconel 718. Always base MRR on the specific material, not a generic category.
MRR = WOC × DOC × Feed rate (IPM)
MRR for turning:
MRR = 12 × SFM × DOC × Feed (IPR)
Cutting time = volume to remove / MRR
Metal Removal Rate Calculator
Calculate metal removal rate, machining time, and horsepower requirements for milling, turning, and drilling. Estimate job time and machine utilization with material-specific cutting energy data.
Non-Cutting Time: The 40% You Forgot to Quote
Non-cutting time includes rapid traverse, tool changes, spindle orientation, part loading and unloading, and air cutting. On a well-programmed CNC mill, non-cutting time is 30 to 50 percent of the total cycle. On a manual machine, 50 to 70 percent.
Tool changes on CNC take 3 to 8 seconds each with an ATC. A part requiring 8 tools has 56 seconds of tool change time. On a manual mill, each tool change is 30 to 60 seconds. Eight changes at 45 seconds each is 6 minutes.
Part loading depends on fixturing. A simple vise job loads in 15 to 30 seconds. A complex fixture with 6 clamps takes 60 to 120 seconds. For short runs, loading time can exceed cutting time.
Setup Time: The Number Everyone Underestimates
Setup time is from "last good part of the previous job" to "first good part of the current job." It includes reading the drawing, gathering tools, loading tools, setting offsets, loading the fixture, running first part in single block, inspecting, and making adjustments. For a typical 3-axis CNC job: 30 to 60 minutes.
For a complex 5-axis setup with custom fixturing and 15 tools: 2 to 4 hours. A 2-hour setup over 2 parts adds 60 minutes per part. The same setup over 100 parts adds 1.2 minutes per part.
Ways to reduce setup time: keep tools in the magazine between jobs, use quick-change workholding, pre-set tools offline, and write setup sheets for every job.
When quoting, never hide setup in per-piece pricing for small runs. List setup as a line item. Customers understand why 5 parts cost more per piece than 50 when you show them the setup cost.
Metal Removal Rate Calculator
Calculate metal removal rate, machining time, and horsepower requirements for milling, turning, and drilling. Estimate job time and machine utilization with material-specific cutting energy data.
Building a Realistic Estimate: Theory to Reality
A complete estimate has four components: setup time (per batch), cycle time (per part), inspection time (per part or per sampling interval), and secondary operations (deburring, cleaning, marking). Per-piece cost = (setup / batch size) + cycle time + inspection + secondary ops.
For cycle time, add up cutting time for each operation, then add non-cutting time using your shop's actual multiplier. If you do not know your multiplier, time 5 parts on a representative job and compare total cycle time to calculated cutting time.
Secondary operations are the hidden time killers. Deburring a part with 20 drilled holes takes 2 to 5 minutes by hand. These operations happen after the machine cycle and are easy to forget in the estimate.
Cost = (Setup / batch size) + Cycle time + Inspection + Secondary ops
Cycle time = Cutting time × non-cutting multiplier (1.4–2.0)
Multiply total time by your burdened hourly rate ($75–$150/hr typical).