BIZOMAT MAKEOS · Manufacturing
Turning Cycle Time and Cost per Part with Tool Life
What does each turned part cost at this speed, and how much would the minimum-cost speed save?
Cycle time and cost per part for a turning operation, combining spindle speed, cutting time, Taylor tool life, tool edges and tool changes per part, handling, set-up spread over the batch and the machine rate, then comparing the cost and time at the minimum-cost and maximum-production cutting speeds.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cut | ||
| Work diameter | mm | Diameter being turned. |
| Length of cut per pass | mm | Length of each pass. |
| Approach and overrun | mm | Extra travel for approach and overrun. |
| Number of passes | Number of passes. | |
| Cutting speed | m/min | Cutting speed now used. |
| Feed | mm/rev | Feed per revolution. |
| Tool | ||
| Taylor exponent n | Exponent of the Taylor tool-life equation V Tⁿ = C for the tool and material. | |
| Taylor constant C (speed for 1 min life) | m/min | Taylor constant: cutting speed that gives one minute of tool life. |
| Cost per cutting edge | $ | Cost of one cutting edge (insert price ÷ edges, plus holder share). |
| Tool change time per edge | min | Time to index or change an edge. |
| Costs | ||
| Machine and labour rate | $/h | Machine plus operator rate. |
| Load and unload time per part | min | Handling time per part. |
| Set-up time per batch | min | Set-up time for the batch. |
| Batch quantity | Parts in the batch. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Cut
What this means: Work diameter
- What it is
- The diameter of the work where it is being turned, which with the cutting speed sets the spindle speed.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- At the same cutting speed a larger diameter lowers the spindle speed and lengthens cutting time in proportion.
What this means: Length of cut per pass
- What it is
- The length the tool travels along the part while cutting on each pass.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Approach and overrun
- What it is
- The extra travel at feed before and after the cut, for the tool to approach the work and run out, added to every pass.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Number of passes
- What it is
- How many times the tool travels the length of cut, counting roughing and finishing passes together.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting time rises in direct proportion to the number of passes.
What this means: Cutting speed
- What it is
- The cutting speed you use now: the surface speed of the work moving past the tool edge.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 5000 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 m/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Surface speed in m/min, not spindle rpm.
What this means: Feed
- What it is
- How far the tool advances along the work in one revolution of the spindle.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 10 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.25 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting time is inversely proportional to feed.
- Get it right
- The Taylor constants hold only near the feed they were tested at.
Tool
What this means: Taylor exponent n
- What it is
- The exponent n in the Taylor tool-life equation V Tⁿ = C, describing how fast tool life falls as cutting speed rises for this tool and work material.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.95. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.25 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take n and C as a matched pair from tool-life tests or the tool maker’s data at your feed and depth.
What this means: Taylor constant C (speed for 1 min life)
- What it is
- The Taylor constant C: the cutting speed at which an edge would last one minute, for this tool, work material, feed and depth.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 100000 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 m/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher C gives longer tool life at the same speed and raises the minimum-cost speed in proportion.
What this means: Cost per cutting edge
- What it is
- The cost of one cutting edge: insert price divided by its usable edges, plus a share of the holder cost.
- Units and format
- Enter a number in $: an amount of money; use one currency for every money input. The form accepts $0 to $100000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $6 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher edge cost lowers the minimum-cost speed.
What this means: Tool change time per edge
- What it is
- The time to index or change one cutting edge, including any offset adjustment, while the machine is not cutting.
- Units and format
- Enter a number in min: minutes. The form accepts 0 min to 600 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- In minutes: 45 seconds is 0.75 min.
Costs
What this means: Machine and labour rate
- What it is
- The combined hourly cost of the machine and its operator, including overheads as your costing method assigns them.
- Units and format
- Enter a number in $/h: money per hour. The form accepts $0.000001/h to $100000/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $90/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Per hour; the tool divides it by 60 to get the cost per minute.
What this means: Load and unload time per part
- What it is
- The handling time for each part: loading, unloading and any gauging done while the machine is not cutting.
- Units and format
- Enter a number in min: minutes. The form accepts 0 min to 600 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Set-up time per batch
- What it is
- The time to set up the machine for the batch, shared across every part in it.
- Units and format
- Enter a number in min: minutes. The form accepts 0 min to 100000 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Each part carries set-up ÷ batch of this time in its cycle.
What this means: Batch quantity
- What it is
- The number of parts made from one set-up of the machine.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 100000000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger batch spreads the set-up over more parts, lowering cycle time and cost per part.
Method and formulas
- N = 1000 V ÷ π D; cutting time t_c = passes × (L + approach) ÷ (f N).
- Tool life T = (C ÷ V)^(1/n); edges per part = t_c ÷ T.
- Cost per part = rate × [t_c + handling + tool change × t_c ÷ T + set-up ÷ batch] + edge cost × t_c ÷ T.
- Minimum-cost speed V = C ÷ [(1/n − 1)(t_ct + edge cost ÷ rate)]ⁿ; maximum-production speed V = C ÷ [(1/n − 1) t_ct]ⁿ.
- Example: at 200 m/min each part costs 3.3586 and takes 1.9507 min; the minimum-cost speed is 203.25 m/min.
Limitations
- Taylor constants are valid only near the tested feed and depth.
- Assumes the feed stays the same when the speed changes.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Quote a turned part from first principles
- Show the saving from running at the economic speed
- Compare insert grades by their Taylor constants
Who it is for
- Manufacturing engineers
- Estimators
- Production managers
What you get
- Cycle time and tool life
- Cost per part breakdown
- Minimum-cost and maximum-production speeds compared
Interpreting the result
- Run between the minimum-cost and maximum-production speeds; go towards maximum production only when the machine is the bottleneck.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1102–1152. Topic reference; no source tables or recommended settings are reproduced.
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