The full cost, not the price
Tool cost per part has four components, and the purchase price is usually the smallest.
| Component | How it is calculated | Typical share |
|---|---|---|
| Tool purchase cost | Tool price divided by parts per tool | Often 5 to 15 percent of the total |
| Edge change time | Machine rate multiplied by time per change, divided by parts per edge | Frequently larger than the tool cost |
| Cycle time effect | Higher speeds and feeds reduce machine time per part | Usually the dominant factor |
| Scrap and rework | Cost of parts lost to unpredictable failure | Highly variable; the reason predictability matters |
| Inventory and logistics | Value tied up in stock and the cost of ordering | Small individually, significant across a tool crib |
Why a cheaper tool can cost more
Two tools cutting the same feature at different parameters produce different cycle times, and cycle time is charged at the machine rate for every part. A tool that allows a 15 percent higher feed rate saves 15 percent of the cutting time on every part, which frequently exceeds the entire tool cost difference.
Predictability matters as much as speed. A tool that fails at random produces scrap and unplanned stoppages, and the cost of a single stopped machine hour usually exceeds the cost of a year's tooling for that operation.
What to measure
The numbers worth recording are parts per edge, cycle time per part, and the reason each tool was changed. Those three make the comparison possible and they are all obtainable from the machine.
- Parts produced per edge, recorded at each tool change
- Cycle time per part for the operation, not for the shift
- Whether the tool was changed on schedule or after a failure
- Scrap and rework attributable to the operation
- Tool changes per shift, which is where the labour cost sits
Where the largest gains usually are
In most operations the largest available gain is not a cheaper tool but a correct one, because the parameters a tool can support determine the cycle time.
The second largest is usually runout. A tool running out of true loses a substantial part of its life and produces inconsistent finish, and correcting the holder frequently delivers more than any change of tool grade.
Frequently asked questions
How do I calculate tool cost per part?
Divide the tool price by the parts it produces, then add the cost of the time spent changing edges and the cost of any scrap attributable to the tool. In most operations the tool purchase price is five to fifteen percent of that total.
Why is the cheapest tool usually not the cheapest option?
Because cycle time and downtime dominate the cost. A tool that enables a higher feed rate reduces machine time on every part, and that saving usually exceeds the entire price difference between tools.
What should I measure to compare tools properly?
Parts per edge, cycle time per part, whether each tool was changed on schedule or after a failure, and any scrap attributable to the operation. Those four numbers make the comparison on total cost rather than on price.
How much does runout cost?
A tool running out of true loads one flute harder than the others, so that flute fails while the rest still have life. The effect is usually a substantial reduction in parts per tool plus inconsistent surface finish, which is why checking holder runout is frequently the cheapest available improvement.
Is it worth buying a more expensive grade?
When the operation is long enough for the tool life difference to matter and the machine rate is significant. On a two-minute operation the tool cost dominates; on a forty-minute operation, cycle time and predictability dominate and the grade choice matters far more.