The cheapest drill bit does not necessarily produce the cheapest acceptable hole. A purchasing comparison should include the tool, occupied machine time, tool changes and losses from rejected work. The right denominator is accepted holes. This article provides a calculation method and a hypothetical example, not measured performance of any Wryno product.
Choose the cost boundary before calculating
For one defined trial or production interval, add consumed tooling cost, occupied machine and labor cost, incremental consumables and attributable scrap or rework costs. Divide that total by accepted holes. Use the same accounting boundary for every candidate. If the hourly rate already includes labor, power or coolant, do not add those charges again.
Cost per accepted hole = total attributable cost ÷ accepted holes. Tooling consumption can include a new tool or replacement cutting edge and any reconditioning assigned to the interval. If a trial ends with usable life remaining, charging the entire tool price produces a trial cost, not a fully measured lifetime cost. Label the distinction.
A worked comparison
The following invented figures represent two completed tool-life intervals. Assume the machine rate is $30 per hour, the recorded occupied time includes tool changes, and all holes are acceptable. Other costs are assumed equal and omitted. These assumptions are for illustration only.
| Input | Drill A | Drill B |
|---|---|---|
| Consumed tooling cost | $12 | $24 |
| Accepted holes | 300 | 600 |
| Occupied time | 120 minutes | 180 minutes |
| Machine cost | $60 | $90 |
| Total assigned cost | $72 | $114 |
| Cost per accepted hole | $0.24 | $0.19 |
In this example, Drill B costs twice as much to buy but produces an accepted hole for five cents less. That result depends on the measured time and output assumptions. It does not mean the more expensive drill will always win.
Account for rejected holes and parts carefully
If Drill B instead produces only 540 accepted holes in the same interval, its cost before additional scrap charges becomes $114 ÷ 540, or about $0.211 per accepted hole. Add any attributable scrap or rework cost before making the final comparison. For parts with multiple holes, a rejected hole may reject the whole component. Define whether the business decision should use cost per accepted hole or cost per accepted component.
Record the conditions behind tool life
Material grade, hole diameter and depth, holder, machine, operating settings and coolant conditions belong in the record. Establish a common acceptance threshold for wear and hole quality. If one tool is stopped at a stricter limit, the life figures are not directly comparable. Compare repeat trials where practical and report the spread rather than only the best run.
Evaluate regrinding separately
A reconditioned drill may have a different usable length or performance from a new one. Include regrinding, coating if applicable, transport and any sorting cost, then measure acceptable output for that cycle. Do not assume the new-tool hole count applies to every regrind. Also check whether the shorter tool still meets reach and holding requirements.
Make a decision that the process can sustain
Consider availability and consistency alongside the calculated average. A small theoretical saving may not survive frequent unplanned stoppages or unstable quality. For a fair comparison, show the purchase price, actual output, time and assumptions on one sheet. Keep a second calculation for a conservative scenario such as lower life or a higher reject rate.
Sandvik Coromant’s application demonstration illustrates the importance of recording material, cutting conditions and measured hole results. Its product-specific settings should not be transferred to a different drill. For related purchasing checks, see our drill inspection checklist, regrinding guide and drill bit range. Bring your actual hole count and cycle-time records when discussing a replacement tool.