Estimating cycle time for deep hole drilling is straightforward if you break it into parts. I have been quoting jobs for over a decade, and the formula below has served me well across gun drilling, BTA, and ejector drilling.
The total cycle time determines whether a job is profitable. Miss it by 20% and you either lose the quote or lose money on every part. Here is how I calculate it.
The Core Formula
Total cycle time = Drilling time + Retract time + Loading time + Idle time
Drilling time dominates. Here is the basic calculation:
Drilling time (minutes) = Drilling depth (mm) / (Feed per rev (mm) x Spindle speed (RPM))
Worked Example: Gun Drilling 4140 Steel
Part: Hydraulic rod, 500mm deep, 25mm diameter Parameters: 0.05 mm/rev feed, 3000 RPM spindle speed
Drilling time = 500 / (0.05 x 3000) = 500 / 150 = 3.33 minutes
But that is the ideal case. Real drilling includes a peck cycle to break chips.
Worked Example with Pecking
Peck cycle: 10 pecks, each retracting 50mm at 5000 mm/min rapid
Peck time = Number of pecks x Retract distance / Rapid speed Peck time = 10 x 50 / 5000 = 0.1 minutes
Total drilling time = 3.33 + 0.1 = 3.43 minutes
Adding Loading and Idle Time
I measure loading time with a stopwatch over 10 parts. Typical values:
| Operation | Time (minutes) |
|---|---|
| Load part and clamp | 0.5 - 1.5 |
| Align guide bushing | 0.3 - 0.5 |
| Start cycle | 0.1 |
| Unload part | 0.3 - 0.5 |
| Deburr and blow out | 0.5 - 1.0 |
| Inspect bore | 0.5 - 1.0 |
For the 500mm hole example, I add 1.5 minutes for loading and 0.3 minutes for idle between cycles.
Total cycle time = 3.43 + 1.5 + 0.3 = 5.23 minutes per part
Factors That Affect Real Cycle Time
The formula gives a baseline. Real cycle time depends on these variables:
| Factor | Impact | Typical Adjustment |
|---|---|---|
| Machine acceleration | Adds 5-15% to peck time | Multiply peck time by 1.1 |
| Chip evacuation issues | Forces feed reduction | Add 10-20% to drilling time |
| Material hardness | Reduces achievable feed | Reduce feed by 10-30% |
| Coolant pressure | Affects chip clearing | Low pressure = peck more |
| Tool wear | Increases cycle time over run | Add 5% toward end of tool life |
| Operator experience | Affects loading speed | ±20% on loading time |
I always add a 15-20% safety factor when quoting. The machine acceleration alone can add 10% to the cycle time on a machine with slow axis drives.
How to Estimate Before Running
When I quote a job I have never run before, I use this method:
- Calculate the ideal drilling time from the print depth and planned parameters.
- Estimate the number of pecks based on hole depth and material. For steel, I peck every 3-5x diameter. For aluminum, every 5-8x diameter.
- Add loading time based on part complexity. A simple shaft takes 1 minute. A complex valve body takes 3 minutes.
- Multiply the total by 1.2 for the shop floor factor.
- Compare against past jobs with similar depth/diameter ratios.
I track actual vs estimated cycle times in a spreadsheet. After 20 jobs, my estimates are within 10% of actual 90% of the time.
For more on how tool selection affects cycle time, see [/process-deep-dives/bta-drill-head-selection](BTA drill head selection). The head type directly impacts achievable feed rates.
Cycle Time by Process
| Process | Typical Feed (mm/rev) | Typical Speed (RPM) | MRR (cm³/min) |
|---|---|---|---|
| Gun drilling, 10mm | 0.02-0.05 | 4000-8000 | 2.5-15 |
| Gun drilling, 25mm | 0.04-0.08 | 2000-4000 | 15-50 |
| BTA drilling, 25mm | 0.08-0.20 | 1500-3000 | 45-150 |
| BTA drilling, 50mm | 0.15-0.35 | 800-1500 | 200-600 |
| Ejector drilling, 40mm | 0.12-0.30 | 1000-2000 | 120-480 |
These ranges assume carbon steel. For stainless, cut feeds by 30%. For aluminum, double them.
Key Takeaways
- The basic cycle time formula is simple: depth divided by (feed x speed). Add peck time, loading, and idle to get the real total.
- Machine acceleration adds 5-15% to peck cycle time. Do not ignore it.
- Track actual vs estimated times. The data improves your quoting accuracy faster than any formula.
- Add a 15-20% safety factor for quoting. Real conditions always add time.
- Material and tool selection directly affect achievable parameters. Match them to your cycle time estimate.
- Use past job data to calibrate your estimates. After 20 jobs, you should be within 10%.