The Basic Calculation
Penetration rate is the speed at which the drill advances into the material, expressed in mm per minute. The formula is straightforward and I use it on every job:
Penetration rate (mm/min) = Feed rate (mm/rev) x Spindle speed (RPM)
For a 10mm gun drill running at 2500 RPM with a feed of 0.04 mm/rev: 0.04 x 2500 = 100 mm/min
To estimate the drilling time for a 500mm deep hole: 500mm / 100 mm/min = 5 minutes of actual cutting time
That 5 minutes is the cutting portion only. The actual cycle time includes peck retracts, dwells, and tool changes. In my experience, the actual cycle time runs 10-15% longer than the penetration rate calculation suggests.
Sample Penetration Rates by Material
I track penetration rates across common materials to set baseline expectations for new jobs. The table below shows typical values I have measured in production.
| Material | Drill Dia. | RPM | Feed (mm/rev) | Penetration Rate (mm/min) | Drilling Time per 100mm |
|---|---|---|---|---|---|
| 1018 mild steel | 10mm | 3000 | 0.04 | 120 | 50 sec |
| 4140 alloy steel | 10mm | 2600 | 0.035 | 91 | 66 sec |
| 4340 high-strength | 10mm | 2200 | 0.03 | 66 | 91 sec |
| 304 stainless | 8mm | 2200 | 0.025 | 55 | 109 sec |
| 6061 aluminum | 12mm | 4500 | 0.06 | 270 | 22 sec |
| Brass 360 | 10mm | 3500 | 0.08 | 280 | 21 sec |
| Inconel 718 | 8mm | 1500 | 0.015 | 22.5 | 267 sec |
I record the penetration rate on the first production run with a new tool for each material. That becomes the baseline for that job going forward.
Using Penetration Rate as a Health Indicator
A drop in penetration rate is the earliest sign of a developing problem. If a drill that normally runs at 100 mm/min drops to 80 mm/min, something has changed. The three most common causes I see are:
- Tool wear: The cutting edge is dulling and requires more force to cut. The feed drops because the control is trying to maintain the spindle load limit.
- Material variation: The material is harder than the previous batch. A hardness increase of 5 HRC can drop the penetration rate by 10-15%.
- Coolant pressure loss: The chips are not evacuating properly and packing in the flute. The packing increases the back pressure and the drill cannot advance at the programmed feed.
| Penetration Rate Drop | Likely Cause | Action |
|---|---|---|
| 5-10% | Tool wear starting | Monitor, plan tool change |
| 10-20% | Significant tool wear or material change | Inspect tool, check material cert |
| 20%+ | Immediate stoppage risk | Stop and inspect before continuing |
| Sudden 30%+ | Chip packing or drill damage | Retract immediately, clear chips |
| Gradual decline | Normal tool wear curve | Replace at threshold |
I set a 15% drop as the alert threshold on most jobs. When the penetration rate drops 15% below the baseline, the control alarms and the operator inspects the tool.
Calculating True Cycle Time
The penetration rate gives me the cutting time. To get the true cycle time, I add the non-cutting elements.
| Element | Time for 500mm Hole at 100 mm/min |
|---|---|
| Cutting time | 5.0 min |
| Peck retracts (15 pecks at 2 sec each) | 0.5 min |
| Final retract | 0.2 min |
| Tool change at start | 0.5 min |
| Part load/unload | 1.0 min |
| Total cycle time | 7.2 min |
The peck cycle adds the most time. I minimize the number of pecks by using the longest safe peck depth for the material and diameter. For a stable cut in steel with a 10mm drill, I use 50mm peck depth instead of 25mm, which halves the peck retract time.
Tracking Tool Life with Penetration Rate
On high-production jobs, I track the penetration rate for every tool across its entire life. The plot of penetration rate against cumulative cutting time tells me when the tool is nearing the end of its useful life.
In my experience, a gun drill shows three phases of penetration rate:
- Phase 1: Initial wear-in. The rate stays stable or drops slightly (0-5%) for the first 10-20% of tool life.
- Phase 2: Steady state. The rate declines gradually at 0.5-1% per hour of cutting. This is the productive period.
- Phase 3: End of life. The rate drops rapidly (5-10% per hour) as the cutting edge breaks down. This is when the tool breaks if I do not replace it.
I replace the tool at the start of Phase 3, not at the end. The cost of replacing a drill that has 10% life remaining is less than the cost of a broken drill and a scrapped part.
Using Penetration Rate for Bidding and Quoting
When I quote a new deep hole drilling job, the penetration rate is the basis for the cycle time estimate. I multiply the penetration rate by an efficiency factor of 0.85 to account for non-cutting time and variability.
For a 400mm deep hole in 4140 with a 10mm drill: Baseline: 91 mm/min Adjusted: 91 x 0.85 = 77 mm/min Cutting time: 400 / 77 = 5.2 min Add 3 min load/unload and retract: 8.2 min per part At $85 per hour machine rate: $11.62 per part in machine cost
The adjusted rate has proven accurate within 10% across hundreds of jobs I have quoted. I adjust the efficiency factor downward to 0.75 for difficult materials like Inconel or titanium.
Key Takeaways
Penetration rate is the single most useful process health indicator in deep hole drilling. I calculate it from feed and RPM, record the baseline for each job, and trigger an alert at a 15% drop. Tracking the rate over the tool life tells me when to replace the tool before it breaks. The same calculation feeds my quoting estimates and keeps the pricing accurate.