Material removal rate tells you how much material is being removed per unit of time. It is useful for comparing tool performance, estimating cycle times, and spotting process problems. I calculate MRR on every new job to establish a baseline.
I track MRR for each job and compare it to the baseline over time. A 20% decrease in MRR indicates tool wear or a process problem. A sudden drop often catches issues before the tool breaks.
The MRR Formula for Deep Hole Drilling
The basic formula for MRR in deep hole drilling is straightforward:
MRR = (Pi x D^2 / 4) x f x N
Where:
- D is the drill diameter in mm
- f is the feed rate in mm/rev
- N is the spindle speed in RPM
The first part of the equation — (Pi x D^2 / 4) — calculates the cross-sectional area of the hole. Multiplying by feed rate and RPM gives the volume of material removed per minute.
Here are some real calculations from my jobs:
| Drill Type | Diameter (mm) | Feed (mm/rev) | RPM | MRR (cc/min) |
|---|---|---|---|---|
| Gun drill | 10 | 0.04 | 3000 | 9.4 |
| Gun drill | 20 | 0.06 | 2000 | 37.7 |
| BTA drill | 30 | 0.10 | 1200 | 84.8 |
| BTA drill | 50 | 0.15 | 800 | 235.6 |
| Ejector drill | 40 | 0.12 | 1000 | 150.8 |
The BTA drill in the 50mm example removes material 25 times faster than the 10mm gun drill. This is why BTA is the standard choice for diameters above 30mm in production environments.
Comparing Gun Drilling vs. BTA MRR
I use MRR calculations to decide between gun drilling and BTA for a given job. The crossover point where BTA becomes worth the setup cost is around 30mm diameter.
For a 25mm hole at 200mm depth:
- Gun drilling: 15.7 cc/min, drilling time about 5.2 minutes
- BTA drilling: 58.9 cc/min, drilling time about 1.4 minutes
BTA is nearly 4x faster at this diameter, but the setup takes longer and the tooling costs more. I use the MRR difference to calculate whether the speed gain justifies the cost.
The same calculation helps me compare different feeds and speeds within the same process. A 10% increase in feed rate gives a 10% increase in MRR, as long as the tool can handle it.
Using MRR to Predict Cycle Times
The MRR also helps estimate cycle time before cutting the first part. For a 500mm deep hole with a 10mm drill, the hole volume is:
Volume = (Pi x D^2 / 4) x Depth = 78.5 x 500 = 39,250 cubic mm
At 9.4 cc/min MRR, the pure drilling time is 39.25 / 9.4 = 4.18 minutes. But the actual cycle time includes non-cutting time.
I add these factors to get the real cycle time:
- Peck retract and re-entry: 30-50% of drilling time depending on peck length
- Tool indexing time: 10-30 seconds per tool change
- Part loading and unloading: 1-3 minutes per part
- Coolant system pressurization: 5-10 seconds per cycle
For the 500mm hole example, the real cycle time is closer to 7-8 minutes including peck time. I use a factor of 1.5x to 2x the calculated drilling time for estimating total cycle time.
Tracking MRR Over Time
I track MRR for each production job and log it by tool serial number. The MRR trend tells me more than a single measurement. A slow decrease over 100 holes is normal tool wear. A sudden decrease means something changed.
Common MRR drops I have seen:
- 10% drop over 50 holes: normal wear, tool nearing end of life
- 20% drop overnight: coolant concentration changed or pressure dropped
- 30% drop after tool change: regrind quality issue, drill not cutting properly
- 40% drop within one hole: chip packing or coolant blockage
When I see a MRR drop of 15% or more, I stop and investigate before the tool breaks.
Key Takeaways
- Use MRR = (Pi x D^2 / 4) x feed x RPM to calculate material removal rate
- BTA drilling offers 3-25x higher MRR than gun drilling depending on diameter
- Real cycle time is 1.5-2x the calculated drilling time when including peck cycles
- Track MRR over time to spot tool wear and process problems early
- A 15%+ MRR drop warrants immediate investigation
For more on process planning, see my guide on calculating cost per hole.