I’ve covered medical implant drilling elsewhere — this is about micro-hole drilling across all industries. The same principles apply whether you’re drilling fuel injector nozzles, medical components, or cooling channels in electronics.
Below 3mm diameter, deep hole drilling changes character. The tools are fragile, coolant flow is restricted, and every parameter matters more than it does at larger diameters.
Where Micro-Holes Show Up
The industries that need micro-hole deep hole drilling:
Fuel injection. Diesel injector nozzles have holes as small as 0.5mm, drilled at angles through the tip. The depth is shallow — maybe 10-20mm — but the diameter tolerance is ±0.003mm.
Medical. Bone screws and trauma nails need through-holes of 1-2mm at lengths up to 200mm. The medical article covers this in detail.
Electronics. Some electronics cooling components need micro-channels for fluid flow. These are typically in aluminum or copper and need to be clean and burr-free.
Mold venting. Injection molds sometimes need gas vent holes as small as 0.5mm, drilled through hardened steel.
The Coolant Problem
At diameters under 3mm, coolant delivery is the limiting factor. A 2mm gun drill has a coolant hole of about 0.6mm. The pressure drop through that hole is significant, and once the coolant reaches the cutting edge, it needs to have enough force left to clear the chips.
I’ve found that coolant pressure needs to increase as diameter decreases. A rule I use:
| Diameter | Minimum Coolant Pressure |
|---|---|
| 3mm | 1000 psi |
| 2mm | 1500 psi |
| 1.5mm | 2000 psi |
| 1mm | 3000 psi |
Below 1mm, you’re at the limits of conventional gun drilling. At that point, EDM or laser drilling might be a better choice than trying to push a mechanical drill that small.
Speed and Feed for Small Diameters
The rule with micro-hole drilling is: high spindle speed, low feed per revolution.
For a 2mm drill in stainless steel:
| Parameter | Value |
|---|---|
| Cutting speed | 50-70 m/min |
| Spindle speed | 8000-11000 rpm |
| Feed rate | 0.005-0.012 mm/rev |
| Coolant pressure | 1500 psi |
The spindle speed needs to be high enough to maintain a reasonable cutting speed. At 2mm diameter, 8000 rpm gives about 50 m/min. That’s a comfortable range for most materials.
I avoid running the spindle at maximum speed. If the machine has a 12,000 rpm maximum, I run at 10,000 rpm. Running at the limit leaves no room for adjustment and increases spindle wear over time.
Pecking Strategy
At small diameters, pecking is essential. I use a peck depth of 5-8 times the drill diameter with a full retract to clear chips. For a 2mm drill, that’s a peck of 10-16mm.
The retract speed matters. I pull the drill back at a fast feed rate — 5000 mm/min or more — to create a suction effect that pulls chips out of the hole. Then I advance at the normal feed rate for the next peck.
I’ve seen shops use short pecks (3x diameter) on small drills, thinking the more frequent retracts will prevent chip packing. In my experience, longer pecks work better because the drill gets deep enough to form a proper chip that clears on retract. Very short pecks just re-cut the same material.
Guide Bushing Condition
At diameters under 3mm, the guide bushing clearance needs to be tighter. I aim for 0.003-0.005mm total clearance between the drill OD and the bushing ID. Any more than that and the drill can wobble enough to affect roundness.
The bushing material matters too. I use carbide bushings for small-diameter work. Steel bushings wear too fast at the high spindle speeds that small drills require.
Tool Material and Micro-Drill Design
For micro-drills below 3mm, tool material and geometry are critical. I use sub-micron carbide grades with a cobalt content of 6-10%. The finer grain size gives the cutting edge the toughness it needs at the high spindle speeds these drills run at.
The carbide grade matters more at small diameters because the cutting edge isn’t supported by a massive tool body. A 2mm gun drill has maybe 0.3mm of carbide supporting the cutting edge. If the grade is too brittle, the edge chips. If it’s too soft, the edge wears and the surface finish degrades fast.
My recommended carbide grades for micro-hole drilling:
| Material Being Drilled | Carbide Grade | Cobalt % | Grain Size |
|---|---|---|---|
| Stainless steel (304/316) | Sub-micron (0.5-0.8μm) | 8-10% | Fine |
| Titanium (Ti-6Al-4V) | Ultra-fine (0.2-0.5μm) | 6-8% | Very fine |
| Hardened steel (45-62 HRC) | Fine (0.8-1.0μm) | 6-8% | Fine |
| Aluminum / Copper | Medium (1.0-2.0μm) | 10-12% | Standard |
Machine rigidity is just as important as tool material. A micro-drill can’t tolerate chatter or vibration. I check the spindle runout before every micro-hole job — anything over 0.005mm TIR causes rapid tool failure at small diameters. I also verify that the machine base is level and the ways are tight. A machine that cuts fine at 10mm diameter may not hold tolerance at 2mm.
For more on coolant strategies in small-diameter work, see coolant pressure in deep hole drilling.
The Main Difference from Larger Drilling
The difference between drilling a 10mm hole and a 2mm hole isn’t just the size. It’s the margin for error. A 10mm drill can withstand some overload without failing. A 2mm drill snaps if you look at it wrong.
On small-diameter work, I’m more conservative with parameters, more careful with alignment, and more attentive to coolant conditions. If I have doubts about the setup, I’d rather spend 30 minutes checking than risk a broken drill in an expensive part.
Key Takeaways
- Coolant pressure must increase as diameter decreases — below 3mm, 1000 psi is the minimum; below 1.5mm, 2000 psi or more.
- Peck depth of 5-8x diameter with fast retract (5000+ mm/min) clears chips effectively; short pecks just re-cut the same material.
- Guide bushing clearance of 0.003-0.005mm total and carbide bushings are required for diameters under 3mm.
- Sub-micron carbide grades with appropriate cobalt content prevent edge chipping at the high spindle speeds micro-drilling demands.
- Machine spindle runout under 0.005mm TIR is critical — a machine that cuts fine at 10mm may not hold tolerance at 2mm.