Gears and splined shafts need oil holes for lubrication. The holes are typically small — 2-8mm diameter — drilled at an angle through the gear body or shaft wall. The position and angle need to be precise to deliver oil to the right location.
I’ve drilled oil holes in gears for industrial gearboxes, automotive transmissions, and heavy equipment drive trains. The work requires careful setup and attention to angle accuracy.
Gear Oil Holes
Gear oil holes are usually drilled from the gear face or the root of a tooth into an internal oil gallery. The gallery is a larger hole drilled through the center of the gear or along the shaft axis.
The challenge is hitting the gallery at the right depth. If the oil hole misses the gallery, the oil doesn’t reach the bearing surface. I calculate the drill depth from the gear face to the gallery center line and add 1mm for tolerance.
For gear oil holes:
| Parameter | Value |
|---|---|
| Hole diameter | 2-8mm |
| Depth | 20-100mm |
| Angle | 15-60 degrees from face |
| Cutting speed | 50-70 m/min (in hardened gear) |
| Feed rate | 0.02-0.05 mm/rev |
The feed rate depends on the gear hardness. Hardened gears — 58-62 HRC — need the lower feed rate. Pre-heat-treated gears at 30-35 HRC can take the higher feed.
Splined Shaft Oil Holes
Splined shafts have oil holes that run from the shaft OD through the spline root into a central through-hole. The holes are typically 3-6mm diameter and 10-30mm deep.
The difficulty is that the splined shaft has an irregular OD. The drill enters at the root of a spline tooth, passes through solid material, and breaks into the central hole. The entry surface is sloped and irregular.
I use a spot drill to create a flat entry surface before drilling the oil hole. The spot drill cuts a small flat at the desired entry point. The gun drill then enters on a flat surface instead of an angled one.
Drill Geometry for Oil Holes
Oil holes are short enough that a standard jobber drill can handle many applications. But for holes over 50mm deep or with high L/D ratios, a gun drill gives better results.
For oil holes under 50mm deep, I use a carbide circuit board drill or a short gun drill. These are rigid enough to hold position without wandering. For deeper holes, I switch to a standard gun drill with a guide bushing.
Deburring
Oil holes in gears and splined shafts need to be deburred on both the entry and exit surfaces. A burr on the entry surface can interfere with the oil flow. A burr on the exit surface can break loose during operation and damage the gear.
I deburr oil holes with a carbide burr on a flexible shaft tool. The burr removes the entry burr and chamfers the hole edge. For the exit burr, I use a deburring tool that passes through the hole and chamfers the far side.
Inspection
Oil holes are inspected for:
- Position relative to the gear face or spline feature
- Diameter
- Depth
- Angle
- Cleanliness
I check the position and angle with a CMM or a coordinate-measuring fixture. The depth is checked with a depth gauge. Cleanliness is verified by blowing compressed air through the hole and checking that air flows freely.
Common Problems
The most common problem with gear oil holes is drill wandering at the entry. The drill starts on a curved surface and slides off center. A spot drill prevents this.
Another common problem is burrs blocking the oil flow. A burr that’s folded over the hole edge instead of removed can restrict the oil passage. I check every oil hole with a wire gauge to confirm it’s clear.
Multi-Hole Patterns and Production Efficiency
Production gears often have multiple oil holes — sometimes 4, 6, or 8 holes around the circumference. Drilling these efficiently while maintaining positional accuracy is a balancing act.
For multi-hole patterns, I index the gear on a rotary table or fixture. The fixture positions each hole location without re-measuring, and the rotary axis holds the angle tolerance. My cycle time breakdown for a 6-hole gear:
| Operation | Time per Hole | Total (6 holes) |
|---|---|---|
| Index and clamp | 15 seconds | 1.5 minutes |
| Spot drill | 10 seconds | 1 minute |
| Gun drill (30mm depth) | 45 seconds | 4.5 minutes |
| Deburr entry | 20 seconds | 2 minutes |
| Deburr exit (manual) | 30 seconds | 3 minutes |
| Total | 2 minutes | 12 minutes |
The deburring step is the bottleneck. I’ve experimented with back-spot-facing tools and one-pass deburring systems to reduce the manual operation. For high-volume runs — over 500 parts — I design a dedicated deburring fixture that reaches both sides of the oil hole simultaneously.
For high-hardness gears (58+ HRC), I pre-drill all oil holes before heat treat and finish them after. The pre-drill removes the bulk of the material while the gear is soft, and the finishing pass in hardened material only removes 0.2-0.3mm per side. This doubles tool life compared to drilling from solid in hardened gear steel.
For more on drill geometry selection for angled holes, see gun drill geometry guide.
Key Takeaways
Oil hole drilling is a simple operation that causes problems when it’s not done carefully. The key points are a flat entry surface, a sharp drill, and proper deburring. Getting these right produces clean holes that deliver oil reliably.
Key Takeaways
- Spot drilling a flat entry surface is essential for angled oil holes in gear faces and spline roots.
- Feed rate must match gear hardness — 0.02 mm/rev for 58-62 HRC, up to 0.05 mm/rev for 30-35 HRC.
- Deburring both entry and exit is critical; folded burrs can block oil flow and cause gear failure.
- For multi-hole patterns, rotary indexing with a dedicated fixture reduces cycle time and improves positional repeatability.
- Pre-drilling oil holes before heat treat doubles tool life on hardened gears.