Why Surface Roughness Units Matter
In my years of deep hole drilling, I have seen countless specification sheets where the customer calls out surface finish in a unit the shop floor does not use regularly. European prints often specify Rz, aerospace jobs use RMS from the old MIL standards, and medical device prints stick with Ra. If you convert wrong, you scrap parts.
I keep this conversion chart posted in the inspection area for quick reference. The conversions here are based on the ISO 1302 standard and verified against actual profilometer readings on gun-drilled and BTA-drilled bores.
Common Surface Roughness Parameters
Ra is the arithmetic average deviation of the surface profile. It is the most widely used parameter worldwide. In deep hole drilling, Ra values typically fall between 0.4 um and 3.2 um for as-drilled surfaces.
Rz is the average maximum height of the profile over five sampling lengths. It represents the peak-to-valley height and runs about 6 to 8 times the Ra value in practice. Some European automotive prints specify Rz exclusively.
RMS is the root mean square of the surface profile. It is roughly 1.1 times Ra, though many shops treat it as equivalent to Ra in inch units. The aerospace industry still uses RMS from the old ASME B46.1 standard.
Quick Conversion Table
| Ra (um) | Rz (um) | RMS (uin) | Surface Characteristic | Typical Process |
|---|---|---|---|---|
| 0.05 | 0.4 | 2 | Mirror finish | Honing / polishing |
| 0.1 | 0.8 | 4 | Precision ground | Fine honing |
| 0.2 | 1.6 | 8 | Very smooth | Superfinishing |
| 0.4 | 3.2 | 16 | Honed finish | Bore finishing |
| 0.8 | 6.3 | 32 | Smooth as-drilled | Gun drilling |
| 1.6 | 12.5 | 63 | Standard as-drilled | Gun drilling / BTA |
| 3.2 | 25 | 125 | Rough as-drilled | BTA / rough drilling |
| 6.3 | 50 | 250 | Very rough | Preliminary drilling |
Conversion Formulas
When I need a quick conversion without the chart, I use these approximations:
- Ra to Rz: Rz = Ra x 7.5 (typical for deep hole drilling surfaces)
- Ra to RMS (uin): RMS = Ra x 39 (for Ra in um)
- RMS to Ra in um: Ra = RMS / 39
- Rz to Ra: Ra = Rz / 7.5
These are engineering approximations, not exact mathematical conversions. The actual ratio between Ra and Rz depends on the surface profile shape. A sharp-peaked surface gives a higher Rz/Ra ratio than a rounded surface. For critical specifications, I always measure in the unit the print requires rather than converting.
Selecting the Right Target for Deep Hole Drilling
I aim for Ra 0.8 um as a standard target for general gun drilling in steel. This converts to roughly Rz 6.3 um or RMS 32 uin. This finish is smooth enough for most hydraulic and pneumatic applications without secondary operations.
For BTA drilling in larger diameters (20 mm to 65 mm), I expect Ra 1.6 um to 3.2 um as-drilled. The larger chip formation in BTA drilling inherently leaves a rougher surface. If the print calls for better than Ra 1.6 um, I recommend a follow-up skiving or roller burnishing pass.
For gundrilling applications below 10 mm diameter, I typically achieve Ra 0.4 um to 0.8 um with sharp tool geometry and proper coolant filtration at 20 um or better.
Industry-Specific Standards
| Industry | Preferred Unit | Typical Spec Range |
|---|---|---|
| Automotive (European) | Rz | Rz 6.3 - 25 um |
| Aerospace | RMS | RMS 32 - 125 uin |
| Medical devices | Ra | Ra 0.2 - 0.8 um |
| Hydraulic components | Ra | Ra 0.4 - 0.8 um |
| Oil and gas | Ra or RMS | Ra 0.8 - 3.2 um |
Measuring Surface Roughness In-Bore
Measuring surface roughness inside a deep hole requires special fixturing. I use a portable profilometer with a 90-degree adapter and a skidded pickup that can reach at least 200 mm into the bore. The cutoff length should match the expected roughness spacing. For gun-drilled surfaces I use a 0.8 mm cutoff, and for BTA surfaces I use a 2.5 mm cutoff.
I take measurements at three positions along the bore length: near the entrance, at mid-depth, and near the exit. The finish can vary along the bore due to coolant pressure drop and tool deflection.
Key Takeaways
- Ra, Rz, and RMS all measure surface roughness but use different scales.
- A quick approximation is Ra x 7.5 = Rz and Ra x 39 = RMS (uin).
- Gun drilling typically produces Ra 0.8 um to 1.6 um as-drilled.
- Always verify critical conversions by direct measurement in the specified unit.
- Post this chart in the inspection area for quick operator reference.