Thin wall stainless steel tubing is used in sanitary, pharmaceutical, and food processing applications. The wall thickness is often 2-5mm with a bore diameter of 20-60mm. The challenge is supporting the tube without collapsing it. I have worked with 304L, 316L, and 2205 duplex tubing across wall thicknesses from 1.5mm to 6mm, and the support method changes for each combination of diameter and wall.
The cutting forces from gun drilling push the tube wall inward. If the wall is not supported, the tube collapses. Once collapsed, the tube is scrap — there is no recovering a collapsed thin wall tube. The support method must be rigid enough to resist the cutting forces but compliant enough not to mark or distort the tube surface.
I use an internal mandrel for support. The mandrel fits closely inside the tube and supports the wall against cutting forces. I machine the mandrel from brass or nylon to avoid marking the tube ID. Brass is my preferred material for production runs because it wears longer than nylon and does not gall against stainless steel. Nylon is better for short runs and prototype work because it is easier to machine and cheaper to replace.
Internal Mandrel Design and Fit
The mandrel must fit the tube ID closely but not so tight that it cannot be removed after drilling. I aim for 0.05-0.1mm clearance between the mandrel OD and the tube ID. At this clearance, the mandrel supports the wall without binding.
The mandrel length should extend at least 50mm beyond the drilling area on both ends. For a tube that is drilled through the entire length, the mandrel must span the full tube length. For a blind hole, the mandrel extends past the bottom of the hole by 50mm minimum.
I machine a groove along the mandrel length to allow coolant to flow past. The groove is 3mm wide and 2mm deep — enough for coolant return without weakening the mandrel. Without this groove, the coolant has no return path and the pressure builds up, pushing the mandrel out of the tube.
For stepped tubes where the ID changes along the length, I make a stepped mandrel with different diameters for each section. The transition between diameters is chamfered to avoid a stress concentration on the tube wall.
| Tube OD (mm) | Wall Thickness (mm) | Mandrel Material | Mandrel Clearance (mm) | Coolant Groove Size |
|---|---|---|---|---|
| 25 | 2.0 | Brass | 0.05-0.08 | 3mm x 2mm |
| 32 | 2.5 | Brass | 0.05-0.08 | 3mm x 2mm |
| 38 | 3.0 | Nylon | 0.08-0.10 | 3mm x 2mm |
| 50 | 3.5 | Brass | 0.08-0.10 | 4mm x 2mm |
| 60 | 4.0 | Brass | 0.08-0.10 | 4mm x 2mm |
| 76 | 5.0 | Nylon | 0.10-0.12 | 4mm x 3mm |
Fill Material Options for Extreme Thin Walls
For tubes under 3mm wall thickness, an internal mandrel alone is not enough. The cutting forces are still high enough to deform the tube. I fill the tube with low-melt alloy or sand. The filling material provides internal support and dampens vibration. After drilling, I melt out the alloy or flush out the sand.
Low-melt alloy (typically bismuth-tin or Wood’s metal, melting point 70-100C) is the most effective fill material. It provides rigid support — almost like drilling solid material. The process is: heat the tube to just above the alloy melting point, pour the molten alloy into the tube, let it cool and solidify, drill the hole, then heat the tube again to melt the alloy out. The downside is the cycle time — the heating and cooling adds 30-60 minutes per part.
Sand is less effective but faster. I use dry silica sand, 80-120 mesh. I pack the sand into the tube and tamp it down to remove voids. The sand supports the wall against radial forces but does not help with axial forces as well as low-melt alloy. Sand filling adds 10-15 minutes per part for filling and flushing.
Polymer fill (urethane or polyurethane foam) is a middle ground I have tested. The polymer is poured as a liquid and expands to fill the tube. It provides moderate support and dampens vibration well. After drilling, the polymer can be pushed out or dissolved with acetone. The support is not as rigid as low-melt alloy, so I only use it for walls above 2.5mm.
| Fill Method | Support Rigidity | Setup Time | Removal Time | Reusability |
|---|---|---|---|---|
| Low-melt alloy | Excellent | 30-45 minutes | 15-20 minutes | Yes, alloy is reusable |
| Dry sand | Moderate | 10-15 minutes | 5-10 minutes | No, sand is single-use |
| Polymer foam | Moderate | 15-20 minutes | 5-10 minutes | No |
| None (mandrel only) | Minimal | 5-10 minutes | 2-5 minutes | Yes |
Parameter Adjustments for Thin Walls
Drilling thin wall tubing requires different parameters than solid material. The cutting forces must be minimized to avoid collapsing the tube, but the feed must be high enough to prevent work hardening on the stainless surface.
For the drilling parameters:
- Cutting speed: 60-75 m/min
- Feed rate: 0.03-0.05 mm/rev
- Coolant pressure: 1000-1500 psi
- Coolant type: Water-soluble oil at 10-12% concentration
- Tool: Sharp carbide, TiAlN-coated
- Tool geometry: Standard gun drill geometry, sharp edge preparation
The feed is lower than for solid material to reduce cutting forces. At 0.03 mm/rev, the forces on the tube wall are about 40% lower than at 0.08 mm/rev. I have measured the cutting forces with a dynamometer on instrumented test parts. The radial force component, which is what tries to collapse the tube, drops significantly with lower feed.
I also reduce the peck depth — 50mm instead of the usual 100mm — to clear chips more frequently. Stainless steel produces long, stringy chips that can pack in the flute. Shorter peck depths give the chips less distance to travel and reduce the risk of chip packing.
| Wall Thickness | Feed Rate | Peck Depth | Cutting Speed | Coolant Pressure |
|---|---|---|---|---|
| 1.5mm | 0.025 mm/rev | 30mm | 55 m/min | 1500 psi |
| 2.0mm | 0.03 mm/rev | 40mm | 60 m/min | 1200 psi |
| 2.5mm | 0.035 mm/rev | 50mm | 65 m/min | 1200 psi |
| 3.0mm | 0.04 mm/rev | 50mm | 70 m/min | 1000 psi |
| 4.0mm | 0.045 mm/rev | 60mm | 75 m/min | 1000 psi |
| 5.0mm | 0.05 mm/rev | 60mm | 75 m/min | 1000 psi |
Workholding and Steady Rests
The external support is as important as the internal support. Steady rest rollers need to be adjusted carefully. Too much pressure collapses the tube. Too little allows vibration. I use nylon or rubber rollers that distribute the force evenly.
I use three-point steady rests with independent adjustment on each roller. The rollers are set to just contact the tube surface — I can spin the tube by hand with light resistance. The roller pressure is checked with a feeler gauge between the roller and the tube. A 0.05mm feeler should not pass through when the roller is properly set.
For tubes longer than 500mm, I use two steady rests — one near the drill entry and one at mid-span. The entry steady rest is positioned within 50mm of the drill bushing to support the tube at the cutting zone. The mid-span steady rest prevents the tube from sagging under its own weight.
The steady rest rollers wear over time. I replace the rollers when I see a groove worn into the surface. A grooved roller marks the tube and causes vibration. Nylon rollers last about 500 parts on 304L tubing. Rubber rollers last 300 parts but are gentler on the tube surface.
Inspection After Drilling
I check the tube OD after drilling for any signs of collapse. A collapse shows as a flat spot on the tube surface. I run a dial indicator along the tube length while rotating the tube in V-blocks. A collapse reads as a 0.05mm or more drop in the indicator.
If the tube has collapsed, I adjust the mandrel fit and reduce the feed for the next part. A collapsed tube means the support was insufficient or the feed was too high. I check both before running the next part.
I also check the wall thickness after drilling. The cutting forces can cause the tube wall to thin out on one side if the drill is not centered. I use an ultrasonic thickness gauge at four positions around the circumference at three depths along the tube. A wall reduction of more than 10% of the nominal thickness means the drill alignment needs correction.
| Inspection Check | Method | Acceptable Limit | Action if Failed |
|---|---|---|---|
| OD collapse | Dial indicator on V-blocks | 0.03mm max deviation | Increase support, reduce feed |
| Wall thinning | Ultrasonic gauge | 10% max reduction | Realign drill, check bushing |
| Surface finish | Profilometer | Ra 1.6um max | Adjust feed, check coolant |
| Diameter | Bore gauge | H8 tolerance | Check drill wear |
| Straightness | Dial indicator through bore | 0.1mm/m max | Check bushing alignment |
Key Takeaways
- Internal mandrel support is the first line of defense for thin wall tubing. I use brass mandrels with 0.05-0.1mm clearance for production runs and nylon for prototypes.
- For walls under 3mm, add fill material. Low-melt alloy gives the best support. Sand works for less critical parts. Polymer foam is a middle ground for walls above 2.5mm.
- Reduce feed to 0.03-0.05 mm/rev for thin wall work. The radial cutting forces drop by 40% compared to standard feeds, reducing collapse risk.
- Use two steady rests for tubes over 500mm long. One near the entry and one at mid-span. Set the rollers with minimal contact pressure.
- Inspect the OD with a dial indicator after every part. A 0.05mm flat spot means the support was not adequate.
- Watch for wall thinning with an ultrasonic gauge. More than 10% thickness reduction means the drill is off-center and needs realignment.