A Ø8 mm hole 400 mm long in a hydraulic block, a 0.5 mm wall in a sensor enclosure, an oil passage with L/D 30 in an injector body: these features overshadow the cost of the rest of the part. As the technical deepening of our DFM article, this article covers deep hole and thin wall challenges from both the supplier's and the designer's perspective.
Part 1: Deep Hole Drilling
When does a hole count as "deep"?
General consensus: L/D (hole length / diameter) up to 5 standard drill, 5–10 long drill and peck drilling, 10–40 gundrill (single-lip drill) or special drill with high-pressure coolant, above 40 BTA/ejector systems or a special machine. Difficulty increases as the diameter shrinks: L/D 20 at Ø3 mm is much harder than L/D 20 at Ø20 mm because drill rigidity falls with the fourth power of the diameter.
Problems
- Drift (runout): The drill wanders off axis as it advances; 0.3–1 mm drift over 300 mm is normal with a standard drill. Drilling from both ends creates a step at the meeting point.
- Chip packing: Chips cannot be evacuated, the drill jams and breaks; the long-chip problem grows in stainless and aluminium.
- Heat: Coolant does not reach the bottom of the hole; tool life shortens, the hole diameter grows.
- Surface and diameter consistency: The diameter varies along the hole; H7 tolerance cannot be held in a deep hole without reaming or honing.
Methods
| Method | L/D range | Diameter | Straightness / drift | Note |
|---|---|---|---|---|
| Carbide drill, through-coolant | ≤ 8–12 | Ø3–20 | 0.1–0.3 mm / 100 mm | Standard on CNC lathes; pilot hole + peck feed |
| Long carbide drill (30xD) | ≤ 30 | Ø2–12 | 0.2–0.5 mm / 100 mm | High-pressure coolant (70+ bar) required; pilot hole essential |
| Gundrill (single-lip) | 10–100+ | Ø1–40 | 0.02–0.05 mm / 100 mm | Straightest deep hole; special machine or CNC lathe with 100+ bar coolant; surface Ra 0.8–1.6 |
| BTA / ejector | 10–100 | Ø20–300 | 0.05–0.1 mm / 100 mm | Large diameter, heavy industry; special machine |
| Drilling from both ends | Any diameter | — | Step risk at the junction | Only for passage function; avoided in precision holes |
| Drilling + reaming / honing | — | — | For H7 and Ra 0.4 | Hydraulic cylinders, valve bodies |
Deep holes on Swiss-type lathes
Swiss-type turning offers a natural advantage for deep holes: the part is supported in the guide bushing, drilling with a stationary drill into a rotating part (the part's rotation helps centre the drill) and high-pressure coolant is standard. Holes with L/D 20–30 at Ø2–12 mm are routinely machined on Swiss-type lathes; injector, valve pin and medical cannula applications rely on this. Details are in our Swiss-type turning article.
Design recommendations
- Question whether the hole really needs to be full length; a stepped diameter (large at the entry, small at the bottom) reduces L/D.
- Give the deep hole tolerance according to function: if it is a passage, H12 is sufficient, do not ask for H7.
- Specify the straightness/drift tolerance separately (for example "axis deviation max. 0.2 mm"); if not specified, the supplier makes an assumption.
- Define the burr and edge standard at the intersection point of crossing passages; deburring is difficult at deep passage intersections.
- Avoid asking for a flat bottom at the end of the hole; accept the drill point angle (118–140°).
- Consider the material: sulphur-bearing stainless (1.4305), free-cutting steel and brass are easy in deep holes; 316L and titanium are difficult.
Part 2: Thin-Walled Parts
When does a wall count as "thin"?
A part is considered thin-walled when the ratio of wall thickness to diameter (t/D) falls below 5% or the absolute thickness falls below 1 mm; 0.8 mm wall in aluminium, 0.5 mm in steel and 0.4 mm in brass are the thresholds where difficulty begins. Long, thin sleeves (L/D > 3) additionally carry buckling and vibration risk.
Problems
- Clamping deformation: Chuck jaws make the part triangular/oval; a round bore machined after clamping becomes oval when the clamp is released.
- Chatter: A thin wall rings like a bell; the surface deteriorates, the tolerance is lost.
- Thermal expansion: Little mass, heat spreads quickly; the part grows during machining and shrinks when it cools.
- Residual stress: When a lot of material is removed from rolled bar, internal stresses distort the part.
- Measurement: The measuring force deforms the part; a thin sleeve measured with callipers does not show the true diameter.
Solutions
| Approach | Application |
|---|---|
| Soft jaws, full contact | Jaws machined to the part diameter; contact area grows, point loading falls |
| Expanding mandrel / internal clamping | The sleeve is supported from inside, the outer surface is machined freely |
| Low clamping force, staged machining | Roughing with high force, finishing with low force in a separate clamping |
| Guide bushing support on Swiss-type lathes | The part is machined while still on the bar, cut-off is the last step; minimal deformation |
| Filler support (low-melting alloy, wax, freezing) | The internal cavity is filled and melted out after machining; for very thin walls |
| Vacuum / adhesive fixture | Flat thin plates and flanges |
| Sharp tools, small depth of cut, high speed | Cutting force falls; positive geometry, polished tools |
| Intermediate stress-relief annealing | Roughing → annealing → finishing; in aluminium and stainless |
| Non-contact / low-force measurement | Optical measurement, air gauge, CMM at low force |
Design recommendations
- Keep the wall at the thickest value the function allows; 0.8 mm instead of 0.5 mm can halve the cost.
- Leave a thick zone (flange, collar) for clamping; it can be cut off after machining.
- Give realistic roundness and cylindricity tolerances on thin walls (0.02–0.05 mm); if 0.005 mm is required, grinding and special fixtures are needed.
- Material selection: brass and 6082 are more stable than steel in thin walls; 316L is prone to distortion due to residual stress.
- Maintain symmetry in the design; asymmetric pockets and holes increase distortion.
- Specify the measurement method: "non-contact optical" or "3-point bore micrometer, force X N".
Cost Impact
Approximate rule: a hole going from L/D 5 to 15 increases the hole cost 3–5 times; if L/D 30 requires gundrilling, 6–10 times. A sleeve whose wall goes from 1.5 mm to 0.6 mm increases machining time and scrap rate 2–3 times. These increases are reflected directly in the part price; our tolerance article gives the general framework and our surface roughness article explains expectations for deep hole and thin wall surfaces. Asking the supplier separately for the share of these features in the price when requesting a quote shows the return on a design change.
Frequently Asked Questions
Can a Ø6 mm, 180 mm deep hole (L/D 30) be made?
Yes; on a Swiss-type lathe or CNC lathe with a long drill with high-pressure coolant or a gundrill. The drift tolerance should be specified.
Is a stainless sleeve with 0.3 mm wall possible?
Yes in short lengths and on Swiss-type lathes; in long lengths the deformation and vibration risk is high and the design should be reviewed.
Can H7 tolerance be held in a deep hole?
Yes with reaming or honing after drilling; no with drilling alone. Cost increases accordingly.
What matters when measuring thin-walled parts?
Measuring force and temperature; a non-contact or low-force method, stabilising at 20 °C and measuring in the free state are required.
Conclusion
Deep holes and thin walls can be produced reliably with the right method and workholding strategy; but the biggest gain is at the design stage: when unnecessary depth, unnecessary thinness and unnecessary tolerance are removed, cost can halve. Send the STEP files and drawings for your parts containing these features via our quote form; together with the quote we also pass on design recommendations that reduce cost.
