Thread this deep
6.0 mm
Drill about 7.5 mm deep so the tap has run out space. You have 3 mm, which is not enough, so add a boss or use an insert.
Engagement rules of thumb scaled by material strength. A joint carrying real load, or one that gets torqued repeatedly, should be checked properly rather than taken from a table.
Engagement by size and material
Engaged thread length in millimetres, not drilled depth. Drill deeper than you tap, typically by two to three thread pitches, so the tap has somewhere to run out.
| Screw | 6061-T6 | 7075-T6 or Ti Gr5 | POM or PA66 |
|---|---|---|---|
| M2 | 4.0 mm | 3.0 mm | 5.0 mm |
| M2.5 | 5.0 mm | 3.8 mm | 6.3 mm |
| M3 | 6.0 mm | 4.5 mm | 7.5 mm |
| M4 | 8.0 mm | 6.0 mm | 10.0 mm |
| M5 | 10.0 mm | 7.5 mm | 12.5 mm |
| M6 | 12.0 mm | 9.0 mm | 15.0 mm |
7075 and titanium need less length than 6061 because they are stronger, not because the thread is better. Plastics need more, and they still wear with every assembly.
When there is not enough material
- Add a boss. A collar of full thickness material around the hole, roughly two screw diameters across, blended into the web with a radius. Cheapest option on a machined part.
- Helical insert. Spreads load over more parent material and is replaceable. Needs its own tapped hole size, so it goes on the drawing before machining.
- Press in or self clinching insert. Designed for thin plate, needs a correctly sized hole and a press operation.
- Through bolt and nut. Not elegant, entirely reliable, and often the lightest answer.
Anodise closes threads
Hard anodise takes roughly four times the coating thickness off pitch diameter, which at 50 micrometres is about 0.2 mm. That is more clearance than most screws have.
Mask the tapped holes, tap after coating, or size for an insert. Work out the number with the anodise growth calculator.
Questions
How deep should a thread be in aluminium?
One and a half to two screw diameters of engaged thread. For an M3 in 6061 that is about 6 mm. Going deeper adds very little strength, because the first few engaged turns carry most of the load, and going shallower is how threads strip.
Do plastics need more engagement than aluminium?
Yes, roughly two and a half to three diameters in POM or nylon, and even then a cut thread wears every time it is assembled. Anything serviced in the field should use a heat set or moulded insert rather than a cut thread.
What if there is not enough material for the engagement?
Add a boss around the hole, use an insert designed for thin material, or use a through bolt and a nut. A boss is the cheapest on a machined part and costs about a gram.
Longer version: threads in thin aluminium, when to cut them and when to use an insert.
Send the part with the fastener list
We check every thread for engagement before we quote and flag the ones that will not survive the field. Reply within 24 business hours.