Guides / Design
Threads in thin aluminium: when to cut them and when to use an insert
August 23, 2026
A drone plate pocketed down to 2 mm with M3 holes tapped straight through it is one of the most common design mistakes we see, and it survives the prototype because a prototype gets assembled twice.
It fails in the field, where the same hole gets opened and closed twenty times by someone with a power driver.
How much thread you actually need
Thread strength in aluminium is about engagement length, not about the number of turns. The usual working rule is one and a half to two diameters of engagement:
| Screw | Minimum engagement in aluminium | Comfortable |
|---|---|---|
| M2 | 3 mm | 4 mm |
| M2.5 | 4 mm | 5 mm |
| M3 | 4.5 mm | 6 mm |
| M4 | 6 mm | 8 mm |
| M5 | 7.5 mm | 10 mm |
Two details behind the table. The first few engaged turns carry most of the load, so a very deep hole buys little. And the strength in 6061 and 7075 is different, roughly in proportion to their yield strengths, so a marginal thread in 6061 may be adequate in 7075.
Four ways to get the engagement
1. A boss. The cheapest answer on a machined part. Leave a collar of full thickness material around the hole, roughly two screw diameters across, and blend it into the thin web with a radius rather than a step. It costs a gram and no process.
2. A helical wire insert. The standard fix for a hole that gets opened often. It spreads load over more of the parent material and it is replaceable if it does fail. It needs a specific tapped hole size, so it must be on the drawing before the part is machined, not added later.
3. A press in or self clinching insert. Designed for thin sheet and plate. Good where there is genuinely no room for a boss, and it needs a correctly sized hole and a press operation.
4. A through bolt and a nut. Not elegant, entirely reliable, and it needs access to the back. On a plate that is thin because weight matters, this is often the lightest option overall.
Design details that decide whether the thread survives
- Never tap into a pocket floor without a boss. The floor is the thinnest part of the plate and it is usually also a stiffness critical member.
- Chamfer the hole entry. It guides the screw and stops the first thread being torn out on a cross threaded assembly.
- Give a blind hole run out space. A tapped depth that equals the drilled depth means the tap bottoms. Drill deeper than you tap, and say both numbers on the drawing.
- Do not put threads where the part flexes. A thread in a member that bends will loosen regardless of thread locker.
- Think about the driver. A recessed hole that only a stubby driver reaches is a hole that gets assembled badly in the field.
Anodise and threads
Anodise grows on both thread flanks. A type II coating at 10 microns is usually fine. A type III hard anodise at 50 microns can stop a screw entering a tapped hole entirely.
Three options, all of which belong on the drawing:
- Mask the tapped holes.
- Tap after anodising, accepting bare threads inside a coated part.
- Use an insert sized to fit after coating.
This is one of the most common reasons a first lot of anodised plates arrives and half the screws will not start. More on the growth numbers in anodising UAV parts.
Plastic parts are different
Cut threads in POM or nylon strip much sooner than in aluminium and they wear every time. On a plastic part that gets serviced, use a heat set insert, a moulded in insert or a through bolt. PEEK holds a thread better than POM but it is still not aluminium. See engineering plastics on a UAV.
What to tell the shop
- Thread size, class and depth, and the drilled depth separately for blind holes.
- Whether a hole takes an insert, and which insert, by part number.
- Whether tapped holes are masked, tapped after coating, or sized for an insert.
- Which holes are serviced in the field, because that is the list that should have inserts.
That last line is not a normal drawing note, and it is the most useful one you can add. It tells the shop, and anyone reviewing the design later, which threads are consumables and which are not.
Send the model with the fastener list through the RFQ page and we will flag any thread that does not have the engagement it needs, before the lot runs, with the quote back within 24 business hours.
Frequently asked questions
How much thread engagement does an M3 screw need in aluminium?
As a working rule, one and a half to two times the diameter in aluminium, so 4.5 to 6 mm for an M3. Less than one diameter is a thread that will strip, and adding more than two diameters adds almost no strength because the load concentrates in the first few turns.
Are helical inserts worth it on a drone?
On any hole a technician opens repeatedly, yes. They cost a little in weight and assembly time and they turn a consumable thread into a permanent one. On a hole assembled once at the factory, a cut thread in enough material is fine.
Can you tap a 2 mm plate for M3?
You can, and it will strip. Either thicken a boss around the hole, use a press in or self clinching insert designed for thin sheet, or move to a through bolt and a nut.
Need this part quoted?
Send the model and the quantity. Price and lead time come back within 24 business hours.