When is a threaded insert preferable to tapping directly into aluminum?

[elowen01Participant @elowen01]

I am making an aluminum component that will need a removable bolted cover.

The cover may be removed several times during maintenance, so I am concerned that directly tapped aluminum threads may wear or strip over time.

At what point is it preferable to use a threaded insert instead of tapping the aluminum directly?

The connection is not heavily loaded, but repeatable assembly and long-term thread durability are important. I am considering thread size, aluminum thickness, thread engagement length, installation difficulty, and whether the insert may loosen when the screw is removed.

I would be interested in practical experience with threaded inserts, helicoils, solid inserts, or direct tapping in aluminum components.

[JasonBModerator @jasonb]

Does it have to be bolted? could you use studs with nuts which would eliminate the problem rather than find a best compromise.

[elowen01Participant @elowen01]

Thanks. A through-bolt with a nut and washer does seem like the best option where there is access to the back side.

The reason I am considering inserts is that some locations may not have rear access after final assembly. For repeated removal of a low-load aluminum cover, would you prefer a solid threaded insert or a wire insert in that situation?

[JasonBModerator @jasonb]

<p style=”text-align: left;”>A stud does not need rear acess. You screw the stud into a tapped hole ONCE, fit the cover and then secure with nuts from the front. So no risk of wear to the tapped hole.</p>

[JohnFParticipant @johnf59703]

This was one of the reasons Helicoil inserts were used on aluminium and particularly so on castings, I would consider it but you will need to purchase the necessary taps and insert tools.

[John HaineParticipant @johnhaine32865]

I think Jason has it right – studs much the best option.  Unless they have to be super strong, cut from suitable size stainless studding (clue is in the name), and finish off external en to look nice. Stainless nuts and washers, loctite the studs into the aluminium body.

[Clive FosterParticipant @clivefoster55965]

Inserts are common is aircraft, military and frequently inspected machinery uses for that very reason. Way back I read an article in one of the trade magazines suggesting that inserts were desirable for smaller threads if more than 10 removals were anticipated in the product lifetime and should be mandatory if more than 20 were expected. The article was technical advice by one of the insert suppliers so obviously some advertising licence involved.

Thread size is clearly important. 3 mm, small BA ad similar are definitely on the risky side if regular unskilled removal is likely. 1/4 Whitworth and up I’d consider pretty safe in a decently hard alloy for lighter loads such as holding a cover against a gasket. For heavier loadings maybe not so much.

One major advantage with inserts is the ability to use a finer thread giving more grip in a relatively thin part. So thread depths similar to a standard nut become safe. Coarse threads, like Whitworth / UNC are reckoned to be less stressful on the part but realistically you need rather more depth than a standard nut. I like double nut depth for 1/4″ Whitworth in alloy. Removing the first thread or two to give a bit of plain bore lead in and protect the opening thread from technical damage is a great help when it comes to clean insertion too.

As ever its a judgement thing.

Clive

[Andrew TinsleyParticipant @andrewtinsley63637]

Helicoils are fine, but when you start adding up the cost of the coils and taps, it becomes less attractive to impecunious model engineers.  Why not make a steel insert, threaded on the outside with the correct internal thread and simply drill and thread your aluminium oversize to take the insert? Secure it with thread lock or loctite and job done.

If you have lots to do then go helicoil, but for the odd one or two, just make your own inserts.

Andrew.

[John Haine]

On John Haine Said:

I think Jason has it right – studs much the best option.  Unless they have to be super strong, cut from suitable size stainless studding (clue is in the name), and finish off external en to look nice. Stainless nuts and washers, loctite the studs into the aluminium body.

Or even acorn nuts for appearance, etc.

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