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.

[Adrian R2Participant @adrianr2]

If we consider something commercial such as a manifold stud on a lawnmower engine it will typically use a stud with a coarse thread into the base casting, a plain section for locating the attached piece and then a fine thread for a nut. The stud is fitted once into the base, access is gained by removing the nut only.

If you can find a suitable sized stud off the shelf then likely easier than inserts.

[Howard LewisParticipant @howardlewis46836]

Our later marine engines had Aluminium components which, from time to time, needed to be separated.

The parent component was helicoiled.

The helicoil will be say 7 x 1 for a M6 bolt, (9 x 1.25 for M8) so will be stronger in the aluminium than the smaller thread would be.

For longevity, helicoil would be ny advice

Howard

 

[JasonBModerator @jasonb]

A lot will depend on the thickness of the components, a helicoil or stud is not going to be much good if the main item is 2mm thick. In a case like that something like a Rivnut would be better

[Adrian R2]

On Adrian R2 Said:

If we consider something commercial such as a manifold stud on a lawnmower engine it will typically use a stud with a coarse thread into the base casting, a plain section for locating the attached piece and then a fine thread for a nut. The stud is fitted once into the base, access is gained by removing the nut only.

If you can find a suitable sized stud off the shelf then likely easier than inserts.

The only snag with the stud system on machine parts subject to high heat and vibration is if one day the torque required to undo the nut exceeds the torque required to cause the stud to turn.

I’ve met this problem more than once on Honda recoil starter covers which couldn’t be removed to replace a broken pull cord or recoil spring because one or more studs going into the engine block were turning in their seats before the nut could be cracked loose. It then becomes a hacksaw/nut splitter job.

[duncan webster 1Participant @duncanwebster1]

When I worked on military aircraft all threads into ally were helicoiled from day 1, but in this case I’d go with previous suggestion of studs. If at some future date you do manage to spoil one of the holes, then you can helicoil it,  or even tap it next size and make a stepped stud

[noel shelleyParticipant @noelshelley55608]

You have given no idea as to size or thread depth + numbers. I have worked on the LARC XV for 27 years. This is an American military amphibious vehicle working in salt water with an alumimium hull. The threads are helicoiled and though built in 1967 seldom give trouble when dis assembled. The bolts are stainless.  This would be my choice.

A helicoil kit is not expensive and if reliability is the criteria then it matters little as to cost.   Noel.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, even Helicoils can pickup and come out with repeated undoing and refastening, and even pick-up and spoil the thread it’s located in. I would also think studs would be a better idea, or if it’s a thin section use rivnuts as JasonB has suggested. You could use socket head studs, if you can get any long enough,  then a hex key can be used so the studs don’t unscrew when you remove  the nut.

Regards Nick.

[bernard towersParticipant @bernardtowers37738]

Surely it very much depends on the grade of aluminium + size + thread rate ?

[Neil AParticipant @neila]

Where I worked, the general practice for cast aluminium components, whether heat treated or not, was as follows:

Tapped holes for studs were not Helicoiled and the stud pulled straight into the aluminium. It was assumed that a stud was not going to be repeatedly removed.

Tapped holes for set screws were Helicoiled. Set screws were expected to be removed frequently.

In each case the thread engagement length would be 1.5 x D.

Plate and bar materials were usually treated the same as castings. There might have been exceptions.

The OP does not indicate what the actual form of the aluminium being used. Some of the plate and bar materials can be very hard and will accept threads without Helicoiling. You could always start off by not Helicoiling and use the insert as a fallback position should a problem occur.

If a Helicoil fails and the thread is damage, there is a company called Tappex which produces a self-tapping solid insert. The drill size for these is just larger than the OD of the Helicoil tap, this allows a repair without increasing the holes size too much. We used the Ensat H type in austenitic stainless steel. Expensive, but very effective. Saved many expensive castings.

Neil

[HOWARDTParticipant @howardt]

You should also consider electrochemical corrosion when choosing the mating metal.

[elowen01Participant @elowen01]

A lot of good advice here. Personally, I’d be happy tapping the aluminium directly if it’s reasonably thick and the cover is only going to come off once in a while.

If it’s a cover that will be removed regularly, I’d probably use studs or an insert so the aluminium thread is not doing all the work every time. Helicoils make sense in a thicker section, while a Rivnut may be better if the material is fairly thin.

The thread size, aluminium grade, engagement length and expected number of removals are probably the things I’d look at first. I’d also be careful with stainless steel in aluminium if the part will be exposed to moisture, as galvanic corrosion can become another issue

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