helicoil on Aluminium 4

[scander]

I must be missing something obvious here but what extra benefit will a helicoil on a standard 5000 or 6000 series aluminium give (or any other insert that is simply tapped on a weaker material for that matter), compared to tapping directly on the AL plate?
In both cases you have a connection consisting of a strong steel thread (either the screw itself or the external thread of the helicoil) against a weaker aluminium thread isn't it? with the helicoil you just "push" the problem to a similar connection of slightly larger diameter. There surely must be another benefit apart from the slightly extra load that the larger diameter on the soft surface can accept for which to opt for helicoils on aluminium?

I can see the gains on inserts that employ a stronger, permanent connection to the weak material (pressed or glued strongly) but ones that simply thread into it? why?

 

[MintJulep]

That change in diameter is a pretty big deal.

Also, for things that get taken apart and reassembled, wear and galling.

 

[SwinnyGG]

The change in diameter is the entire point.

For example- for an M12 Keensert, the installed thread is M16. The difference in load capacity of M12 female and M16 female is between 30% and 50%, depending on material and depth.

The benefits to thread wear and galling as mentioned by MintJulep are also significant if an assembly is non-permanent.

 

[scander]

Cheers. I hadn't looked into the exact dimensions of inserts so I thought their increase in diameter was minimal and not enough to offer a significant advantage that will justify the extra cost and assembly time. 30 to 50% is pretty big though I agree.

The wear/galling improvement is an obvious benefit on connections that are designed to be reassembled several times so that feature alone could justify the need to use them.

 

[Tmoose]

Keensert threads are larger than Helicoils to repair the same tapped hole.

Sometimes that is no problem whatsoever.
But, This can be a significant disadvantage when I have been dealt a damaged tapped hole in a boss etc that results in minimal "wall" thickness, or dealing with a blind tapped hole that must not "break thru".

https://www.motorcyclecruiser.com/s...ped-threads-teaser.jpg?itok=JhfistMM&fc=50,50

https://www.youtube.com/watch?v=r7LU3sM_btk

=====.=========

The tap drill for an M12 Helicoil repair is close to Ø12.5 mm. The tap drill for a new M14 thread is 12.0. So the Helicoil is likely roughly equal to or maybe even a bit stronger than an M14 would be in the parent material, after making adjustments for reduced engagement length of the 14 mm tapped hole.

As the screw/bolt Ø changes, the relative strength of the Helicoiled hole changes too.
I'm still working to understand these tables.

http://www.olander.com/contentonly.aspx?file=images/Tensile_Strength_HC68-2_rev4.pdf

 

[SwinnyGG]

I'm still working to understand these tables.

They are confusing at first, but once you learn how to read/use them they become extremely useful for quickly making decisions.

The purpose of the tables is to tell you what the ultimate load of your assembly is based on base material and helicoil insert length for a given fastener size, so that you can select a length which creates the failure mode you want (almost always bolts failing before the insert).

The solid angled lines plot insert pullout load vs. parent material shear strength. Each available insert size has its own chart. Each available insert length has its own line on each chart.

"X Dia" equals insert length versus nominal diameter. So an M12 3 Dia insert is 36mm long. M12 1.5 Dia insert is 18mm long, etc.

Vertical dashed lines show a few common base material shear strengths, as a reference.

Horizontal dashed lines show common bolt tensile strengths, again as a reference.

So, if I'm selecting an insert for a design...

1) Draw a horizontal line at the tensile strength of the chosen fastener (or use the one that's already there)
2) Draw a vertical line at the shear strength of the base material (or use one of the ones that's already there)
3) Starting from the point where those two lines intersect: If I want my screws to fail before the insert, follow the vertical line up until it meets a solid, angled line. Use the insert for that line. If I want the insert to fail first (which you almost never do...) follow the vertical line down to the next solid angled line and use that insert instead.

 

[drawoh]

scander,

If I am tapping a hole in aluminium and all I am worrying about is shearing the threads, I can make the hole deeper, and call up a longer screw. I have designed some structures out of 1/4" aluminium 6061[‑]T6 plate where I called up 8[‑]32UNC screws in preference to 6[‑]32UNC screws and inserts. The structure was intended to be permanent, I wanted the bigger screws and I did not want thicker plate.

Aluminium threads do not withstand repeated use. If your device will be taken apart frequently, I strongly recommend thread inserts.

--
JHG

 

[BUGGAR]

I'm designing some race car spindles. I have to thread a fastener into aluminum so I'm using a steel stud threaded into a helicoil insert in the aluminum. Next design I will design a through-bolt instead.