Hello everyone,

There is no set way to do it as it varies with design.  ideally on a straight forward sheet you would need to consider the load direction and place the metal plate such that the load is spread over as much carbon as possible.  Often this is why they are fine to bond on afterwards. 

Positioning is the hard part as you rightly say it would be necessary to locate it somehow so it cannot move during the laminating process. If the nut is sticking out into the mould surface, then a simple recess for the nut would stop it moving. Or you could fabricate a piece of strip to go across the mould which is bolted or secured firmly in a set place. You can then have pre-drilled holes for you to put a bolt into then wind it into the insert thread. Thus the inserts are held in place by bolt and in turn the strip. 

Plenty of other ways with a bit of imagination and design specific to your mould. However in all cases ensure that any mechanism to hold the insert doesn't interfere with the bagging stack.  Most "off the shelf" inserts will have a perforated load spreading plate for better bond which will usually be fine as there is a route for infusion resin to flow.

Also consider using wax to seal the thread of the insert so it is not blocked with resin during the process.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I should think the insert I'm the picture is fitted after.

the cf would be drilled and then the insert is actually loose fitted and then crimped with a tool similar to a press stud used in the clothes industry.

Similar fixings are used on the motorcycle industry to put fixings into ally frames, swing arms etc.

I have actually conducted several tests of bonding in and laminating in metal fasteners (made from Grade 5 titanium) for a small shipyard that builds CF pre-fab parts for medium sized vessels.

I can tell you that there is a very small advantage in strength of the fasteners binding to the actual part, from laminating it inside (vac-bag) or infusing it into the CF laminate.  It is simply only cost effective in terms of the work you need to put into it, unless you need to produce a fair amount of copies of the same part.

Bonding the fasteners in with patches of cf fabric laminated on top retains 90 -95 % of the strength of an "infused in" fastener.

Bonding in a simple standard nut will not be very strong, its much better to have some threaded inserts made, that have a larger surface area to bond to and to spread the load over a larger area of the cf laminate.

I can give an example:   an M5 threaded insert machined as a flat disc 18 mm in diameter has more than 600% higher strength, compared to a standard M5 nut laminated into the CF sheet.

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In the past I've scored the surface of some small bits of sheet metal and then put that in the stack.  Once cured I drill and tap into the piece of sheet.  How effective the scores are for the resin I'm not sure but I had no problems.  This way you don't need to be ultra accurate with your insert placement.

How thick a piece of metal are you using? I always think of sheet metal as being like 0.5 to 3mm thick, which isn't really enough for fasteners... Depends what loads the fastener sees, but the rule of thumb is you need at least 4 turns of thread for the fastener to be doing anything. 

Well .. thickness of the metal insert with relation to the strength of the fastening depends entirely on what size of threading you use.

There are very good resources for calculating the force you can apply to such and such a length of threading of a certain size .. the "ballpark rule" of at least four turns of a given thread is probably not far off for high tensile materials like Hard aerospace alu alloys, steel and titanium..  Bear in mind that the tightening torque of the nuts and bolts SUBTRACT from the available strength... this is also given in various tables, some of which can be found on the internet.  A lot of people seem to forget that tightening the nuts and bolts do NOT make for a stronger "joint"  It just helps to keep the bolt from undoing..

The force that a given thread can hold also depends very much on the thread tolerances, and proper sizing of the holes that you thread.  Too small a hole drilled in Titanium WILL break your tap, even slightly too large a hole drilled in aluminium will produce a weak thread.

The rule I've learned is: for softer alloys the thickness of the fastener has to be twice the drill size for the thread (for standard Metric threads)  Metric Fine and UNF / UNEF can go down to 1 - 1,5 x for thickness.
For harder alloys the rule says:  1:1  i.e. : 5mm Metric requires 4,2 mm thickness to keep the inherent strength.

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Apart from when ive been laying up prepreg tooling ive never put inserts into the laminate stack. Its always been the case to cold bond in inserts after the composite has cured and usually post cured. A decent structural adhesive will create a bond that will not fail before the laminate, but you must ensure that the metallic insert is properly abraded and cleaned afterwards, the same with the composite.