mounting screws in cored materials

[schoonerdog]

Our boat has cored fittings throughout (divinycell) to save in weight. Hinges and other fittings were drilled and screwed into the cored fiberglass. However the foam core doesn't present a very good material for screws to firmly bed and sooner or later the screws come loose. I've tried a few things now, including sheetrock screw anchors (too wide for placing into thin doors) and drilling out a larger hole, filling in collodial filler mixed with epoxy and then redrilling for the mounting screw (the hardened mix though often shatters with the screw as it's very brittle). What do others do, perhaps teak plugs?

[ssullivan]

As with nearly all through-the-hull projects, the secret is to use bolts and backing plates. Forget trying to anchor your screw into the cored hull. Not only will you end up with water in your core possibly, but you will always have to re-drill and fit a larger screw as time goes on.

Do it right this time and forget about it for the life of the boat. (Make sure to seal out water too)

[GordMay]

Originally Posted by ssullivan

... Do it right this time and forget about it for the life of the boat. (Make sure to seal out water too)

1. Screws should never be used as a structural fastening on a boat.
2. Screws will not hold (more than a label/nameplate) in fibreglass.
3. A sandwich cored assembly (FRG-Foam-FRG), is, for fastening purposes, as fibreglass (see 2 above).
4. A veneered fiberboard, chipboard, and/or particleboard (cabinet door?) is, for fastening purposes, as fibreglass (see 2 above).

As Sean indicates; use a bolted fastening wherever possible, and a structural adhesive (5200) where nuts & bolts not practicable.

[Sailorguy]

As others have noted use through bolts w/ backing pads wherever possible. In addition its very desirable to remove the coring around the fastener and replace with a material which will resist water penetration, such as west systems epoxy mixed with high density filler. This also resists being crushed when torquing the fasteners and adds greatly to the rigidity of the deck surface surround the fastener. The decks on Kristali are balsa cored, to remove the coring around a 1/4" bolt I first drilled the 1/4" hole, then inserted a tool (nail bent 90 degrees, short end = 1/2"), insert long end into drill pass short end through hole and turn with drill until material has been removed. A vacuum placed on the underside will remove the material as the tool loosens it. This process creates a 1" diameter cavity surrounding the fasterner in which the epoxy mix is worked into. Allow to harden and redrill. I have not tried this with divinycell and don't know if it will work in a similar fashion. Possibly you can adapt this idea and experiment....good luck.

Steve

[Microship]

The bent-nail trick works well with Divinycell. I use compressed air and a pair of hemostats to get the chunks out, then fill the toroidal space with thickened epoxy. If it is a vertical surface, tape over the holes and drill smaller hole at the top to allow filling without bubbles using a syringe (an extension tube helps a lot to fill from the bottom), then re-drill the fastener hole when it is well set. It is also sometimes useful to cast threads directly into this material, but only for fasteners that won't be cycled too frequently and are mostly loaded in shear. To do that, wax the thread, fixture it in place, then fill the annular ring as above.

The key is engaging both skins and avoiding air spaces. A backing nut/plate is ideal if the loading requirements demand it, but not necessary for lighter jobs.

One of my favorite tricks is to bond in a proper fastener, essentially adding stainless threads directly into a cored composite. I like to use PEM nuts, the long kind, grooved on the grinder or with a dremel so they have tooth. But you can even do it with stacked hex nuts (not jammed); just rough 'em up, thread onto the waxed male, fixture in place, and cast filled epoxy around the whole mess using a bonding filler (not fairing... you want it hard and abrasion-resistant, not particularly sandable).

Cheers,
Steve

[delmarrey]

Sometimes pictures help. But there is a routine to the whole process.

My procedure is to drill the upper layer the size of the prospective fastener down thru the core to the inside layer and stop.

Next I use a cut-off allen wrench mounted in a drill motor because they are heat treated and resist bending better. By spinning the allen in the hole it breaks up the core leaving a hollowed out area. I use a vacuum and a pick to get out the waste material.

Then mix up epoxy with a structural filler (I use West systems 105/205 w/ the 404 filler). With a syringe suck up the mixture and then inject it into the hole from the bottom up.
There will be micro bubbles from the mixing and to keep that to a minimum use a large shallow bottom container like a cottage cheese container, and tilt it over to one side to create a depth to withdraw from.

After filling the hole it will start to shrink, either from bubbles coming up or just natural shrinkage. About every 5 minutes or so check it's condition and keep it over full. Actually, create a dome over the top of the hole so when it shrinks you just fill the crater.

After it cures, sand it flush with the deck and drill all the way thru both decks with a clearance drill. I like to go 1/64" over for alignment and space for sealer to ooze in.

Nothing to it. After much practice you have your own routine that works for you!

[schoonerdog]

That sounds like an interesting technique. This is for inside cupboards, not outside fixtures, so water penetration isn't the issue. Specifically the biggest issue is trying to get the hinge on the doors itself which are 5/8" thick to get something inside it which has holding power. I like the idea of the PEM nuts, I will give that a shot next.

Originally Posted by Microship

The bent-nail trick works well with Divinycell. I use compressed air and a pair of hemostats to get the chunks out, then fill the toroidal space with thickened epoxy. If it is a vertical surface, tape over the holes and drill smaller hole at the top to allow filling without bubbles using a syringe (an extension tube helps a lot to fill from the bottom), then re-drill the fastener hole when it is well set. It is also sometimes useful to cast threads directly into this material, but only for fasteners that won't be cycled too frequently and are mostly loaded in shear. To do that, wax the thread, fixture it in place, then fill the annular ring as above.

The key is engaging both skins and avoiding air spaces. A backing nut/plate is ideal if the loading requirements demand it, but not necessary for lighter jobs.

One of my favorite tricks is to bond in a proper fastener, essentially adding stainless threads directly into a cored composite. I like to use PEM nuts, the long kind, grooved on the grinder or with a dremel so they have tooth. But you can even do it with stacked hex nuts (not jammed); just rough 'em up, thread onto the waxed male, fixture in place, and cast filled epoxy around the whole mess using a bonding filler (not fairing... you want it hard and abrasion-resistant, not particularly sandable).

Cheers,
Steve

[delmarrey]

Originally Posted by schoonerdog

That sounds like an interesting technique. This is for inside cupboards, not outside fixtures, so water penetration isn't the issue.

Now you tell us You'll still want to drill all the way thru due to the weight of the doors. But no filler needed.

[schoonerdog]

By the way, great pictures. I've rebedded about 4 hatches on my boat and I dremel out the core surrounding the hatch itself as well and fill that with the colloidial filler and epoxy mixture to create a solid epoxy gasket around the entire hatch. It is a pain but I found that my hatches when they leaked would penetrate into the foam from around the gasket. I'm also going to be putting shims underneath the hatches when they are rebedded to insure I've got a good amount of sealant. I hadn't done this before and the weight of the hatch coming down I think removed too much of the sealant around the rim and a few may have to be rerebedded.
I like your technique with the syringe, what consistancy do you make the mixture? It looks far neater than smearing in a peanut butter consistancy mix (which is what I was doing).

Originally Posted by delmarrey

Sometimes pictures help. But there is a routine to the whole process.

My procedure is to drill the upper layer the size of the prospective fastener down thru the core to the inside layer and stop.

Next I use a cut-off allen wrench mounted in a drill motor because they are heat treated and resist bending better. By spinning the allen in the hole it breaks up the core leaving a hollowed out area. I use a vacuum and a pick to get out the waste material.

Then mix up epoxy with a structural filler (I use West systems 105/205 w/ the 404 filler). With a syringe suck up the mixture and then inject it into the hole from the bottom up.
There will be micro bubbles from the mixing and to keep that to a minimum use a large shallow bottom container like a cottage cheese container, and tilt it over to one side to create a depth to withdraw from.

After filling the hole it will start to shrink, either from bubbles coming up or just natural shrinkage. About every 5 minutes or so check it's condition and keep it over full. Actually, create a dome over the top of the hole so when it shrinks you just fill the crater.

After it cures, sand it flush with the deck and drill all the way thru both decks with a clearance drill. I like to go 1/64" over for alignment and space for sealer to ooze in.

Nothing to it. After much practice you have your own routine that works for you!

[schoonerdog]

Sorry! But the hinges are mounted to the side of the cupboard door, not the outer or inner panel (like a regular house door hinge), so going completely through isn't an option...ok, maybe it is if I change the mounting......

Originally Posted by delmarrey

Now you tell us You'll still want to drill all the way thru due to the weight of the doors. But no filler needed.

[schoonerdog]

Number 4, cabinet doors and cabin doors. Sounds like bedding a proper fastener is the trick...

Originally Posted by GordMay

1. Screws should never be used as a structural fastening on a boat.
2. Screws will not hold (more than a label/nameplate) in fibreglass.
3. A sandwich cored assembly (FRG-Foam-FRG), is, for fastening purposes, as fibreglass (see 2 above).
4. A veneered fiberboard, chipboard, and/or particleboard (cabinet door?) is, for fastening purposes, as fibreglass (see 2 above).

As Sean indicates; use a bolted fastening wherever possible, and a structural adhesive (5200) where nuts & bolts not practicable.

[delmarrey]

I use about two tablespoons of filler to one pump of the dispensers or about one oz. just so it's still runny, a bit like cold syrup or cake mix.

At first with the hole already thru I put tape on the back side but that was for holes that already existed and just needed filler..................._/)

[Oceansandmts]

I am using threaded inserts with machine screws for my hinges and latches attached to foam core cabinetry and doors. I am also replacing the core with 1/4" thick solid glass where the inserts will be epoxied in.

If I had to repair I would epoxy in inserts, but would remove some foam core as explained by delmarrey above before gluing the inserts. The more foam you remove the better the bond to the existing structure. I would want the epoxy to bond to the skins, not just the foam core.

I got brass inserts made for thermoplastics at McMaster-Carr online when I couldn't find stainless. Brass should be OK for the interior. I believe there are "aircraft" threaded inserts but I couldn't find what I wanted using Google.

[bill good]

Schoonerdog,
G'day You might try a plug made of PVC rod & glued in with epoxy glue. I use 10mm round for small jobs. If it is not structual just drill into the panel using a drill stop to the depth of the core & not thru the second skin. A screw driver blade or a dremel to clean out the hole. The plug can be drilled & tapped for bolts or drilled to alow clearance for the self tappers. The rod is roughed up to key the glueing. This method allows one person mounting of bits on the boat & nothing seen on the other side. For stronger structual jobs the drilled layer should be glassed over. Also helps where water may be involved. The strength is from the inner being glued & the outside of the plug being glued with the top joined by glass to the other layer. Water proofing by the fact the tapped hole is blind. I am fitting a windlass with a backing plate of 10mm thick block & the whole lot tapped some 40mm & that is strong. Plastic engineering shops is where I got my PVC. Try & test a bit on a scrape piece of composite & let me know how much force you applied before destruction you will be amazed. ps I have been told of the aircraft type which decores then has resin injected thru 2 holes to fill & hold. Not sure about the metal & salt water + expence. All my hatches done without any leaks!!
Regards Bill Goodward

[GordMay]

Use a cross dowel with connector bolt.
Details for Solid Brass Connector Bolts, Cap Nuts & Cross Dowels - Rockler Woodworking Tools

Similarly, I’ve used a simple wood dowel (inserted face to face, about ½" from edge), then edge screwed into it.