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My sister recently gave my daughter a toy train set. It's sturdy enough —the axels are thick and strong— but they have screw-in brass eyes and hooks to join the carriages together.

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These only take 5 full rotations before they're out and that seems well within the possibility of something getting thrown around by a young child.

Is there a good way to bond wood to brass, in strong and permanent way?

Running Notes:

  • Cyanoacrylate appears to set too fast while screwing in. It also doesn't withstand much torque. Possibly related. Better than nothing but something that sets slower and stronger would be better.

  • The picture might give the wrong impression but that block is only 15mm (⅝") across. The screw hole is only around 0.5mm (~20thou) in diameter. Whatever we're putting in this either needs work through with the screw or be thin enough to go in.

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    In addition to the problem of a glue bond the screw is also going into end grain and screw threads are notorious for stripping out of end grain with just a little force applied. Your best bet in terms of a glue is almost certainly going to be an epoxy. I'll expand a bit more on this later if someone else doesn't provide a good Answer in the mean time. – Graphus Aug 15 '17 at 12:59
  • If I were to re-do it I'd use an eyelet normally used for shoelaces, like this: cut a slot in the wood using a dremel or an oscillating multitool, push the flat piece in, then screw right trough the hole from the bottom of the toy. – pawel Aug 16 '17 at 15:17
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Wood glue does not bond well to metal. I would either try a thick CA glue or a high-quality epoxy.

You mentioned CA setting too fast, but there are different types of CA glue. You probably have a "thin" CA glue. Try a "thick" CA glue and see if that is still setting too fast. They normally give you 30 seconds to a minute of open time.

My other suggestion would be a high quality 2-part epoxy like West or Entropy. They bond well to both wood and metal, but they are a bit more complicated to work with. In addition to mixing the 2 parts in the correct proportions you also need to add an appropriate filler. Depending on the hardener you use you should have 10 minutes to half an hour of open time.

  • +1 for using 2 part epoxy – ewm Aug 16 '17 at 16:27
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Is there a good way to bond wood to brass, in strong and permanent way?

In this context, no. There's too much available torque to break a glue bond and once you get over the initial resistance you're in much the same position as if you'd applied no glue*.

It would be very different if you were bonding a flat piece of brass to a wooden surface. Even PVA and other glues not known for their ability to bond metal can work surprisingly well in that way.

There are I think only three reasonable candidates for adhesive here, superglue (cyanoacrylate), foaming polyurethane and epoxy.

Superglue
Very strong in a thin layer but doesn't resist shear well as you note yourself and even the smallest rotation of the brass ring results in a shear force being applied to the glue line. While it could be strong enough to resist turning by the fingers of a child one sharp knock would be enough to crack the glue and render it nearly valueless here*.

Polyurethane
Worth trying if you already own a bottle since you lose nothing if it doesn't work well enough.

If you don't already own any and had to buy it for this job though you'd have to make a point of using it up quickly after breaking the seal as it can, and does, go off in the bottle sitting on the shelf — it's not at all uncommon for the great majority of a bottle of polyurethane to go to waste in this way and the glue is not cheap.

Although polyurethane is known for its strength I don't know how secure the bond would be here and anyway I think there's a better candidate.

Epoxy
Your best bet of these three I'm sure. Even a basic 5-minute epoxy is surprisingly strong but generally the slower the setting time the stronger the epoxy.

Slow-setting epoxies tend to be more expensive but at least epoxy has a long shelf life (nearly indefinite IME as long as it is tightly capped) so the unused portion left in the syringes is sure to still be usable somewhere down the line for any future jobs that might crop up.


*I say nearly because almost any glue will give some benefit in that it will strengthen the short wood fibres present in the threading in the hole, as well as acting to fill tiny gaps. So even if the glue bond is broken together these will make the brass screw eye much harder to rotate which is a definite benefit.

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Wood glue. In the hole and possibly around the edges afterwards. The latter looks ugly.

Maybe too late but change the loop to something that isn't threaded and does not turn.

On a side note fasten a piece of string to the loop. Make the knot ununtiable through heating, plaiding, gluing or whatever means. It makes the loop harder to swallow and if it is, there is a chance the string is hanging out. Especially if you fastened something (I am thinking of a wooden ball) to the pulling end of the string. I So if the kid runs around with the toy in one hand and a red ball hanging out of the mouth just pull the string. carefully.

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    Wood glue doesn't bond metal worth a damn in this context. I've reinforced holes with PVA many times and the screws remained approximately as easy to turn as if there were no glue at all in the hole. – Graphus Aug 15 '17 at 12:57

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