First post on here!
I’m creating a piece that requires a M12 threaded rod driven permanently into some endgrain on Japanese cedar (soft), which will in turn be used to attach a removable part.
I experimented with drilling a 10mm hole, then tapping the endgrain (yes, I know!) and glueing in the rod. However, because the tapping (predictably) churned up the hole so much the rod ended up being a bit loose in the hole.
So, on my next test I threw caution to the wind, drilled a 10mm hole, then drove the threaded rod straight into the hole, with a bit of wood glue to hopefully provide enough resistance to stop it from being turned inside the hole when turning bolts etc on the rod. It went in beautifully, nice and snug, but with a fair bit of resistance and creaked a little on the way in.
My question is, is that 12mm diameter rod now applying too much outward pressure to the piece? Would you think it would crack sometime down the line, with changes in humidity etc?
Want to get some opinions before I risk it on the final piece. Right now thinking about going out an buying a bit for an 11mm hole, just to give the wood a bit of breathing space.
Any thoughts would be much appreciated.
Replies
I would be concerned about splitting. The piece is under pressure and over the course of a year with typical wood shrinkage, I would expect it to split.
I would personally drill a 12mm hole and epoxy the rod in place. You may also be able to drill an 11mm hole and screw the threaded rod in place but special care will be necessary to not mar the exposed threads.
The article by Tim Coleman describes how he used threaded rod and epoxy to connect legs to a bench. A quote under a photo of him drilling the hole says, "Holes for the rod. Using a handheld drill
with a bubble in the handle and a shopmade
depth stop, Coleman cuts the four holes for the
threaded rod. He uses a brad-point bit sized to
make a hole the rod will slide into easily."
https://www.finewoodworking.com/2020/10/14/285-threaded-rod-and-epoxy-a-versatile-option-for-leg-joinery
If there is a way to hide it, another option is to drill and insert a large dowel (or square piece) of a much denser wood into the face of the piece where the rod will screw into it. It's pretty easy to tap dense woods like maple or beech with bolt type threads across the grain. I also use this when using lag screws into end grain, like into the ends of pedestal table legs. I can insert the beech dowel into the bottom of the leg, where it will never be seen.
If you're worried about splitting, jere is what I would do.
Simply drill a 12mm hole to proper depth and epoxy in place. If tou're also concerned about it rotating, you can file off the threads on one side of the rod to create a flat before you epoxy it in place.
Modern epoxy is incredibly strong, I use it on all my turning tool handles which are subjected to many different forces and they have never failed.
I have done that for knock down cabinets in endgrain. The threads on bolts and rods are not suited for wood, you would be better with inserts made for that purpose, they have external threads that are designed to cut deep in the wood and the internal threads to suit bolts and threaded rods.
https://www.leevalley.com/en-ca/shop/hardware/jig-and-fixture-parts/100589-1-4-20-and-5-16-18-brass-insert-nuts?item=00M9401
How thick is the board you are boring into the end grain of, and how close to the edge of the board? Will you be bearing down heavily to tighten the removable part, or just spinning it on and off? Tough to really give a solution to a challenge we do not fully understand.
Using the screw-in inserts to take a threaded rod, as referenced by another poster, is the strongest option. The Lee Valley ones are typical of those found in North America - with a slot to screw them in employing s bladed screwdriver.
The European type, with a hexagonal centre to take an allen key, are much less prone to damage from the screw-in tool, especially if the thingy is brass rather than steel. You can buy a special tool to screw in the slot variety. It generally contains the end of the thingy as you screw; and has a blade exactly matched to the slot.
But what you're screwing into is also a limitation to the strength of such a joining technology. As another mentions, if there's insufficient wooden "meat" around the hole, there's a greater chance of it failing. The metal screw-in thingies reduce the risk - except when you're first screwing them into the newly-drilled hole. The self-cutting outer gripping threads can cause the hole to fail. Once in successfully, they do add resilience.
The type of wood also matters. You're screwing into cedar, which is quite lightweight and also brittle. More chance of a failure than if you were screwing into a denser and more elastic timber.
Lataxe
Thought I’d replied to this but looks like my reply had just got sent out into the ether.
Thanks for all the suggestions!! The reasons why I was tempted by the snug fit were 1. Accuracy. As long as I drill a nice straight hole, and the hole is a little too small I can be sure that the bolt is going to be centred and square. If there’s any slack I was imagining a lot of bother getting it in the right place. 2. To be honest I think I’d forgotten about good ‘ole epoxy! Was using this odd wood glue which is formulated to also work with metal, supposedly. Epoxy would be better though.
I think what I’m going to try tomorrow is a hole which is just a tiny bit small, so it’s automatically accurately placed, with a smear of epoxy to keep it fixed in place.
Wooden inserts are a great idea, however the project’s already got way more complicated than it started out, and would rather not add any more steps. Definitely will be using that in the future though.
Metal inserts, I’ve used quite a bit in the past. Love them. Haven’t been able to find ones big enough, in the style with an outer thread (just ones with a flange and pins, that are hammered in, wouldn’t work in this situation). The rod also doesn’t need to be removable this time. They are great though.
I tried to keep my post as short and simple as possible, but yes MJ, I should have included some details like the use and thickness of the board, as that makes a big difference. Got only a cm or so on each side, so a bit tricky, and will be taking some fairly fierce use. Going to have to be careful and do lots of tests before doing the final thing. It’s a bit tricky to explain what it is as it’s a bit of an odd one. Maybe I’ll upload a new post of it finished when it’s done!
Thanks again for all the advice everyone. Such a big help.
As Lataxe says, the screw in inserts are the strongest totally hidden solution. And they still rely on threads in end grain. The strongest solution is a captured nut nestled in a snug mortise, with the mortise plugged with a piece of matching wood. Almost invisible, and really strong. I've used this to attach bed rails to headboards/footboards.
When using the threaded inserts, it is helpful to chamfer the edge of the hole before inserting the inserts. It makes starting them straight easier, and prevents the thread lifting a chip of wood that keeps the parts from fitting tightly together. To insert them, I use a long bolt (that threads into the insert's inner threads) with two nuts jammed together with enough thread sticking out to thread well into the insert. The long bolt really helps get the insert started straight as well, compared to a screwdriver. And when inserting with a screwdriver, the part that creates the screwdriver slot on the brass ones can break off, leaving you with a challenge getting them out. Don't ask how I know that!
Those are some very nice tips. You’ve done this before! I’ve had loads of problems getting them straight, and with taking chunks of wood out on their way in. I normally post a bolt through the other side (if it’s accessible), put the insert on it, and pull it through the whole while screwing it in with a screwdriver. But that only works when the other side is accessible, which a lot of the time it isn’t. And the head strips so easily, as you say. I’ll be doing it your way next time.
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