T nuts/threaded inserts?

[Fred Voorhees]

Gentlemen,

Its been a while. I have a question. I am building a river table...have the pedestal parts completed and am beginning to focus on the tabletop surface. Prepping for the epoxy work soon, but looking beyond that to how I am going to attach table surface to the pedestals. I thought this would be a great opportunity to use those screw in threaded inserts. I think they would offer the strongest option and it would be my first experience with them. I am using ash...which I have found to be a fairly hard, dense wood. I first bought brass 3/8th" inserts. I drilled the prescribed pilot hole for the insert into a scrap piece of the ash and I didn't get far before the thing just stopped and refused to go further. The brass eventually broke. I know there are also steel inserts of the same hardware and I want to try them...maybe with a slightly larger pilot hole to help with the installation...maybe use a bit of epoxy to cement it in. Would this be a good idea? Also, I wanted to avoid them, but how about the T nuts? Much simply installation, but I am concerned about the strength factor of them. I will still use a 3/8th size. I will be using a total of eight bolts - four on each end. Thoughts on either method? Advice?

[Jim Becker]

I would go with threaded inserts (good quality) like Blacktail Studio recommends for this application. Consider using the C-channel he also uses to insure the surface stays flat across the span.

[Tom M King]

I use EZ-Lock stainless steel inserts in 18th and 19th Century window jambs to make the stops easy to put in place, and take out for future work on sash. They're made for metal, and fit in a tapped hole. I darken the slotted heads on brass machine screws, and so far, not a single person has ever noticed them.

These old window jambs are Southern Longleaf Heart Pine around here, and very difficult to drill an accurately placed, and square to the surface hole, so I made a guide for the drill bit that uses metal guides. That shouldn't be necessary for your use, but I wanted the spacing to be the same on all the windows of the same size, so didn't have another good choice.

A shopmade wooden guide should do enough holes for what you want to do. You will need a bottoming tap for stopped threaded holes for the inserts though. A metal cutting tap might not cut perfectly clean holes in wood, but they're good enough. I've had difficulty driving the brass ones made for wood too, and decided they just weren't worth the trouble.

https://www.ezlok.com/e-z-lok-solid-wall-threaded-inserts-stainless-steel

[Fred Voorhees]

I would go with threaded inserts (good quality) like Blacktail Studio recommends for this application. Consider using the C-channel he also uses to insure the surface stays flat across the span.

And that was my first choice. Considering that I need to insert them into ash...if I have trouble installing them into the ash using the recommended pilot hole size, what do you think about stepping up a size for the hole until I can install them completely and use some epoxy to help solidify the insert in the ash?

[Fred Voorhees]

I would go with threaded inserts (good quality) like Blacktail Studio recommends for this application. Consider using the C-channel he also uses to insure the surface stays flat across the span.

Oh Yeah Jim.....I am also going to "submerge" some sort of stiffener in the underside of the wood sections to help in alleviating any cupping or twisting. I was thinking about 3/8 rebar, but I was going to ask questions where I am purchasing the epoxy. That is just one of the questions I will be inquiring about. I am just about ready to sit down and writing down all of my questions because I am almost ready to order the epoxy and any of the other stuff that I will need for the process. Nevertheless, I will be using something to help keep things stiff and flat.

[Steve Jenkins]

When I install threaded inserts I scrape them across a piece of beeswax. They go in much easier.

[Adam Herman]

Oh Yeah Jim.....I am also going to "submerge" some sort of stiffener in the underside of the wood sections to help in alleviating any cupping or twisting. I was thinking about 3/8 rebar, but I was going to ask questions where I am purchasing the epoxy. That is just one of the questions I will be inquiring about. I am just about ready to sit down and writing down all of my questions because I am almost ready to order the epoxy and any of the other stuff that I will need for the process. Nevertheless, I will be using something to help keep things stiff and flat.

I have no experience on a river table, but rebar seems like a poor choice. its low quality steel and it will only hold the epoxy, not the wood and is similar to a wet noodle unless its in something like concrete.

I have used steel angle iron let into a saw cut on the underside of projects with a good number of screws to help support butcher block bar tops over the seating for example, and this has worked out very well. be sure to think about the wood movement when implementing though. easy to "submerge: with just a circ saw as well, no router needed and less wood to remove.

[Maurice Mcmurry]

I made a tap for brass inserts by threading a nut and two inserts onto a threaded rod, I ground them so that the two nested together with the threads lining up. I shaped a taper and cut chip slots with a hack saw and file. It looked like a tap when it was done. I glued everything together with JB weld and put a T handle on the end. It worked well for pre threading the holes (for a while). Eventually the JB weld gave out. There should be real steel taps for those.

A few drops of thin super glue helps lock those in.

[Don Stephan]

"Hard" connections like those being discussed do not accomodate seasonal changes in width of a tabletop. Metal "figure 8's" are an option to do accomodate seasonal movement. Years ago I learned to make wooden "buttons" as they were called, about 1" x 3/4" x 3/4" with a tenon at one end flush with the surface, which would mate with a groove running around the inside surface of the rails. A screw through the button into the underside of the tabletop will accomodate movement if the button's tenon is not fully inserted into the groove. Sorry, I don't have a photo. Experiment a bit if you decide to explore the button approach. I make a dozen or so at a time so I always have enough for a tabletop.

[Lee Schierer]

A neat trick when installing threaded inserts is to scrape the external threads across a lump of bees wax. Then screw the insert into your wood. Paraffin will not work for this and soap is likely to leave a stain on your wood. Any bee keeper in your area will gladly give or sell you a near lifetime supply for very little cost.

[johnny means]

Why use inserts when an equal number of large screws will do the job?

[Curt Harms]

I use EZ-Lock stainless steel inserts in 18th and 19th Century window jambs to make the stops easy to put in place, and take out for future work on sash. They're made for metal, and fit in a tapped hole. I darken the slotted heads on brass machine screws, and so far, not a single person has ever noticed them.

These old window jambs are Southern Longleaf Heart Pine around here, and very difficult to drill an accurately placed, and square to the surface hole, so I made a guide for the drill bit that uses metal guides. That shouldn't be necessary for your use, but I wanted the spacing to be the same on all the windows of the same size, so didn't have another good choice.

A shopmade wooden guide should do enough holes for what you want to do. You will need a bottoming tap for stopped threaded holes for the inserts though. A metal cutting tap might not cut perfectly clean holes in wood, but they're good enough. I've had difficulty driving the brass ones made for wood too, and decided they just weren't worth the trouble.

https://www.ezlok.com/e-z-lok-solid-wall-threaded-inserts-stainless-steel

There at least used to be two styles of threaded inserts. One had two slots in the top and used a large straight screw driver. Another had a hex socket formed into it that accepted an allen wrench. The ones with the hex socket were much better from a driving standpoint.

[Jim Dwight]

I would experiment in scrap with larger holes. Going up 1/64 may make a big difference. I think putting a little epoxy on the threads during installation will ease installation and also avoid it coming out. I find threaded inserts to hold significantly better than t-nuts. T nuts depend on the little prongs to bite in (although there are also ones that take little screws). Threaded inserts are like using a lag screw - which might make a tap if the threads work out. Lots of gripping area in the wood. To drive threaded inserts I double nut them. I put a short bolt with a nut on it into the insert and then tighten the nut against the insert. That lets me use a wrench on the insert so you can exert a lot more torque. But the wood will fail if they are too tight. If you can drive them without lubricant in scrap they should go in smoothly in the table top with a little epoxy on the threads.

[Jim Becker]

I agree with Jim D...do some testing in some of your scraps of the ash with varying the pilot hole diameter. They need to go in very snug but not so much that it takes major effort. Be sure to do the little chamfer on the holes, too...it really does make a difference with getting them to seat flush. And be sure you are using quality inserts, not the crappy zinc ones, but you already know that. :)

[Lee Schierer]

One other hint for installing inserts. Find a hex head or socket head cap screw that fits the thread inside your insert and a matching nut for that bolt. Thread the nut onto the screw, run the screw into the insert so it doesn't stick out of the bottom of the insert. Run the nut down against the top of the insert and snug it up with a wrench against the insert. Then use a socket or hex wrench to screw the insert into your wood. Once the insert is seated use the socket or hex wrench and a second wrench to loosen the nut and remove the screw. This method does no damage to the insert.

[Patrick Varley]

When I use epoxy on the threads, I'll wax the threads/tip of a screw and let it sit in the insert while the epoxy sets. May not be necessary, but it gives me piece of mind.

[Fred Voorhees]

Why use inserts when an equal number of large screws will do the job?

I have always used lag screws...but for this project...a new direction for me as a woodworker... I thought I would go with better mechanical fastening and step up my game. PLUS, they will be stronger than screws in my opinion.

[johnny means]

I have always used lag screws...but for this project...a new direction for me as a woodworker... I thought I would go with better mechanical fastening and step up my game. PLUS, they will be stronger than screws in my opinion.

You might want to test your strength hypothesis. I have and threaded inserts didn't come out on top. Yes, the bolt is stronger than a screw, but neither is like to fail. What fails is the thread to wood connection. Specifically, the wood holding the threads. The limited depth of inserts means the fastener is only bitting into ½"-¾" of material. Meanwhile a 2" thick top can have 1¾" of screw buried into it.

[Mike Monroe]

I used hex socket threaded inserts on the legs of recent dining table bench build. I built two benches for a friend at work. The owner wanted to be able to remove the legs from the benches for easy storage. Legs were 3" x 3" red oak tapered on the inside down to 2" x 2". I cut a small flat on the inside corners of the legs, easing the flat back into corner with a lamb’s tongue. I epoxied the 5/16"-18 threaded inserts into pilot holes drilled into the flat area. I put a small piece of green masking tape over the end of the threaded inserts during installation, so no epoxy got into the inside threads of the inserts. I also experimented with different size pilot holes until I found the Goldilocks fit, not too easy to screw in, but not too hard to screw in either. The owner wanted some rock-solid benches as some of his in-laws are on the large size and the benches would be used for family gatherings. He was impressed on how solid the benches were and the threaded worked great in this application. The trickiest part of the build was matching the color of the commercially stained/toned dining table. That task turned into a week of samples and trying different stains, dyes, and techniques to get the color just right.

[Jonathan Jung]

Rampa 8mm tri-thread inserts are the best. I have tried nearly every brand and type available and these are all I use. If you can't find any online, let me know and I can get you a baggy of them.