One of the challenges in using plywood as a material for projects is fastening to the plywood. While glues, screws and nails (the traditional fasteners) work, many of the newer fasteners developed for the furniture industry are considerably better. The only problem is that they tend to be rather expensive to buy, if you’re not a furniture manufacturer.
One of the features of many of these newer fasteners is the ability to remove them, taking the assembly apart for moving or modifications. While not every project has that need, it is a nice option to have available, when needed.
Another fastener option, which has been around much longer and is considerably less expensive, is T-nuts. These are a type of threaded insert, specifically designed for use with plywood products, especially those which do not have a good wood grain for other threaded fasteners to mount securely into.
What makes T-Nuts different from these other threaded inserts, is that it is the clamp force provided by fasteners running into the T-Nut, which holds the nut in place. Literally, the tighter the screw, the more solidly the T-nut is secured in place, as the head of the T-nut will be pulling on the opposite side of the sheet of plywood.
This makes the T-nut an ideal fastener for many different types of plywood projects. They are easy to install and as long as they are installed properly, there is very little chance of them falling out, without someone driving them out. That little bit of chance is eliminated by the clamp force of a screw tightened up in the T-nut.
Since the only way the T-nut can come out of the wood is to be pulled through it, it is an incredibly strong fastener. T-Nuts installed into soft plywood have a pull out strength of 1378 pounds, while those installed in hardwood plywood have a pull out strength of 1850 pounds. When used with other plywood products, such as MDF and OSB, these numbers will be lower, because of the lower strength of the materials the T-nuts are being installed into.
In many cases the limiting factor in the strength of the T-nut isn’t the nut itself or even the wood; it’s the screw that are driven into the T-nuts. A ¼” diameter #2 grade screw has a shear strength of 260 lbs. while a #6 screw only has a shear strength of 72 lbs (shear strength is the amount of force that has to be applied across the axis of the screw to cause failure, usually breaking or at least deforming).
Even so, it should be noted that it is actually not hard to knock a T-nut out of the hole it is mounted in, if one drives it out with a punch from the screw side.
Because of the shear strength, T-nut sizes should be selected based upon the load which will be put on the screw running into them. When in doubt, go larger, giving more of a safety margin, especially when critical loads are being mounted or the attachment will be supporting the weight of a person.
T-nuts are extremely easy to install. To start with, it’s a good idea to drill pilot hole through both the object to be mounted to the plywood panel and the panel at the same time, clamp the pieces together, so that they can’t slide while drilling. In the case where the piece to be mounted already has holes in it, use the piece as a template to mark the panel it will be mounted to.
The hole in the piece to be mounted and the panel that the T-nut will be installed in will need to be enlarged slightly to allow clearance for the machine screw and T-nuts that will be used for installing the device or part to be mounted. Hole sizes can be determined by holding up the screws and T-nuts to the drill bits, but standard hole clearances for common sizes are:
|Screw size||Diameter||Screw Clearance Hole||T-Nut Clearance Hole|
The T-nut is installed from the back of the panel, placing the barrel of the T-nut into the hole drilled for it. The plywood panel itself should be laid on a solid surface or floor for this installation. Once the T-nut is in place, its flange should be hit with a hammer to drive the prongs into the back of the panel, until the bottom side of the flange is flush with the back side of the panel.
T-nuts should not fall out, if properly installed. However, it is possible for the nut to twist, while being installed. Should this happen, the prongs could bend, rather than being driven into the panel. In such a case, the flange will not lay flush with the back of the panel and the T-nut should be replaced.
Even though T-nuts are secure when installed, they will not reinstall well in the same hole, if they are driven out. The prongs count on the compression factor of the wood fibers to hold them in place. Once the T-nut has been removed, the wood fibers won’t provide as much compression. However, if the T-nut is reinstalled in the same hole, but with the prongs twisted to drive into a fresh point in the plywood panel, they will hold well.
Installing T-nuts Blind
It is possible to encounter situations where T-nuts might need to be installed, but there is no way of hitting the back side of the T-nut with a hammer. This is called a “blind installation.” It is still possible to install T-nuts in this manner. There are two ways of doing so.
The faster, but not as effective way, is to use a pair of channel locks, clamp the panel and the head of the T-nut, to apply pressure to press the prongs into the backside of the wood. This works fine for MDF, but does not work so well with OSB or plywood. It is easy to bend points using this method.
The slower, more secure way of installing them blind is to place the T-nut into the back of the hole and draw it in with a screw. A couple of ashs should be placed under the screw head for this and a lot of torque will need to be applied. It would be better to use a hex-headed screw, so that a ratchet could be used to apply leverage. Care must be taken to ensure that the T-nut’s prongs go all the way into the panel.
Using T-Nuts to Make a Rock Climbing Wall
One very useful place for using T-nuts is in the building of a rock climbing wall. Typically, 72 T-nuts are installed in one 4’x 8’ sheet of plywood, with the T-nuts being roughly 8” apart. This allows for lots of positions to mount each “rock” to, creating a variety of challenges to overcome, while climbing the wall.
A minimum size of ¼” bolts should be used for attaching holds to a climbing wall. Anything smaller just doesn’t have the weight capacity to provide a good margin for safety. Larger bolts aren’t a problem.
Lay out a roughly diamond shaped pattern for the T-nuts, spacing the rows 8” apart and the individual hole in each row 8” apart. Drive T-nuts into the panel from the back side, before mounting the panel. It’s also a good idea to sand and finish all the panels used in the rock wall with varnish, to protect from splintering.
Most people buy their holds, the “rocks” used for a rock climbing wall commercially. However, there are two ways you can make your own.
Making Handholds Out of Rocks
The first is to use real rocks, drilling clearance hole through them for the mounting bolts to go through. Before drilling the holes, clean the rocks and then level the back side by applying a layer of Bondo (a two-part body filling compound used in auto-body repair) thick enough to provide a flat back. Set the rock down, Bondo side down, on waxed paper, for Bondo to cure.
The hole should be drilled through the rocks with a masonry drill bit and a hammer drill. It is necessary to keep the drill bit cool, as heat will cause it to dull quickly. Submerging the rocks in water, while drilling them, is a good way to protect the drill bit.
While masonry drill bits work well for rocks, they aren’t all that great for Bondo, so the final part of the drilling, through the Bondo, may have to be done with a standard twist drill. However, don’t try drilling through the rock with the twist drill, as it will just dull the bit.
Finally, clean up any excess Bondo that squirted out the edges on the belt sander or grinder, grinding the Bondo flush with the edge of the rock, so that it is not very visible.
Making Handholds Out of Plywood
Handholds can also be made of plywood or even scrap dimensional lumber. To start, laminate three layers of ¾” thick plywood together. Scrap pieces of the same sort of birch plywood or marine grade plywood used for the wall itself will work fine. Be sure to spread glue over the entire surface before clamp the pieces together.
Once the glue dries, cut the laminated plywood into irregular shapes; triangular, crooked rectangles, trapezoids and freeform shapes. Almost anything will do, as long as they are large enough to grab hold of and/or use as a toe hold.
Round the edges and corners of those shapes with a belt sander or grinder, making them more irregular. The idea here isn’t symmetry, as real rocks aren’t symmetrical. Be sure not to leave any sharp edges or corners anywhere around the shape, as those could cause splinters or cuts. It is a good idea to grind a roughly ½” wide by ½” groove into the top side of each handhold or to undercut the back edge about the same amount, to act as a grip.
The attachment hole should be countersunk if flat head screw are to be used for mounting the handholds to the climbing wall or counterbored if round head screws or hex head bolts are to be used. Make sure that the countersink or counterbore is deep enough so that the head of the fastener will sit below flush, eliminating any risk of the user’s hands being cut on a burr on the edge of any hardware used to install the handholds. If hex head screws or bolts are used, it is necessary to make the counterbore wide enough not only for the screw or bolt head, but also the socket that will be used to install it.
When drilling a screw hole with a countersink, it is usually easier to drill the clearance hole first, and then the countersink, allowing the clearance hole to act as a guide for the countersink bit. However, when drilling a counterbore, it is usually easier to drill the counterbore first and then the clearance hole. That way, the center indentation in the hole made by the counterbore provides a centering point for the drill bit used to drill the clearance hole.