Custom Table Tops for Restaurant Table Bases
I’m hiring a local woodworker to make some custom tabletops for me. I’ve specified that I would like them to work with some commercial restaurant table bases that I am buying.
I’d like to be able to take the furniture apart, so I asked if “threaded inserts” would be useful. An insert is a metal socket with external holding threads AND internal machine threads. It allows one to use machine bolts with a softer material like wood. The insert is installed into a pre-drilled hole to a certain depth. The advantage of an insert is that joined parts can be easily unfastened without stripping the screw hole, as would happen if just screwing directly into the material. Inserts are used for making jigs and furniture designed to be taken apart for moving.
We are looking into ways of using inserts with these tabletops. Strength is a concern, so one idea is to use a piece of hardwood glued and screwed into the tabletop, and use the inserts there, but this also increases the height of the tabletop. The ideal height is 29 inches. The alternative is to use the inserts directly in the plywood, which we need to test.
Information about Inserts and Suppliers
Our application is veneered 3/4″ plywood, so we are looking for inserts that are designed for soft woods that take a machine screw.
- McMaster-CARR has the most useful summary about using threaded inserts. They sell a variety of “tapping inserts”
- The general rule of thumb seems to be that for hard woods, use a slotted-drive insert (also called a knife thread insert). They can be quickly installed using a slotted power driver.
- The general rule for soft woods, particle board, and plywood, use a hex drive insert. These have larger external threads to increase gripping power. Can be installed with a hex power driver.
- The internal thread size varies, and can be “standard” or “metric”.
- Installing the threaded insert requires drilling a 90-degree hole to a certain depth with precision. Since I don’t have a drill press, using a drill guide will help with that. To control the depth of the drilling, a drill stop can help with that too.
DIYers use these kinds of fasteners for making jigs and portable projects, particularly ones who are using phenolic-coated plywood or are making A/V cabinets. Makers of electric guitars that have “bolt-on necks” find these kind of fasteners useful. Makers of knock-down furniture also use these kinds of fasteners heavily.
There are two kinds of sizes that appear relevant: machine screws which are measured in numbers from 0-14, and “fractional taps” (also known as “hand taps”) that are measured in fractional inches and threads-per-inch. This is a standard between the US and UK from WWII, UNC or NC for “unified standard coarse thread” and UNF or NF for “fine thread”. The particular parts that seems to be in common use for jigs and have a lot of parts available are the 1/4-20 parts, which are available in various lengths. The wood inserts are
I’m starting to look into suppliers. I’d like to avoid using zinc alloy parts because I hate that metal, though it seems common for plywood and MDF applications.
- Lee Valley Hardware. I like their informative catalog for its above-average selection of high quality and clever tools that never show up in Google searches. They don’t carry a wide range of inserts, but they do sell a system of fasteners for knock-down furniture use, their 1/4-20 Quick-Connect Hardware. It’s neat because it’s a system of parts that work together.
- Rockler sells a variety of threaded inserts. Their hex drive threaded inserts seem to be designed for this application. They also sell an interesting low-profile threaded insert which might be useful.
- McMaster-CARR has a fantastic website that is easy to search and filter. Tapping Inserts for Wood
- Companies named EZ-Lok and StaFast make a variety of threaded inserts and related fasteners. EZ-Lok, StaFast
I have purchased the following sample parts for testing
- E-Z Lok Threaded Insert, Brass, Knife Thread, 1/4″-20 Internal Threads, 0.500″ Length (Pack of 25)
- E-Z Lok Threaded Insert, Zinc, Hex-Flush, 1/4″-20 Internal Threads, 0.512″ Length (Pack of 100)
- E-Z Lok Threaded Insert, Zinc, Hex-Flush, #10-24 Internal Threads, 0.512″ Length (Pack of 50)
- The Hillman Group 57117 1/4-20 x 0.472-Inch Slotted Insert Nut, 12-Pack
- 12K7990 149 pc 1/4-20 Jig Parts Kit
- 00N1630 30mm Regular-Head Bolt
- 00N1113 13mm Plain Insert Nut
- 00N1013 13mm Flanged Insert Nut
Using the inserts
While the threaded inserts can be used with a flat-headed screw driver or hex driver, it’s challenging to drive them by hand without damaging the work. There are a few specialized tools that I’m looking into:
- For use with slotted insert, an appropriately-size “drive tool” can help hold onto the work when used with a drill or drill press. Rockler sells their power drive threaded insert tool [(amazon picture)[http://www.amazon.com/Power-Drive-Threaded-Insert-Tool/dp/B001DT1EY8]), and EZ-LOK’s version is similar. It’s basically a fancy screwdriver head for a powered drill/driver that costs around $9-$30 depending on where you get it from. User reports indicate it’s not the most precise tool, requiring firm pressure to prevent the insert slots from stripping.
- A T wrench for 1/4-20 threads is the hand version of it. It’s relatively cheap at $5.50+$5 shipping, and its advantage is that it screws into the insert and holds it tight. However, the threads are apparently not the greatest quality. Woodcraft and other places also sell some variant of this.
- Cutting down an 8″ eye bolt with 1/4-20 threads would give you rod that would work with an adjustable-chuck drill. It might not be perfectly straight. You’d have to cut-off the bolt part and then at a 1/4-20 nut to screw-down on the insert and hold it in place. An installation video shows one guy made his own driver tool in this way.
- The Groov-Pin company makes a series of inserts called Tap Lok; they sell a threaded hand installation tool that is custom made for $60. You can get them on ebay for half price, which is what I ordered. Once I get it, I’m going to look for a tap wrench that may be useful.
- Here’s a cheaper Heli-Coil 1/4-20 Thread Insert Hand Installation Tool
Drilling a straight hole is also challenge without a drill press. It’s essential to being able to drive-in the insert. This discussion on DIY StackExchange goes over some solutions. The Gator Tools drill guides look like my best bet if I want to buy something, otherwise going to someone’s house with a drill press is preferred.
Drill Press Alternatives
While there are several inexpensive “drill guides” available for aligning a hand-held drills (see this 2008 drilling straight holes without a drill press guide from Rockler), they aren’t generally well reviewed, earning only an average rating as in this Wolfcraft Drill Guide. I ended up buying a 20-year old device called a PortAlign off of Ebay for about $29 including shipping. The PortAlign is all-metal machined and made in San Diego by a long-defunct company, and seem sturdy enough. It came with a 3/8″ chuck already on it too. My preliminary tests seem to indicate it will work.
Some mysteries about drill chucks. The three things to know about a drill chuck is the min/max opening, the mount type, and whether it is keyed or keyless. For small hand-held drills, the min/max is 0 (closed) to 3/8 or 1/2 inch. The mounting type is a 3/8″-24 threads shaft, which is screwed onto the back of the chuck and often also held firm by another screw at the end, accessible inside the chuck with a screwdriver. Keyless chucks are popular. Larger drills can handle larger tool shanks (drill bit diameter), and use a mount called a Jabob’s Taper (JT). It’s an unthreaded precision-machined shaft that has a slight taper/pointedness, and a bit of pressure holds it in place because the more you press down on it, the more tightly it grabs. It’s kind of ingenious!
Anyway, the PortAlign was originally designed to work with your power drill’s original chuck. You have to remove the chuck from your drill and screw it onto the end of the PortAlign, then screw your drill’s power shaft onto the top of it. I have a Panasonic Cordless drill, so I have to remove the chuck. From videos I’ve seen on YouTube, removing the chuck is essentially unscrewing the counter-threaded (righty-loosey screw inside the chuck that secures it to the end of the drill’s drive shaft, then unscrewing the chuck itself by turning it counter-clockwise (lefty-loosey) with the aid of a big allen wrench to provide torque from hitting it with a hammer. The Panasonic has the 3/8-24 arbor (the name for the connecting mount?) so it SHOULD work with the PortAlign.
There’s two operations I need to perform with this rig:
- drilling the hole for the EZ-LOK brass threaded inserts into a 3/4″ bit of hard wood I’m using for the intermediate mounting board.
- threading the insert into the pre-drilled hole straight.
I have just about all the pieces I need, although I ebayed another NOS PortAlign to have for purely aesthetic reasons. I’m curious if the tolerances are tighter on a newer one.