Tapping Threads in Wood?

[Mike Goetzke]

Mike I feel that in order to give you better advice, a quick run down on what you are actually considering constructing. A quick sketch on a piece of paper will be helpful. Perhaps an estimate on how often this thing would be assembled/disassembled as well

Ron

Ron - I'm building my second crib. On the first one I used EZ-Lok inserts. Just wondering if there is a better method 8 years later.

I just did an experiment using a 1/4-20 bolt as a tap and was amazed how strong the threads held. I drilled a blind hole about 1/2 way into a 1" piece of ash and used my impact driver. The screw never stopped so I thought the threads were stripping but to my surprise the end of the bolt broke through the board and the threads looked great!

Reason I thought of Domino connectors is that they would be MUCH easier to access to tighten. When bolts are used you need to fight with the maximum distance between slats to access the bolt head.

 

[tsmi243]

Mike I feel that in order to give you better advice, a quick run down on what you are actually considering constructing. A quick sketch on a piece of paper will be helpful. Perhaps an estimate on how often this thing would be assembled/disassembled as well

Ron

Ron - I'm building my second crib. On the first one I used EZ-Lok inserts. Just wondering if there is a better method 8 years later.

I just did an experiment using a 1/4-20 bolt as a tap and was amazed how strong the threads held. I drilled a blind hole about 1/2 way into a 1" piece of ash and used my impact driver. The screw never stopped so I thought the threads were stripping but to my surprise the end of the bolt broke through the board and the threads looked great!

Reason I thought of Domino connectors is that they would be MUCH easier to access to tighten. When bolts are used you need to fight with the maximum distance between slats to access the bolt head.

I was gonna suggest that in a really hard wood, the bolt itself will pretty much be it's own "form tap".  If you don't anticipate needing to un-do the bolt by hand, that'll work great.

For jigs and stuff where you DO want to thread in/out by hand, I found that metal taps always made loose threads, but a bolt with a groove cut into it (Dremel time!) will cut threads that are just about perfect. 

I've also found that

 

[mino]

As has been stated, the main thing is how many times it will be disassembled.
...

This.
One-time use is fine, anything beyond is questionable. A couple times may be possible really only a high-quality threaded rod/bolt which does not damage the wood. A casual threaded rod is just too low quality to not destroy the wood thread almost immediately.

2) a long is thread inside is needed, at least 2" at the absolute minimum, ideally 3"+
3) some compression is needed, so if using a "metal" tapping tool, only the first (from a three piece set) should be used. Using a cheapo one-pass metal tapper will make the hole too big/wobbly and too short at the same time.
4) If doing this in softwood, no tappin is needed, actually it nees to be avoided - the screw is just screwed into a hole with the right diameter and makes its thread.
5) for any side-force, you need a very strong bolt. The bolt needs to be as strong as to not deflect - to be the strong point.
6) use increased strength (8.8, 10.9 is better) bolts/rods, they have a better quality thread and are more resistant to bending

In any case, I would not use this for knock-down assembled by customer - when tightening a bolt directly in wood one must not be overtightened even one bit else the thread is stripped and all is lost. It is unlikely you can guarantee the customer, or worse, the installer, to not mess it up.

Practice:
I commonly use M16, M20 and M24 for simple adjustable feet for (not only) shop furniture. Anything below M16 (5/8") I would not even entertain, and even M16 is cutting it close for lateral forces.

 

[Michael Kellough]

Long time ago on a job site I made a knock down router table from melamine coated particle board.

The top is attached to the sides with 1/4-20 flathead screws. The screws thread into 3/4” poplar dowels going through the side of the melamine (cross dowel concept). The grain of the dowel is perpendicular to the screws so less chance of splitting and maybe better integration with the thread. That gives the screws 3/4” of real wood side grain and that allows making the screws tight. I ran the screws into the dowel without a tap, probably into a slightly oversized pilot hole). I’ve set it up a bunch of times and the screws remain serviceable.

Today I’m resetting part of a balustrade that a stair guy did a poor job installing. The spindles attach to the rail with dowel screws (about 5/16” double ended wood screws). Had to take the spindles off the rail and the dowel screws are about ten times more secure in the side grain of the red oak than in the end grain. Making a further eight turn to get the square spindle correctly oriented probably breaks most of the female wood thread right away.

 

[Crazyraceguy]

[member=44099]Cheese[/member] Sure I can explain it, but the pic might be easier.

I "assemble" the rectangular holes, never cutting them from a larger piece. This is mostly because of the rabbet along the top edge of the bottom rail. The rabbet is always the same, 3/8" x 3/8". Then the bottom edge of the panel gets a 5/16" x 1/2" rabbet, both on the router table. This gives a 1/8" reveal at the bottom edge. The panel itself is 1/4" smaller in the width and 1/8" in height, leaving the appearance of a 1/8" gap everywhere.
There is a 4" wide 3/4" ply strip backing the vertical seam, centered to expose 2". This gives the panel something to land on and closes the opening, plus providing a screw location. This is one of the standards that I worked out with the engineering team, years ago. They know that this is what will happen, so they don't have to bother with dimensioning it on the drawings. These are some of the things that the apprentice needs to learn though, for exactly the same reason, it's not on the drawing.
I drill a 5/16" hole 3/4" x 3/4" away from the upper corners, for the screws. I don't recall the brand name of the screws I use, but they are very similar to the "Powerhead" screws from FastCap. They have a thicker shank/threads though. The FastCap ones are something like a #8 woodscrew? The ones I use are 1/4" with a much more coarse thread. I predrill for them at 3/16" and they drive right in.
The edges of both pieces get painted black in most cases, some get painted white, but that is rare.

As far as the epoxy idea, How would you get the screw back out? Wouldn't the epoxy stick the screw in place too? I can't see the epoxy staying in place well enough w/o the screw being inserted while it hardened? Mould release on the screw and hope?

 

[Svar]

As has been stated, the main thing is how many times it will be disassembled.
...

This.
One-time use is fine, anything beyond is questionable. A couple times may be possible really only a high-quality threaded rod/bolt which does not damage the wood. A casual threaded rod is just too low quality to not destroy the wood thread almost immediately.
2) a long is thread inside is needed, at least 2" at the absolute minimum, ideally 3"+
3) some compression is needed, so if using a "metal" tapping tool, only the first (from a three piece set) should be used. Using a cheapo one-pass metal tapper will make the hole too big/wobbly and too short at the same time.
4) If doing this in softwood, no tappin is needed, actually it nees to be avoided - the screw is just screwed into a hole with the right diameter and makes its thread.
5) for any side-force, you need a very strong bolt. The bolt needs to be as strong as to not deflect - to be the strong point.
6) use increased strength (8.8, 10.9 is better) bolts/rods, they have a better quality thread and are more resistant to bending

In any case, I would not use this for knock-down assembled by customer - when tightening a bolt directly in wood one must not be overtightened even one bit else the thread is stripped and all is lost. It is unlikely you can guarantee the customer, or worse, the installer, to not mess it up.

Practice:
I commonly use M16, M20 and M24 for simple adjustable feet for (not only) shop furniture. Anything below M16 (5/8") I would not even entertain, and even M16 is cutting it close for lateral forces.

That's because you probably use those M16+ in end grain in softwood. My experience is that smaller machine screws hold very well perpendicular to the grain, comparable to wood screws and are good for disassembly many times. The only concern is it is easier to cross thread when starting, hence I wouldn't use those for a random customer.

P.S. FW endorses using machine screws in knock down furniturehttps://www.finewoodworking.com/1997/10/01/threading-wood-for-machine-screws

P.P.S. Basically my experience is consistent with this experiment, although CA glue was a poor choice (too brittle):

 

[Packard]

Mike I feel that in order to give you better advice, a quick run down on what you are actually considering constructing. A quick sketch on a piece of paper will be helpful. Perhaps an estimate on how often this thing would be assembled/disassembled as well

Ron

Ron - I'm building my second crib. On the first one I used EZ-Lok inserts. Just wondering if there is a better method 8 years later.

I just did an experiment using a 1/4-20 bolt as a tap and was amazed how strong the threads held. I drilled a blind hole about 1/2 way into a 1" piece of ash and used my impact driver. The screw never stopped so I thought the threads were stripping but to my surprise the end of the bolt broke through the board and the threads looked great!

Reason I thought of Domino connectors is that they would be MUCH easier to access to tighten. When bolts are used you need to fight with the maximum distance between slats to access the bolt head.

I was gonna suggest that in a really hard wood, the bolt itself will pretty much be it's own "form tap".  If you don't anticipate needing to un-do the bolt by hand, that'll work great.

For jigs and stuff where you DO want to thread in/out by hand, I found that metal taps always made loose threads, but a bolt with a groove cut into it (Dremel time!) will cut threads that are just about perfect. 

I've also found that

Confirmats can allow for disassembly and reassembly and they offer a significant amount of structure (probably more structure than a 3/8” dowel).  They are specifically made to be assembled and disassembled.  However I have never seen any data that says how many cycles they can go through.

They are faster and easier to use than E-Z locks. 

I use them in conjunction with dowels and in place of clamps.  I assemble in the shop with no glue, then break it down and reassemble on site with glue (and no clamps) with the Confirmats.  You do need a special drill bit, but no special jigs or fixtures.  The Confirmats have inserts that “hide” their appearance.  I don’t like the appearance of those inserts and I never use them.

I used the EZ-locks for a router plate insert so that I could level the plate by adjusting the screw. 

But if the entire assembly is going to get glued, I would skip the EZ-locks.

 

[Crazyraceguy]

The situations where I use metal inserts, they are not intended to be hidden. I like them for adjustable feet on table legs and mounting metal legs to wooden tops.

 

[squall_line]

The video that [member=15585]Svar[/member] posted from Suman is the one that I've seen on this topic.

I'm going to say that, for something like a crib, it may not necessarily be how many times it's assembled/disassembled (potentially at least a dozen times to change rooms or move houses), but also the type of stresses the particular joint is subjected to by the activities of a toddler, especially if it's intended to be a crib that converts into a safety-rail toddler bed or trundle/day bed later in its life.