Machine screws in wood

[Brian Holcombe]

I see a few threads on this but have not seen anything which points to a guide for drill and tap size. I have a few common taps (1/4-20, 3/8-16 and 1/2-13) and bought the smallest recommended drill bits for each one, however these are guides dedicated to use in metal and the threads seem to leave more clearance when used in wood than I would prefer.

Wondering if there are any useful guides for choosing drill bit sizes for tapping wood?

[Ralph Boumenot]

I've used the 1/4-20 and 5/16-18 metal taps in maple and poplar for years without any problems. They aren't meant to be substitute for a through bolt and nut and not to be constantly threaded in and out. They do hold up reasonably well.

[glenn bradley]

In a word, no; I don't know of a guide converted for use in wood. I use the guides that are geared to metal and just drop one size (or there-abouts). That is one of the luxuries of tapping wood, the precision is much looser.

[Jim Koepke]

What I do is experiment with scrap wood.

For an often used size after the threads were cut and fit well, the piece was cut through the threaded parts, one that worked and one with a larger pilot hole that didn't, labeled and saved for future reference.

jtk

[Thomas Hotchkin]

Brain
I use one size smaller drill bit then recommended, and use paint thinner as cutting fluid. Has worked well for me in hardwoods. If heavy loads are needed, I coat threads with a thin layer of super glue. Tom

[Tony Shea]

Brain
I use one size smaller drill bit then recommended, and use paint thinner as cutting fluid. Has worked well for me in hardwoods. If heavy loads are needed, I coat threads with a thin layer of super glue. Tom

I love the super glue idea, never thought about that one. I have before filled a hole with epoxy and then thread the epoxy. I usually don't get great results on the end grain portions of the hole when threading wood. I use Jojoba oil as my cutting fluid when threading wood. I remove the tap often to clear the chips and just work slowly. I use at least one size smaller than is recommended for metal.

[george wilson]

Commonly,drill sizes for tapping metal threads leave about a 70% thread. Use the next size smaller to get closer to a 100% thread. Using metal threads in wood is not the very best idea. They won't be real strong in wood. What you can get away with depends upon how much thread strength you need for the application.

If decent strength is needed,you can buy screw in brass inserts that have wood screw type threads on the outside,and metal screw threaded holes on the inside.

[Brian Holcombe]

Thanks fellas. I have issue with one project which required screws only into walnut and they seem to be loosening consistently, which brings about my interest in machine screws for the application. They seem to work well enough that Lie Nielsen uses them for their vise installations.

[george wilson]

Are you sure that LN doesn't use the inserts I mentioned?

[Jim Koepke]

Thanks fellas. I have issue with one project which required screws only into walnut and they seem to be loosening consistently, which brings about my interest in machine screws for the application. They seem to work well enough that Lie Nielsen uses them for their vise installations.

If there is a lot of movement or stress at the joint it may be a problem that needs a different solution.

Screws seldom consistently loosen all by themselves. If they do, you may be more in need of an exorcist's help than of a woodworking forum.

Pictures or a description of use may be helpful to those trying to help you.

jtk

[Brian Holcombe]

This is for stair treads and the amount it has loosened is about 1/4 turn over a period of a year. What I've read suggests that machine screws have a higher pull-out strength than wood screws in wood, especially in shorter thread engagements. So the reason I would like to use a machine screw in this situation is so that I can put a bit more tension on the bolt with less risk of pulling the threads.

[Brian Holcombe]

George,

Scroll down to page 7 on this.

http://www.lie-nielsen.com/pdf/TViseInstructions12-28-12.pdf

I also found this:

https://umdrive.memphis.edu/jthieme/www/Using%20Machine%20Screws%20in%20Wood.pdf