machine screws in tapped holes vs. wood screws

[Barry O'Mahony]

Hello,

Lee Valley sells tap and drill bit "kits" for making threaded holes in hardwood for machine screws:

http://www.leevalley.com/hardware/page.aspx?c=1&p=40057&cat=3,41306,41330&ap=1

The claim that in their tests, the machine screws in threaded holes form a stronger connection than wood screws.

Has anyone tried this? I'm sure I'm not the only one that's had problems putting brass wood screws in certain woods, like White Oak. Even using steel wood screws to "tap" the threads can be problematic; I've snapped the heads off a #4 now and then. I think I might rather try it with a hardened steel tap instead. Heck, I've developed enough of a "touch" that I can avoid breaking a small tap even in something like stainless steel (usually!); I should be able to avoid breakage in wood. ;) But it would only be worht it if the resulting connection was strong.

So would one still use tapping fluid? ;)

[Bruce Page]

Barry, that’s interesting. I’ve been drilling & tapping metal for years but I never thought to give it a try in wood. I think I’ll do some experimenting with it, there's times when it would be real handy!
Thanks for the info!

[Rob Will]

It sounds like a good place for an extra-coarse thread (if there is such a thing). Also I would imagine that the start hole could be slightly smaller than would be required in steel. A threaded insert sounds better to me, perhaps with a touch of epoxy as it goes in. (?)

.02 Rob

[Jim Dunn]

Barry, Bruce and Rob, I've drilled and tapped holes in oak, and hard maple. Just did it like steel but I used fine threads instead of course. I thought about it and decided that fine threads would just be stronger. Also the machine screws were 8-32 and 10-32 as I recall. Not what you would call large taped holes at all. But it did the job and really tapped nicely.

Jim

[Don Frambach]

Here is an example of a commercial use of tapped machine screws into wood:

My SawStop fence faces are secured by machine screws tapped into the plywood. The faces seem very secure and are very flat. I presume that these guys know what they are doing. It occured to me that tapped machine screws would not lead to as much dimpling of the face as would regular wood screws.

added 12/30/05 1:20 AM PST - OOPS!! Just looked again more closely. The machine screws attach to metal inserts. Sorry for the misinformation.

[Ian Barley]

I have tapped 6mm holes in Iroko (like Teak) and Oak. Works fine and while I have no tests I have never had a failure. I would think that the connection is at least as strong as a wood screw, depending on application.

[Bill Antonacchio]

Hi All,

Pat Warner, the router guru, wrote this article for Taunton and he has some more on his web site that I can't quite find again. But evidently he is a proponent of machine screws in hardwoods.

http://www.taunton.com/FWN/SkillsAndTechniques/SkillsAndTechniquesPDF.aspx?id=2489

If Pat thinks it is OKwith the meticulous approach he takes to everything he does I would think it is a winner.

Regards,
Bill Antonacchio

[Doug Shepard]

I bought the LV taps, but so far only tested it out on some scraps, but was VERY impressed. I mangled the screw head pretty good trying to pull it out with a pair of channel lock pliers. Just wanted to see how good it held. IIRC I think Jamie Buxton has some threads here with some detailed writeup of tapping wood for machine screws. You might want to try an advanced search.

[Mark Singer]

I think it works fine ....it is just another step that takes time over just using wood screws. In the right application...like inside a fine cabinet to add something or for a jig or fixture that gets repeated use it seems great!

[Lars Thomas]

I've used this method in the past and have been very impressed with the strength. Lars

[Randy Meijer]

Posted this last night; but it apparently got lost during the update??

If you are having problems, seems like the simple thing to do is use a slightly larger pilot hole?? Do you lubricate your screws?? Bee's wax, candle wax or even a touch of bar soap really helps. According to some posts that I've seen, the brass screws you get at the local home centers are pretty soft. You might look for a better quality of screw if you are not using top grade hardware.

Other difficulty I see is the extra time invloved. If you are only driving 6 screws, it's no big deal; but on a larger project with 50 or 100 screws, you will be adding a lot of time to the job??

[Don Baer]

I have been taping wood and using machine screws for some time now. To speed things up I use a 2 fluted tap and instead of a tap wrench I use one of my corless drills. This is a trick I learned years ago when I worked in an electrical panel shop and had to thread hundred of holes a day.

:D

[George Summers]

Other than a couple of jigs, I have not used machine screws for wood fastening. When I did use them for jigs, they worked really well. But, I have made hundreds (well maybe not hundreds, but tens) of ZCI of wood and plywood. In every one I have ever made I use set screws for leveling screws. I tap the four holes and screw in the set screws. I've never experenced any of them vibrating out of adjustment.

George

[Jim DeLaney]

Lee Valley ...claim that in their tests, the machine screws in threaded holes form a stronger connection than wood screws.
Has anyone tried this?... )

Yes, I've used 10-24 and 10-32 metal screws (bolts) in wood on several occasions, with good results. I use a regular metal tap and the appropriate drill (#21 for 10-32), then put a drop of CA glue into the tapped hole. When the glue is hard, I run the tap in again to clean up the threads. I've never had one come loose/strip out yet.

BTW, about fifteen years ago, Fine Woodworking had an article about tapping wood. I hadn't tried it until then. I find it useful for jigs, etc., but haven't used it on any projects that would have a lot of long-term stress on them. I really don't see why it wouldn't work just as well for that, though. I just haven't tried it...

[Frank Chaffee]

Jim,
Thanks for the tip about CA glue.

Frank

[pat warner]

"BTW, about fifteen years ago, Fine Woodworking had an........"
__________________Whoa, maybe Sept/97_________

1/4-20 machine screw in ~ 1/2" of dry maple has a pull strength of about 500 pounds! A serious fastener indeed, in wood. Drilling schedule can't be willy-nilly, however; use 13/64" (tap) pilot.
See picture link (http://www.patwarner.com/images/mag3.jpg) for tight threads in MDF. Expect ~ 100-150 pounds of pull in furniture grade MDF.

[Barry O'Mahony]

Thanks for all the responses.

re: drilling larger pilot holes for wood screws; yes I can do that, and I really don't snap that many heads these days, especially now that I know to drill the top of the hole as a clearance hole for the larger (unthreaded) shank of the screw that's near the screw head.

But making the pilot hole for the threaded portion too large can reduce the strength. In most cases this doesn't matter. However I have one applicatipon coming up where I'm concerned about strength. It's a large-ish chest that I will be attaching two stout lifting handles to, one on each size. Each handle attaches to the chest with six #6 screws. Using wood screws, especially with pilot holes large enough to ensure they don't snap, seems like it may not be so strong to stand up to long-term use. Even though I generally use good-quality screws from Lee Valley.

So the alternatives I've been thinking of are threaded inserts, or tapped holes. I've never heard of anyone else trying the tapped holes before this thread. Once again, thank you to those that chimed in. I finally subscribed to the FWW site to get that Pat Warner article.

I happened to pick up the LV tap sets when I was at their retail store in Vancouver B.C. this summer. They look just like standard taps, but the drill bits are metric and may be a bit smaller than the usual sizes recommended for metal. I'll do some experimenting with them after the holiday.

[Alan Turner]

Given your application, I might think in tems of a T-Nut, although this, of course, depends entirely upon the total design, and whether there is enough room in the design to accomodate this type of fastener. #6 screws seem pretty small for lifting a large piece.

I have used a threaded 12-24 flat head machine screw, using standard metal taps, into Bubinga, for a scratch stock, where a fair amount of pressure is applied, and it has been working for about a year with no problems. Bubinga is quite strong, of course. Previously I used this same fastener with a countersunk square nut, but the chiseling of the mortise for the nut is much slower than the threading.

I can imagine using the Lee valley product for hinge installation since,a s I read the listing, the threads are specifically cut for wood screw threads, a product I had not noticed previusly. Oh well, just another credit card bill, I guess.

[Dave Richards]

I'm impressed with the 500lb strength in maple. I need to remember to try using tapped holes in wood.

Barry, another thing you might consider with your application if you want to use wood screws is to drill the pilot and shank holes and inject a tiny amount of epoxy in the hole before driving a screw lubricated with beeswax or pastewax.

I doubt you'd need more strength in your application than you'd get from six machine screws threaded in properly drilled and tapped holes. If you did, you could drill oversized holes, fill them with thickened epoxy, redrill and tap for the machine screw.

This is done frequently in marine applications such as for rigging on sailboats. It isn't uncommon to see loads many times higher than 500 lbs applied to the rigging in the form of shock loads.

Edited to add: check a marine supplier for better grades of brass wood (or machine) screws. Jamestown Distributors is good. You could also use silicon bronze screws with are harder and stronger than brass. The color is slightly different though.

[Jim DeLaney]

"BTW, about fifteen years ago, Fine Woodworking had an........"
__________________Whoa, maybe Sept/97_________

...

Actually, I'm pretty sure there was another article much further back than that. I recall first trying it when I lived in my first California house, and that was over twelve years ago...

[Randy Meijer]

[quote=Barry O'Mahony].....especially now that I know to drill the top of the hole as a clearance hole for the larger (unthreaded) shank of the screw that's near the screw head....

Brian: Your comment makes it sound like you are using "standard" drill bits to drill your pilot holes?? Are you aware that there are tapered pilot bits made for wood screws that compensate for the difference in diameter of the shank portion of the screw and the threaded portion??

[Frank Chaffee]

Barry,
There is much information presented here and elsewhere regarding the drilling and tapping of holes in many materials for screws of many types.

What I have not seen addressed here is the design of the handle that you are constructing.

The further above (gravity wise), the handle center the body of the handle goes, the greater is its resistance to rotating due to compression or bearing forces.

The further below the handle center a screw is located, the greater is the resistance of the screw to forces that would pull it out.

A handle that is symmetrical about its horizontal axis, that is secured by screws only on this axis, and has little body above that axis, will quickly overcome the bearing strength of the above material, and will put weak screws into shear where they will fail.

You describe a handle connected via six screws. I assume three screws per side. The best configuration I see for lifting here is for two of those screws to be as far above the handle as possible, and four of those screws to be well below that handle.

If you show us a pic we could say more.

Frank

[Randy Meijer]

Hi All,

Pat Warner, the router guru, wrote this article for Taunton and he has some more on his web site that I can't quite find again. But evidently he is a proponent of machine screws in hardwoods.

http://www.taunton.com/FWN/SkillsAndTechniques/SkillsAndTechniquesPDF.aspx?id=2489

That link wouldn't work for me. They wanterd me to pay $60 for a membership to view the article.

[Barry O'Mahony]

Frank,

These are stock brass handles: http://www.leevalley.com/hardware/page.aspx?c=1&p=40881&cat=3,43520,43566,43569 The ones I'm using are "C", the biggest.

The two screws on the bottom (1 on each side) will experience the greatest withdrawal forces. I'll experiment on some scrap.

I tapped a few holes in some scrap White Oak tonight. I'm impressed. The wood taps extremely easily, compared to metal. I tried to strip the threads by tightening the screw as much as I could, but ended up chewing up the brass slot head first. The screws go into the tapped holes very easily; no worries about snapping the head off while screwing them in.

[Frank Chaffee]

Hi Barry,
Do those handles lift fully upwards in use or do they stop at 90° to the case sides?

Nice handles by the way, but the screw on the horizontal axis is extraneous, and would be better placed below.

One other question, what is the weight of the object being lifted?

Fun talking with you Barry,
Frank

[Mike Wenzloff]

Add another vote for machine screws in wood.

I almost always use them for my hinge screws, especially when using woods like Bubinga, Paduak and Bloodwood. Takes less time to drill and tap than it does to remove a broken screw, plug the hole and start over.

I especially like them when using small hinges in say a jewelry box lid that is only 1/2" thick--which leaves 3/8" that screws would need to thread into.

Before I did my first ones, I did a simple test. Screwed two 1/2" pieces together and another pair with machine screws and pried them apart til failure occured. Haven't used screws for hinges since.

Take care, Mike