Threaded inserts for hardwood

[Jim Becker]

Presuming that you don't cross-thread the screw, what's the mechanism for hole wear? You're sliding a nice metal screw into a recess which fits it.

Any time one surface moves against another, as certainly happens with a screw in a threaded hole, there is wear and friction. The more you want to do it...the more durable you want the accommodation.

[Jamie Buxton]

Any time one surface moves against another, as certainly happens with a screw in a threaded hole, there is wear and friction. The more you want to do it...the more durable you want the accommodation.

Hmmm... Okay, let's say we have a 1/4-20 tapped into a block of maple. We run it in and out without crossthreading it. How many in-and-out cycles do you think it will take to wear out? Ten? A thousand? A hundred thousand? A million?

[Cliff Rohrabacher]

Cliff --

Try drilling and tapping a machine screw into hardwood -- say a 1/4-20 into maple. Orient the hole so that you're going into face grain -- that is, not end-grain. Make the tapped hole at least four screw diameters deep. Insert a screw. Try your favorite method of pulling out -- claw hammer or whatever. You'll find it doesn't come out.

It isn't really too surprising. The biggest difference between a machine screw and a wood screw is that the latter is tapered so that it cuts it own thread, while the former needs a separate thread-cutter. Yeah, there are differences in thread depth and shape and pitch, but that's the small details. They both have threads to grab the wood, and that's the important part.

Jamie

Yah huh? Isn't that cool? I've done exactly that lots of times. Both side and end grain - the finer the grain the finer the thread you can use. I don't ask it to give much strip out resistance though. As you observed you gotta have one hell of a lot of thread engagement to do very much. The wood screw's thread depth shape and pitch are however substiantial factors and they weren't arrived at by accident.

[Jim Becker]

Hmmm... Okay, let's say we have a 1/4-20 tapped into a block of maple. We run it in and out without crossthreading it. How many in-and-out cycles do you think it will take to wear out? Ten? A thousand? A hundred thousand? A million?

Another scenario...I think you have to consider the stress you place on the threading when you tighten things up. For example, let's say you are going to use said 1/4"-20 screw/bolt to fasten a jig to your maple workbench top. (Something I actually do for my D4 and Kreg jig in several places in the shop) Drilling and tapping the wood will certainly work, but over a reasonable period of time, I believe, you would begin to stress the wood fibers merely by tightening things down snuggly...err, tightly...as we are all so apt to do. The fibers will give at some point just from that stress. I think you'll be fine for situations where there is minimal change by tapping (with a big enough screw/bolt size for some "bite"), but for things that you regularly use, I think that the inserts are a better idea. When installed properly, they provide a secure and long-lived fastening point.

'Just my opinion. No problem either way...use the method that best suits your needs.

[Cliff Rohrabacher]

Hmmm... Okay, let's say we have a 1/4-20 tapped into a block of maple. We run it in and out without crossthreading it. How many in-and-out cycles do you think it will take to wear out? Ten? A thousand? A hundred thousand? A million?

Sounds like a worthy project Jamie. Not a small project by any means - actually a throughly daunting project. But it's about how the Engineering standards for thread types and applications are derived.

To do it justice one would need sample runs from all the principal species and grades.
There'd need to be a thread class as the standard I'd use 2-B. Not sure how you test a 2-B in wood but what the hell it's a project.
Prolly best to run a wide range of threads 2-56 through 1/2-13 ought to be good.
Humidity will also need to be factored. You'll need to control humidity for a slew of samples under conditions that are constant as a few places in the range to some that experience variations in humidity.

There would be several classes of testing ranging from
Number of times the threads can be run in and out
Pressures used to simulate hand pressure
Pressures used to torque the things down.
Stress loads vibratory, and pull out, and side loads on each class (whew that'd be a lot of samples).

And ofcoiurse the number crunching would be another issue. Count me out I don't crunching like lots and lots of numbers just give me the results.

Then once that is done and a few others have repeated the test protols and found the results to repeat in their tests there'd be a basis for belief that machine threads will perform to some defined objective level of expectation in wood.

[Ben Roman]

I use a old fashioned Brace for inserting threaded inserts.. Its cheap and it works well.

Ben

[Arvind Srivaths]

Has anyone tried these zinc alloy threaded inserts? How do they compare with the brass inserts?

https://www.grainger.com/product/GRA...nc-Alloy-4ZU77

Arvind

[Mark Bolton]

"What do you use for threaded inserts"

Real threaded inserts, Steve. Click link.
Drill, machine tap, then gently screw insert in x fingers. No drill press driving, plum trivial.
After the insert is where you want it, press in the lock pins and get on with it.

Routers

Those machinist style thread-serts are way overkill for any wood application and have very shallow threads compared to a wood insert. The main issue is price. You can buy 50-100 comparable inserts for what a couple of those inserts cost forget about if you buy the kit with the tap, tool, and so on. They are phenomenal for what they are made for, but thats not wood.

Installing standard brass inserts in solid surface is a bit of a chore sometimes and I always opt for the slightly averaged hole size already mentioned. Right in the middle of the major/minor diameter of the insert seems to work well. And make sure the slots are down. A lot of people think the slots are for installing with a slotted screwdriver. Not so.

[Arvind Srivaths]

Mark, have you tried these zinc alloy threaded inserts? How do they compare with the brass inserts?

https://www.grainger.com/product/GRA...nc-Alloy-4ZU77

Thanks,
Arvind

[Carl Beckett]

I almost always tap. Unless using T-nuts. Heck, I even heli coiled once (stanley makes a heli coil specific for wood). They are cheap. I do not have good luck with 'self tapping' inserts, I find the larger diameters of these tend to stress the wood and bulge it up, or push it out the back, etc.

Tapping wood is easy, just chuck the tap into your drill/driver and run it at slow speeds. Faster than dealing with getting the self tapping insert aligned/started/etc.

Sometimes I just use the wood itself no insert, if not needing lots of cycles. Lets face it, there are MANY threaded vice screws made out of wood over the years, which sees repeated cycles. So wood is viable even if the threads are not perfectly 'designed for it'. More often I tap for the insert threads (I also use them for plastics such as delrin, teflon, phenolics, etc), so have them laying around. Sometimes I glue the insert in with epoxy. T-nuts are great if acceptable (fixtures, etc).

[Jim Becker]

Mark, have you tried these zinc alloy threaded inserts? How do they compare with the brass inserts?

https://www.grainger.com/product/GRA...nc-Alloy-4ZU77

Thanks,
Arvind

Those are available from a variety of sources in addition to Granger and yes, I've used them for utility purposes.

BTW, this is a thread from 2006...

[Arvind Srivaths]

Those are available from a variety of sources in addition to Granger and yes, I've used them for utility purposes.

BTW, this is a thread from 2006...

And the zinc alloy threaded inserts do work well in hardwoods? I was wondering whether to get these or stick with brass ones. I tried using T-nuts in neem and they come out very easily. I am hoping that the friction will hold the threaded inserts better.

Yes, I noticed that the thread is from 2006 😀, but I thought it’s better to keep related questions on the same thread so that it’s convenient to access.

Thanks,

Arvind

[Gordon Stump]

inserts.JPG I do hundreds of the brass knife threaded inserts for my products. I use the jig in the picture with a cordless drill. They go in straight and it works great. I drill 13/32 holes in hardwood for 10-24 inserts. But the price has almost doubled lately. Tariff? Idunno. So for some of my stuff I changed over to those zinc inserts with the flange. Yes they do work in hardwood. I drill a 5/16" hole for both 1/4-20 and 10-24. I dab the threads with a little T88 and drill them in with a cordless drill and driver. I have not had a product come back in 12 years due to zinc or brass threaded inserts. The flange helps keep the zinc inserts square. Both kinds are counter sunk well below the surface.

[Jim Becker]

I've not used them in hardwood, Arvind...they were sunk in MDF. However, they theoretically should work just fine as long as your pre-drilling is the correct size for hardwoods. There is typically one pre-drill hole size recommendation for hardwood and a different one for softwood/composites so that the insert doesn't get over stressed when trying to cut the threads as you spin them in. The correct sizing also helps keep them from pulling out. So you need the specifications for the inserts you choose for that pre-drilling piece and it may not be a common drill size.

[Arvind Srivaths]

Thanks for the input Jim.

Arvind