Threaded insert vs. machine screws directly into hardwood
bautastein
| Posted in General Discussion on
I need to attach a piece of softwood (Norway spruce) onto a piece of hardwood (Hard Maple/Sugar Maple). I want it to be firm, and possible to disassemble/reassemble freely. I had bought a set of threaded inserts and some epoxy for the task. The threaded insert would be inserted into a flatsawn piece of hard maple, across the grain. Then I stumbled across this video:
https://youtu.be/S4SnrbGwGpI?si=YSrKzfi7VI5oRd1B
(I couldn’t figure out how to make the link active)
Essentially, the video suggests that for multiple reassemblies, just using a metal threading tap and machine screws directly into hardwood is actually superior to a threaded insert. His tests have a few flaws:
1) He doesn’t unscrew the machine screws completely for each “disassembly”. This means there is no risk of misaligning the machine screw threads. In the real world, this misalignment could possibly wear out the threads in the hardwood.
2) He uses superglue to fortify the threaded insert, while I planned to use epoxy, which most people suggest, and might be better(?). Is it?
3) He uses a drill to screw/unscrew, and we don’t know how much torque his setting #5 is, it might be way too much.
4) Comments below the video suggests his threaded insert was made for softwood, however, so are the ones I bought (probably).
Nevertheless, I couldn’t dismiss the results, so I searched for similar tests, but found none. So I’m thinking the following: I could try the wood tap method, since it’s easy to convert to a threaded insert if it doesn’t meet expectations. However, then I might as well go for a wood screw, perhaps? Or would a wood screw not be so good for multiple cycles of disassembly/reassembly?
Thoughts?
Replies
I was doubtful about the wood tap method until I got a spokeshave kit from Lee Valley Veritas that came with one. You have to unfasten and refasten whenever you sharpen the blade. It has held up fine in hard wood. The main thing is to be careful and do it correctly. A drill press is best.
Sme additional considerations:
1) The best tap for cutting thread directly into hardwood for a machine screw is the screw itself. You'll get an exact fit of the threads. I measure the diameter of the inside and the outside of the thread and then drill a pilot hole with a drill having the diameter of the inside; then use the machine screw to cut the thread in it. If you want a really tight fit, drill the pilot hole around 0.25mm diameter smaller than that. Use some wax to help cut the thread with the machine screw. You're cutting it by compressing the wood into a thread shape rather than by cutting it.
2 Consider which way any forces acting on the joint are acting. Any significant force pulling along the bolt is going to be more of a strain on it than forces acting tangentially to the bolt. However, a longer bolt in a longer thread cut into the hardwood will make it all more resistant to being pulled out.
3 Consider the grain of the hardwood that you cut the thread into. Some grain patterns are more prone to a thread crumble. Straight and even grain tends to be more resilient than swirly grain for cutting a thread into.
I've mounted many a drawer knob directly into a hardwood drawer front using a relatively short machine screw (around 12mm) They seem to be stronger than using a similar length wood screw, perhaps because the wood cut by the finer threads of the machine screw is less damaged than it would be by the larger threads of a wood screw?
I have some bench stops on the end of the workbench that are held on with machine screws 30mm direct into the hard maple bench aprons. They're loosened to turn the bench stops to different heights. One of them must have been loosened and tightened hundreds of times yet is still strong enough to hold the stop firmly as I plane a plank resting against it, without any loosening or movement of the stop.
Lataxe
If you want it to last go with a threaded insert that is either barbed for press/ hammer installation or has knife/ screw threads for a thread-in install. Wood vs metal will fail eventually on the wood side, and epoxied fasteners will work loose over time and a few movement cycles.
I have made several things where the attachment is tapped directly into hardwood. I use a 1/4" x 24 thread minimum (generally just coarse threads). None of these connections are load bearing beyond a few pounds. None have ever failed. I would not expect threaded hardwood to be good for heavier loads due to seasonal changes and wood movement. That is; cabinet to panel should be fine. Cabinet supporting another cabinet without additional support, not so much. You can reinforce the threads with CA glue. Apply wax to a sample bolt or threaded rod, apply CA to threaded hole, wind the waxed rod in and out several times.
Measured screw holding forces for doors. Mind you these are wood screws.
Veneered particle core 200ft/lbs
Veneer engineered core 700ft/lb
Hardwood stile and rail doors averaged at 400 ft/lbs with a giant caviot for different species.
Manufacturers recommend thru-bolts or blocking on particle doors. Threaded inserts as a secondary recommendation. I've noticed reinserting screws is fine as long as care is taken. If these are going to be taken apart by average Joe 6 pack use metal. Putnit this way, I have never regretted using a threaded insert. Just my take.
Screw capacity in wood depends on the screw diameter, the thickness of the pieces being joined, the embedment, the allowable bearing stress of the wood in question parallel and perpendicular to the grain, the direction of loading (shear, tension, or shear+tension),the load duration and if there are metal side plates helping out. Also dry and or wet service conditions can factor in along with the MC of the timber. Engineering wise there are a set of equations that can calculate screw capacity (or lag bolts too) for a given situation. See the 2018 NDS for Wood Construction for values and equations. You can calculate the capacity of most all wood/screw connections directly.
Yes sir. So many variables. These are test performed on commercial grade wood veneered doors and they are for holding hardware to the door. Specifically. Just a reference for material differences and the incredible holding force achieved with just a plain wood screw.
It is amazing how much load a small screw can carry!
Thank you all. After reading the replies and done some research I've decided to go for machine screws directly into the wood. Turns out there is a guru named Pat Warner who swears by them and has written about it in this magazine. He tends to go for 1/4 inch screws with 20 threads per inch. As I live in metric-land (Norway), the closest standard size would be M6 screws. The outer diameter is only slightly smaller (6mm vs 6.35mm), but the thread pitch is much smaller (1mm vs 1.27mm). Therefore I will use M8 screws which have a thread pitch of 1.25mm, as I believe thread pitch is more important than outer diameter. As a reference for other metric guys out there, the pilot hole for M8 screw taps will be 7mm, which is part of almost every metric drill bit set.
I've put quite a few screws into tapped holes. They hold amazingly well!!
They work well
Tapped holes in hardwood hold screws amazingly well!
Late to the party but...
What this video missed was that there is a little play in any screw fixing, and when that is taken up, the threads move very slightly against the constraining material.
Although raw strength is undoubtedly the same initially, the test neglected to consider the possibilities of:
1. Overtightening being more likely in wood alone;
2. Small amounts of movement over time leading to tearing of the wood.
3. durability on repeated reassembly.
I can't say what impact these will have over time, but suspect that very small movements will cause much more weakening in the screw without the threaded insert.