for woodworking joinery screws are easy, dowels look nicer, but which one is stronger?
I'm going to test these by measuring how much force it takes to pull them straight apart
I actually made a lot of test samples with different types
of dowels and such and these are all very small to fit in my test apparatus
so I made these metal brackets for my pull test apparatus and the test pieces just slide
right into those first one is a 3/8" dowel glued with regular yellow Carpenters glue
and that failed at 219 kg it didn't pull out of this but did pull slightly out of this part here
next test piece held together by a number eight screw
I can see this is starting to open up but it hasn't failed yet
okay still holding but that Gap is not very nice looking so looking at
this it appears that the screw head is just sinking into this piece of wood
the screw is still holding in this part here even though it's screwed into end grain well
and it finally failed just splitting apart
now the screw head is kind of a wedge that helps us split the wood apart.
drywall screw's have a much flatter head so let's try that next
So at 100 it really hasn't opened up very much yet and this is as much as the other one held
now we are opening up here now and by the looks of it we're also pulling the
head through the piece of wood and the force is now actually quite a bit less
Force has gone up I guess it just depends on how hard the grain is that I'm pulling
through at any one moment because that force keeps going up and down
200 but not much use with it opened up this far
and I think with that bang the thread let go of this piece here
so that head got way sunk in but not as far as the head went in on
this one which probably went to about here before it split the wood in half
whereas the dowel had the higher force and it didn't really pop
apart until it got very close to the maximum Force
so that joint stayed good for the longest but with the head sinking in being such a
problem I'm wondering about pocket hole screws where the head is on end grain
so maybe this will do a little bit better
so at 100 kg we're already quite opened up so that would be pretty ugly for furniture
oh and now the force is dropping off
so I think what's happening is the screws yielding out of this bottom piece here
and I made this so that the screw penetrated almost all
the way through the piece of wood to give it maximum engagement here
but still it started to pull out and you can see this piece is starting to crack
here so it definitely pulling out of the bottom part here
so dowel joint wins then right?
well I got a lot more pieces to test still 3/8-inch dowel joined with TitebondBon 3
all right my uh clamping thing broke here
all right second TightBond 3 sample same test
oh no same problem
now this next one is dowel joint of sorts with just a little piece of bamboo skewer
and that failed with a dowel starting to pull through the bottom piece
here at 126 kg which is still more than the pocket hole pieces I made
32 test specimens in all here's a few more
this one's joined with a 5 mm skewer
this one's a 3/8" in fluted dowel pin
and a 5 16th in fluted dowel pin
and a 1/4" fluted double pin
now with all my dowel joint ones of course there's always
glue squeeze-out and that makes for a butt joint with the rest of the wood
so I rubbed a little bit of salad bowl finish on the piece of wood
to make sure the glue wouldn't actually stick to the flat surface
I broke all my test specimens. now let's look at the results
the 3/8" dowel is clearly stronger than all the screws especially the pocket hole ones
but for the 3/8" I had a lot of them where it actually failed where the wood was clamped
so I have a little yellow thing at the end of those lines because I
don't know how long those lines actually would be
but then I had the idea of re-clamping those pieces using a different metal bracket that I made
and pulling on them again and on retesting none of those seven samples failed where I was grabbing it
so my other gripper for those samples was probably less than ideal
and here's my results numbers and the graph the dotted area here those are the retests
so I just used a retest here but I marked it separately in
the graph to show which ones I had to test twice
weakest was the pocket holes and that's because there's only so much thread that
goes into the wood here and that's just failed by yielding out of here
next was the number eight wood screw and that tended to fail by the head
just pulling into the soft wood whereas the drywall screw that's these ones here
did better because that head is harder to pull in
but most of these screws even before they failed there was a
gap that opened up so at 80 kg we could see the number eight wood screws and the
pocket holes showing a gap whereas with the drywall screws it was a bit later
but still this ultimate fail strength isn't really
that useful because there's quite a gap already there
the next group is the 3/8" dowels and I used
homemade dowels these ones and fluted dowel pins that I bought
these are inch and a half long the homemade ones I made a tiny
bit longer because I think these ones are just too short but surprisingly the dowel
pins actually rated overall a little bit higher in terms of the ultimate failure
and that's even though the usable length of
these is actually a little bit less because they're chamfered on the end
which is to say the fluting does help a little bit possibly as much as 20% in the softwood
next Group after that is 5/16-inch dowel pins which are a little bit
smaller than the 3/8 but uh they did almost as well so that's pretty good
then quarter inch dowel pins which are also a bit shorter
these are just an 1 1/8" long or about about 28 mm
and that's actually a pretty good showing considering these dowal pins are so much
smaller than the big ones and then going further down in diameter I went
to 5 mm bamboo skewers and made these a bit longer because these ones are just too short
and these did about as well as the quarter inch dowel pins even though they're thinner
so the length probably helped on those and impressively these ones are more
than half the strength of a 3/8" dowel, even though there's so much less to them
but surface area wise these have half the surface area as these ones
but before I started filming this video I did this
pull test and here the skewer itself failed at just 80 kg.
the grain was crooked the thing with bamboo is any single piece of bamboo material is likely
to have a knot every 30 cm or so, so the front edge piece is got a knot here another one here
next one is here here and bamboo skewers are the same and across their length.
they typically have one knot so I just lined up three of them here
here and here if that's part of your dowel pin that'll really weaken it
now I didn't look out for knots at all when I
made my test samples and these two had the lowest fail forces
and with both of these I can see that the dowel pin itself pulled through the cross-grain piece
here which is to say I see no sign of the skewers themselves pulling apart in any of my test samples
so maybe I just got lucky or maybe this test piece was just unusually bad
looking at the averages of my fail forces the 5/16" dowel pin actually
did surprisingly well compared to the 3/8".
next we have the quarter inch dowel pin and the bamboo skewer which did almost as well
as the quarter inch dowel pin, but also made my bamboo skewer pins a little bit longer.
worst was the pocket hole but all the screw joints opened up a fair bit before their ultimate failure
but an interesting take-away from this is that just a piece of bamboo
skewer will hold things rather better than a pocket hole screw
other than the pocket holes my screw samples relied on screws holding in the end grain so
I screwed some more screws 32 mm into blocks of wood end grain in side grain
and test it out on my strength testing machine to see how well the end grain actually holds
and these tests were all without pilot holess
and for the drywall screws the end grain held 88% as well as the cross
grain for the number eight screws it wasn't quite as consistent but
on average we got 78% of the holding power in end grain than in Cross grain
so surprising screws and end grain work almost as well as screws and cross grain