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nickyburnell View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nickyburnell Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 10:20am
Originally posted by bee bee wrote:

100% with mdf, I also pva any cut edges once assembled pre painting. This helps stop the furring you get on edges.


Cellulose putty good for this too, stops the (different edges) look under paint.

Mind you, when you use MDF for cheapness, then add PVA, 8 corners......comes up a bit
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 11:42am
Originally posted by SouthwestCNC SouthwestCNC wrote:

Theres certainly a balance, adding excessive amounts of screws will compromise the structure as panels can then split through its thickness however I agree a minimal amount of well placed screws does increase joint strength over just being glued.

+1

Its something that I cant get my head around, the thought that screws dont help a joint (if done properly)

I screw at 50mm c/c with a pilot but in opposing panels and so that leaves 100mm c/c between them.

I looked at BEE'S corner joints, it never occurred to me that having a rebate with an additional 45 mitre was doable, nice touch, I wonder what the router bit is called?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 12:03pm
Originally posted by 70,s hero 70,s hero wrote:

Screws clamp the joints together and distribute any forces further in to the opposing planes of material effecticevly increasing the area of distibution of force should any lateral or compressive forces be applied.

To increase the contact area for the same purpose and alignment, rebating is better than butt joints,the grain really has no effect as the grain in ply is set upon each layer at 90 degrees in opposition. It is questionable if there is any appreciable soaking in of glue as the ply (grade bb Birch) is compressed greatly with glue at manufacturer making it very dense and the viscosity of PVA and bubble glue is a deterring factor.

I do not agree that a screw does not add to the strength of a corner joint, for sure, the glue does the majority of work in stabilization but screws will help as they help distribute forces.

The idea that glue is the only requirement for the strongest joint is true in say glue beams or ply which are manufactured with hydraulic presses, I am open to be convinced that a corner joint in say 18mm ply does not benefit from additional screws, that's my take on it, hoping to be convinced otherwise.

imagine you have a perfect glued joint using a perfect material. the joint will bend to some extent without breaking. as you bend it the stress is uniformly distributed being equal at every point along the joint. now imagine you put a screw in at one point. the joint here now becomes stiffer and less able to bend. so when force is applied all the stress is focussed on the point where the screw is. the screw fixing is not able to take all the stress that was formerly distributed evenly along the joint and the wood begins to split starting where the screw is. it is possible to make metal joint reinforcements that strengthen the joint but they would look like brackets, not screws.

this situation with ply is slightly different as the layers of ply are not that well stuck together. the screws strengthen the joint not by holding the two pieces together, but by stopping the piece that is glued side on from delaminating internally.

try breaking some joints apart. if you have done it properly you will find that it is always the wood that has split apart, not that the glue has failed between the two pieces.

if you find your glue is failing you have either not cut the wood straight and there are big gaps in the joint, or you have a very close fitting joint and it has gone dry due to over-clamping.


Edited by snowflake - 13 March 2017 at 12:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 12:06pm
a good plywood joint that can be made with a rail saw and how I would make joints if I could find the right router bits. rebated joints are often good enough and quicker to make though.




Edited by snowflake - 13 March 2017 at 12:22pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 12:52pm
Originally posted by snowflake snowflake wrote:

Originally posted by 70,s hero 70,s hero wrote:

Screws clamp the joints together and distribute any forces further in to the opposing planes of material effecticevly increasing the area of distibution of force should any lateral or compressive forces be applied.

To increase the contact area for the same purpose and alignment, rebating is better than butt joints,the grain really has no effect as the grain in ply is set upon each layer at 90 degrees in opposition. It is questionable if there is any appreciable soaking in of glue as the ply (grade bb Birch) is compressed greatly with glue at manufacturer making it very dense and the viscosity of PVA and bubble glue is a deterring factor.

I do not agree that a screw does not add to the strength of a corner joint, for sure, the glue does the majority of work in stabilization but screws will help as they help distribute forces.

The idea that glue is the only requirement for the strongest joint is true in say glue beams or ply which are manufactured with hydraulic presses, I am open to be convinced that a corner joint in say 18mm ply does not benefit from additional screws, that's my take on it, hoping to be convinced otherwise.

imagine you have a perfect glued joint using a perfect material. the joint will bend to some extent without breaking. as you bend it the stress is uniformly distributed being equal at every point along the joint. now imagine you put a screw in at one point. the joint here now becomes stiffer and less able to bend. so when force is applied all the stress is focussed on the point where the screw is. the screw fixing is not able to take all the stress that was formerly distributed evenly along the joint and the wood begins to split starting where the screw is. it is possible to make metal joint reinforcements that strengthen the joint but they would look like brackets, not screws.

this situation with ply is slightly different as the layers of ply are not that well stuck together. the screws strengthen the joint not by holding the two pieces together, but by stopping the piece that is glued side on from delaminating internally.

try breaking some joints apart. if you have done it properly you will find that it is always the wood that has split apart, not that the glue has failed between the two pieces.

if you find your glue is failing you have either not cut the wood straight and there are big gaps in the joint, or you have a very close fitting joint and it has gone dry due to over-clamping.

Thanks for the explanation,I have seen glued joints take the wood with them, PVA, Cascamite, Poly, I think that its true for general wood glued along a grain which is well clamped but boxes would be open to distortion of thier shape if dropped... say a tendency to collapse although internal bracing and the front and back panels would help stop this, I dont see the forces being applied to a right angle joint as bending as such, more an opening or closing of the angle.We also have to consider that gluing end grain is a great deal weaker than along the grain. Perhaps the only way to be sure would be to mock up two joints, one with just glue and one with screws as well.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote SouthwestCNC Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 1:05pm
There is a joint stress test floating about somewhere on the net, You are right about a screw then becoming the weakest link if the joint has a certain degree of flexibility.

In the test of newtons of force being applied to the joint glue alone scored something like 80, Screw and glue 120, Mortise and tennon 300 and screw and washer no glue also 300.

which backups what you say about distributing pressure.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 1:42pm
Originally posted by SouthwestCNC SouthwestCNC wrote:

There is a joint stress test floating about somewhere on the net, You are right about a screw then becoming the weakest link if the joint has a certain degree of flexibility.

In the test of newtons of force being applied to the joint glue alone scored something like 80, Screw and glue 120, Mortise and tennon 300 and screw and washer no glue also 300.

which backups what you say about distributing pressure.

Thanks
, that makes sense. For the joint on a cabinet Gluing and screwing is stronger than just glue, for simple lateral (long joints say two pieces of 100x50) gluing alone with clamping is strongest.

I wonder if the OP has any idea of what filler is bestLOL




Edited by 70,s hero - 13 March 2017 at 1:47pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SouthwestCNC Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 4:14pm
Metrolux 2 part is pretty good imo, Behaves a bit more like wood than automotive fillers. sadly the closest to birch colour is light oak which is not a great match.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 4:57pm
Originally posted by SouthwestCNC SouthwestCNC wrote:

Metrolux 2 part is pretty good imo, Behaves a bit more like wood than automotive fillers. sadly the closest to birch colour is light oak which is not a great match.

Sounds good, I wonder if it stains well, I need to fill some of my oak wood at home this year.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SouthwestCNC Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 8:36pm
Yes it stains quite well for a 2 part. Sorry its called metolux, comes in decent sized tins for a good price too :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 9:18pm
Originally posted by SouthwestCNC SouthwestCNC wrote:

Yes it stains quite well for a 2 part. Sorry its called metolux, comes in decent sized tins for a good price too :)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote bee Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 9:31pm
Originally posted by 70,s hero 70,s hero wrote:

Originally posted by SouthwestCNC SouthwestCNC wrote:

Theres certainly a balance, adding excessive amounts of screws will compromise the structure as panels can then split through its thickness however I agree a minimal amount of well placed screws does increase joint strength over just being glued.

+1

Its something that I cant get my head around, the thought that screws dont help a joint (if done properly)

I screw at 50mm c/c with a pilot but in opposing panels and so that leaves 100mm c/c between them.

I looked at BEE'S corner joints, it never occurred to me that having a rebate with an additional 45 mitre was doable, nice touch, I wonder what the router bit is called?


 
there all hand done, I'm very lucky to have some big boys wood working tools, my mafell 85c can cut upto 60 degrees on a track saw, my main track saws are the kss300 and kss400 again both mafell. I got rid of my big table saw when I get the cnc, but still use my Mafell Erica table saw all the time. Hand router wise I have a few of the trend routers 2 x t11, and a smaller t5.
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