Bowed long boards - any remedy?

[Adam W.]

Ah, thanks Richard.

Can't they just be planed down to an inch ?

 

[Sgian Dubh]

Ah, thanks Richard.

Can't they just be planed down to an inch ?

Assuming he can get them more or less straight taking the boards from ~32 mm rough sawn to an inch (~25 mm) thick, yes. But if a bit more has to be taken off to achieve straightness I think that would be okay too, as would flatness coupled with a thicker end result, e.g., 27 mm, which is unlikely in this case, obviously. It would just mean adjusting construction a bit to accommodate the change from whatever was the planned dimension. As I'm sure you know, with your experience, unforeseen variations from the initial plan are pretty normal as a response to the material being different to what was expected, or perhaps planned/hoped for. Slainte.

 

[tibi]

Adam, he's concerned about the four boards required for the headboard and the board between the legs at the footboard, five in total.

A possible solution for the footboard end is to flatten the board as best he can, which might leave it somewhat bowed, but of consistent thickness throughout its length. Then he could glue and screw a roughly 60 X ~20 mm piece on to the inside face to straighten the footboard rail: effectively this piece would become one of the mattress supporting slats. The same strategy could be employed for the lowest rail between the headboard legs. That takes care of two boards out of five.

This leaves three rails above the lowest headboard rail. In this case they're mostly going to be hidden by the mattress and pillows anyway. Overall I'd say tibi might be getting overly concerned about being able to get a usable thickness for these three parts after flattening because he's starting with 32 mm thick stuff and I suspect he'd have enough stiffness even if he had to end up with boards that are only 18 - 20 mm thick. Even with this thickness the rear face of these three boards sit back from the rear face of the headboard legs, and if stiffening and/or straightening is required a similar stiffening piece of suitable width, i.e., not protruding beyond the rear face of the headboard legs, could be glued and screwed to the rear face of each board.

I think tibi may be somewhat anxiously anticipating difficulties that may just disappear once he gets the basic machining for true and square completed. Slainte.

PS. Edit. I forgot to mention he's also concerned about the two side rails. Straightening these, if required could be accomplished much as above with a return (bearer) glued and screwed to the inside face upon which the slats sit, as seen below where the side rails are ~20 mm thick. Also a judicious screw or two at the end of a couple of slats where/if the side rails bow outwards would also serve to straighten them. I did something similar with this bed where the slats are stapled to a couple of hessian strips. The slats come in two halves, one at the headboard end stretching to the side rail to almost the mid-point, and the second set roll out again almost to the mid-point from the footboard end. The last slat at both ends of each set slip over a stop screwed to the bearer and it's at the mid point where a screw goes through a slat into the bearer below.

View attachment 137307​

Thank you very much for the answer. You gave me some great ideas.

It is a good idea to support footboard and headboard lowest boards with additional board (I will use 60x40 mm spruce/fir for side board slat supports, so I can glue/screw those to the footboard and lowest headboard as well. I will also screw two boards from behind to the headboard (dividing the length to thirds) to align the bows to uniformity

My overly anxious reasoning (for which I am famous for - my wife tells me that I always look for what can go wrong and why things cannot be done) asks if 20-25 mm oak board (after machining) will give in to the 40 mm spruce board and will straighten or 40 mm board will bow in alignment with 20-25 mm oak board, due to greater density of the wood, or whatever reason? I personally think, it will be OK, as the thickness difference is bigger, but I still have a shadow of a doubt.

 

[tibi]

Assuming he can get them more or less straight taking the boards from ~32 mm rough sawn to an inch (~25 mm) thick, yes. But if a bit more has to be taken off to achieve straightness I think that would be okay too, as would flatness coupled with a thicker end result, e.g., 27 mm, which is unlikely in this case, obviously. It would just mean adjusting construction a bit to accommodate the change from whatever was the planned dimension. As I'm sure you know with your experience unforeseen variations from the initial plan are pretty normal as a response to the material being different to what was expected, or perhaps planed/hoped for. Slainte.

I always make two designs. Initial design to get a cutlist. And after handplaning/thicknessing I will alter the design based on actual widths and thicknesses of the boards that I got from the raw lumber.
So If I plan to use 10 mm chisel for morticing, then I might need to go down to 8 mm, if I took away more material than I initially planned, so that the walls are not too thin.

 

[Sgian Dubh]

My overly anxious reasoning (for which I am famous for - my wife tells me that I always look for what can go wrong and why things cannot be done) asks if 20-25 mm oak board (after machining) will give in to the 40 mm spruce board and will straighten or 40 mm board will bow in alignment with 20-25 mm oak board, due to greater density of the wood, or whatever reason? I personally think, it will be OK, as the thickness difference is bigger, but I still have a shadow of a doubt.

Stiffness is more important than density for the return or bearer to be able to straighten a bowed board. Having said that, there is a relationship between stiffness and density in a piece of wood, but it's a little more involved than I'm willing to go into here in what I hope will be a succinct answer. Oak is a stiffer wood than spruce so spruce glued and screwed as a bearer or return to the oak has a lesser ability to take a bow out than using a stiffer wood of equal dimensions to the spruce you propose. The alternative to finding a stiffer material for the purpose of straightening a rail as you propose is to increase the width and/or thickness of the spruce.

We're basically talking about beam theory, another topic I don't have enough time or space to go into here. To give a simple example, doubling the thickness of your stiffening spruce doubles its load carrying capacity, i.e., its ability to straighten your oak board. Similarly, doubling the width of your spruce piece doubles its ability to straighten your oak. A combination of increasing your spruce piece's thickness and width by randomly chosen amounts increases its strength, but by how much is a bit of a guessing game without doing some fairly complex calculations.

Oak is generally stiffer than spruce by some margin, but just how much stiffer depends upon which oak species and which species of spruce you're using. In conclusion I'm not saying don't use spruce for your bearers or straightening returns, but if you find a test dry fit of your chosen spruce bearer size (screwed but not glued) doesn't do the job you'll need to increase the width and/or thickness of your bearers or/and use a stiffer species of wood. In other words, try it and see what happens, and go from there. Slainte.

 

[tibi]

Stiffness is more important than density for the return or bearer to be able to straighten a bowed board. Having said that, there is a relationship between stiffness and density in a piece of wood, but it's a little more involved than I'm willing to go into here in what I hope will be a succinct answer. Oak is a stiffer wood than spruce so spruce glued and screwed as a bearer or return to the oak has a lesser ability to take a bow out than using a stiffer wood of equal dimensions to the spruce you propose. The alternative to finding a stiffer material for the purpose of straightening a rail as you propose is to increase the width and/or thickness of the spruce.

We're basically talking about beam theory, another topic I don't have enough time or space to go into here. To give a simple example, doubling the thickness of your stiffening spruce doubles its load carrying capacity, i.e., its ability to straighten your oak board. Similarly, doubling the width of your spruce piece doubles its ability to straighten your oak. A combination of increasing your spruce piece's thickness and width by randomly chosen amounts increases its strength, but by how much is a bit of a guessing game without doing some fairly complex calculations.

Oak is generally stiffer than spruce by some margin, but just how much stiffer depends upon which oak species and which species of spruce you're using. In conclusion I'm not saying don't use spruce for your bearers or straightening returns, but if you find a test dry fit of your chosen spruce bearer size (screwed but not glued) doesn't do the job you'll need to increase the width and/or thickness of your bearers or/and use a stiffer species of wood. In other words, try it and see what happens, and go from there. Slainte.

Thank you,

I have some leftover 50 mm thick beech boards from my workbench, I can use them as bearers provided they will be long enough. I will just try and see. But I have cut another set of long side boards and they are a bit straighter than previous ones, so maybe I will get a flat board just by planing away the thickness. What do you think is the minimum thickness of the boards for such a bed, so you would not go any thinner?

Thanks.

 

[Sgian Dubh]

... so maybe I will get a flat board just by planing away the thickness. What do you think is the minimum thickness of the boards for such a bed, so you would not go any thinner? Thanks.

The bed I showed an image of on the previous page had side rails approximately 18 - 20 mm thick. There are mattress supporting bearers screwed and glued to the inside face, and that naturally stiffens those long side rails a bit, but I can't say by how much because I've never thought to do any calculations on that. The mattress supporting slats are of similar thickness, i.e., 18 - 20 mm.

In both above, there's plenty of strength even if the occupants (two or more perhaps?) wish to undertake some horizontal and energetic gymnastics, ha, ha. Slainte.

 

[tibi]

The bed I showed an image of on the previous page had side rails approximately 18 - 20 mm thick. There are mattress supporting bearers screwed and glued to the inside face, and that naturally stiffens those long side rails a bit, but I can't say by how much because I've never thought to do any calculations on that. The mattress supporting slats are of similar thickness, i.e., 18 - 20 mm.

In both above, there's plenty of strength even if the occupants Two or more perhaps?) wish to undertake some horizontal and energetic gymnastics, ha, ha. Slainte.

Nice to know that 20 mm oak will be stiff enough for gymnastics friendly bed, 18mm and less will be for sleeping only (sport activities prohibited).

 

[Jones]

For strength the depth of the rails is more important than thickness.I would suggest a 100x22mm minimum a deeper rail could go a bit thinner. The slat support can add alot of strength depending on size. I would recommend a support lengthways under the middle of the slats ,if this is dovetailed into a block fixed mid rail on foot and headboard it will stiffen it all up and hold everything straight. As the bending force is greatest in the middle it can taper at the ends so it's not seen unless you look under.

 

[Sgian Dubh]

Nice to know that 20 mm oak will be stiff enough for gymnastics friendly bed, 18mm and less will be for sleeping only (sport activities prohibited).

Naturally, if you can get the rails out thicker, at say 22 - 25 mm thick, they'll be stiffer. Subsequently, of course, if you need to take a bow out of them with bearers (battens, returns, whatever you want to call them) it will be somewhat more difficult, but hopefully you'll not really need to do any significant straightening.

As an aside, the side rails of the bed I showed before are approximately 160 mm wide, i.e., their top to bottom measurement as seen from the side. Slainte.

 

[tibi]

For strength the depth of the rails is more important than thickness.I would suggest a 100x22mm minimum a deeper rail could go a bit thinner. The slat support can add alot of strength depending on size. I would recommend a support lengthways under the middle of the slats ,if this is dovetailed into a block fixed mid rail on foot and headboard it will stiffen it all up and hold everything straight. As the bending force is greatest in the middle it can taper at the ends so it's not seen unless you look under.

All my boards will be 120 mm wide (they are already cut to approximately 130 mm, so I cannot make them wider), legs will be 60x60 or a little less, depending on the thickness that I can get from two boards and glue them together to form a leg.

I was considering a sliding dovetail for central bearer (support) boards (as I have two dividing the width of the bed in thirds), but I am not sure if I could cut it right, so I will use this hardware instead and try first sliding dovetail on some smaller stuff or practice pieces.

 

[tibi]

Naturally, if you can get the rails out thicker, at say 22 - 25 mm thick, they'll be stiffer. Subsequently, of course, if you need to take a bow out of them with bearers (battens, returns, whatever you want to call them) it will be somewhat more difficult, but hopefully you'll not really need to do any significant straightening.

As an aside, the side rails of the bed I showed before are approximately 160 mm wide, i.e., their top to bottom measurement as seen from the side. Slainte.

I will try to get them flat naturally by planing away the material. If I get to some critical thickness, e.g. 22 mm, I will stop planing further and focus on straightening them with bearers.

 

[Sgian Dubh]

I was considering a sliding dovetail for central bearer (support) boards (as I have two dividing the width of the bed in thirds), but I am not sure if I could cut it right, so I will use this hardware instead and try first sliding dovetail on some smaller stuff or practice pieces.
View attachment 137359

That's the same hardware as I've used for centrally positioned longitudinal bearers. It's simple to install and effective and just needs the bearer to have a profile approximately 60 X 32 mm.

As for attaching the side rail to the headboard I've mostly used this type immediately below. I haven't come across this exact pattern of bed fitting here in Europe, but I used to buy them when I lived in the US. The nearest equivalent is something called Noval K bed fittings seen at bottom which I've used a couple of times. In my experience they seem to work well enough, are easier to install than the type in the upper image, although I don't think they're quite as robust and bulletproof. The image, incidentally, is sourced from Häfele as follows: https://www.hafele.co.uk/INTERSHOP/...le/en_GB/pim/images/default/ppic-00630241.jpg Slainte.

 

[Doug71]

That's the same hardware as I've used for centrally positioned longitudinal bearers. It's simple to install and effective and just needs the bearer to have a profile approximately 60 X 32 mm.

As for attaching the side rail to the headboard I've mostly used this type immediately below. I haven't come across this exact pattern of bed fitting here in Europe, but I used to buy them when I lived in the US. The nearest equivalent is something called Noval K bed fittings seen at bottom which I've used a couple of times. In my experience they seem to work well enough, are easier to install than the type in the upper image, although I don't think they're quite as robust and bulletproof. The image, incidentally, is sourced from Häfele as follows: https://www.hafele.co.uk/INTERSHOP/...le/en_GB/pim/images/default/ppic-00630241.jpg Slainte.

View attachment 137398

This is a very timely post as I'm building a bed next week and now realise I was overthinking the joinery details, I presumed metal brackets were a no no but if they are good enough for @Sgian Dubh they are definitely good enough for the small budget model I'm building

 

[tibi]

That's the same hardware as I've used for centrally positioned longitudinal bearers. It's simple to install and effective and just needs the bearer to have a profile approximately 60 X 32 mm.

As for attaching the side rail to the headboard I've mostly used this type immediately below. I haven't come across this exact pattern of bed fitting here in Europe, but I used to buy them when I lived in the US. The nearest equivalent is something called Noval K bed fittings seen at bottom which I've used a couple of times. In my experience they seem to work well enough, are easier to install than the type in the upper image, although I don't think they're quite as robust and bulletproof. The image, incidentally, is sourced from Häfele as follows: https://www.hafele.co.uk/INTERSHOP/...le/en_GB/pim/images/default/ppic-00630241.jpg Slainte.

View attachment 137398

Hello Richard,

Is the central bearer placed in a horizontal position? I have the same type of connector on my current MDF bed, but it has two supporting legs under central bearer, which is placed in a horizontal position. If I want to omit the supporting legs, would not be it better to rotate the central bearer vertically, so it will not bow as much under load? If yes, maybe I should screw together a T-shaped bearer so that the upper part will be horizontal and the lower part will be vertical.

Something like the picture below. The connector will be attached to the end of the upper board.

Those side board connector that you have shown me can be bought easily here in Slovakia. I have opted for a different system, as I am afraid of some play in the connector joint and resulting squeeking. I want to control the tightness of the joint as the wood dries and moves, so I would use this system of headless bolt, nut and dowels. I will cut out square hole for nut that will be big enough for a wrench, so no special round washer will be needed, just a regular one.

 

[Orraloon]

As said above, to support weight depth of section is more important than width. Double the width and you double the load bearing but double the depth and its 8 times the load. I also favor a center leg in the middle rail as you can then use a much lighter rail. One center leg still allows plenty of under bed storage methods.
I also just used standard knockdown hardware for the side and middle rails so you can move the bed with just a mallet to knock down and assemble. 10 plus years on and no problems. With that kind of hardware the weight itself keeps things tight. They are simple and they work. I had had enough of M&Ts by the time that headboard was done in any case.
Regards
John

 

[Sgian Dubh]

Hello Richard,

Is the central bearer placed in a horizontal position?

Those side board connector that you have shown me can be bought easily here in Slovakia. I have opted for a different system, as I am afraid of some play in the connector joint and resulting squeeking. I want to control the tightness of the joint as the wood dries and moves, so I would use this system of headless bolt, nut and dowels. I will cut out square hole for nut that will be big enough for a wrench, so no special round washer will be needed, just a regular one.

Yes, the central bearer is positioned horizontally, i.e., the wide faces are parallel to the ground. You could certainly strengthen it by adding a piece on the underside (glue and screw), but I really don't thinks it's needed. I say that because in the case of one the beds I've made the mattress supporting slats are 20 mm thick poplar (tulipwood) on top of the ~60 X 32 mm poplar longitudinal bearer carrying an 80" X 60" (2030 X 1520 mm) mattress, and after nearly twenty years there's no appreciable sagging of either the slats or the longitudinal bearer. This bed is in our bedroom, it being being one of a pair I made, one for a customer and the extra one for us. There's also no additional middle leg, and time has proved it's not necessary, but if you want to include one on your bed, that's your choice. Adding one does rather clutter up the under bed space, but it's your choice.

I wouldn't choose your side rail connection method. If I was going to put a bolt of some sort in I'd want the bolt to pass all through the leg. With yours there's a small chance the brass threaded insert could pull out from the leg leading to looseness at the rail leg intersection. In addition you need a spanner to assemble the bed, whereas with the Noval K type fittings the two parts lock together as the side rail hook part drops into the leg mounted socket part. In general I'm not a fan of bolt together beds, but that's just my personal bias. If it works for you and you're happy with it you'll be in line with a lot of other people that have used the same assembly method. Slainte.

 

[Sgian Dubh]

As said above, to support weight depth of section is more important than width. Double the width and you double the load bearing but double the depth and its 8 times the load. Regards, John

You've highlighted a very good point about beam strength. Certainly, increasing the depth of a beam is the most significant means for increasing its load bearing capacity. I think, however, you might have got the increased load carrying capacity factor a bit out when you double a beam's depth. You've said it can carry 8 times the load, but I think it can carry only four time the load.

I may be wrong, but I hope I'm not. Slainte.

 

[Orraloon]

You've highlighted a very good point about beam strength. Certainly, increasing the depth of a beam is the most significant means for increasing its load bearing capacity. I think, however, you might have got the increased load carrying capacity factor a bit out when you double a beam's depth. You've said it can carry 8 times the load, but I think it can carry only four time the load.

I may be wrong, but I hope I'm not. Slainte.

Going back a few years I made a few wood archery bows. That was a figure quoted by a lot of bow makers so I took it as a working guide. Not something I ever measured. I also think they were talking about loaded to destruction not safe working load. (get a fair bit of destruction making bows) Anyhow I will dig out and blow the dust off some old archery books to check. I'll get back with a source.
Regards
John

 

[Sgian Dubh]

Stiffness is more important than density for the return or bearer to be able to straighten a bowed board. Having said that, there is a relationship between stiffness and density in a piece of wood

We're basically talking about beam theory. To give a simple example, doubling the thickness of your stiffening spruce doubles its load carrying capacity, i.e., its ability to straighten your oak board. Similarly, doubling the width of your spruce piece doubles its ability to straighten your oak. A combination of increasing your spruce piece's thickness and width by randomly chosen amounts increases its strength, but by how much is a bit of a guessing game without doing some fairly complex calculations.

The following is an edit to correct an error I made in the post above at post #25. I'm not quite sure why I made the error, but probably a brain f art, so apologies extended.

In the text quoted above I said, "doubling the width of your spruce piece doubles its ability to straighten your oak" which is incorrect. I should have said doubling the width of your spruce piece quadruples its ability to straighten your oak. To almost double the strength of a beam or a board through increasing its width you only need to increase the width by 40%. As an example, to almost double the strength of a 100 mm wide X 20 mm thick board increase the board's width to 140 mm, and to almost double the strength of a 150 mm wide board increase its width to 210 mm.

Notice that I've consistently said 'almost double the strength' in the above paragraph. The reason is that it's quite easy to remember the idea that adding 40% to the width of a piece of wood, or the depth of a beam doubles the strength. In truth, the required extra width is approximately 41.25% to double the strength. So, for easy recollection of the guidance, but with slight inaccuracy, 40% it is. Slainte.