Bowed long boards - any remedy?

[Sgian Dubh]

Make the upper mortice as normal, i.e., just maybe 0.5 - 0.75 mm longer than the width of the tenon. Make the lower mortice perhaps 1.5 - 2 mm longer than the tenon's width. This concentrates any width change in the rail at its bottom edge. You could, of course, reverse the lengthening of the mortices to concentrate such movement to the top edge of the rail because in your case it doesn't really matter as far as I can see which way any width change occurs. In the example of the 300 mm wide rail I sketched earlier I wanted to keep the top edge of the rail flush with the top of the post (leg) which is why the bottom tenon wasn't glued and the mortice was longer than the tenon width to allow for any dimensional change to be concentrated towards the rail's bottom edge.

As to dowel/peg hole boring you'll just need to make the hole slightly slotted across the width of the tenon that goes into the elongated mortice, whichever mortice it is that you choose to extend a bohair. Generally, you should be able to achieve the slotted effect by carefully wriggling the drill bit back and forth a bit across the width of the tenon after you've initially bored the tenon's clean circular hole.

In truth, the movement your rail is likely to experience is tiny because it's only 120 mm wide so you shouldn't let yourself get overly concerned about it, really. Slainte.

 

[tibi]

Make the upper mortice as normal, i.e., just maybe 0.5 - 0.75 mm longer than the width of the tenon. Make the lower mortice perhaps 1.5 - 2 mm longer than the tenon's width. This concentrates any width change in the rail at its bottom edge. You could, of course, reverse the lengthening of the mortices to concentrate such movement to the top edge of the rail because in your case it doesn't really matter as far as I can see which way any width change occurs. In the example of the 300 mm wide rail I sketched earlier I wanted to keep the top edge of the rail flush with the top of the post (leg) which is why the bottom tenon wasn't glued and the mortice was longer than the tenon width to allow for any dimensional change to be concentrated towards the rail's bottom edge.

As to dowel/peg hole boring you'll just need to make the hole slightly slotted across the width of the tenon that goes into the elongated mortice, whichever mortice it is that you choose to extend a bohair. Generally, you should be able to achieve the slotted effect by carefully wriggling the drill bit back and forth a bit across the width of the tenon after you've initially bored the tenon's clean circular hole.

In truth, the movement your rail is likely to experience is tiny because it's only 120 mm wide so you shouldn't let yourself get overly concerned about it, really. Slainte.

Thanks a lot Richard,

I will do as you advised. I have made a guide for morticing that will keep side of the chisel perpendicular to the workpiece and the resulting mortice is now much cleaner than I used to make without the guide. It makes 12 mm offset from the edge. I will make a complimentary guide for sawing tenons that will make tenon central on my rail and 12 mm thick. As my rails are 25 mm thick, the offset on the complimentary guide will be (12 mm tenon width + 6,5 mm)=18,5 mm

Unfortunatelly, the edge of the mortice is only 8 mm away from the top of the leg and I managed to crack it a bit. I have repaired it and it will not be visible in the finished piece. But I have learnt the hard way that I need to hold leg clamped top to bottom when morticing.

 

[Sgian Dubh]

Unfortunatelly, the edge of the mortice is only 8 mm away from the top of the leg and I managed to crack it a bit. I have repaired it and it will not be visible in the finished piece. But I have learnt the hard way that I need to hold leg clamped top to bottom when morticing.

It sounds like you're talking about the rail into the footboard leg, and you possibly haven't allowed a wide enough shoulder at the top edge of the rail, which has led to that 8 mm bit splitting out. With M&Ts there's the common guidance of the 'Rule of Thirds'. Basically, where a rail joins into the top of a leg you should leave a substantial shoulder to prevent plugs of wood shearing out at the end of the mortice, and this rule of thirds, whilst not needing to be adhered to slavishly can offer some guidance. Your rail is, I believe, 120 mm wide, which would suggest the shoulder should be somewhere between about 30+ - 40 mm. If you refer back to the sketch of the forked tenon I showed earlier here, you can see the top shoulder is 50 mm, plus a haunch.

An illustration of shear failure caused by racking is below. This kind of failure at the end of the morticed member can also be caused by excavating the mortice - there's not enough material there to provide long grain shear resistance so that failure is more likely. Slainte.

 

[tibi]

It sounds like you're talking about the rail into the footboard leg, and you possibly haven't allowed a wide enough shoulder at the top edge of the rail, which has led to that 8 mm bit splitting out. With M&Ts there's the common guidance of the 'Rule of Thirds'. Basically, where a rail joins into the top of a leg you should leave a substantial shoulder to prevent plugs of wood shearing out at the end of the mortice, and this rule of thirds, whilst not needing to be adhered to slavishly can offer some guidance. Your rail is, I believe, 120 mm wide, which would suggest the shoulder should be somewhere between about 30+ - 40 mm. If you refer back to the sketch of the forked tenon I showed earlier here, you can see the top shoulder is 50 mm, plus a haunch.

An illustration of shear failure caused by racking is below. This kind of failure at the end of the morticed member can also be caused by excavating the mortice - there's not enough material there to provide long grain shear resistance so that failure is more likely. Slainte.

View attachment 138924


View attachment 138925​

Thank you for your explanation. I have always thought that rule of thirds refers only to the thickness of the tenon, not the width. I have 25 mm thick rail. It would account for an 8 mm mortice hole. As it sounds weak for a bed to me, I have opted for a 12 mm mortice hole, as my footboard leg is 51x51mm. My mortice is offset from the edge by 12 mm.

I have read a rule that mortice width should be maximum 5x mortice thickness. As I wanted to use forked mortice and wanted the mortices to be wide, I did not account for the top shoulder. I made 2x 40 mm mortices with 30 mm middle haunch between them. Only 5 mm wide shoulder is on the tenons from the top and bottom of the rail. Hopefully, there will not be much sideways racking on the bed that would make the top part pop out.
This is the drawing of my footboard legs. When I assemble the leg to the rail, the leg top is 3 mm above the top of the rail. It was not made to be flush in my case.

and here is the first leg that I have made. As you can see part of the wood splited also sideways when I was making the groove between mortices. Hopefully it will not affect the strength of the mortice. It will not be visible, as it will be covered by the shoulder. I will surely clamp the leg top to bottom when I will dry fit the tenon and then also assemble the rail, because the top part would surely pop out, if the tenon is a bit oversized.

 

[Sgian Dubh]

Thank you for your explanation. I have always thought that rule of thirds refers only to the thickness of the tenon, not the width.

Your drawings confirm what I'd guessed. As I said in my last message I would have aimed for a shoulder at the top edge of your footboard rail of about 30+ mm. The 8 mm beyond the end of the mortice at the top of the leg is too small really and it is a weakness, as you've found. But it's done now, so you have two choices of either carry on or, secondly, modify or remake the parts. Ideally, you'd modify or remake, but given what I guess is your time and investment so far, I'd take the gamble and carry on. If there's later failure, that I suppose will have to be the time you address the issue - with a bit of luck you'll be okay; I've seen quite a few constructions much like yours where the maker has just about got away with it.

As to the rule of thirds I think of it as a loose guide rather than something that should be adhered to strictly, and there are many circumstances where it doesn't really apply. For example, regarding tenon thickness, if your tenoning an 18 mm thick rail into the middle of a 60 mm thick leg then you can legitimately make the tenon thickness ~14 mm with just a 2 mm wide shoulder on either side assuming you've got the means to execute the job. As for the rule of thirds applying to the width of a tenoned rail, it really only applies where the rail joins a stile, leg or post at a corner. So, for example, if you have a 60 mm wide rail forming a corner at the top of a table leg, then the shoulder towards the top edge of rail should be something like 18 - 20 mm, and might incorporate a haunch, something like below. Slainte.

 

[tibi]

Your drawings confirm what I'd guessed. As I said in my last message I would have aimed for a shoulder at the top edge of your footboard rail of about 30+ mm. The 8 mm beyond the end of the mortice at the top of the leg is too small really and it is a weakness, as you've found. But it's done now, so you have two choices of either carry on or, secondly, modify or remake the parts. Ideally, you'd modify or remake, but given what I guess is your time and investment so far, I'd take the gamble and carry on. If there's later failure, that I suppose will have to be the time you address the issue - with a bit of luck you'll be okay; I've seen quite a few constructions much like yours where the maker has just about got away with it.

As to the rule of thirds I think of it as a loose guide rather than something that should be adhered to strictly, and there are many circumstances where it doesn't really apply. For example, regarding tenon thickness, if your tenoning an 18 mm thick rail into the middle of a 60 mm thick leg then you can legitimately make the tenon thickness ~14 mm with just a 2 mm wide shoulder on either side assuming you've got the means to execute the job. As for the rule of thirds applying to the width of a tenoned rail, it really only applies where the rail joins a stile, leg or post at a corner. So, for example, if you have a 60 mm wide rail forming a corner at the top of a table leg, then the shoulder towards the top edge of rail should be something like 18 - 20 mm, and might incorporate a haunch, something like below. Slainte.

View attachment 138943​

Well, I have looked at my drawings and I have decided to leave the already made leg as it is (with the risk of splitting later) and alter the rest (as the discrepancy will not be seen).

The impacted places are the other leg of the footboard and the top rail of the headboard. For the top rail of the headboard, there is 25 mm of material between the top edge and the uppermost mortice. I can make the width of the uppermost mortice 35 mm instead of 40 mm and I will get 30 mm from the edge.

For the other footboard leg, I have several options.
I can either make it like this

Or just one single tenon, that will be 85 mm wide and 30 mm from the top. What is better? Thanks.

 

[Sgian Dubh]

Or just one single tenon, that will be 85 mm wide and 30 mm from the top. What is better? Thanks.

Go with that. No need to fork it. Slainte.

 

[tibi]

Go with that. No need to fork it. Slainte.

I was also thinking about filling the gap at the top of the already made leg with 22 mm long and 40 mm deep piece of oak rectangle. I would glue it inside the mortice. Maybe it would prevent splitting the top in the future. I would shorten the width of the tenon to accommodate the change.

 

[Sgian Dubh]

I was also thinking about filling the gap at the top of the already made leg with 22 mm long and 40 mm deep piece of oak rectangle. I would glue it inside the mortice. Maybe it would prevent splitting the top in the future. I would shorten the width of the tenon to accommodate the change.

You could do something like that. Done well it should help reinforce the top end of the leg beyond the existing mortice. But I'd compromise a bit and make your 'plug' only about 10 - 15 mm long X 40 mm wide X a thickness to match the mortice width (12 mm?) because that still leaves you with your already cut, and then modified tenon (I presume it's cut) at ~25 - 30 mm wide. You can, of course, make your plug somewhat longer than your target length, glue it in place, and then chisel the plug back to get the length you need. Making your finished plug 22 mm long would leave your reduced width top tenon only 18 mm wide, which I'd say is a bit too delicate. Slainte.

 

[tibi]

You could do something like that. Done well it should help reinforce the top end of the leg beyond the existing mortice. But I'd compromise a bit and make your 'plug' only about 10 - 15 mm long X 40 mm wide X a thickness to match the mortice width (12 mm?) because that still leaves you with your already cut, and then modified tenon (I presume it's cut) at ~25 - 30 mm wide. You can, of course, make your plug somewhat longer than your target length, glue it in place, and then chisel the plug back to get the length you need. Making your finished plug 22 mm long would leave your reduced width top tenon only 18 mm wide, which I'd say is a bit too delicate. Slainte.

Thanks for the advice. Fortunately, I did not make anything yet, just this single leg. Other components are just dimensioned to size (not trimmed to exact lengths yet). So no tenons are cut yet. So for the existing leg, I can insert the plug, so that I can have 30 mm shoulder on the top. I will excavate the middle haunch in the mortice, so I get 85 mm single mortice and the bottom shoulder will be 5 mm as it was originally. If it is a bad idea and longer shoulder is needed, then I need to make a plug for the bottom part of the mortice as well.

It would be more feasible to change the forked mortice to a single mortice because it is easier to produce and I need 10 single mortices for the bed in total. If a single mortice will compromise the strength, then I need to stay with a forked mortice.

What proportions do you suggest for a single mortice? I can alter the design, because only 1 mortice and no tenons were cut yet. The mortice thickness is 12 mm. The width of the rail is 120 mm. The left and right shoulder will be 6,5 mm. Now I have to decide for the top and bottom shoulder and if any haunch is necessary on the top or bottom part. I would like to use identical mortices on the whole bed, if possible.

It will also be a little bit tricky to cut tenons, as the rails are 1,89 m long, so I need to clamp them in the vice and cut them at an angle to support the other end on the floor. A bandsaw would be handier now, but I do not have one, so I need to do it by hand.

 

[Sgian Dubh]

What proportions do you suggest for a single mortice? I can alter the design, because only 1 mortice and no tenons were cut yet. The mortice thickness is 12 mm. The width of the rail is 120 mm. The left and right shoulder will be 6,5 mm. Now I have to decide for the top and bottom shoulder and if any haunch is necessary on the top or bottom part. I would like to use identical mortices on the whole bed, if possible.

Generally I don't think about incorporating a forked M&T until the tenoned member gets up to roughly 150 mm wide, or greater. There have been occasions where for aesthetic reasons I've included a forked tenon in narrower tenoned parts, but this has always been, as far as I can recall, where the tenons came through and were reinforced with wedges in a different wood colour. So, basically a bit of highlighting of the joinery more than anything.

I can't see any good technical reason why you can't use single tenons almost up to the full width of your 120 mm wide rails, e.g., with shoulders only five or six mm wide at the narrow edges. But, of course, some rails ought to have bigger shoulders than that especially at corners, or near corners, such as the top of the legs at both the footboard and headboard ends. That just leaves the one joint you've already cut and needs a bit of rescuing. You can, incidentally, include more than one drawboring dowel or peg into a 100 mm wide tenon. Two are not really needed, but if it preserves a consistent appearance with what you may have already done I'd say it's justifiable. Slainte.

 

[tibi]

Generally I don't think about incorporating a forked M&T until the tenoned member gets up to roughly 150 mm wide, or greater. There have been occasions where for aesthetic reasons I've included a forked tenon in narrower tenoned parts, but this has always been, as far as I can recall, where the tenons came through and were reinforced with wedges in a different wood colour. So, basically a bit of highlighting of the joinery more than anything.

I can't see any good technical reason why you can't use single tenons almost up to the full width of your 120 mm wide rails, e.g., with shoulders only five or six mm wide at the narrow edges. But, of course, some rails ought to have bigger shoulders than that especially at corners, or near corners, such as the top of the legs at both the footboard and headboard ends. That just leaves the one joint you've already cut and needs a bit of rescuing. You can, incidentally, include more than one drawboring dowel or peg into a 100 mm wide tenon. Two are not really needed, but if it preserves a consistent appearance with what you may have already done I'd say it's justifiable. Slainte.

Thanks,

So I will create top tenons of the footboard rail and headboard rail with a 30 mm top shoulder, 85 mm tenon, and a 5 mm bottom shoulder. All other tenons will be 100 mm wide with 10 mm top and bottom shoulders. All tenons will be 40 mm deep and will have two drawbore pegs to clamp the workpiece during glue-up, as well as for giving an accent to the legs. I will add a plug to the already made mortice so that I can have a 30 mm top shoulder on that footboard leg as well.

Thank you again for recovery coaching. I will post some pictures in this thread, once I have the footboard and headboard finished.

 

[Sgian Dubh]

Thank you again for recovery coaching. I will post some pictures in this thread, once I have the footboard and headboard finished.

Glad I could help a bit. I'm not quite sure how this thread ended up being an almost one-to-one conversation between us. There's usually plenty of people drop into threads like this with contributions, but not here at the latter stage for some reason. Anyway, all I can say is do the best you can, and maybe let us know how it all goes in the end. Slainte.

 

[tibi]

Glad I could help a bit. I'm not quite sure how this thread ended up being an almost one-to-one conversation between us. There's usually plenty of people drop into threads like this with contributions, but not here at the latter stage for some reason. Anyway, all I can say is do the best you can, and maybe let us know how it all goes in the end. Slainte.

Thank you again for all your help. I will have some more questions later about the slats, once I have the bed frame built .

I think that this thread became boring for some people to join. But I know some dirty trick, how to get more people involved:

I would like to cut all the mortices quickly and thus I would like to know what is the best way to sharpen my chisel.

And now we can both wait if someone takes the bait

 

[profchris]

I think that this thread became boring for some people to join. But I know some dirty trick, how to get more people involved:

I would like to cut all the mortices quickly and thus I would like to know what is the best way to sharpen my chisel.

And now we can both wait if someone takes the bait

Please Sir, Sir, I know the answer!

Get someone else to do both these for you.

 

[MikeK]

I would like to cut all the mortices quickly and thus I would like to know what is the best way to sharpen my chisel.

And now we can both wait if someone takes the bait

Not likely. This topic of sharpening does not come up here often, and when it does, the responses are usually the same since everyone agrees on the way to do this.

 

[morqthana]

Bench grinder, angles by eye, surely?

 

[tibi]

Bench grinder, angles by eye, surely?

It is also possible to sculpt an edge with an angle grinder. You just hold the chisel in your non-dominant hand and angle grinder in the dominant hand. It is best done near a river or a pond with a battery-charged angle grinder so that you can have constantly submerged tip of the chisel in the water to avoid overheating. Bigger water sinks are swimming pools are also permitted if your facility is equipped with them.

 

[Inspector]

The thread isn't boring but when you have one of the forum's best helping you there isn't much to add.

As for the chisels you need a professional sharpener to sit beside you and with pairs of the same sized chisels sharpening one while you work with the other. That way you only need to stop to sleep, eat or the other thing.

Pete

 

[tibi]

The thread isn't boring but when you have one of the forum's best helping you there isn't much to add.

As for the chisels you need a professional sharpener to sit beside you and with pairs of the same sized chisels sharpening one while you work with the other. That way you only need to stop to sleep, eat or the other thing.

Pete

I got it. You must first buy a sharpening machine similar to the machine that is used for copying key profiles

Then you must borrow a chisel from your favourite sharpening guru and copy it in the machine.

So the best idea is to buy a spare set of chisels that would be used as a benchmark, where you would copy the profile from the favourite sharpening guru for each size. Then you would use those benchmark chisels to copy the profile to your regular chisels. Problem solved. Forever.