Lignum
20th January 2006, 12:11 AM
This is re posted for anyone who is interested in having a go at making a Torsion Box for some veneering and didnt get to read it on another thread.
It was initionaly aimed at making a 1500mm dia x 50mm box. but towards the end it was kindly pointed out the table is to be square:o The basics are the same, but you get a guide on how to do the round torsion frame and apply the solid edging just before the veneer is applied. The way the solid edge is applied you can then rout profiles if you want as the whole top will have similar propeties as a solid one. The sections in bold are questions that were put to me to reply to:)
Arron my advise would be (and this applies to anyone else out their who would like to have a go at a torsion box) is to get two 4mm MDF coversheets and tape them together and draw a 1500mm (dia) top onto it. That will be your template for the positioning of the torsion ribs.
On the template work out where the legs will be fixed to the top and mark those positions.
The thickness of your ribs will be determined by the skin thickness. I would favor 9mm both sides that will alow 32mm slats x 19mm and they are best in pine because of the lightness in weight.
First, draw your leg positions on the template and include a generous size there, as it will be of hardwood not pine so you have good strength for anchoring. As i dont know your design ill just assume its in four points near the edge and you can alter it to suit.
Draw 14 @ 19mm strips parallel down the length of the table starting dead center. My quick calculations are 14 strips x 19 thick = 266mm . 1500 top - 266 divide by 14 = 95mm. thats your spacing. Its closer than what i would use but again your first torsion should be overkill until you fully understand it. They might alter from 95 when you get to your leg spacing but thats ok. If possible 4 @ 95 x 32 x 300 Tas-oak would be good for leg blocks.
Once all the parallel ribs have been drawn, then its time to mark your edging on the template. Their will be two edgings here. The first "inner" edging 65 (w) x 32 (th) Tas-oak. That will secure your ribs strongly and allow the 9mm skins to be pressed over the entire top. The second lot of "outer" edging will be the same timber as the top veneer that is being layed and same thickness of your finished top, and will be glued around the edge of the top after the mdf and Tas-oak has been trimmed. That way when your veneer has be layed you will have a substantial solid edge that "wont" be sanded through and can also be routed with a profile if you wish.
(the 65mm edging isnt the final size. Once it has been routed parts of that will end up around 20 or so mill, you may find when you draw the template you need 70mm wide or maybe more, but you will work it out, because im telling you all this with out drawing one up and having exact measurements, so keep that in mind)
So you will have drawn two circles on the outer edge. The first the finished table size (1500) and the second the finished torsion size wich will be 30mm less in dia. Divide the second size into 12. (If it ends up to short making your edging to wide, just increase the amount to 18 and see what its like) As long as the final results in edgeing of around 65-70mm. So cut a strip of MDF to the final worked out length length x 65mm (or more to suit) for the thickness. Place that at the top of the drawn center rib, with the center top of the template overhanging the the line of the inner circle by around 4mm and trace around it. Then repeat all the way around just overlapping so you end up with 12 (or more) long segments x 65 (or more) that form a circle. Then with your rule mark a straight line through where they meet at the end as that will be the mitered angle.
Now you are ready to make the ribbing. First cut the Tas-oak edge exactly as its on the template. once you have it sitting on the template and happy with the angled mitered joins (butt joined) then join it with what you have at hand I find staples are great, or skew nail. As they will be end grain to end grain, only certain glues are any good, i use titebond 111 as it is terrific for end to end gluing. But any good glue will be fine as its a non stress join so no need to panic. Its all about getting a good strong initional frame built up to take the skins which is where the strength comes from
Once the inner circle framing has been done and glued and fixed, cut and lay all the pine ribs into it including your tas-oak pieces for the legs.(dont glue or staple yet) Then cut a heap of 32 x 19 x 95 for the internal bracing. But first rotate the Tas-Oak frame slightly so the ends of the ribbs as drawn wont intersect with a join in the miter. When the ribs are cut to fit very snug, glue and staple/nail the first rib (the one in the center) then working to the sides place the cross braces around 200mm apart next to the first rib with a dab of glue on the end and then put the second rib in and draw it tight and staple/nail, keep repeating it till you get to the last rib, then you will just adjust the final cross brace to fit snug. Also stagger them as its easy then to pin from the back of the rib into it. Just like the frame work on a wall in a house.
If all went well you will now have a very good strong ribbing. But the MOST IMPORTANT thing is it HAS to be flat, so punch down all nails/staples and plane or sand all ridges of and drag a straight edge over it to make sure its as flat as you can possibly get it. this is the most crucial area.
Now you are ready to lay the skins. If you havnt access to a vacuum bag (1500 is wide) this is best done in two goes. And just as important, if the skins are glued and you have a twist or anything with the frame it will STAY forever. Torsions are just impossible to alter once they are glued. And resist any temptation to secure your ribbs directly to a skin as it "isnt" strong. It appears to be a logical quick way but it defeats the whole purpose and will result in a weak torsion.
So prepare your top (you will need a 3600 x 1800 x 9 sheet of MDF cut in to two) cut them around 1500 (dia) each and get as many off cuts of mdf or chipboard etc to use as weights. Place your sheets on top and bottom and mark with pencil the positioning of the ribs.(always good to know where they are) Then using a roller, starting on the underside first roll on some good PVA all over the ribs and place the MDF on and carefully pin all over (dont staple as they dont penetrate properly) I do this when im sitting on it as extra weight is good. Once that is done place as many off cutsand weights as possible as evenly as possible all over and leave for a few hours. Then repeat the process on the top. but first do a final check with a straight edge to make sure its flat. I dont pin the top at all, i like it to be super clean and smooth so no blemishes show through the veneer.
When this is completed turn it over and with trammel and router trim it to its final diameter (excluding the outer edging)
The last stage is the fiddliest, is to apply your solid outer edging. The main thing to look for here is to get the ends to mate as good as you possibly can. If you dont get it perfect on the curve of the torsion dont stress, you can use some good epoxy.
Lay your selected edge timber onto the MDF template and cut and (no glue yet) butt join end on end. When you are happy with the joins wrap a large piece of rope with a stick, just like a tourniquet, around to get good firm pressure to keep it stable. Then place the finished torsion on top of it and draw the profile to the edging. Take the edging apart (keep them numbered) and bandsaw very carefully the inner profile. Spokeshave and sand untill they all fit around the edge of the torsion. The main thing here is to make sure the end to end joins are spot on. If their are gaps where the edge mates with the torsion, dont stress, just use epoxy here as its gap-filling. Making a torsion should be quick, flat, accurate and tough, not pretty. Just go for it.
When ready to glue just follow the rope-tourniquet method brushing on glue inner edges and endgrain joins. and leave to dry (overnight if epoxy was used)
Also take into account if you intend to route profiles later be carefull if you nail it from the back or anything they dont interfear with the cutter. When dry use the same technique as the Tas-oak edging, use a trammel and router and trim to final 1500 dia. Then its only a matter of laying the veneer.
If it all sounds complex and to much stuffing around, well its not. Once you have done one, you will find its just as quick as preparing a solid top.
If all goes well, you will have a light and super, super strong table top that will have almost zero movement and excellent looking edging ready for veneering, with good strong hardwood inlayed blocks for securely anchoring your preferred legs to:)
It was initionaly aimed at making a 1500mm dia x 50mm box. but towards the end it was kindly pointed out the table is to be square:o The basics are the same, but you get a guide on how to do the round torsion frame and apply the solid edging just before the veneer is applied. The way the solid edge is applied you can then rout profiles if you want as the whole top will have similar propeties as a solid one. The sections in bold are questions that were put to me to reply to:)
Arron my advise would be (and this applies to anyone else out their who would like to have a go at a torsion box) is to get two 4mm MDF coversheets and tape them together and draw a 1500mm (dia) top onto it. That will be your template for the positioning of the torsion ribs.
On the template work out where the legs will be fixed to the top and mark those positions.
The thickness of your ribs will be determined by the skin thickness. I would favor 9mm both sides that will alow 32mm slats x 19mm and they are best in pine because of the lightness in weight.
First, draw your leg positions on the template and include a generous size there, as it will be of hardwood not pine so you have good strength for anchoring. As i dont know your design ill just assume its in four points near the edge and you can alter it to suit.
Draw 14 @ 19mm strips parallel down the length of the table starting dead center. My quick calculations are 14 strips x 19 thick = 266mm . 1500 top - 266 divide by 14 = 95mm. thats your spacing. Its closer than what i would use but again your first torsion should be overkill until you fully understand it. They might alter from 95 when you get to your leg spacing but thats ok. If possible 4 @ 95 x 32 x 300 Tas-oak would be good for leg blocks.
Once all the parallel ribs have been drawn, then its time to mark your edging on the template. Their will be two edgings here. The first "inner" edging 65 (w) x 32 (th) Tas-oak. That will secure your ribs strongly and allow the 9mm skins to be pressed over the entire top. The second lot of "outer" edging will be the same timber as the top veneer that is being layed and same thickness of your finished top, and will be glued around the edge of the top after the mdf and Tas-oak has been trimmed. That way when your veneer has be layed you will have a substantial solid edge that "wont" be sanded through and can also be routed with a profile if you wish.
(the 65mm edging isnt the final size. Once it has been routed parts of that will end up around 20 or so mill, you may find when you draw the template you need 70mm wide or maybe more, but you will work it out, because im telling you all this with out drawing one up and having exact measurements, so keep that in mind)
So you will have drawn two circles on the outer edge. The first the finished table size (1500) and the second the finished torsion size wich will be 30mm less in dia. Divide the second size into 12. (If it ends up to short making your edging to wide, just increase the amount to 18 and see what its like) As long as the final results in edgeing of around 65-70mm. So cut a strip of MDF to the final worked out length length x 65mm (or more to suit) for the thickness. Place that at the top of the drawn center rib, with the center top of the template overhanging the the line of the inner circle by around 4mm and trace around it. Then repeat all the way around just overlapping so you end up with 12 (or more) long segments x 65 (or more) that form a circle. Then with your rule mark a straight line through where they meet at the end as that will be the mitered angle.
Now you are ready to make the ribbing. First cut the Tas-oak edge exactly as its on the template. once you have it sitting on the template and happy with the angled mitered joins (butt joined) then join it with what you have at hand I find staples are great, or skew nail. As they will be end grain to end grain, only certain glues are any good, i use titebond 111 as it is terrific for end to end gluing. But any good glue will be fine as its a non stress join so no need to panic. Its all about getting a good strong initional frame built up to take the skins which is where the strength comes from
Once the inner circle framing has been done and glued and fixed, cut and lay all the pine ribs into it including your tas-oak pieces for the legs.(dont glue or staple yet) Then cut a heap of 32 x 19 x 95 for the internal bracing. But first rotate the Tas-Oak frame slightly so the ends of the ribbs as drawn wont intersect with a join in the miter. When the ribs are cut to fit very snug, glue and staple/nail the first rib (the one in the center) then working to the sides place the cross braces around 200mm apart next to the first rib with a dab of glue on the end and then put the second rib in and draw it tight and staple/nail, keep repeating it till you get to the last rib, then you will just adjust the final cross brace to fit snug. Also stagger them as its easy then to pin from the back of the rib into it. Just like the frame work on a wall in a house.
If all went well you will now have a very good strong ribbing. But the MOST IMPORTANT thing is it HAS to be flat, so punch down all nails/staples and plane or sand all ridges of and drag a straight edge over it to make sure its as flat as you can possibly get it. this is the most crucial area.
Now you are ready to lay the skins. If you havnt access to a vacuum bag (1500 is wide) this is best done in two goes. And just as important, if the skins are glued and you have a twist or anything with the frame it will STAY forever. Torsions are just impossible to alter once they are glued. And resist any temptation to secure your ribbs directly to a skin as it "isnt" strong. It appears to be a logical quick way but it defeats the whole purpose and will result in a weak torsion.
So prepare your top (you will need a 3600 x 1800 x 9 sheet of MDF cut in to two) cut them around 1500 (dia) each and get as many off cuts of mdf or chipboard etc to use as weights. Place your sheets on top and bottom and mark with pencil the positioning of the ribs.(always good to know where they are) Then using a roller, starting on the underside first roll on some good PVA all over the ribs and place the MDF on and carefully pin all over (dont staple as they dont penetrate properly) I do this when im sitting on it as extra weight is good. Once that is done place as many off cutsand weights as possible as evenly as possible all over and leave for a few hours. Then repeat the process on the top. but first do a final check with a straight edge to make sure its flat. I dont pin the top at all, i like it to be super clean and smooth so no blemishes show through the veneer.
When this is completed turn it over and with trammel and router trim it to its final diameter (excluding the outer edging)
The last stage is the fiddliest, is to apply your solid outer edging. The main thing to look for here is to get the ends to mate as good as you possibly can. If you dont get it perfect on the curve of the torsion dont stress, you can use some good epoxy.
Lay your selected edge timber onto the MDF template and cut and (no glue yet) butt join end on end. When you are happy with the joins wrap a large piece of rope with a stick, just like a tourniquet, around to get good firm pressure to keep it stable. Then place the finished torsion on top of it and draw the profile to the edging. Take the edging apart (keep them numbered) and bandsaw very carefully the inner profile. Spokeshave and sand untill they all fit around the edge of the torsion. The main thing here is to make sure the end to end joins are spot on. If their are gaps where the edge mates with the torsion, dont stress, just use epoxy here as its gap-filling. Making a torsion should be quick, flat, accurate and tough, not pretty. Just go for it.
When ready to glue just follow the rope-tourniquet method brushing on glue inner edges and endgrain joins. and leave to dry (overnight if epoxy was used)
Also take into account if you intend to route profiles later be carefull if you nail it from the back or anything they dont interfear with the cutter. When dry use the same technique as the Tas-oak edging, use a trammel and router and trim to final 1500 dia. Then its only a matter of laying the veneer.
If it all sounds complex and to much stuffing around, well its not. Once you have done one, you will find its just as quick as preparing a solid top.
If all goes well, you will have a light and super, super strong table top that will have almost zero movement and excellent looking edging ready for veneering, with good strong hardwood inlayed blocks for securely anchoring your preferred legs to:)