# Calling all engineers!



## AandCstyle (Mar 21, 2012)

My son's Christmas present to me is that I get to make a desk for him.  Here is the inspiration piece except he doesn't want the file cabinet part. It will be an inverted "U". The dimensions will be 75"L x 30"H x 30"D and will be made from 8/4 walnut keeping it as thick as possible. I estimate that it will approach 200 pounds when complete. Therefore, it needs to be able to be dismantled for moving from my basement shop to his upstairs office.

My primary concern is the wracking since it will have no apron or modesty panel. My thought is to cut a number of mortices in the legs for corner braces and epoxy them in place. Then cut matching mortices to receive the upper arm of the brace and screw those to the top. How many of these should I put on each end and how long should the arms be? Am I correct to think that larger screws (1/4" lags) will hold better than smaller ones (#6)? I see this as the most likely point of failure so more screws are better? Can anyone suggest a better option?

The secondary concern is that the top might want to bow or twist. I can cut dados to accept angle iron and screw them to the top through slotted holes to permit seasonal movement. Any thoughts on this?

I greatly appreciate your comments and suggestions, Thank you!


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## PPK (Mar 8, 2016)

Wow! I would liken this to music, in which sometimes the fast/ornate is actually easier to create than the slower/simple.

I'm not really sure how one would go about it, except that yes, I'd try to use some angle iron or steel of some sort to prevent sagging & twisting.

I wish you good luck with it! I hope that others more knowledgeable chime in.


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## Sigung (Nov 20, 2013)

Art, I wouldn't build this on a bet, not out of a solid slab of walnut. Having already made a huge cutting board for my kitchen island out of walnut and hickory, even though I cut the walnut slab and the hickory boards into 1" thick pieces and rotated them a half turn before glue up ( thereby imitating quartersawn ) the darn thing still twisted about 1/8". I'm not sure a piece of angle iron that long would be strong enough to resist the forces the walnut could put on it. As an experiment, get a 75" long piece of angle iron, clamp it in your vise, grab the other end with a vise grip and see if you can twist it. If you can, then the wood can too. I'm sorry I'm not offering solutions, just problems, but I don't have any.


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## AandCstyle (Mar 21, 2012)

Thanks for the responses. Jerry, the angle iron would run across the underside of the top perpendicular to the boards so it would only be about 28" long and I can add as many as needed. Also since it would be morticed in it would effectively reduce the top to, maybe, 3 or 4 shorter sections.


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## splintergroup (Jan 20, 2015)

Wow Art, such a generous kid! 8^)

Ahh to be needed, I can see how you are chomping at the bit to take up the challenge!

To avoid any braces that are outside the plane of the top and sides, I think your idea has merit.

Let me get this straight 8^)
You are thinking of a slot, the thickness of your angle iron, in the top edge of the legs. The angle iron drops into this slot, providing an "L" through which to screw into the top. Basically you are creating the corner between the top and leg with angle iron.

Without thinking everything through, I'd probably get some 3 or 4" angle and just flush mount it into a rabbet on the underside ends of the top and inside top edge of the leg. This would give the best strength if a sufficient number of lag type screws are used.

I'd still worry about racking since the lever arm produced by the leg length is significant. Perhaps some 1/4" thick angle, 2"x2" with dowel pins running through the slot on the top edge of the legs? I'd worry about the epoxy separating from the slot if tweaked enough.


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## sras (Oct 31, 2009)

I'm with Jerry on the challenges of stability of a piece that big. I would consider laminating up layers of high quality plywood and covering that with thicker veneer - none of that paper thin stuff.

As for assembly, the angle iron suggestions sound good to me.


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## Snipes (Apr 3, 2012)

I would have a piece of plate steel fastens to leg mortised into the top.


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## theart (Nov 18, 2016)

Not sure about stability of the top, but your best bet for attaching it to the sides might be some heavy duty floating shelf supports like this:
https://www.walnutwoodworks.com/wp-content/uploads/2018/05/24-HD-DIMS.png

Mortise the wall plates into the bottom of the top and drill down into the sides for the rods.


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## AandCstyle (Mar 21, 2012)

Let me try to describe my thoughts better. I would use 8" corner braces with one arm morticed and epoxied into the desk legs and the other arm screwed into the underside of the top. These would, hopefully, be sufficient to prevent wracking.

The angle iron I mentioned above would run from the front to the back and be set into the wood to be invisible and screwed to the top through slotted holes. The purpose of this is to prevent the top from twisting.

I like the look of the shelf supports but I am concerned that they wouldn't provide enough anti-wracking insurance.


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## wormil (Nov 19, 2011)

The leverage of such a large desktop combined with the weight will almost certainly fail over time without aprons, modesty plate, or heavy steel plate angles which could be let in to minimize their visibility but that will mean shorter fasteners (edit, those little construction corner braces you linked won't be heavy enough IMO). It will need take down joinery so it can be moved. And I would choose QS walnut that had been around for awhile, not fresh from the lumberyard, to minimize the busyness of cathedral grain and give it the best chance of remaining stable.

But the example desk isn't solid wood and I would consider a veneered torsion box. Building it lighter will allow you to use lighter joinery.

From the link:

*Specifications*: 
The Kefa model is available in many different versions and finishes that uses mainly the following materials: leather, saddle leather, black walnut veneer with wax natural finishing, Titanium Oak veneer, brushed eucalyptus veneer with wax natural finishing. ...

The tops and legs of desks and conference table are made of honeycomb fiberboard with edges and crossbars in beechwood, honeycomb cardboard with thickness 42 mm and two panels each one 8 mm on the 2 faces available in the finishing leather, saddle leather or wood veneer. Tops and legs are available in the version partially in leather or saddle leather and partially in wood veneer. Tops of the desks are equipped with a plug box with 2x universal power sockets, 2x USB, 1x Audio 3.5, 1x HDMI, 2x RJ45 with top covered in leather or saddle leather.

The legs are fixed to the top with some cylindrical wood dowels and with some eccentric screws. Furthermore under the top, between the two legs is fixed a cross with thickness 36 mm in order to give solidity to the desk or to the conference table. *The modesty panel has a thickness of 19 mm*. and is available in all the finishing. 
The leg of the desk, in its function to support the top, can be replaced with a fixed pedestal cabinet or by a fixed sideboard cabinet.


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## AandCstyle (Mar 21, 2012)

Thanks, Rick, if I had the ability to veneer I would have gone that route. The torsion box construction certainly avoids numerous pitfalls.


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## pitchnsplinters (Dec 26, 2008)

Art,

I like the idea of making the top out of high quality plywood, like Steve suggests. have you ever seen the Mr. Sawdust table for the radial arm saw? The top of that long span could be constructed like the Mr. Saw dust table. The table is constructed of two pieces of plywood laminated together. each slab of plywood has a series of shallow 1/8" wide slots cut the length of the top … 5/16" deep. Then the strength of the top comes from embedding, I epoxyed mine in, lengths of 1/8" steel flat bar. Glue the two slabs together and you have an incredibly strong surface that will not sag. The structure is "hidden" in the top.

As for attaching the leg … you will have to use some steel or aluminum angle as suggested above.

Good luck.


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## bilyo (May 20, 2015)

> But the example desk isn t solid wood and I would consider a veneered torsion box. Building it lighter will allow you to use lighter joinery.
> 
> - Woodknack


100% agree with WoodKnack. The only way to do it (IMHO) is a torsion box using premium quality walnut plywood. You then have the opportunity to make the top and legs as thick as you wish without twisting or sagging concerns. I also agree that making it knock downable would be wise. You could perhaps make some removable panels on the underside adjacent to the legs to access assembly bolts.


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## AandCstyle (Mar 21, 2012)

This plywood suggestion is growing on me.


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## Unknowncraftsman (Jun 23, 2013)

This is a plywood design and a good one. I believe some designs don't cross over to both solid and made man substrates very well.
It's not impossible for it to be made from solid walnut.
It's kinda backwards to make something from solid to look like plywood. All the hard lines on the edges give it a feel.
Good Luck


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## sawdust1whisperer (Nov 11, 2014)

Art. Looks like a great modern era project.

I didn't have time to read all the suggestions so I don't know if anyone has mentioned this.

The angle iron sounds like the best solution. You could paint it black. If you don't want to mess with cutting and counter sinking steel you could make an approximately 3"x 3" solid length of walnut. Place it just out of view when standing or sitting at the desk where the top and side meets. Screw it to the top and and side of the desk.

I'm sure there's a brace like this one on the manufactured Nunziati nspired piece.

If have done this before and it works well.

For the top cut 4 or 5/4 walnut slats beveled on the ends and screw them at a right angle to the bottom of the top. I have a large wooded table from a Scandinavian mfg. that uses this technique.

Can't wait to see the finished piece. Walnut, to me seems like the best wood for this project.


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## splintergroup (Jan 20, 2015)

You basically have a choice, lighten up the materials/construction or beef up the structure at the corner. Any corner/90 deg type bracket is actually quite weak without any form of diagonal structure (triangle). The only way to avoid using a triangle is to thicken up the legs (hence going to .250" or larger)


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## DS (Oct 10, 2011)

In all likelihood, the example desk has a modesty panel that is fairly short, probably about 12" tall and it is not visible in the photos.

Certainly, this is how I would design a desk for production like that one.

Also, the example desk is built using HPL high pressure laminates.
You can buy HPL pre-laid up onto VC core plywood at most hardwood vendors that lay up veneers.

You can edge the piece with some basic laminating tools, etc.

For dis-assembly, mending plates and small angle brackets are customary.


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## sras (Oct 31, 2009)

I checked out the website a little more. Most of their desks use the file cabinet for stability. Ones without a cabinet use wider legs and a small apron. It seems you could add an apron and not impact the design negatively. If the apron was set back and didn't extend too far down it would be hard to even see it.

Just a thought…


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## AandCstyle (Mar 21, 2012)

I am really torn. Do I go with the torsion box construction or the beefy solid wood route? The torsion box presents a learning curve for an old dog. Solid wood is more challenging from the design perspective.

Please vote on how YOU would construct this project.


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## corelz125 (Sep 23, 2015)

If you are going to go the steel route I wouldn't use angle. The angle unless it's 3/4" thick it flexes and twists pretty easy. When we need to add more strength to a steel beam we use tee's. Which is like a beam cut in half. That will add even more weight though.


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## sras (Oct 31, 2009)

+1 for torsion box.

Torsion boxes provide a lot of stiffness for a not much weight. Add blocking to any attachment points.


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## a1Jim (Aug 9, 2008)

I think that most of the people with concerns are because of the fact that you intend to use solid wood,this project would be much less challenging if a stable man-made product like ply, or melamine or even if the top with the long span was built as frame and panel (like a door) perhaps with some glass over the top. Then the credenze could just be screwed to the top with bolts or screws from underneath.


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## wormil (Nov 19, 2011)

It went unnoticed but the specs mention a modesty panel. That may not matter since it's only an example desk but I think it's a good idea.


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## LeeRoyMan (Feb 23, 2019)

This is how I would do the legs. 12" L-Brackets They are very heavy duty. 
The brackets should be sturdy enough to stop racking.


















For the top I would have no problem being 8/4 solid. I made a table top 84×40 out of 8/4 Walnut and it stayed flat and was definitely stiff enough not to sag. It did sit on a center base so I did inset some angle iron the 40" direction to keep the ends flat. I can show you a pic of it if you want to see it.
I have also done 3/4" ply top with a build up underneath the edge and trimmed with hardwood, I can show you pics of that as well if you want.


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## AandCstyle (Mar 21, 2012)

Lee, it sounds like you have done exactly what I am contemplating. Did you just drill the holes for the arms of the braces? How did you ensure a tight fit? I would appreciate the additional pix if it isn't too much trouble. Thank you!


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## HokieKen (Apr 14, 2015)

I'll throw another log on the fire  I think this is do-able. Of course, the sag in the center is the biggest hurdle with that big of a span unsupported. But, what causes the sag? The weight. The weight of the top that's directly supported by the legs on the sides doesn't contribute to the sag though, only the unsupported portions.

So, with that in mind, what if you take your solid 8/4 top and route a pocket out of the center section 1-1.5" deep to reduce the weight?









Just a thought. It's not going to be as stable as a torsion box or veneered plywood but, it is solid wood and simple. You could still add a hidden modesty panel under there as well for added support if you felt it was needed.

Good luck with it!


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## AandCstyle (Mar 21, 2012)

Kenny, that is a good suggestion. Thanks.


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## JimYoung (Jan 20, 2014)

A couple more suggestions….

From LeeRoyMan's brackets, have a small triangle welded inside the bend and slot the legs for them. This will greatly increase the bend strength of the brackets and you hide the triangle inside the leg, or at least only have a small triangle showing underneath. I have a "breakfast bar" granite counter top which over hangs about 12" and uses 2 brackets made of 1/4" steel with a small triangle and it is rock solid (pun intended).

From Kenny's suggestion on routing out the bottom, maybe replace the routed out portion with plywood or steel angle to further support it.

Finally, I think you solve all of your racking problems if you convince your son to go with the "L" shape of the inspiration piece. Maybe not with the file cabinet, but it could be used for a printer or something.


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## JimYoung (Jan 20, 2014)

One more idea of using just wood to solve your problems. Think about where the desk will be placed and at what angles people will see it. You are trying to create the illusion of this heavy wood being able to support itself only at the corners. If people can't see the supports normally, the illusion is maintained. My personal desk is 36"x68", and you really don't see the structure underneath it when you are in the room with it.










One or two arches could support the top and prevent racking. You could do a dovetail to join the arch to the legs and screw the top to the arches. Or embed bolts in the legs that insert into the arch, and have a slot in the arch to tighten a nut on the bolt to pull it all together (like knock down hardware on a bed, but in reverse).


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## DS (Oct 10, 2011)

The description on the website link states the panels are honeycomb panels for light weight.

This is basically 1 1/2" thick cardboard with veneers or laminate on each face.
This is surprisingly strong and light weight. (And a bit expensive, and a bit of a learning curve)


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## mnguy (Feb 4, 2009)

IMO, this desk is a modern design with modern materials and methods required. Stretchers and other visible structure didn't evolve for pure form / aesthetic reasons; they are required to make solid wood furniture "work". This desk is the same - the form was only considered possible when new materials (high quality plywood, honeycomb cores, etc.) became available. My recommendation is like several others; torsion box with walnut plywood skins and walnut edge banding.

If you don't have the bench space to glue up the torsion box, I recommend using a solid core door as a work surface; they are heavy, solid, and pretty dead flat. They make pretty good utility workbench tops.

I'm a chemical engineer, which isn't the best qualifications for this challenge, but I do work in product design ;-)


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## AandCstyle (Mar 21, 2012)

Jim, more good suggestions. I especially like your arch design. Do you think that if I can get Greg to agree to 2 arches dovetailed in that I would even need to do the braces?


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## Lazyman (Aug 8, 2014)

Is he planning to put it against the wall or positioned in the middle of the room so that he is facing out when in use? If against the wall there is no reason not to put an apron on the side against the wall to make it more stable.

BTW, unless I am reading it wrong, according to the Sagulator, a 75×30x2 inch "shelf" will only sag .02" total with a 50 lb per foot uniformly distributed load and only .03" if center loaded. Unless you are planning to stand on it, that sort of load is unlikely. I assume that the sagulator adds the weight of the slab (~100 lbs) into the calculation but it doesn't actually say. It doesn't seem like the weight of the slab itself is a problem for any significant sagging so racking would seem to be the primary issue.

Note that I am a software "engineer" but you didn't specify which type could answer. ;-)


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## Tony1212 (Aug 26, 2013)

I plan to do something similar (eventually) with a torsion box. That seems to be the best way to get a long span without much sag. Ron Paulk's workbenches are 8 ft long without any discernable sag. (My desk will look more like a Paulk workbench with open sides).

My plan for attaching the legs is to do plywood boxes at each end - 3/4" thick on the outside. The desk top would rest on the top of the box while the 3/4" side would extend up and connect to the side of the desk.

I was going to look at the stability and add some triangles in the corners for stability if needed.

If you go with the torsion box, you can think of the cross members as the apron.


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## theart (Nov 18, 2016)

> This is how I would do the legs. 12" L-Brackets They are very heavy duty.
> The brackets should be sturdy enough to stop racking.


Back of the envelope, assuming those brackets are 1/8" thick and 1 1/2" wide structural steel, four of them together will only support a 20lb side load on a 28" tall table. That's not nearly enough. If you can get them in 1/4" thick it gets you up to 80lb, which is still marginal.



> From LeeRoyMan s brackets, have a small triangle welded inside the bend and slot the legs for them. This will greatly increase the bend strength


Only if the gussets extend out past the first attachment point on the top of the bracket. Otherwise, the failure point just moves.


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## LeeRoyMan (Feb 23, 2019)

Theart, Do you have a link to where you got those specs. I would be interested to look at it.

@80lbs each x 4 =320 lbs x 2 = 640lbs side force? 
How much side force do you recommend?
Do you have a suggestion that works better?


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## theart (Nov 18, 2016)

> Theart, Do you have a link to where you got those specs. I would be interested to look at it.
> 
> @80lbs each x 4 =320 lbs x 2 = 640lbs side force?
> How much side force do you recommend?
> ...


It's just from bending stress in the bracket. The issue is that the bending moment applied to the joint is the side load times the length of the leg. Bumping into the side of a table, or dragging it across the floor puts a large bending moment on the joint. Stress in the bracket came from sigma=Mc/I, with c=thickness/2 and I=width*thickness^3/12. If you assume a sigma value like 36000psi, you can solve for a maximum side load per bracket. The forces I gave above are total, not per bracket.

The shelf supports I suggested above might be a slightly better option. Again, the spec on those is 150lbs, but that assumes that the load is being applied at the center of a 12" deep shelf rather than at the end of a 28" table leg.

Ultimately, a knockdown joint for a right angle like this with no aprons is going to be very hard to pull off. The only way to reduce the bending stresses in the hardware is to add a compressive preload to the joint, but it's a long way coming up from the bottom of the leg.


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## wormil (Nov 19, 2011)

Look into steel corbels. They make some that look L brackets except they're thicker steel and a lot stronger but they'll also cost probably 20-30 bucks a piece instead of a couple dollars.


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## LeeRoyMan (Feb 23, 2019)

The bracket you mentioned looks good however there are lots of drawbacks to getting the strength.
Even if you drilled perfect holes, they have to be loose enough to get them in and out which creates racking in itself.
Also, there is only 1 1/2" wide plate mounting to the table so no mater how strong the connection is from the rod to the plate you lose strength in only having 1 1/2 wide surface. Holding a shelf on the wall is different than the racking.

I have used mine in the real world, never did the math, only speaking from experience, 
I would use mine over yours. 
(Although I have never used the ones you mention, so I have no experience with them.)

I have said all I will say,

Whatever method you use Art, I wish you the best, I'm sure you will figure out a way that works.

Cheers


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## jacksdvds (Jun 13, 2015)

Can I give you a Christmas present too? I have in mind an 8' laminated top work bench with tool drawers along the length, a vise and many dog holes? Hopefully in time for my lavish Christmas eve party so I can show it off to all.


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## sawdust1whisperer (Nov 11, 2014)

Torsion verses solid wood. Art you really started a lively conversation here.


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## JimYoung (Jan 20, 2014)

> Jim, more good suggestions. I especially like your arch design. Do you think that if I can get Greg to agree to 2 arches dovetailed in that I would even need to do the braces?
> 
> - AandCstyle


Hi Art,
I would definitely think that 2 arches would eliminate the need for brackets. My taliesin desk has two arches with through tendons, and it is very solid. 
If you are going to dovetail them, are you going to glue them in or try to make them slide so it can knock down? If they are not glued in place, you might need wedges or some hardware to help hold them tight.

I'm an Electrical engineer, but had to take a few mechanical engineering courses.

Let me know if you need any drawings.


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## AandCstyle (Mar 21, 2012)

Thanks for all the comments and suggestions. I have emailed Greg asking if he can tolerate the arches Jim suggested. Once he replies I will move forward or regroup.


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## sawdust1whisperer (Nov 11, 2014)

Art,. Will you be using the classy Kreg pocket holes on the arches?


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## AandCstyle (Mar 21, 2012)

Sorry, James, I don't own a Kreg jig.


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## sawdust1whisperer (Nov 11, 2014)

Art that would be interesting. Mixing a sort of arts & crafts (Tailiesin ) design with a modern era desk.

Kevin Rondel has very nice work In which he uses arts & crafts designs mixed with elements of Charles Rennie MacKintosh.

Do you think it will work?

If anyone could pull it off it will be you. 
Good luck.


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## AandCstyle (Mar 21, 2012)

I hope to hear back from Greg later tonight. I have explained the twin arches suggestion and I think he will agree that it will avoid the potential issues of sagging and twisting. At this point, his main concern is legroom but by pushing the front arch back a bit that should be a non-issue. I will post his decision and the final solution. We are going to their house for Christmas so it may take until then to iron out any wrinkles.


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## bndawgs (Oct 21, 2016)

I didn't think your tools were worked on anything other than QWSO Art?


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## AandCstyle (Mar 21, 2012)

Steve, I have had such a shortage of projects lately that my tools are happy to work on anything.


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