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MCAD Furniture Class #4: Building a Scale Model

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Blog entry by Jeff posted 05-03-2008 05:46 AM 6922 reads 0 times favorited 14 comments Add to Favorites Watch
« Part 3: Design Presentation and Final Modifications Part 4 of MCAD Furniture Class series no next part

As mentioned in the previous entry, at the suggestion of my instructor, I modified the design such that the whole top would be a torsion grid. This approach did present its own problem though. Since the design called for the ‘leg’ to act as a massive tenon seated in the underside of the top, I would need a lot of bulk around the leg. I considered laminating solid wood pieces to build it up. I decided against that; I can’t recall why.

Instead, I opted to take what I call the weight machine approach. I would laminate several pieces of dense materials together like the stacks of weights on a machine at a gym. The thought was I could get 1) a solid area to bore out a mortise and 2) more weight by putting together several pieces of MDF together (nasty, toxic material, I know). I knew I would be making some jigs for this so I went with it and Willie confirmed it would be an okay approach.

I modified the SketchUp file so I could refer to it later for actual measurements on the cut list as well as knowing what the dimensions would be so I could scale it accurately. I was off an running.

Willie, our instructor, wasn’t particular about the materials to use for the model or how ‘tight’ the model should be. I told him I wanted to build the model pretty accurately because I thought it would give some good insight to the actual process. He suggested Basswood since it’s tight grained even though it is soft. Actually, he thought I should build the torsion out of Basswood too since it was so light and would not be seen. I was surprised but did a little research and discovered it weighs 26 lbs per cubic foot making it one of the lightest domestic woods readily available. It’s also pretty cheap. I went to Rockler and got a small stick.

As it turned out, building a detail model was a great idea. The first realization I made was the number of cuts that would be necessary. Lots. Also, the realization that there were going to be a lot of setups. Not only were there few right angles but there were few parts with the same dimensions. The cutting of the parts for the model was tedious but necessary to see if this concept had a chance of succeeding. I really didn’t want to invest the funds in expensive wood and cut it all up only to become firewood.

Why so tedious you ask? Well, this was in February and being in Minnesota, my shop doesn’t get used between September or October through April because its in a detached, unheated garage. So, the bulk of all this fun and games was being done with exacto knives, exacto saws, a small bench hook and a block plane… I could have gone to the shop at school and used the bandsaw but by the time I got out of work, drove to Minneapolis from St. Paul in rush hour traffic I had about an hour and a half of shop time. Between gas prices, lack of familiarity with their machines, and competing with full-time students for ‘open shop’ time, it was problematic.

The situation was what it was. Here is the result of a weeks worth of evening work on the kitchen counter:

3/4 view from left end

3/4 view from right end

When I was working on the leg/main support, I second guessed myself on the thought of a simple friction fit between the top and the leg. I for some reason thought cutting an actual tenon at the top of the leg would be a good idea. I think I reasoned it would make fitting it in the stacks of MDF easier.

detail #1 of tenon on model leg

detail #2 of tenon on model leg

In my previous post there is a detail shot of the tenon inserted in between the stacks. All in all, despite my experiment with the tenon, the model was solid! I got brave and tried squeezing the top/torsion frame from both the sides and the end. I even tried twisting it. It was stable. The one part I couldn’t test was the beam/stretcher strength. I didn’t actually cut a through mortise for that on the leg. I just glued it so technically it wasn’t completely accurate for judging the physical strength of the design. I felt relatively confident with it though…

model supporting a 14oz iPod

My iPod weighs 14 oz. I didn’t do any math to determine it’s real world weight. Frankly, I don’t know the necessary formulas to accurately calculate what the real-world weight of an object that size would be. I do know that the deflection of the top from level was minor and I attributed a lot of that to the fact that the fit between the tenon and the stacks was not as snug as it would be when actually built. Also, I did put my 17oz #90 shoulder plane on the model but didn’t have the camera close by at the time. (I know. No pic, didn’t happen…) The model supported the plane but I was nervous about the glue holding the beam to the leg so I didn’t push my luck more than 10 seconds or so.

In summary, I truly believe building a model is a valuable undertaking when the design before you is involved or unproven. Were I to do it again, I would perhaps make it at 1/3 scale so the process was less finicky. At that size, a person could also use something like corrugated cardboard or foamcore but that might not provide accurate feedback as to the physical abilities of the design but would likely be adequate for seeing the piece in 3D so you could do as many walk-arounds as it took to be sure you were satisfied.

As always, thanks for following along and your critique is truly invited.

-- Jeff, St. Paul, MN



14 comments so far

View Karson's profile

Karson

34876 posts in 3057 days


#1 posted 05-03-2008 05:53 AM

Jeff a great post. Nice job and a great discussion item.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Southern Delaware karson_morrison@bigfoot.com †

View Greg3G's profile

Greg3G

815 posts in 2742 days


#2 posted 05-03-2008 06:30 AM

very good post Jeff. excellent work on the model. I tried modeling a few times, it not as easy as it looks. I never was satisfied with my model, but it did help with the full size project.

-- Greg - Charles Town, WV

View John Ormsby's profile (online now)

John Ormsby

1280 posts in 2393 days


#3 posted 05-03-2008 06:44 AM

Hello Jeff, Interesting design and concept. I would like to make one suggestion. Using a tenon as you have will put an incredible strain on the tenon. By using the entire upper portion of the base, you will have full structural integrity of the upper base. It would be easy to just rip the edges of the top stacks at the proper angles to accommodate the base. You could possibly use through bolts to attach the base and top together. I also think it would be a good idea to add the torsion box ribs to the other side of the top to add more strength and rigidity for the base\top connection. This way you would be trapping the base to the top.
I like the fact that you are researching other ways to play with wood.

-- Oldworld, Fair Oaks, Ca

View Scott Bryan's profile

Scott Bryan

27251 posts in 2478 days


#4 posted 05-03-2008 01:51 PM

Jeff,

Thanks for the update. This is a nice post. For most of us, the idea of a model, is somewhat foreign. We would rather just jump in and start making cuts and worrying about whether it is actually going to work later on. Good advice.

Thanks for sharing.

-- Challenges are what make life interesting; overcoming them is what makes life meaningful- Joshua Marine

View jjohn's profile

jjohn

390 posts in 2370 days


#5 posted 05-03-2008 02:46 PM

If i made a model I’d get away from it somehow during the actual project. With me If I can draw it out the rest is just blind luck. But I also don’t veer to far from conventional designing either.

-- JJohn

View jjohn's profile

jjohn

390 posts in 2370 days


#6 posted 05-03-2008 02:47 PM

oh…yea, The design looks great. ;)

-- JJohn

View Jeff's profile

Jeff

1011 posts in 2750 days


#7 posted 05-03-2008 04:06 PM

Good morning! Thanks for your feedback. I think the modeling toping was discussed once ‘back in the day’ here on the site. I’ll see if I can find that thread and bring it back to top. It would be interesting to hear others experiences.

John Ormsby, I agree with your assessment and I ultimately decided to go with the original plan without the bolts. There is a method to my madness regarding the absence of ribs in the torsion on the other side of the leg. That area is reserved as a box for counter weights.

-- Jeff, St. Paul, MN

View GaryK's profile

GaryK

10262 posts in 2645 days


#8 posted 05-03-2008 06:18 PM

That is definetly an interesting design. Models are always a great idea.

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

View johnjoiner's profile

johnjoiner

160 posts in 2550 days


#9 posted 05-03-2008 06:46 PM

That’s cool, Jeff.

My understanding of torsion boxes is that the skin plays a major role in their strength. So your top should even more rigid when it’s complete with it’s skin.

-- johnjoiner

View Harold's profile

Harold

310 posts in 2504 days


#10 posted 05-03-2008 07:50 PM

First, I love the design. Can you shift the tenon cut in the rear leg further to the rear, not further back in the table top, but actually in the leg section itself. this would create the ledge or bench in which the top sat in front of the tenon as opposed to the rear. It would put this portion of the joint in compression when weight was placed on the long end of the table. I don’t see why the tenon would have to extend all the way thru as you have it currently, you could acually create a mortise in the “sub” top and then use long mecanical fastners to pull the top down on and thru the tenon.

-- If knowledge is not shared, it is forgotten.

View Jeff's profile

Jeff

1011 posts in 2750 days


#11 posted 05-04-2008 06:10 AM

Hello Harold. Thanks for your input. I actually decided against the cut tenon. The whole leg is being treated as a tenon into the top. Check out the drawings below. These are from the final design.

detail of the leg as its own tenon from below

detail of the leg as its own tenon from above

John, you’re right. It did take on an increased rigidity after I attached the skin.

Gary, I’ve already started thinking about another model to work out the new dining room table design I have in my head. I’d like to incorporate some the same elements from the desk. The design I have in mind won’t be as tricky as this desk though I don’t think. Should make for quicker execution of the model. Ha! Famous last words…

-- Jeff, St. Paul, MN

View Dorje's profile

Dorje

1763 posts in 2653 days


#12 posted 05-05-2008 07:03 AM

I’m still intrigued with all the work you’ve done!

-- Dorje (pronounced "door-jay"), Seattle, WA

View grumpycarp's profile

grumpycarp

257 posts in 2402 days


#13 posted 05-05-2008 10:54 AM

This load of this design is pretty much based on the interface between the (most red) rear section and the sides. Once the skin is on drop something on it . Dollars to doughnuts that is where it will fail. All that torsion box is just filligree. The short end of that “joint” will fail. And making the lowest element only provides an additional wedge, it should be rectangular, not triangular.

View Jeff's profile

Jeff

1011 posts in 2750 days


#14 posted 05-06-2008 04:38 AM

grumpycarp, I’m not sure I follow you. What are you referring to when you say, “making the lowest element only provides an additional wedge…”?

-- Jeff, St. Paul, MN

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