Branching Out Wood

Modern Functional Home Decor by David Wertheimer

CNC & Torsion Boxes

David WertheimerComment

I've really enjoyed working with my CNC so far, though I'll confess, I've been doing mostly prototyping - and one giant commission of twenty four oval mirror frames - with no finished products in my catalog yet to show off. But a lot of the time I've been spending with the CNC has been setting it up just right.

As mentioned in an earlier post, many of the next posts will be about the nuances of getting this thing set up, and with the first in the series, I'll talk about "torsion boxes". A torsion box is just a fancy word for a common construction technique where two thin layers of material are applied to either side of a lightweight core.

Flat, Light, & Strong

torsion_box_door.jpg

The most common example that we interact with every day is a "hollow core" door, that is, probably the type of door inside your house. They're super flat, fairly light, and yet very strong. If you were to peel back the thin wooden skin on one side of that door, you'd see a grid of material perpendicular to that skin made of something light and cheap- thin wood, or perhaps even cardboard, depending on the door quality. But they're also used in ships, furniture, some bridges, and many other places as well.

A few months ago, I bid on a kitchen cabinet project that included some 2" thick 15" deep shelves edge mounted on the wall. A 2" thick shelf, if made of solid wood of any variety, would be really heavy and expensive. For instance, a walnut shelf that large would be 0.7 pounds per inch of length, or 25 pounds for a 3 foot section. Mounting three of these on the wall with hidden hardware would be challenging to say the least. Solid wood also expands and contracts with moisture and temperature, making it more challenging to use for large solid wood furniture - see this blog for some more details about that. However, if made with 1/4" veneered MDF (medium density fiberboard) - a very smooth engineered product used as the core within a lot of furniture - the weight would be cut by more than half, to about 12 pounds for that same shelf.

Gaining Extra Height

Now let's return to the world of the X-Carve CNC. I added some extension brackets that allow me to work with much thicker blocks of wood with the machine, up to about 4.5" thick. But the X-Carve z-axis only goes up & down about 3", so that means that for the more common thinner stuff that I'll be cutting and carving, such as a cribbage board or a customized cutting board, the cutting head wouldn't be able to reach the material.

 You can see one of the silver brackets giving my machine an extra 2" of height in the lower left

You can see one of the silver brackets giving my machine an extra 2" of height in the lower left

Here's where the torsion box comes in. By adding another 2" layer to the bottom - kind of like a high chair for my CNC - that negates the effect of those brackets, so that I can work with standard-height materials again. But it's critical that the surface be perfectly flat, so that it doesn't introduce any errors into the CNC.

Bonus: Vacuum Clamps

 Instant 60 pounds of clamping force

Instant 60 pounds of clamping force

One other great bonus of this setup is that I can now easily use my vacuum clamps. The vacuum pump I purchased is great for veneering, but with a vacuum, you can also easily clamp flat items - like that cribbage board, cutting board, or a sign - if the underlying surface is also flat.

In fact, every square foot of clamping surface exposed to these vacuum clamps produces about 1300 pounds of clamping pressure. The vacuum clamps aren't a square foot - they're closer to 2.5" x 2.5", which produces only about 60 pounds of clamping force, but that's still huge - especially given that they do not mar the surface of what you're clamping, provide complete access to the top of the item, and are nearly instant to insert and remove, with the aid of a foot pedal.

The X-Carve "waste board" - the gridded board you see in some of the pictures - is perfectly flat. But it also has about one hundred holes drilled into it, so that you can use screw clamps. You can see those in that picture above as the green and blue plastic inserts sticking out of the board. They're very useful in their own right, but they limit the use of the vacuum clamps since, unless they are very carefully placed, the vacuum just goes right out the other side of the board, through those holes.

Wrapping It Up

So there ya have it - now you know what a torsion box is, and you could even make your own door if you so desired! But if you'd rather have me make the doors - or anything else for your home or business - look me up! I'd be delighted to explore your project with you.

How to Build a Cabinet in Ten Easy Steps

David Wertheimer1 Comment

I’ve been fortunate to get a number of commissions, both large and small, in the past few months, which has kept me from making a more recent post up, but it has also given me a bit more to write about.

Today, I’ll drill into one of those commissions in detail, share the construction considerations, and show some of the steps along the way.

Interested in discussing a piece? Send me an email or give me a call!

Entryway Shelf & Mirror

The request from the client was for hanging entryway shelf for their recently completed remodel, to fit between the angled walls. After a little more discussion about what woods might look nice together and complement their decor, they decided on walnut and birdseye maple, and for a wall-mounted mirror frame above.

The client provided a template for the top and the desired width. We had to go the template route - rather than me personally taking detailed measurements - as the piece would be installed in a condominium in Florida. After going through a few more detailed questions about design - top surface overhang, depth, etc. - I was able to produce this rendering that the client signed off on.

Rendering.png

Veneer or Solids?

There are two main construction techniques that were possible here: solid wood, or veneer. As touched on in an earlier post, though solid wood has some advantages, for something of this size - especially as it will be installed in a home in Florida - would deteriorate over time if made of solid wood with glued joints. Not to mention finding or joining together boards of that size would be either a lot of money, or time, or both. So that steered me towards a predominantly veneered MDF (medium density fiberboard, an incredibly smooth and stable engineered wood product made of wood fibers glued together under considerable heat and pressure) construction technique for the large top and bottom.

However, the front edges would not be great as veneered: as they would get a bit more abuse from bumps, I decided to use a thin strip, approximately ⅛” thick, of solid walnut as the “edge band”, and for the veneer to cover the top and bottom of that as well so that it would be indistinguishable as an edge band. Much thinner than ⅛” , and it might as well be veneer; much thicker, and you run the risk of “telegraphing”, where that edge band of solid walnut “moves” (expands and shrinks) with the changing temperature and humidity, but with the more stable veneer and MDF glued on the top and bottom, gets stretched.

I also decided to make the three vertical uprights of solid wood, for a few reasons. As they were much smaller, I had neither the challenges of cost of availability, or weather stability, of the top and bottom. And I decided to remove about half of the material on the outer edges of the two sides to make fitting to drywall that might be bowed slightly, much easier. This removal of material would pose problems for veneered MDF, such as significantly reduced strength in comparison to solid walnut, and the fact that you typically need to veneer both sides of a board to prevent stresses from the veneer from warping the board over time but yet veneering such a shaped piece would be a lot more challenging.

The drawer fronts and drawer boxes would also be of solid wood, walnut and poplar, for appearance and durability, and similarly, because as much smaller pieces, they would not have the same cost or stability problems as the top.

Drawer Boxes

Finally, the drawer boxes and slides posed a little bit of a challenge: given the shape of the drawer fronts and overall cabinet, I could do either rectangle drawer boxes with conventional drawer slides and overhanging drawer fronts that would significantly reduce the size of the drawers, or trapezoid drawer boxes that would have to use simpler undermount slides. The conventional side-mounted slides would allow for very high quality soft-close slides, and “push to open” slides, that a lot of clients like, but the client here opted for the trapezoidal larger drawer boxes with these wooden drawer slides.

So there we have the logic and thought process in the construction and material choices. Now let’s get to work!

Construction Steps

The rough step-by-step is below, with a few steps where I learned a bit more via trial and error, or to explain more nuance, covered below. But first, some pictures along the way!

  • Cut the the ⅛” walnut strips, and glue them to the long edges of an MDF panel; sand to make sure the top and bottom of the MDF-and-walnut piece is still perfectly flat
  • Measure & cut the trapezoidal MDF panels for the top and bottom
  • Cut the veneer pieces for the top and bottom

Three of the four pieces were simple trapezoids of walnut veneer, cut about 1/4" larger than the MDF surface, and so very quick to prep. However, the top had the five pieces of veneer joined together to form the principal element of the design, so I spent a lot more time cutting this out precisely. As I cut each piece, I verified that it would fit seamlessly with the previous pieces cut so far, and then used painters tape to temporarily hold the pieces in place.

However, the painters tape would not work in the vacuum press: though it peels off quite easily under ordinary usage, when clamped under 1000# of pressure for a few hours, some of that adhesive transfers to the wood and is very tedious to remove. So once the cutting was complete, I then used veneer tape - very similar to brown kraft paper tape - to keep the set together.

  • Veneer the solid walnut field of the bottom of the top piece and the top of the bottom piece
  • Trim - via a router with a flush-cut trim bit - the edge of that oversized veneer
  • Route out the back edges of the top & bottom pieces so that the back - attached as the last piece - can fit flush inside to give the piece additional stability and strength
  • Cut the three uprights (left, center, and right), and route out the outside edges of the left and right uprights
d Uprights Routed.jpg

This detail - which you can see at right here - came about in discussion with the client after following up with the template in detail. In particular, the angles and some of the lengths written on the template did not perfectly align with the angles and lengths of the template. Which were correct? We decided to follow the template, but in case there were minor deviations - or the drywall bowed out - I would a) route out the sides so that if the cabinet needed to be sanded down to fit, there was much less material to sand, and b) provide 1/4" radius quarter round solid walnut trim pieces for the top to allow for the opposite possible error, that the shelf was too small. Fortunately, the concern ended up being moot: the shelf fit perfectly requiring neither further sanding nor the quarter round.

  • Install the drawer slides on the bottom of the shelf - much easier to do precisely now before the top gets permanently attached
  • Assemble - via glue and screws - the uprights to the top and bottom, making sure the screw heads are below the surface of the MDF
  • Cut and install the drawer slides

  • Use wood filler and sand paper to make those countersunk screw holes disappear; now the “interior” of the shelf looks like solid walnut
  • Veneer the top of the top, and the bottom of the bottom

This was my "oh shit" moment in the assembly. The veneer glue I use has roughly a 20 minute "open time" - that is, I need to be applying the clamping pressure within twenty minutes of mixing the glue or I run the risk of the glue not holding. This is more than enough time for a small piece, still easily manageable for the unassembled top and bottom, but a bit more rushed for the outer surfaces of the assembled piece, both because the piece was heavier and thus harder to maneuver in the vacuum bag, and also because the top had to be placed precisely so the "joints" of the four walnut borders intersected with the corners of the top.

So, I was scrambling a bit, but breathed a sign of relief when I closed the bag and flipped the pump switch at about minute 15. The bag started contracting as expected, I started to clean up the mixing bowl, ... and then...

download-1.jpg

The vacuum bag burst! And while you can think of this as just a giant ziplock bag, they're a bit more expensive: this one cost $300!

The sharp corners of the trapezoid, combined with the thickness of the assembled shelf, popped the bag, as the vacuum press stretched the material across the edge. This isn't a problem for simple panels - like the unassembled trapezoids - because that stretching is only over about 3/4", and the polyurethane bags have more than enough give for that. But they don't have enough give for a 7" thick shelf!

In retrospect I should have put some spacers in the bag, or skipped the bag altogether, but I didn't have time to think through it now or even find the hole: I had about four minutes left of glue open time! Fortunately, I had plywood laying around already cut to about the size of the surface, heavy woodworking cauls handy, and enough bar clamps to clamp down on something that size. Not quite as ideal as a vacuum press, which provides far more even pressure over the entire surface, but - unless I wanted to potentially start over with cutting the outer veneers and lose a day or two sanding down the glue that would dry on the surface, I made do with the equipment I had on-hand.

Fortunately, it indeed worked out pretty well. Now we have a delicate piece of furniture with veneer / finished surfaces on the exterior, which means work has to proceed much more carefully as I can no longer sand out or as easily repair any blemishes that I might introduce as I move the piece around.

  • Trim - via a router with a flush-cut trim bit - the edge of that oversized veneer
  • Make the drawer boxes with poplar & walnut-veneered lauan bottoms
  • Cut and install the drawer slides to the drawer boxes, notching out the bottom rear rail of the drawers
  • Cut and attach the drawer faces to the drawer boxes
  • Cut and install the back
  • Finish with three coats of spar polyurethane
  • Pack, ship, and wait!

The crate alone was about 20 pounds; fortunately, the size and weight of the crated furniture fit just within FedEx's limits, so I was able to drop it off at my local carrier without too much hassle!

OK the title of this post was a little misleading - there were more than ten steps, and while a few of them were easy, most were a bit more involved. Nonetheless, I hope this gives you a better sense of what's involved in designing and delivering a custom piece of furniture, and so when you talk with a craftsman about their trade, there's a lot more that happens behind the scenes in delivering a beautiful finished piece than just a little sawing and gluing!

Other Commissions

Since the beginning of the year, I've done a handful of other commissions, including:

  • A pair of modern replacement drawers for an antique heirloom cabinet
  • A solid walnut shelf for a boat house in Emeryville
  • A set of 24 oval oak mirrors for a carousel refurbishment at the Spectrum Center in southern California
  • An entryway tray featuring purpleheart, walnut, and maple

Is yours next? I'd love to hear about a project you're considering!