SSBO Wrap Up

Last weekend was a great experience. First off, I want to thank Chris Wong for initiating this event; without doing so, undoubtedly I would have trudged on with my meh shop stool for many more years. Having participated, there were many positives.

• I have gained clarity on what makes a mo' betta' task seat by really looking at the various functional features of even the "simple" backless stool.
• I gained confidence and practice cutting odd chair angles. Chairs are pretty close to the top of the woodworking pyramid and can now see having one or more chairs in my normal project list (vs. a bucket list)
• In seeing how successful others were in going the eyeballing angles, round/octagon legs route, I feel like giving that a shot too.
• Having a deadline forced me to press on. Often on a project, I get stuck either daydreaming about what it will look like, or over analyzing what is next when really the next step is inevitable and I'm just stalling. Prior to the build, I wrote up a coarse process map, that helped eliminate "what's next?" hang ups.
• Be careful what you consider scrap. The walnut I had was short, narrow, punky and had other defects. I could not have made a chest with it, but a chair or stool doesn't take much material, and I could work around the defects with some effort. I ended up with nice air dried material with consistent coloring for a great price, nothing. At the same time I used a piece of wood that several people had hung on to for a number of years. That feels good to put something to rest, once and for all.
• It's OK to make scrap. I had to work hard to highlight great figure in the seat. In the middle of this log was a small amount of burl grain, it took quite a bit of finagling to get it into a place that would put it on display. My juvenile sense of humor also enjoys the word play of "crotch".
• "All time greatest tool of all time this week" (FWW STL) Hands-down my plastic handled Veritas striking knife. This thing really punched above its weight. I've had it for a while and have used it a bit, but this was my first major layout project replacing an old reground kitchen knife. This little guy was inexpensive, yet it fit everywhere and never failed to leave a perfect line. 
• Subtle differences make a big difference with chairs. This has been fit to my bum like a custom saw tote, and I like it that way since it is purpose built. However, my petite wife didn't think it very comfortable due to the width at the top relative to the cutaways. While it was only 2-3" taller (prior to final height adjustment) I thought it felt a bit odd too.

The obligatory things I'd consider doing differently... I probably shouldn't, but it is compulsive, so here goes. Also, if you are a judge, please skip along, nothing to see here.

• Don't forget about vector clamping. With weird angles, it can be near impossible to  pull surfaces together. I found this out about midnight Saturday when I needed to glue the saddles to the top. Ideally, I should have hide glued on some blocks to allow the joint to be clamped normal to the joint. I hadn't time or lucidity required at that point and glued it together with tape (yellow glue) Because of this, I have some gaps that I will need to fill next time I get a chance. Not a huge deal but... it happened.
• The "wings" on the seat come forward a bit much aesthetically IMO, but it "sits" perfectly for my tastes. My prototype seat was shorter, and I found it caused a minor pressure point on the thigh. I will give this a fair chance,  but I may end up altering this shape in the future and feather the lower edge,
• I think I could have simplified the structure with joinery akin to a Morovian stool and combined with hexagon legs and had a faster constructed stool.

All in all this was a great experience. Based on the response this year, Chris will surely organize another, and can't wait to sign up sight unseen, hmmm that's an idea... Sealed orders build-off.

Shop Stool Build Off (Day 0) Waste Not

Not a Stool

In preparation for the SSBO tomorrow, last weekend I prepped some stock. I had committed myself to using what was on hand and one of the options that seemed viable was a pair of smallish walnut logs I had been given a while back. 

These particular logs had been cut probably 30 years ago and stored in a shed for "someone" to do "something" with "someday". It had passed hands a few times and finally ended up at my house after pending some time outdoors (Thanks Jeff!) 

Hopefully there's an award winning stool in there

Well a year or so ago I had just replaced the motor on my band saw (long story) and was itching to cut something, so I slabbed them up (heavy 4/4). They were in sorry shape, quite a bit of environmental damage, narrow & short anyway but I figured I might get "something" out of it and leaving it in the garage was easier than doing something else with it. The SSBO seems like a good use now.

I'm always amazed at how much waste comes from working with trees/logs/lumber. While trimming our trees I note that I do little to alter the tree's appearance, yet there is another tree left on the burn pile. Here, I had only 2 small, short logs to start with, but created 35 gallons of chips, an evening's worth of fireplace scraps (it is crazy cold here right now) and hopefully one shop stool (or possibly another fire.) Looking forward to tomorrow morning (and grateful for insulated boots, and fleece lined pants!)

Stool Design Guide: My SSBO Napkin Sketch

So after the last string of posts I owe it to share my personal design plan. I was actually going to make a CAD model to illustrate and talk through it but my CAD laptop lost it's mind last weekend and shot that plan. So instead I am sharing my most recent doodle from during some corporate rah blah blah meeting. I hope you can make out the idea and obviously it's rough. I intend to shape beyond the basic structure but would rather let that bit be organic and in-the-moment. Also, while I discussed important design criteria before, I failed to mention time as a factor, and this project definitely has that, which colored my choice of joinery etc. (see other excuses & disclaimers below)

Material

Walnut. I have from a few random freebie scores over the years. It is not great stuff, but since most of my parts are small and I have more than enough to pick through.  I love walnut, but usually find it too dark for our house. I'm not buying any new stock for this, so it's whatever I can scrounge. There is a chunk of Bois D'ark I'd like to use a bit of as an accent but it might not be worth the time to man-handle it. Hopefully this weekend I get some stock prepped.

Update method

I'm going to have a single post here and just post random photos throughout the day on the same post so you'll have to refresh if you are following along at home. Then I'll tidy up a week or so later.

Base 

I chickened out, and went with a tripod. I didn't devote the time to come up with a flex-body structure for 4-5 legs since this would inevitably have ever increased joinery. (But it made me miss out on the requisite FEA heat maps, which would have been very cool). I have flared the rear legs out & back a bit (not equilateral triangle) which will lessen me tipping backwards. Falling forward and at an angle is a lesser risk as my feet will have defined positions.

Seat

Definitely directional and wood, (though I liked the idea of 1" nylon webbing or string as a seat too.) In the end, I wanted to try out a saddle chair to see if they are ergonomic. I like the idea of the divided saddle for airflow and it seems like a nice handle and maybe even a tool slot for some tasks if not too wide. I mocked up a seat a few weeks ago and think it's going to work out. I had originally tilted it forward, but in reality, a flat top was more comfy plus surely a flat datum surface wouldn't hurt...

Joinery/Structure

The over-thinker in me struggles with this item;.I'm not afraid of screws, but I also love joinery. I've tried to design this piece to feature quick to create mechanical joints. Look at the sketch for dovetail laps, bridles, draw boring and fake mortises (where a joint is made without chopping). I'm sure this is ambitious for my speed. Weird angles abound so there will be some head scratching and I may end up using pocket screws somewhere to get it done. I incorporated a shelf at the bottom for stepping, foot resting, lowering the CG, and stabilizing the single front leg side-to-side.

Shaping

I'm sure there will be some round-overs/chamfers used and final shaping with rasps shaves etc.

Finish

Ha! like I'm going to get that far... Still eventually I'm pretty sure it'll be burnished oiled & waxed.

Workshop

I insulated the garage shop this summer, but I didn't get a heat source and it's been pretty cold here lately. That could impede progress. The shop is unused and in disarray while I've been remodeling other spaces. Hopefully the weather cooperates. The shop will be below glue temp, so that could complicate things.

Concerns

• I'm worried I'm going to need to glue something as a sub assembly and that's going to take time in clamps and it'll mess with the schedule
• I'm worried I won't be able to get some of the angles right and it will look unintentionally asymmetrical.
• I'm worried it will work, OK but look bad.
• I'm worried I'll get derailed shaping the seat and not finish the legs.
• I'm worried the shelf won't sturdy the front leg or be a good foot rest
• I'm worried that as a step stool it might be tippy

Stool Design Guide: Structure

As a warm up to the upcoming Shop Stool Build-Off, I'm putting together some points to help design an ideal shop stool. We've looked at both ends, which leaves only the intervening structure to consider. I apologize for the lack of images for this segment.

Structural considerations are going to revolve around mass, stiffness & degrees of freedom (motion) requirements relative to your stool's functions.

Degrees of Freedom

We discussed DOF* in how we want the base to move relative to the world. Still we may wish to leave a DOF free or only partially constrained within the structure for instance elevation and yaw (swivel). You may want a height adjustment on your stool so pneumatic cylinders may be an option, also threaded wooden rods (piano stools). Cylinders allow some cushioning potential as well in that direction. Either one usually also means you will have a swivel, which may be desired. Some sort of 4 bar linkage could be created to allow 2 elevations or folding away, but can't seem to find any examples for that beyond the convertible kitchen stool/step combinations.

Mass

Mass is going to be a consideration if your stool is to be mobile and especially if you intend on picking it up to move it (as opposed to rolling) Woodworkers tend to overbuild everything, which means too much mass. To illustrate, most some pieces I've made, I do a final check by standing on it and jumping... This is excessive for sure, but the adage "When in doubt, build it stout (from stuff you know about)" comes into play, So think about removing some mass if it's not needed for a function (or adding if you are going to use it as a in-feed support.) But be careful to watch for buckling scenarios on overly thin components. Windsor chairs are a pretty good example of minimal mass usage. Any sort of optimized for mass/stiffness layout is going to end up being some massively complicated truss type system like Chris Hall's amazing Treteau (which I would have no hope of completing for the build)

Another mass factor to consider is where the Center of Gravity will be, especially relative to any handhold (should be vertically in line with CG for minimum frustration) or if there will be tipping forces present.

Stiffness

Stiffness is related to mass, as whatever material we use has a given density and modulus of elasticity. Still, design geometry plays the more significant roll. Consider standing carefully on an empty aluminum can vs an equivalent mass of aluminum foil arranged as you wish. Likewise a structure that keeps wire members in tension is very stiff relative to mass (suspension bridge.) or the Eames Molded Chair frame

Ideally, you could simulate all expected load cases on your design with some sort of finite element analysis (FEA) model optimizing it so none of your materials go beyond their elastic limits. In reality, this would be a ton of work and a bit of unreliability is involved given the inconsistency of sawn wood as a material for razor edge designs (if you can even really understand all the potential use cases; is a 300lb teenager rocking on the back legs a valid design criteria?), So most are going to just overbuild the geometry a little and if it flexes, then they'll add a brace etc. The problem with this is every structure flexes and acts like a spring to some extent, it's just a question of how much, and adding that brace can cause flexing in other directions/locations...

Flexing components and especially joinery past their limits will cause catastrophic failures, but flex-body design can be used to good effect when kept within reason. Consider the difference between the structure of 4 leg pedestal base table vs. 4 leg table with aprons. A pedestal base is very stiff and so it wobbles on an uneven floor. The apron table typically might touch on diagonals initially (setting up for the wobble) but if the top and aprons together aren't too stiff (likely given their size), it will flex like a hinge a small amount (staying within the elastic range and allowing it to bounce back when moved) and make contact on all four legs under it's own weight. A similar thing happens with the typical 5 legged rolling office chair, note how the legs are on arms (that sounds weird) when on an un-flat floor (within reason) the "high point" arm will flex a bit until the other arms start to take some of the flex until finally most or all will bear the combined weight (and stay within the elastic limit). If the 5 legs were more like tripod legs (going straight toward the swivel) then in addition to the legs getting in the way of curling your legs under you, I suspect it would be too rigid and you'd get the dreaded wobble. You could also use the flexing of a springy wood to give a slight suspension effect mimicking the spring pole lathe or bow's principles.

Hopefully I've given you some things to think about in designing your shop stool and beyond. In my next post, I'll discuss how I plan on combining these variables (along with the short build time-line) into my own design.

*If you've enjoyed this sort of technical engineering stuff, I recommend this short design reference, it's excellent:
Exact Constraint: Machine Design Using Kinematic Processing
Douglass L. Blanding  
ISBN-10: 0791800857

Stool Design Guide: Have a Seat

As a warm up to the upcoming Shop Stool Build-Off, I'm putting together some points to help design an ideal shop stool. After looking at the base, it might make sense to continue up through the structure and arrive at the seat. However, defining the end points illuminates the intervening structure design. Since we have a looked at the connection to ground, now we can look at the connection to your... occupant.

A seat is as unique as the person connecting. Still, it's good to consider options prior to finalizing a personalized design. A few choices to be made:

Functionality

While a stool seat obviously is a place to store your keister, some might also stand or step up on theirs (guilty) or use them as an out feed table, deadman, carry handle, cup holder or bottle opener. These ancillary functions deserve your consideration. 

Bum Interface Materials

Think a bit about what material you want to actually sit on. For many it will be straight wood, but others might prefer leather, cushioning, or webbing. These materials provide some "give" which can add to the comfort of the chair. Still there is nothing wrong with a wood surface especially if like me you have recently installed additional padding to the other side of the interface...

NINNA suspended seat
By Adentro (French)

Risom Stool 
by Knoll

Brian Bogg's 
Webbed Stool

Ergonomics

This is highly subjective, I am not an ergonomics expert, but I do have lots of experience from sitting around...</rimshot>  Based on my own issues sitting at a desk for years and learning from passionate people, I've found some good ideas along the way, though my bad habits are hard to unlearn. (Two TED talks from Niels Diffrient and Esther Gokhale come to mind.)

Playing the angles

It's Brown & Sticky
(and sort of a saddle)

One angle to consider is what angle you can sit on the stool (yaw). Some may want to be able to just set down without thinking. This is convenient, but limits the seat shape to circular and flat/concave/convex. Convex may work (especially around the rim), but concave and flat often have a tactile "edge" along the thigh that I'm not fond of.
If you are willing to go from omnidirectional to directional, you may gain a more comfortable seating position(s). For instance, the Schwarz Roubo stool offers 3 (identical) seating angles. Another option is to have 2 different seating positions like the cock-fighting chairs Mary May recently featured on her blog (I'm not sure if adding a backrest automatically turns the stool into a chair or not, but I like this concept).

Plank (Italian design company)  Miura Stool (sort of saddled)

Rolling Divided Saddle Chair from Salli

Another angle  that keeps coming up from ergonomics folks is the angle of your hips to torso (pitch). Opening up from <90 degrees (hunched over in a flat stool) to something more open ~120 degrees, allows for you to breathe easier, naturally straighten your back, and sit longer without the "hunching begets hunching" spiral. The problem with this waterfall angle effect is that there is a tendency to slide forward, so you might need a stop. "Kneeling" stools are one solution (shins prevent sliding) as are saddle chairs (feet prevent sliding). The later also opens up your hips and may allow you to get closer to your subject, since your knees are less likely to be in the way.

Kneeling Chair

Biker's Saddle Stool

In the end, it boils down to what you want to sit on for long periods of time.

I hope that you've found this useful. Next I'll address the structure of the stool, and then wrap up by sharing the design that is taking shape for my build.

Stool Design Guide: Drop the Base

As a design warm up to the upcoming Shop Stool Build-Off, I'm putting together some design points to help you along in designing of your ideal shop stool. The first thing to address is how to attach to the ground, and how it moves. You may think that you don't want your stool to move, but unless your stool is a boulder or bolted down, it's going to move in some way or another as you interact with it. You should give some consideration to how/where you want that movement to occur.

Degrees of Freedom

Degrees of Freedom

Many engineers will be familiar with applying constraints to the needed 6 degrees of freedom. The concept is that a body (stool) needs to be restrained from translating (scooting) in 3 directions (forward/back), (left/right) & (up/down) it also must be prevented from rotating in 3 directions roll, pitch, and yaw. What motions might be (un)desirable? Only you can say what is needed in your shop. Some activities might benefit from motion, others might be impaired. But here are some possibilities for constraining your stool to the ground.

Some methods that deliberately leave degrees of freedom unconstrained and allow movement:

• Swivel casters allow you to slide in 2 directions and rotate in the yaw direction. (-3) 
• Spring loaded casters can retract when weight is applied Library stool
• Wheels might allow you to move in one direction only (side to side maybe) (-1 Translation)
• A swivel allows yaw  (-1 Rotation)
• A rocker allows pitch or roll (-1 Rotation)
• A sphere would allow pitch & roll (and an intense core workout) (-2 Rotations)

In addition to bases intended to move all the time via rolling, there are also bases intended to be static, but end up moving when the nesting force or center of gravity (CG) shifts. You've likely experienced this with a four legged stool. Because 3 contact points establish a plane. 4 points establish 2 planes. Either the ground or the stool prevent it from being perfectly co-planar. When your CG goes over one of the lines of the contact triangle, the stool shifts to the other plane or tips over. This can be annoying (Crossing the grey lines in the animation pitches/rolls a small amount) or dangerous (crossing the black lines may mean tipping over and on your head.) The annoying bit is exacerbated since all possible contact triangles in a 4 legged stool cross right through where your CG likely is (near your navel when standing, heart when sitting), in the center. Wobbling/shifting can be used to good effect, shifting between two distinct stable working postures as demonstrated in the tip-ton chair or to move around as in the Lee Valley walking stool (But why doesn't my 5 legged office chair wobble? I'll discuss that in "structure")

So annoying is this problem of 4 legs intermittently making contact with the ground that many people decide to make a tripod stool to eliminate that, with varying degrees of success. A problem with a 3 point connection to the ground is the ratio of inscribed circle (the tipping over on your head point) to circumscribed circle (how big of a footprint required) is quite large. Another negative is the tip-over point is significantly different based on what angle you are sitting on the triangle. Compared with 4,5,6 legs (regular polygons) it must be significantly larger to be as stable when leaning.

You can also go less than 3 and rely on your balance (legs) to constrain the remaining degrees of freedom. Strap-on, one-legged Milking stools exist on this principle. These force concentration on what is at hand and stay "in the moment" which might be useful for some tasks (Some autistic kids have found these helpful for focus.)

Tipping Zones of Various # Legs
(Stay out of the red zones with your CG)

But I would guess the majority here want nice stable stools, so how many contact points should your static stool have?

• You could have a single round base that also has an optimal tip over ratio (and can be moved by like rolling a barrel) but with a rigid base you are inviting another safe but annoying tip.
• Two points would be dangerous like a teenager tipping back a dining chair (-1 rotation pitch) or very like a bicycle at a stoplight (-1 rotation roll) 
• Three, discussed above
• 4,5 & 6 quickly improve on the stability, getting a close approximation of the stability of a pedestal.
• 7+ isn't going to be worth the effort, improved stability is marginal at best (see chart)

Gratuitously Geeky Chart

From a construction perspective, even numbers may have an advantage.  This explains why a 4 legged stool is common. It provides an easily manufactured, compact, safer-from-tipping over base, but unfortunately the annoying tipping point between stable positions requires only a tiny CG shift.

Another point to consider is that this assumes regular polygons (equally spaced legs) but you could design your stability envelope via irregular spacing to protect you in certain directions and allow closer access (with a little more tipping risk) in a direction you know your legs can assist or CG shifting is impossible. If you are thinking in this direction then you need to have a directional seat. You wouldn't want a surprise stability issue depending on how you sat down. (A tripod stool has this problem.)

However, if you are thinking about going more than the exactly constrained 3 legged stool, you are going to want to manage the extra constraints with your structure, otherwise the rocking will likely drive you crazy in time. I'll discuss more of this next time.

Stool Design Guide

I'm jazzed up by Chris Wong's stool build idea for Jan 25. Why?

• It's a "simple" project that might be finished in a single day.
• It doesn't need to be Smithsonian worthy since it's not for "the house"
• An opportunity to experiment with any wacky idea since I'm the customer.
• To replace that horrid thing I've made do with in my space.
• A chance to do something "together" with other woodworkers & see how others solve a similar problem.

I want to add to the buzz and design discussion by examining the variety possible with this form. Putting aside aesthetics & material choices, there are still many geometric configurations possible that will be right for some and horribly wrong for others.

Boiling down the simple stool you have 3 major variables

1. The base (where & how it touches the floor)
2. The supporting structure
3. The seat (where & how it touches you)

Pretty straightforward... still I can't wait to dive into design options and pros & cons of those choices from a functional perspective. Hopefully this will help someone design a stool that will function exactly as they need. I'm also hopeful that I'll learn a few things in the comments, blogs and build notes from being a part of a fun shared experience.