Friday, July 26, 2013

Chiavari Chairs

Italian chiarmakers and designers have made a huge impression on me.  Gio Ponti's Superleggera chair blew me away when I first saw pictures and read about it; light, strong, elegant, visually dynamic and simultaneously simple.  Chairs like this set the bar very high for would be chair designers.  Two winters ago I hunted down every word written about Gio Ponti in the English language, primarily through interlibrary loans.

Superleggera ad from the 50's.

Gio Ponti frequently cited the chairs of Chiavari as a significant source of inspiration for his design.  Subsequent research about Chiavari chairs yielded these two videos which I thought others might be interested in.  Don't expect drawknives and steam bending, but there are at least a million other things to learn from watching these videos over and over again.

The first is a short history of the Chiavari chair.  Watch for Franco Casoni's friendly treatment of his statue.

Fratelli Levaggi is a factory in Chiavari that makes these classic chairs.  This video details a new design.  The music makes you feel like you are on an epic adventure, which is exactly how the workshop feels when everything is going right.  Enjoy!

Monday, July 15, 2013

Taming Bandsaw Vibration

I love my bandsaw.  It is a rugged 12" Atlas with a cast iron frame and good blade guides that was made in Kalamazoo, MI in the early 1950's.  When I bought the saw a few years ago it was in rough shape, but it had good bones and an almost new 1HP Baldor motor (which alone was worth more than the asking price for the whole setup).  One of the things I like about buying vintage machinery is that in order to get it to perform well, you often have to take the whole thing apart, clean and lube it, and then put it all back together again.  The process is far from plug and play, but by the end I know the machine and all of its quirks inside and out and that base of knowledge helps me to properly tune and maintain my tools at a very high level.

My Atlas 912 bandsaw.

When I was tuning up this saw I did all of the standard things to minimize vibration.  I replaced and crowned the tires, aligned the wheels, aligned the motor pulley and drive pulley, and upgraded from a standard v-belt to a link belt.  That initial tune up got the saw into good shape and it has performed admirably for me for three years now, but it has always been a little nosier than I would like and there was a noticeable vibration in the table while the saw was running.  Every now and then I would try something small to eliminate a little more vibration and noise, always coming shy of the quiet vibration free bandsaw of my dreams.

I have known for a while that the thin sheet metal wheel covers were creating a lot of noise, so most of my vibration reducing efforts have focused on them.  Last week I finally had a the vibration destroying breakthrough that I have been seeking for the last couple of years. 

While the saw was running I placed my hand on the center of the lower wheel cover and the noise and vibration were immediately reduced.  I took my hand off.  The noise and vibration came right back.  I took off both wheel enclosures and ran the saw without them for a few minutes and it ran smooth and quiet.  It turns out, the thin sheet metal was picking up vibrations in the saw and amplifying them. 

There were two distinct sources of noise that the saw was producing which had to be addressed separately; the hum of the center of the covers and the rattle of the edges.

Tracing the covers.

The centers of the wheel covers were humming with harmonic vibration and needed something to absorb it and keep the vibration from radiating throughout the rest of the saw.  I picked up a piece of 1/2" MDF to make inserts to attach to the inside of each wheel cover to add mass and absorb the vibration.  I traced the covers and then cut out the shapes on the bandsaw.

Traced and ready to cut.

Fitting the inserts took a couple of attempts.  The corners of the sheet metal are rounded and the MDF inserts needed to be 1/2" smaller than the outer dimensions of the sheet metal to rest flat on the center of the covers.

Inserts fitted.

I used double sided tape to secure the inserts to the wheel covers.

Double sided tape.

With the inserts in place the wheel covers had a solid, reassuring weight.

Edge wrapped in vinyl tube.

Where the edges of the wheel covers meet the cast iron frame or other pieces of sheet metal they vibrate and rattle against each other.  These areas needed a barrier to prevent the two pieces of metal from contacting.  I slit a line in a roll of 1/4" vinyl tube and slipped that over the edges of the wheel covers.

Cover installed.

With the MDF inserts and the vinyl tubing installed my little bandsaw runs like a top: quieter than I could have imagined and only the slightest vibration on the table.  Simply a joy to use.

Monday, July 8, 2013

Build a Better Shaving Horse

The first woodworking I ever did was on a shaving horse.  I had just turned twenty and a twelve year old sat me down at a shaving horse and taught me how to make a spatula from a piece of red maple firewood with a drawknife.  I was hooked.  The simple elegance and intuitive feel of the shaving horse and drawknife completely drew me in.

The shaving horse.

I've had the good fortune since that day to spend many hours and days on an array of shaving horses and work with a few other shaving horse aficionados.  I want to emphasize credit to Carl Swensson for the clamping mechanism that I used on my horse.  His winter of experimentation with weights and pulleys testing the holding power of various shaving horse configurations forms the basis of my understanding of how shaving horses function.  My shaving horse draws heavily on Carl's ideas.

My horse looks similar to a traditional dumbhead style shaving horse, but the design offers increased holding power and better ergonomics than  many of the other horses that I have used.  If you have ever had the wind knocked out of you by a piece of wood slipping from the jaws and slamming into your stomach, read on!  The clamping geometry of this horse and the reduced racking of the swing arm keep your workpiece in the jaws and away from your sternum.

The horse consists of four components; the base, the platform, the swing arm, and the seat.  Dimensional southern yellow pine lumber is the material of choice for this horse.  It is rigid, relatively light, affordable, and just hard enough for all of the components.  Also if you are willing to sort through the pile, you can buy nearly perfect, knot free boards.  Medium soft hardwoods like tulip poplar would also work well.

Shaving horse base.
The base consists of two rails, three legs, and a few spacer blocks.  The front leg angles forward at 25 degrees, and the rear legs splay out 20 degrees and rake back 10 degrees.  The two rails make for a very rigid base that does not flex at all under the heaviest use. Three legged horses are stable on even the most uneven floors and outside.  The top of the rails are 18 1/4" from the floor.

Rear leg detail.
The rear legs are rabbeted into the rails and bolted in place to form the back of the horse.  I unwisely use this section  as a sometimes chopping block when I am lacking something more suitable.  My shaving horse is always nearby.

The business end.
Wedged tenon.
The working platform is secured to the base by an integral tenon that is secured with a wedge from the bottom.  This allows the whole horse to be disassembled in a few minutes for easy transport in the back of my small pickup.  The front edge of the working surface is 10 inches from the top of the base rails and the platform is angled at 7 1/2 degrees.  This arrangement works well for my body.  I am 5'5". For ergonomics your shaving horse should have the platform at a height and angle that puts your forearms in a straight line with the piece of wood being held in the horse's jaws.

Forearms form a straight line that is continuous with the workpiece.
Look at the picture above.  Now, imagine that the work surface were five inches lower.  In order to keep my forearms in line with the workpiece the angle of the work surface would have to be sloped to a greater degree.  If the work surface were raised it would have to be less tilted to maintain the proper relationship.  The platform height and angle are easy to mock up if you have a mirror, camera, or friend to check your forearm alignment.  When mocking up the platform height, use a piece of wood of a thickness that you will commonly be shaving.  Most of my shaving is done with pieces that are close to one inch in thickness.

More than a 15 degree work platform slope starts to feel awkward to me and greater angles also reduce the gripping power of the jaws.  For an arrangement of optimal holding power the platform should be parallel with the jaw of your horse.  In fact, if I were making this horse again I would either make the platform parallel to the rails, or angle the head of my swing arm to match the slope of the platform.

Laminated platform assembly.
The base of the platform is a three part lamination.  The longer center piece, which forms the integral tenon, has a mortise angled at three degrees that locks the assembly in place on the base.  The center lamination also has a low angle cutaway at the top to allow the swing arm to pivot all the way forward.  The two outer laminations form the giant tenon cheeks and are pulled tight against the rails of the base when the wedge is driven into the tenon.  The outer laminations also form the channel that the swing arm assembly travels in.  The swing arm is planed to fit this channel with minimal slop.  A nice fit here prevents the swing arm from racking to the left or right when a piece of wood is under only one side of the jaw.

Notice the orientation of the growth rings on the outer laminations.  The pieces are oriented so that they cup away from the swing arm.  If the wide boards cup (which they will) and they are oriented with the pith side of the board to the outside of the lamination they will pinch the swing arm and prevent it from swinging.  No fun at all.

Some shaving horse users like to leave their horses outside in the rain and snow.  The tighter tolerances of this horse will not fair so well in those conditions.  This is an inside horse when the weather turns foul.  Repeated soaking and drying will make the fit of the swing arm sloppy and increase the racking of the head from side to side.  The more the head racks when you press on the treadle, the harder you have to push to hold your workpiece securely.

Swing arm and treadle.
Adjustment holes and groove.
The most notable feature of the swing arm is the groove that the holes for the pivot pin lie in.  This groove is one of the biggest improvements over the arrangement in the basic dumbhead style horse that many people use.  As you draw the pin from the hole to adjust the height of the head, the pin remains in the groove which keeps it in line with all of the other holes.  This makes it a simple matter to slide the pin in the next hole and adjust the head up and down.

Shaving horses have better holding power the closer the height of the head is adjusted to the workpiece.  For the greatest power, the head should be in the lowest position that the thickness of the workpiece allows.  Simplifying the pin mechanism encourages more frequent adjustment of the head.

Adjusting the head.
For the greatest possible holding power the front edge of the head, which actually grips the stock, should be directly in line with the pivot pin holes in the swing arm.  For a dumbhead style shaving horse this is not exactly a practical arrangement.  However, for power, the distance between the front edge of the head and the pivot holes should be minimized.  This is accomplished by placing the pivot holes as close to the front edge of the swing arm as possible.  Mine are one inch on center from the front edge.  I believe they could be moved further forward without jeopardizing the integrity of the swing arm.  Using a harder wood with greater shear strength for the this piece would allow the holes to be safely moved even closer to the front edge.  In the same vein, the front edge of the head should protrude only minimally in front of the swing arm.  Mine extends one inch forward.  The greater the distance between the front edge of the head and the pivot holes, the less holding power the assembly has.

Pinned head joint.
The joint that connects the swing arm and the head is a pinned mortise and tenon.  The steel pin passes all the way through the tenon and locks it in place.  The orientation of this joint is unconventional, but has shown no signs of movement in three years of hard use.  The mortise is cut across the grain of the head.  This orientation allows the head to be short from front to back without danger of splitting in use.  The low profile head is easy to reach over and facilitates working on the other side of the head.

Literally a pain in the butt.
I won't say much about the seat of my shaving horse except that it slides in the groove created by the rails (which I like) and it causes physical discomfort after hours of continuous use (which I don't like).  I have been meaning to carve a new wider stool seat or sackback seat and I just never get around to it because I have this one which functions (barely).  My recommendation is not to carve a bicycle seat shape.  Make something wider that you will want to sit on all day.

With all of this clamping power optimization, both the work platform and the head of the shaving horse need some form of padding to prevent them from denting the wood being so firmly held.  I lined both with thick leather.

Expect more entries about this shaving horse design in the near future.  A good friend of mine is coming to build one with me next weekend, which will provide a great opportunity for me to offer more specific dimensions and construction details for this style of shaving horse.