Saturday, March 28, 2009

Irregular Situation VI

Today I'll wrap up my account of making an irregular sawhorse with a look at the final steps in completing the cap and assembling the structure. Last time, I showed how the sliding dovetail mortises were cut, and all that remained after that step was to fit in a locking mechanism. This is the same type of mechanism I used to secure the paneled top on the Vanity project described in a previous thread.

First I fit the Larch top into position, selected the location where the locking pin would be fitted and then made a mark to index both pieces:

Marks were then transferred across the face of both the cap and the beam, and the trenches were fully marked out. Then, out came the dozuki and I started the cut out:

These steps are identical for both the cap and the beam, so I'll only detail the steps for the cap. After the trench shoulder was crosscut, I used a chisel to remove most of the waste:

Then I fine-tuned it with a LN skewed corner cutting plane, a tool I have since lost somewhere in Kimberley B.C.:

The nearly completed trench in the cap, just another pass or two (done when the fixing pin gets fitted) are remaining:

What can't easily be seen in the above picture is that the shoulder of the trench does not run 90˚ across the surface - it is sloped inward a couple of degrees, as is it's partner on the beam. The shoulder cut, however, is 90˚, which means that the fixing pin is a tapered parallelogram.

Here's how the cap and beam come together then:


And three, with the fixing pin started:

This is the completed sawhorse after assembly:

The short side, where the legs splay 3 in 10:

And the long side, where the legs splay 2.25 in 10:

This sawhorse was major layout triumph for me, and I consider it, in technical terms, an equal or better of any piece of furniture I have made to date. I know that may not be saying much, but I guess my point is that the world may be found in a teacup - the sawhorse is a great vehicle for layout study, and I felt a great sense of accomplishment to have figured out the layout for this structure.

This sawhorse has been put to plenty of use since I made it 4 years ago, and has proved it's strength and durability, holding well over 1000 lbs when moving a large lathe one time. I have left the mortise and tenon stretcher connections without pegging, wedging, or glue, and the construction holds together just fine. I have, alas, found the top with a saw blade on one occasion, and dropped a chunk of wood on it another time, so the cap has a couple of Yellow Cedar patches now. It's serving it's purpose well, in other words.

I am extremely thankful for the gentle pointers that Togashi-sensei provided me at the beginning of this study. When I completed the sawhorse, I sent him pictures and a small token of my thanks. He seemed pleased with what I had made, and asked me to send him my drawings, which I considered an honor.

Friday, March 27, 2009

Irregular Situation V

Continuing on with the account of the making of the irregular pitch, regular plan sawhorse...

Once the nuki were all cut, I did a trial assembly with the legs to see how things come together. The ideal is to cut everything once, put it together once, and have everything fit perfectly, but I haven't got there yet. I usually need to make a couple of trial fits and slight adjustments before the fits are satisfactory. In this type of structure with splayed legs, all the parts need to snug up simultaneously - here's the mid-way point:

The fit looked pretty good at the initial fitting in this case, which I put down to being extra fussy with stock prep and leg shaping:

Then I spead the works apart again and fitted the top beam in and began to bring it all together:

The fit was fine, so I moved onto the next step - processing the dovetail tenons on the tops of the legs. While I have a few different types of dovetail planes, I prefer to cut sliding dovetails by router:

I used a simple jig, the router referencing off the outer edges of the MDF:

This was the result:

Then I started working on the top cap, which I chose to make out of some Larch. In the region of the Kootenays (a region of S. Eastern British Columbia) I was living, there were loads of standing dead Larch for some reason, so it was possible to obtain some really good material - this was layout around the shop as scrap in fact. I had never worked with Larch before, and I found it to be a forgiving material and easy to work cleanly.

First I established the layout, then mortised the entry points for the dovetails and cleaned them up with a chisel and paring block:

Then I used a kebiki to score the lines for the dovetail mortises:

Then out came the bulk of the waste using a Forstner bit in the drill press:

That was followed by a pass with the router, and here's the result of processing half the dovetail:

Then I cleaned up of the inside corners with a 5mm paring chisel after completing the dovetail routing on both sides:

It looks like there will be a part 6 to this series....

Thursday, March 26, 2009

Irregular Situation IV

Before I continue on with the details of the leg tenon cut out, I wanted to put in a photo I omitted yesterday: a close up one of the mortises on the legs. This one is mostly cut out, just the left-side shoulder to trim flat and some clean up inside:

Back to the leg tenons - the next step was to cut the inside shoulders, using a medium-size ryoba nokogiri:

And then I define the outside face which is to be housed. The French refer to this part as the 'barbe' (beard), and they are a little tricky to cut given the acute inner angle - again, a sharp rip blade on my ryoba noko was the ticket:

Here's a close-up of two of the tenons, cut-out largely complete:

The next step was to individually fit each leg to the top:

Once each leg was fully in position, I could mark out the inside face of the tenon for the small slice that would be removed:

This allowed a pair of these tenons to nestle tightly together:

Then I turned my attention to the nuki. I don't have detailed photos of the cut out sequence, but what I processed were haunched and shouldered tenons, slightly smaller than 1/2 height:

When cut-out was complete, then followed a round of planing on the nuki:

Then I planed the end-grain on the beam and chamfered the three arrises (leaving the top arris since the cap was yet to be fitted):

--> On to part V

Wednesday, March 25, 2009

Irregular Situation III

The next steps in the process of making the irregular sawhorse were the legs. Determining the diamond-shaped profile for a leg which slopes differentially took a fair amount of head-scratching and multiple contacts with Togashi-sensei, as mentioned in the first post in this thread. There were several drawings involved, and they get a bit complex - here's a stripped-down illustration, showing in red the shape of the leg required:

You might notice that the corners of the leg do not meet the diagonal axis line of the piece.

With the shape determined, I set about preparing the rough stock, in some lovely dense Kootenay-region Douglas Fir, and then making a sled to process the leg shapes in the planer. The jig is essentially two pieces of MDF connected with a piano hinge:

The wedges can be seen protruding from one side. These wedges are made as precisely as I could make them. I then made up another MDF jig to fix the blank from which the wedges are cut:

Here I'm running the wedge-maker jig through the table saw:

Once the wedges are made and the sled configured, I do a test run on some scrap stock and check the results carefully, then adjust the jig as necessary. Then I run the legs through the planer, one round for one side, then I run the leg through the planer without the jig to make a pair of parallel faces, then the jig is reset with new wedges, and the complimentary faces are processed:

With a different amount of angle requiring removal from two different faces, and only a slight difference between the angles, it would be easy to get confused, so I mark the end grain of the legs before planing to keep everything in order:

The legs came from reclaimed stock, and I chose to patch one of the more egregious flaws - which happened to be right where a through tenon would emerge. Here is the patch after gluing but before final planing:

Then I lay out and knife the legs for the stretcher (nuki) mortises, along with the bottom cuts and top cuts, and of course the top tenons:

Then I chop out and pare the mortises, cut the feet, and rough cut the tenons. After all that, I then finish plane the legs:

Then I chamfer the arrises:

Here's the beginning of the processing of the tenons on the tops of the legs, the rips of the side tenons done on the table saw with a tenoning jig:

The arrows identify the outside corner of the legs - it's important to take extra steps to avoid confusion - this I have learned by experience.

--> go to post IV

Tuesday, March 24, 2009

Irregular Situation II

After I had decided upon the main design details for the irregular sawhorse project, I had concluded that I wanted a means of using joinery to lock the legs to the top beam of the sawhorse. I also wanted to use a 'sacrificial' cap on the top of the beam which would take abuse from errant saw cuts and material dropped onto the beam. In previous sawhorses, I had used floating dovetail keys in one set, and the glue trick with newspaper on another, and this time I wanted to improve upon both of these methods. The glue trick worked fine, but in outdoor conditions humidity can cause the glue bond to fail prematurely - that's what happened to my sawhorses done in that manner.

In this sawhorse, I decided to form 1/2-dovetail tenons on top of each leg, which when brought into position together would protrude through the top and form a single dovetail. This dovetail would also lock the sacrificial cap into position as well.

Once I had decided upon the joinery system, the next step was to clarify the respective irregular slopes - I went with a slope of 10/3 for the short side splay, and 10/2.25 for the long side splay - through a slight trial and error process to determine how it would all come together at the top. Here's the finished cross-section view of the top:

As you can see, the legs are to be 2.5" in section, and the top beam, with cap fitted, forms a 4"x4".

I then spent some time working on the descriptive geometrical drawings necessary to determine the legs shape along with the cuts for the top and sides of the stretcher tenon shoulders. That done, I set to work on cutting out the top beam for the leg mortises.

Once the layout was complete, and the mortises knifed at their cross grain positions, I used a General International tilt-head hollow chisel mortiser, in concert with a sloped bed jig thrown together from some MDF scraps to rough out the mortises:

All of this work on the sawhorse I performed on weekends and evenings in the shop at the College of the Rockies in Kimberley B.C. (now in Cranbrook, B.C.), where I was a timber framing program instructor for a term.

When I was done with the mortiser, the result looked like this:

Here's the underside after the rough cut out:

Then I used paring blocks and chisel to clean up the inside of the mortises:

Unlike the French joiner's work practice, in Japanese joinery the housing is preferred to reinforce most connections. Without the housing, the connection is limited only to the strength of the smallest part, which is the tenon, typically at around 1/3 the thickness of the member.

I made these housings quite shallow, at only 0.25" deep. I used a router to deck the bottom of the housings, and then an MDF block with chisel to pare the shoulders:

When the cut out was complete, the mortises with housings looked like this:

The next step in the process was the processing of the legs into the irregular diamond-shape form. The diamond effect, by the way, is termed 'ku-se' in Japanese, which means 'peculiarity'. I'll detail the work undertaken on the legs in the next post in this thread.