Wednesday, June 4, 2008
Bench #3 - Leg details and vises
I didnt follow the Schwarz "automatic tenon" technique on the legs since I wasn't starting with blanks of equal thickness. These tenons are 2-1/2" wide, by 2" long.
I cut the shoulders on the table saw with a sled. I butted the bottoms of the legs to the saw's fence for length registration.
I set the fence on the bandsaw to leave the tenon a few thousandths over 2-1/2" for final fitting with a shoulder plane.
I drilled holes in the legs for Gramercy holdfasts with a 3/4" bit. I used the locations from the Schwarz book.
One thing I'm concerned with is the ability of the Gramercy holdfasts working properly in such a thick leg. In the Schwarz book (or perhaps his blog, I don't recall) there was some talk that the only holdfasts that work in 5" thick legs are the Veritas Hold Downs. I want to be able to use my Gramercy holdfasts in these legs. My solution is, in effect, to reduce the thickness of the legs in the location of the holdfast holes. I accomplish this by simply drilling a 1" hole from the back of the leg, to within 3" of the front of the leg. The rest of the hole (3" long) is the typical 3/4" that the holdfast requires.
Here's a shot from the back of the leg with the holdfast shank poking through. I tested a 1" hole to see if it would allow enough clearance for the shank once the holdfast was tightened down. It's perfect. The holdfast doesn't contact any part of the 1" hole.
A couple whacks and it's holding fast, like a holdfast should.
I'm using wood vise screws from Big Wood Vise. Joe Comunale started making these screws late in 2007. Correction: Joe reports he's actually been making these screws for over a decade. He started selling them last year. I heard about him through a post at one of the woodworking forums in January. Joe's workmanship is excellent. The threads are flawless, and the heads are very smooth. I couldn't detect any sanding scratches in the two screws I bought. The nuts are also very robust, and Joe leaves plenty of extra length to accommodate different setups.
One thing that is intriguing about these screws is their rapid movement. One revolution yields a full 1/2" of travel. My Record iron vise is exactly half that. I suppose that sacrifices some fine control, but I doubt I'll care. I have other vises for fine work, and I can always slide over to the Record on the old bench.
I had Joe mill a garter slot into the shaft of my screws for the garter design in the sketch above. It's right behind the hub. At Joe's suggestion, I also had him mill his standard slot in case I want to use a different garter system in the future. (see the previous pic)
The Schwarz plan calls for placing the leg vise screw 9" down from the bench top. Roubo's drawing (pl. 279) shows the screw a bit lower. Here I've placed the screw at 12" from where the bench top will be. It's not bad, but it felt a little low to me. I think I'd rather trade a little depth for less bending.
I moved the screw up to 9" (on center) from the bench top. That's more like it. Not so much bending. Besides, I've already gained almost two times as much capacity versus my Record 52 1/2. Its metal guide rods are about 4-1/2" from the top of bench #2.
Simply bolting the nut to the back of the leg would seriously reduce the capacity of the bench screw. I suppose the most economical route would be to inlay the nut into the front of the leg, bolting it to the leg itself. But that would rely completely on the strength of the bolts opposing the vise's clamping action. If the mortise in the back of the leg would be two inches deep, plus the thickness of the vise chop, I'd end up with about 10" of vise capacity. That's plenty for my work. I decided to mortise to block into the back of the leg, the strongest method I could come up with. I also considered cutting a dovetail socket into the back of the leg, but this would remove material completely across the width of the leg, effectively making the entire leg that thickness. The mortise leaves a lot of material in place. Locating the screw at 9" from the top, the nut is a little long for this application. I didn't want to mortise the leg any closer to the tenon's shoulder than necessary, since this area will get some serious chopping and pounding stress.
So I cut the nut a bit shorter to leave the leg as robust as possible. Here I'm marking for the mortise with a pair of skew chisels.
I used a router and fence with a straight bit to cut the perimeter of the mortise about half way down. This would establish a nice registration surface for paring the mortise.
Back to the drill press to get rid of most of the waste.
After paring the waste and squaring the corners, I was ready for a test fit. The mortise is 2" deep.
The fit is a little tight, as planned. I was diligent to make sure the end grain surfaces were as close to a piston fit as I could manage, since a tight fit here would reestablish the vertical strength of the leg. I fit a little at a time, then pared the burnished areas from the nut instead of touching the mortise.
After a few minutes and some light mallet taps, I had the fit I wanted.
The screw is 2" in diameter, so I bored a 2 1/8" clearance hole through the mortise to the front of the leg and chamfered the opening.
This an 8" wide beam of 16/4 Ash that will become the vise chops and the back half of the sliding vise.
The beam yielded three nice pieces for the vises.
I suppose some would enjoy flattening boards like this just for the fun. I had my fill of it about 12 years ago. But when you only have a 6" jointer, those old-school skills sure come in handy.