Friday, August 8, 2008
Getting to Know Roubo - Part 2 - Tail Vise Update
Just a quick mention here of the fantastic capabilities of the sliding leg vise. Holding longish workpieces is a snap. This is a 56" long pair of shelves for a wall hung unit I'm building for the shop. Jointing them in tandem using both vises makes for effortless workholding.
A 99" long bench also doesn't hurt. I can work on long projects quite easily.
Now on to the more interesting topic of this entry. The updated tail vise.
A couple weeks ago I mentioned my dissatisfaction with the operation of the tail vise. I didn't want to let this go, so I was diligent about developing a proper solution in short order. After some recommendations from my machinist, I decided to plunk down the money for a left-hand acme-threaded screw and matching nut. I liked the speed of the German bench screw I used originally, so I picked a thread pitch to match, 4 tpi. Once I received the screw I noticed two things. The tpi was a bit more than the German screw, so I got about 1/32" less travel per revolution. I wasn't too concerned with this, since it would allow a bit more controlled clamping, at the cost of a little speed. The other thing was the quality of the screw. The threads look to be rolled instead of milled or cut into the shank, and they were very smooth and almost polished. This meant that the screw worked much smoother than the German version. Granted, the German screw worked fine, and I was quite happy with it (except for the backwards action), but I was still impressed by the quality of the Acme screw.
Another thing with the German hardware that seemed a bit unnecessary was the handle. The German screw has a typical "T" style sliding handle that one sees on most woodworking vises. For a tail vise, and especially for clamping between dogs, this is overkill. And not good overkill either. Clamping between dogs requires the right touch, lest the workpiece become distorted. So most of the time when I was opening or closing the vise (gross movements) with the T handle, I was spinning it round with my index finger against the wood. And once I got close, I would just grab the short end of the handle, slide it out just a tad and tighten down. I was never cranking down the vise with the long end of the handle, so most of it was useless, and frankly, in the way. Especially when the screw would stop in a position that would cause the long end of the handle to protrude past the front of the bench. Chris Schwarz, when he retrofitted his Roubo bench with a tail vise (he calls it a wagon vise) used a veneer press screw that features a handle that most of us are used to seeing on Jorgensen style bar and pipe clamps. On a clamp this is a functional handle, but for a bench, I wanted something a little more friendly to touch, which is why I ruled out using Chris' press screw idea in favor of the German tail vise hardware. After coming to the above conclusions, I decided that a heavy handwheel would be nice to try. It's no coincidence that handwheels are used to adjust machinery settings, like table saw blade height and tilt. They are ergonomic, compact, and their weight is an advantage when spinning long distances.
The position of the screw dictated the maximum diameter of the wheel I could use, if I didn't want the wheel to extend past the top surface of the bench. This ended up being 5", with about 1/4" to spare. This will allow several flattenings of the bench top before I reach the diameter of the screw. I'm not sure what will happen when I get that far. Maybe I never will. I tried to find an inexpensive wheel, with good ergonomics and a nice finish. I ended up with chrome plated cast iron. The edge is thick enough to grasp easily, and the rotating handle spins freely for rapid adjustment, although this replaceable part could be a little larger. The screw was machined down to 3/4" dia. and the wheel's hub bored out to match. The hub rests against a flange of cold-rolled steel that in turn presses against a washer abutting the shoulder of the screw. My machinist thought the washer would be more functional on this side of the flange given the directional forces of the vise. We thought about painting the collar, or having it powder coated, but we both decided gun bluing would do for now. The screw is fastened to the wheel's hub with a tapered pin.
The German bench screw nut was removed from the angle and the new acme screw nut was screwed to the plate with cap screws. We considered welding the nut in place, but didn't want to commit to a permanent placement before testing the vise.
The nut is 1-7/32" thick.
The vise components.
The new flange bolts to the end cap using the same bolts and holes as the German screw.
The hand wheel is really nice to use. It spins freely, and thanks to the mass of the cast iron has a bit of momentum to keep it going. It's a tad slower than the German screw, but this has an important advantage for all but gross movements.
Here's where the slower movement of the screw really shines. With my leather-lined jaws and dog faces, all I need to do to clamp firmly is crank the vise to contact the workpiece, then give the handwheel another little turn. It holds extremely well with little effort. Opening the vise is just as easy. I don't have to really crank down on the wheel, and I never need more than one hand to tighten or loosen the vise. I think this is a combination of the pitch of the screw, the fine polished finish of the threads, and the leather faces of the jaw and dogs. It's just about perfect.
In the video I'm clamping first between jaws, then between dogs. I really put my entire weight into trying to dislodge the workpiece, and I'm not small. That's 250 pounds wrenching about. Between jaws, the workpiece shifted forward about 1/2"after a couple shakes. Between dogs it didn't budge. And the work piece wasn't distorted. Granted, it's a short piece in the video, but longer pieces hold just as well. I'm putting enough force into the test to visible move the bench, so this obviously a torture test. No woodworking task would ever stress the bench to this extreme.
As I mentioned earlier, one thing that could be possibly improved (and it's a small point) is the size of the handwheel's handle. It could be a little larger. The handwheel itself could also be a tad larger in diameter. This would make the clamping effort even more effortless, but might make gross adjustments a bit more work due to the larger diameter. It's really a toss-up. I'm totally satisfied with the tail vise at this point.
And finally, here's the really interesting part. When I asked my machinist if he was interested in building some of these vises for friends (I've had a few inquiries), he said yes. The complete assemblies, since they are not being mass-produced, will be more expensive than typical tail vise hardware, which runs around $150. But the parts are also of superior quality. If you are interested in buying this vise hardware, please drop me an email and I'll keep you updated on pricing and availability.