Roubo workbench questions

[overland]

I have a 6" oak slab that's about 19" wide and 6' long. I'd like to make a Roubo-style bench with a wooden-screw leg vise, but I have some questions:
1. Could I build it so the top could be removed from the base? (That's how my existing bench is--the top comes off and the base can be broken down.) If I do that, can I still use a leg vise? 
2. The wooden screw I have is 2 1/4 inches in diameter. How do I bore a hole that big in the bench leg?

[iclark]

I have a 6" oak slab that's about 19" wide and 6' long. I'd like to make a Roubo-style bench with a wooden-screw leg vise, but I have some questions:
1. Could I build it so the top could be removed from the base? (That's how my existing bench is--the top comes off and the base can be broken down.) If I do that, can I still use a leg vise? 
2. The wooden screw I have is 2 1/4 inches in diameter. How do I bore a hole that big in the bench leg?

1a) yes. easier if you are not of the school that requires the trapezoidal leg through-tenon through the top. I think that it would be easiest with a top rail between the end legs that the top sits on with one blind round loose tenon at each end and some table top clips between the top rails and the bench top to allow for wood movement.
1b) yes. definitely. putting a face board on the top and on the leg with the leg vise will make it less painful to tweak the alignment each time you re-assemble. (less psychologically painful to plane a bit off the face boards than off that slab. if you are doing an end vise, you could make the face board for the top thick enough to contain the dog holes. That way, if you decide you want different types of dogs or dog spacings later, you can replace that face board and you have not messed up your slab.

2) Forstner bit - preferably in a drill press. I would drill a snug fit t in the leg and in the chop and then use a rasp to elongate the hole in the chop to allow for some in and out tilt of the chop.

edited to add: congratulations on that slab. I have had the vises for my split top Ruobo for way too long but have not found the right wood or time yet.

[titanxt]

I have a 2 5/16 forstner bit for sale in the classifieds....

Just curious - why the need for the bench to break down? I wonder if building it to come apart might cause some wobbling if you aren't careful, but I am projecting my lack of skills into that thought.

[theoldfart]

I have a largish Roubo and I moved it from Mass to CA this past May. I designed it to be broken down. I used the through mortise and dovetail tenons on the top. I used BenchCrafted bench bolts to secure the end leg assemblies to the stretchers. The bench weighs around 400-500 lbs so getting it out of the cellar in one piece was out of the question. it did take bottle jacks to get the top of the legs.

[cputnam]

I'm going to opine that a 6" x 19" x 72" oak slab will largely stay right where it is put. You need a tight fit of a large mortise and tenon at the leg vise corner as that vise is probably all that is going to move the top. If you do not want to use bench bolts in the base, consider the use of tusk tenons. Bob Lang's use of dovetails for his bench was thought provoking.

[5thumbs]

I built a split top Roubo out of flame birch and mortised the top to the legs. This lets me remove each portion of the top as desired. As mentioned before, this bench is heavy. Mine has a 4" thick top and probably weighs around 400 - 500 lbs. I installed a leg vice; the suggestion about a Forstner bit is a good one.

Enjoy!

[overland]

Thanks all for the excellent advice. But please clarify. I realized after my initial post that using a leg vise forces me to have a stronger connection between the base and the top than my current bench, which has a steel vise mounted to the top and pretty much just sits on the base, held in place only by two lug bolts underneath. But I'd still like to be able to disassemble the bench in case I have to move it.  I know how to make a base that comes apart, but I'm not sure how people here have created a mortised connection between the legs and the top that can be taken apart. How is that done?

[iclark]

My mortising skills lack the precision for a removable top that can be removed any time of the year. Hence, my suggestion for the round loose tenons (near the front of the top) between the top and base coupled with table top brackets on the rails.

To deal with the strength of the leg vise, I would do a rear chop that came up the leg and flush with the bench top. Putting a face board along the top that goes over to flush with that rear chop. Making that rear chop and the face board 8/4 or 10/4 (can be laminated), gives strength and room in the face board for the dog holes. If you want square dogs (I do not, but to each their own), you can do that face board in 2 parts and cut the square dog recesses before you do the glue up.

Those face board and rear chop to not have to be glued in place. Bolting them on lets you remove them later for transport.

If the leg with the leg bench is not all the way at the end of the bench, you could consider making 2 different face boards for left of the leg vise: one just flush with the rear chop and one with a crochet for planing.

If you make the face board a bit taller (deeper) than the bench top, you can use it as the front guide for a sliding deadman.