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Thread: How to ensure a straight edge/front face on glued up bench top & related questions...

  1. #1
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    How to ensure a straight edge/front face on glued up bench top & related questions...

    Hi Guys. Now that my dust system is nearing completion I'm thinking about the Roubo style bench that's next on the list - the timber for it (European beech) has been stacked for about a year now. It'll be 8ft x 3ft with a 4in+ top thickness - along the lines of the Schwarz design - but with a Veritas twin screw vise on the face, and a large Veritas front vise with probably full width jaws at the end. Both vises will have dog holes in the moving jaws lined up with the grid on the bench top.

    Pardon my bringing up the topic again, but before sitting down to draw it up properly there's a few issues bothering me that don't seem to be covered in the write ups:

    (1) The first is how best to get the edge/front face straight when gluing up the top - the laminations at 1 1/4 in + are thin enough that it seems they won't necessarily clamp up dead straight. One option may be to clamp maybe four laminations at a time up along a length of steel box section - which should leave only a little adjustment to be done with a hand plane afterwards.

    The same question arises with the hand plane though - how best to ensure a straight and flat edge other than to lay a reference straight edge along it?

    It's a significant issue in that it looks to me like the fixture to place and drill the dog hole grid is best lined up off this face/edge - so it needs to be straight - if the grid isn't true it risks pushing frames etc out of square when they are clamped up between dogs. Work will be clamped off this face in use too. (one way to kill all these issues (and others like ensuring the holes are vertical) would I suppose be to park the bench top under a friend's CNC router rig, but transport and handling would be a big deal due to the weight)

    (2) The second is the choice of dog hole layout grid. My instinct is a fairly comprehensive grid of maybe four rows of holes on about a 4x4in spacing down the full length of the bench for clamping flexibility while gluing up the laminated curves etc. I want to experiment with.

    Lots seem to go for the traditional single row down the front edge though, with a few widely spaced holes to the rear for hold downs, but this seems limiting except for traditional straight edged/rt angled cabinetry - and to be an ideology based decision as much as a practical one. Am I missing something?

    (3) The final issue is whether or not to split the top down its length as Benchcrafted and Bob Lang do - and fit a lift out infill/stop strip. The major 'pro' seems to be the additional clamping possibility, but the 'con' is that by splitting the top the structural intergrity is compromised and cross members are needed to link the legs and hold the halves in alignment. This versus the way the traditional Roubo uses the top as a structural member, with the legs set directly into it.

    Right now I'm thinking 'not', as dog holes seem likely to provide whatever clamping capability may be needed. Plus it's not a particularly useful stop location. Thoughts?

    Thanks,

    Ian
    Last edited by ian maybury; 08-24-2011 at 9:59 AM.

  2. #2
    Ian,
    I'm completing a Benchcrafted Split Top Roubo right now. I used about 4 biscuits / laminate (stay away from the ends, if you need to cut the tops to length.) My tops all ended up within 1/16" flat. They went through my planer with only very light passes needed. I made one row of square dog holes, per the Benchcrafted instructions. It's not really hard, but I was about neck deep in router chips when I finished. I am using both the Benchcrafted leg (Glide) vise and the Benchcrafted wagon vise. Jameel at Benchcrafted has been great as a source to turn to for help! I had to widen the split, since the new clamps are wider than the drawings show. Not a problem. I will warn you, those tops are heavy buggers, especially if you're working by yourself. My bench is 87" long. I'll try and help if you have any questions.
    Cheers,
    Tom

  3. #3
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    Do you have a jointer? Glue several of the 1 1/4" strips together to get a 6"-wide chunk, and face-joint it. It'll be heavy, so you'll probably need outboard support for the ends. If you don't have a jointer, there's a router-based scheme that you'll probably use for flattening the top surface. You could use it for the front edge too.

  4. #4
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    Thanks for the offer of help Thomas. It'll be a few weeks until I'm into the nitty gritty, but I'll bear you in mind for a PM when I get there - your bench is pretty similar to what I'll be doing.

    My concern is not so much about keeping the top/working surface flat, as with ensuring I end up with straight and flat faces in the vertical plane - so that the joints between the three or four glued up sections that will make up the top and the edge of the top (at right angles to the working surface) will end up straight.

    I have a Hammer A3 410 that's just been fired up and some heavy duty roller stands Jamie. My experience has been on lighter and shorter stuff over the jointer on a Robland combo where straightness wasn't really an issue though.

    The problem I'm thinking of is the possibility that the 4 x 1 1/4in on edge strips (that each of the four the sections that go to make up the top is glued up from) may be inclined to flex/curve a bit across their narrow dimension during the glue up - if only under their own weight (i.e. even if they are straight coming off the jointer/thicknesser), or due to being disturbed during gluing and clamping. i.e. they may move side to side viewed from above.

    I guess you're suggesting that the 'four packs once glued may be run over the jointer again to true the aside faces and leave them ready for jointing - that by now they ought to be rigid enough not to deflect while being handled.

    That's the plan, but I'm keen to ensure the laminated four packs are already pretty straight re. the above before they go back over the jointer. Maybe people just go ahead and glue up the strips and don't experience problems severe enough to be an issue?

    I guess I was also being a bit cautious as to (a) how consistently straight a face the jointer (it'll be tested and adjusted to perform to its best) was likely to produce over the 8ft or so involved, and (b) how anyway is straightness over this sort of length checked?

    Please pardon the lack of clarity...

    ian
    Last edited by ian maybury; 08-24-2011 at 12:34 PM.

  5. #5
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    As for the split top - I did mine that way with a flip-able center piece that stays about 1/2" proud when in the up position. I use it as a stop all the time and find it very convenient (frequently dont have to clamp this way, just let the piece hit this center rail on one side, and hit a raised dog as a stop on the next side - trapping it in a corner so to speak). Would definitely do it again.

    The structural integrity is handled by cross members on the side rails between the legs.

    Generally speaking, these things are so far overbuilt that structural discussions are more philosophical than practical (just $.02 - have you ever heard of one of these benches failing structurally??? I am pretty sure mine would support my pickup truck if I drove up on it)

  6. #6
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    Ian,

    A neat solution that I saw on an old Roubo build thread on the North Carolina Woodworking forum, is to build a beam from mdf as a reference surface. The beam in question was a type of I beam, with the web and flanges constructed of 1" mdf. There were vertical pieces dado-ed in-between the flanges (about every foot) on either side for extra support. The beam is laid across a couple of sawhorses and the laminations are glued up clamped to the beam. Hope that makes sense.

  7. #7
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    Ta guys, that leaves me clear to start drawing it up. Hear what you are saying about the split Carl - I guess that what works best depends on the sort of work you do mostly. My caution wasn't so much about the possibility of the bench failing. More that a big slab top has such attractive qualities in terms of absorbing impacts from hand tool use without bounce, and I wouldn't want to compromise that.

    What you say more or less validates the thought that that an alignment beam/reference surface may be worthwhile when gluing up the strips for the top David. I'm thinking of using a steel beam though - less work, plus it can easily be narrow enough (less than the thickness of the top) to permit the use of cauls top and bottom when gluing up too.

    On the dog hole grid pattern question. Thoughts anybody? My instinct is to head for a multi row grid as I envisage doing work that involves gluing up laminations and frames with curved parts. It looks on the other hand like the more traditional single row of dog holes with another more widely spaced to the rear works fine for work holding for hand tool use - as in this video from a few years ago by Jameel Abraham (?) : http://khalafoud.com/media/roubo.wmv

    It's impressive how much grip he's getting from a single row of dogs while planing cross grain - he's using a Benchcrafted wagon vice. Just listen to the solid 'clunk' when wood contacts that bench - that's the sort of stability/no bounce feel I'm after....

  8. #8
    Ian,
    I glued up the laminates two at a time, then joined the pairs, etc. I also glued them up horizontally, with parallel clamps on both sides not more than 5-6 inches apart. That and using the biscuits kept everything lined up. I'm looking forward to being able to use the split to place clamps through and thereby using the bench as a "monster caul" for larger pieces, like chests and so forth. Be sure to use those rollers! If one of those monster slabs gets away, nothing good will happen. Feel free to PM anytime.
    Tom

  9. #9
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    Thanks Thomas. It'd be something of a 'monster caul' all right - I'm guessing that by clamping through you may get more force than is available using hold downs and the like.

    Here's another thought/issue while we're batting this stuff around. I can't for the life of me see the point of the combined through tenon and dovetail joint used on the traditional versions of the bench. http://www.popularwoodworking.com/te...il-tenon-joint.

    As in the through tenon is fine - lots of glue area, can be wedged if needed and is contained within a mortice = lots of lengthwise racking resistance.

    But the half enclosed through dovetail is hard to figure - glue on one side only, the wedging action of the dovetail under racking forces may tend to split the side of the top away, and no face at right angles to resist racking.

    Use a second through tenon by all means if it looks like the glue area is a bit minimal - but in practice there's probably not too much wrong with a single large through tenon either????

    Yet it seems a bit unlikely that it was done for sport back in the day. Wonder why???

    ian

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