How Flat does the bottom of the bench need to be?

[Randy Bonella]

Hi All,
I've got a Roubo Bench build thread going but questions get lost in the longer threads besides I don't have much of an update yet but I do have this question.

How Flat does the bottom of the bench need to be?

My take is that the bottom where the legs mount needs to be flat with no winding and ideally parallel to the top. The rest of the bottom just needs to be reasonably flat, with reasonably defined as good enough to clamp to.

Am I off base on this or ok?

I don't mind spending time getting it flat but don't really want to waste the time or energy doing something that isn't needed.

Your input is much welcomed.
Randy...

[george wilson]

You could just add shims underneath the top until the sets of legs are parallel to each other. Crude way to do it,but it would work.

[John Schreiber]

I made mine nearly as flat as I could as practice for planing the top. It's not at all necessary though.

[Jim Koepke]

It seems it would not be necessary to have the bottom anywhere near flat.

As long as you ca add shims, as George suggests, make a leg slightly longer or trim a bit from a high spot you can make the top sit level. If the face of a clamp can get a hold, the rest is not important.

jim

[Randy Bonella]

Thanks for your input. I kind of figured that to be the case but thought it was worth asking about before moving on.

Now on to leg mortices and stretchers. I'm beginning to think that I'll actually get this done.

Randy...

[David Keller NC]

Randy - If you're making a true Ruobo design (where the legs will be mortised into the top), then it doesn't matter at all how flat the bottom is. However, what is important is that the legs be close to perpendicular to the surface of the top, and that the shoulders on the tenons mate well with the bottom edge.

Typically, most cabinetmakers ensure this alignment by 4-squaring the top, so making sure the shoulders fit is as easy as making sure the legs are 4-squared and marking out the tenons with a square and a knife.

But that's not the only way to do it - you can rub the bottom surface of the top with graphite in the vicinity of the legs, rough cut the tenons and mortises, and insert the legs into the mortises and give them a good whack with a mallet. Dissasembling the leg and observing which areas are marked with graphite tells you where to pare (the black marks need to be removed).

[Josh Bowman]

I made mine nearly as flat as I could as practice for planing the top. It's not at all necessary though.
+++ for John. I used it only for practice.
Now here's something to think about. If you're going to do round dog holes, you'll want to champer the holes on top, which I did. But I did the holes after the bench was together and I also needed to champer the holes underneath. I find when I put one of Jr's wonderful hold downs in I sometimes splits some wood off the bottom of the hole as the hold down is inserted. And champering it now is going to be a pain.