Box bottoms

[Wend]

Hey folks,

I've been working on a project for which I didn't know what height I would need, so I made it oversize and cut the bottom off. This has left me with this:

The dovetails are far from perfect, but they'll certainly hold the sides together - in fact, I was surprised at how well they went. Certainly a lot easier than getting the wood flat and square!

I have some 6mm plywood, so I figure I may as well turn this into a box. It's 180x380mm, 70mm tall, and the walls are 12-14mm thick. It is not yet glued together.

The strongest box bottom would presumably be to make a groove a few mm from the bottom and hold the plywood captive. I have a router table and 3mm and 6.3mm bits. However, I'm not confident about getting the grooves to start and stop before the ends, and I think it would look quite ugly if they extend through to the faces.

So I think a rebate would be easier, and just rely on glue to keep it together. Hopefully that will be strong enough for anything it's likely to have to cope with. I think I could use those same bits to rebate the sides separately, with the faces flat on the router table, and starting and stopping short seems like it might be doable, but still a little scary. Would probably also benefit from squaring off with a chisel.

So I was thinking that the easiest way would be to glue the box together, and then get a trim rebate cutter (the default 4mm sounds reasonable) and add the rebate with the router table. I'd need to finish the corners with chisels/router plane - or I guess I could round off the plywood corners to fit the shape the router cuts.

Does that sound sensible, or am I heading off in the wrong direction?

Thanks for any advice!
Wend

 

[sunnybob]

It depends what you will be putting in the box. a rebate is strong, but if the box will have heavy stuff and be repeatedly lifted the bottom may (MAY) come loose.
Your original plan is the strongest. its quite easy to make stopped end rebates on a router table, it just needs a bit of maths and a couple blocks of wood.
Clamp the stop blocks to the fence to limit travel. Practice on scrap wood first. I always do a scrap wood run first because I usually forget the width of the cutter :roll:

 

[Jacob]

Depends what you are going to use the box for. Basically the strongest bottom would be simply to clap the ply on with glue and pins. No rebates nothing. Then the bottom sits on the runners (if it's a drawer) or on whatever's beneath it (if not a drawer), taking all the wear, with no load being taken by the sides.
Grooving the sides makes for the weakest bottom as all the load and wear is taken by the thin remains of the side at the bottom of the rebate. OK for light use.

 

[Marineboy]

It depends what you will be putting in the box. a rebate is strong, but if the box will have heavy stuff and be repeatedly lifted the bottom may (MAY) come loose.
Your original plan is the strongest. its quite easy to make stopped end rebates on a router table, it just needs a bit of maths and a couple blocks of wood.
Clamp the stop blocks to the fence to limit travel. Practice on scrap wood first. I always do a scrap wood run first because I usually forget the width of the cutter :roll:

This is bang on. The key is to practise on scrap until you’re sure that the stop blocks for the grooves are in the right place. Stronger and neater than rebating.

 

[Wend]

Depends what you are going to use the box for. Basically the strongest bottom would be simply to clap the ply on with glue and pins. No rebates nothing.

That's a fair point, but of course would leave the ugly plywood edge showing. I suppose the upside would be that it distracts from the gaps in the dovetails!

But of course, the wider the rebate, the closer it approximates this. I do actually already have an 11.1mm guided rebater, but that sounds mighty close to the wall thickness. Looks like Axminster do a couple of rebater sets that can do 8mm and 9.5mm, so maybe that's the way to go.

Then the bottom sits on the runners (if it's a drawer) or on whatever's beneath it (if not a drawer), taking all the wear, with no load being taken by the sides.

Until it's lifted up, of course!

Grooving the sides makes for the weakest bottom as all the load and wear is taken by the thin remains of the side at the bottom of the rebate. OK for light use.

Good point.


Thanks
Wend

 

[ED65]

Wend, as usual the truth lies somewhere in between the extremes presented by Jacob. He's making no allowance for the strength added by the glue in the grooves or rebates, which is not inconsiderable (remembering that glue joints can genuinely be stronger than wood).

That said, if you don't mind the aesthetics then the bottom simply glued and nailed on will be super strong. And it's certainly the fastest option!

 

[Jacob]

Wend, as usual the truth lies somewhere in between the extremes presented by Jacob. He's making no allowance for the strength added by the glue in the grooves or rebates, which is not inconsiderable (remembering that glue joints can genuinely be stronger than wood).

Would glue the bottom on too - with a bigger glued area then the slot.

That said, if you don't mind the aesthetics then the bottom simply glued and nailed on will be super strong. And it's certainly the fastest option!

 

[ED65]

*sigh* one option being strongest doesn't make the other one weak.

It's not like compromises aren't a regular staple of woodworking: bridle joints where M&Ts would be stronger; glued tenons when glued and pegged or wedged would be stronger; box joints when dovetails are stronger. I could go on. Many choices are strong enough, and that's okay.

 

[samhay]

You've made some lovely dovetails, so you should show them off.
Put the bottom in a groove. You don't have to do stopped grooves for the front and back, just the sides and this is not as difficult as you think it is. You also get to hide any (small) mistakes in the middle of the joint.

 

[AndyT]

There's another option worth mentioning - as samhay says, one through groove is hidden anyway, but if you make the other groove go all the way through, you only get a tiny square hole showing on the outside. If you patch this with a sliver of the same wood, with the grain the same way round, nobody else will ever notice it. So you don't have to do stopped grooves if you don't want to.

 

[Wend]

Thanks for the advice everyone! I think that despite your assurances, I would feel more comfortable rebating the glued box than trying to do stopped grooves, and it seems likely to be strong enough, so I think I'll go ahead and order the Axminster variable rebate set.

One quick follow-up question, though: I assume that I would be better off doing the rebate in steps rather than all at once. I could do that by setting the cutter height to say 2mm, then 4mm, and finally 6mm. But I could presumably also do it by starting off with the 3mm rebate bearing on, then 6mm, then 9mm. I'd guess I can do that without removing the cutter from the router. (I guess I'd probably need to finish off with a small height adjustment anyway, to get it just right).

So is one approach preferred over the other?

Thanks!
Wend

 

[sunnybob]

Why keep changing bearings? Lots of extra work if the material is flat sided. Just start low, and increase the height after each pass.If its soft wood, 3 or even 5 mm at a time is fine. If its hardwood, 2 to 3 mm.
As youre cutting flat wood, you dont actually need a bearing guided cutter, you can just use the fence to keep the wood steady and at the correct distance. In fact using a bearing guided bit on high sided wood free hand is quite risky as you could easily let the wood lean in or out, which would ruin the cut and maybe throw the wood across the room.
Use the fence.

 

[Wend]

Why keep changing bearings? Lots of extra work if the material is flat sided.

But if it isn't then I might get a step. Still, it's probably easier to assume that it is, and if I get a small step then deal with it later.

As youre cutting flat wood, you dont actually need a bearing guided cutter, you can just use the fence to keep the wood steady and at the correct distance. In fact using a bearing guided bit on high sided wood free hand is quite risky as you could easily let the wood lean in or out, which would ruin the cut and maybe throw the wood across the room.
Use the fence.

Except I plan to glue the sides together first, to make it easy to get everything at the same depth, so I wouldn't be able to start the cut if I used the fence.

 

[sunnybob]

If you use a bearing guided bit inside a completed box, the bearing will not go in to the corners, you would have to square the corners with a chisel.

 

[Wend]

The rebate set came with a myriad of washers, spacers and extenders, and no instructions. I assembled it in a way that made sense to me, and I guess it was good enough, as the routing then proceeded without problem.

I tried rounding the corners of the bottom to fit inside the routed area. Matching the shape didn't go too well, but with Christmas fast approaching and the box being full of other imperfections anyway, I decided to just go for it rather than trying my hand at squaring the corners with a chisel and cutting a new base.

From the top it looks fine anyway. I had been planning to plane the box sides down flush to the base, but in retrospect, I'm not sure there's much reason to do so. If anything, it would probably just emphasise the gaps.

Thanks everyone for the advice!

 

[sunnybob]

Fill all the cracks with polyfilla and then cover the entire base with either green or red sticky backed plastic felt.
It will look very seasonal and hide EVERYTHING. =D> =D>

 

[Wend]

Fill all the cracks with polyfilla and then cover the entire base with either green or red sticky backed plastic felt.
It will look very seasonal and hide EVERYTHING. =D> =D>

Nice idea, thanks!