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I'd like to try an effect where corner keys for a decorative box show on both outside and inside (typical approach is the cut does not go all the way through the corner with the key only showing on the outside of the box).

So it's easy to make the cut depth needed and fashion a key piece big enough, removing the outside waste from the keys as usual, but given that the bottom of the box could be close to a key, even within an inch, I can't come up with a straightforward way to remove the key waste on the inside. Even using a delicate sawing technique it seems like one could all too easily damage the bottom and/or sides and/or corners.

I have thought about pre-shaping the key with a corner before glueing in, so that just sanding is needed, but not sure what is best way to go.

Any thoughts?

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    Terminology 101: these aren't splines (no matter how commonly woodworkers today refer to them as such). These are keys, see previous Answer. Splines run along a joint, keys go across it.
    – Graphus
    Jan 1, 2019 at 8:13
  • Yeah...you are right. Kinda like dados and grooves...for whatever reason everyone calls them all dados. I blame it on the coarsening of our culture.
    – mblatz01
    Jan 5, 2019 at 15:29

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I can't come up with a straightforward way to remove the key waste on the inside. Even using a delicate sawing technique it seems like one could all too easily damage the bottom and/or sides and/or corners.

Sawing may be one of the better methods to remove the majority of the excess, although it does come with some danger a great many operations in woodwork pose a certain amount of risk of doing some damage to the wood (including sanding). This is an aspect of David Pye's "the workmanship of risk".

I was thinking I would probably try doing this first by notching the key with a sharp knife, then by doing some careful work with one or more chisels, but the size of the box may make this impossible unless using skews and furthermore there's always a chance of marring adjacent surfaces when paring flush even in ideal circumstances.

So back to sawing, a quality flush-cutting saw with a flexible blade is ideal for this kind of work. An option is to tape one side of small pull saw, and in a pinch you can also use a hacksaw blade. These two options will leave more wood proud of the surface which will require more effort to clean up.

Cleanup
Once you have removed all or almost all of the excess, you will be left with some cleanup. Sanding into a corner is tricky to say the least. Scraping is a better option in this situation, the work can be tedious but it is what it is.

A small bullnose plane may be the ideal tool to deal with the remainder and make the keys perfectly flush, but not everyone has one of these or can afford one, so careful scraping may be the preferred solution.

I have thought about pre-shaping the key with a corner before glueing in, so that just sanding is needed, but not sure what is best way to go.

I think this is actually a viable option, and actually one of the better ones if you can figure out how to do it accurately and repeatably.

Accurately aligning the cutouts in the keys to the inside of the corner may turn out to be the most critical aspect of this method. Small blocks inside the corners to clamp against (covered in tape or well waxed to protect them from glue) might prove useful.

If you want the box finished inside as well as out pre-finishing the inside of the box* would make cleanup of any glue squeeze-out easier (glue doesn't bond to finished wood). I would use either paste wax or a light coat of shellac for this myself, but spray lacquer could also be perfectly suitable. See this recent Q&A for why you might want to avoid using any oil-based finish here.

If you prefer to leave the inside of the box bare protect adjacent surfaces using tape. Packing tape is great for this as it's wide and very thin, and costs a lot less than equivalent painters' tape.

*If you pre-finish the box pieces before assembly be very careful not to get any finish on joint surfaces.

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  • If you make the key oversize, it almost doesn't matter what orientation your notch is relative to the key. As long as the notch is square, you can line it up as Graphus suggests, and then trim the outside of the key. Trimming on the outside of the box will be much easier than trimming the inside.
    – Katie Kilian
    Jan 4, 2019 at 21:38
  • I think I am going to experiment with some Dremel accessories, too...one can usually get some fine detail out of them with a little practice and patience. @charlie: yep...but the whole point of the question was about trimming the inside, because that is where the real challenge is. I was hoping for a slick tip/trick, but it may just all come down to "be careful".
    – mblatz01
    Jan 5, 2019 at 15:33
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    @mblatz01, the trick is arguably the use of a bullnose/chisel plane (assuming the box is large enough that one will fit inside and can then be moved back and forth!) but these tend to be pricey for a start, plus care is still needed. I've used Dremel-type accessories a fair bit (never Dremel branded by the way, far too overpriced) and I would only use them for the gross material removal here, if that as it's far easier to screw up badly with a power tool! Once I got close to flush I'd switch to hand methods no question, no matter how slow — this is a job where it takes as long as it takes.
    – Graphus
    Jan 6, 2019 at 5:32
  • "Once I got close to flush I'd switch to hand methods no question, no matter how slow — this is a job where it takes as long as it takes." --Agreed! My current 1st attempt plan will be to protect inner bottom with foam pad (e.g. mousepad) and the inner walls around the corners with layer or two of blue tape, and then use a flush cut tool, possibly jimmied out of a hacksaw blade, to remove waste leaving may 1/32" - 1/16" of material proud, then finish with some sanding. Will probably need to reapply finish/clear coat after all that, but there are worth things in life, I guess...
    – mblatz01
    Jan 7, 2019 at 14:55
  • I would try to aim for the excess key to be proud by approximately the thickness of two layers of masking tape at most. It's easily doable to get closer to flush than you've said, somewhere around 1/64" at max. The more wood you leave the harder removing the excess becomes..... this goes double if you end up using sanding to do the job!
    – Graphus
    Jan 7, 2019 at 15:29

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