The problem you might be facing is the parts bow when being cut. Or the box isn’t square after glue up have you checked to see how square it is after a glue up? I make boxes and can confirm they very rarely stay square.
The solution is to flatten the lid and base after the cut. Use one of the many techniques
Box making is a satisfying woodshop ritual. Keep at it.

 

As noted by Aj2, the square issue may be impacting the cut. Is this on all the corners, or just the final cut/side? I typically don't cut all the way through, and then separate with a hand tool (saw, knife), like you mentioned trying. Any movement or drift would allow that small gap. Understand how frustrating this is, as the whole purpose of this method is to get that "continuous" look between the box and lid.

 

I saw that once before on a box a friend used a table saw to cut the lid off.. He declared the issue cause by the table saw blade. Anytime I make a box I cut the lids off with a Japanese saw and never saw that issue.

 

This is likely because the box isn't square, if your table saw was off by that much you would probably see it on other cuts. Go back and check your box assembly techniques for areas where you can improve accuracy. Another thing to look at is the blade insert/guard on the table saw. If that little orange thingy isn't set flush with the table saw top you can get cut-outs like that on the last cut. Meanwhile, get out a block plane and plane the box to fit.

 

Thanks everyone so much for taking the time to write these suggestions! I will keep at it!

 

I used to use the table saw to cut lids but became frustrated. I now use my bandsaw was a sharp blade and much better for me.

 

I agree with what’s been said above, especially what BB1 said about not cutting all the way through. If you carefully set the table saw to not cut all the way through but instead leave just a paper thin section inside, the whole box stays stable throughout the cutting process and you can easily cut it apart with a sharp utility knife run down the center of the kerf afterwards. It‘s a safer cut for both you and the box and it also avoids any grain tear-out on the inside of the box from the saw blade breaking through. After I do this I just lay a sheet of sandpaper gritty side up on the tablesaw and sand both trimmed areas dead-flat to this reference surface. This whole trimming/sanding process only takes two minutes and it gives me a perfect fit on the first try. You could also do this with your example above, .01” will sand away in no time. I also have a large square of floor-sander paper for flattening much larger boxes. I keep that around just for this purpose and it’s lasted for years.

Jack

 

After the utility knife to cut through the paper thin last bit of kerf, I use a card scraper to dress the mating edges.

 

A couple of swipes with a block plane will eliminate the "step".

 

Did this yesterday on a cellarette case 18x13"

 

So many great comments above.

Not yet mentioned, but less likely:
Just curious, is the problem at the end of the cut? Could you be rotating the box slightly at the end of the cut?

 

Sandpaper adhered to a flat surface is a good way to hand this. Even if the cut is perfect you would want to take care of the saw marks.

 

The other day I saw a video from Steve Ramsey about a pretty slick way of making perfectly fitted box lids using the router table (and I think you could adapt it for a table saw too, though that’s in a whole nother zip code from my wheelhouse). It requires some planning beforehand and you probably will still need to touch up the meeting edges with a plane or sandpaper as mentioned above, but seeing as you’re obviously making some beautiful boxes this might be worth adding to your arsenal.

btw for anyone in need of a totally flat reference surface and not in possession of a table saw, get yourself a decent size picture frame with real glass in it (not the plastic crap). Even just the glass will do, if you can find someone getting rid of an old worn-out frame where the glass is still intact. Glue it to a plywood or MDF backing cut 1” larger than the glass (rout out a recess for the glass to sit down into if you wanna get super fancy, or run duct tape around the edges for a quicker solution) and you’re good to go!

 

Possibly wobbly blade of a few thou. Stabilizes when fully inside the cut. Consider checking the blade for wobble with saw off and unplugged. Set something on the table adjacent one of the teeth and SLOWLY advance the next and following teeth towards the sensor object. ATB teeth should touch...miss...touch...miss the object by exactly the same amount. Try using blade stabilizing washers, and cross your fingers

 

I use double stick tape attach blocks on the inside of the box. Then set table saw to just cut through the box material the blocks will support the kerf through the cut then simply remove the blocks and tape.

 

I make a LOT of boxes and I have often encountered this. It always frustrated me and required the after-the-fact corrections mentioned before. ...Planing the remainder of the lid or body to match the low spot. Or sanding everything - a real pain.

Then one day I noticed my box shift ever so slightly as I was cutting a lid and pushing it between the fence and the blade. Upon closer inspection, it was apparent that my zero clearance insert was not perfectly flush with the cast iron table top on both sides of the insert. Fortunately, this insert has set screws for adjustment. Working with a straight edge and square, I got it perfectly level and it solved my problem.

BTW - I use the Bob Dickey method of cutting and removing the lid. (He has a You Tube site with some really nice box building tips.) I set the fence height so that it does not cut through the stock leaving about 1/64" of wood. I then cut the lid off with a flush cut saw or just a matte knife. Doing this means that the wood can't shift as it passes by the blade.

Finally, I always plane a small chamfer on the body and lid. It adds an elegant relief and masks any unevenness you couldn't eliminate with the steps mentioned by others.

See Bob Dickey instructional here : Art N a Box

 

I have a slightly different interpretation: You're likely cutting through the full thickness of the material and the last side cut is freeing the top, but the now free piece is hitting the blade, whereas the other three sides were still supported by the other box sides.

The fix: Don't cutoff the box top. Leave a slight amount of material so that the box top remains attached to the bottom. Use a hand saw to make the final separation. This eliminates that snip you're getting, plus it's safer.

The alternative: Still following the above advice about not cutting fully through the top and use a thin slot cutter bit on your router table. The kerf is MUCH thinner. You'll have much better control because the box sits flat on the router table rather than perpendicular to your table saw. The table saw option has the potential of kickback whereas the router table option is much less likely to get you in trouble. Here's router bit that I use and if you follow my suggestion, the box top comes out perfect every time - Amana Tool - Timberline - 5/64 Kerf 3 Wing (270-20) - Three Wing Slotting Cutters - Amazon.com

 

Next time ,use a featherboard to keep the box uptight to the fence. just a thought...

 

Use a bandsaw

 

Or, just go and watch Paul Sellers do that job with just a Backsaw....

This seems to work nicely at leveling the edges up...

 

Something is not square on your saw. Check the front and back of the blade so see if it is toed in and that is spot on check the fence to see if the front to the back is square. If both of these are good then check the run out on your arbor.

 

There are many good suggestions here but badic table saw issues cone to mind. When you finish there is virtually no way to keep the good side with pressure against the fence as no feather board can work through the blade reaching over the blade to finish the cut often results in undue pressure on the forward end of the piece causing the tail to migrate into the blade I would suggest a clamp sled or double sided tape to a longer piece of stock that will allow the box cut to complete before you run the sled or guide board all the way through

 

Lid is to the outside, NOT the fence side of the tablesaw's blade..

 

The Paul Sellers method to get the edges coplanar (as pointed to by Bandit571) can be briefly seen here at about 35'48":

In other vidéo, one can see that after having turned his plane around the corner to remove the obvious steps, he makes full turns which ensure the edges are coplanar.

IMO a plane long enough to span at least the width of the box is preferable, so a block plane is not the best choice.

 

I have run into this several times. Checked all the usual culprits and evrything seems is fine. I use spacers held in place with tape.

I am concerned that all the pressure holding the box against the fence is somehow influencing the process, since I am making large boxes and am paranoid about having something slip.

On the next one, I will position the body of the box against the fence and have the lid as the offcut.

Opinions?

 

I have run into this several times. Checked all the usual culprits and evrything seems is fine. I use spacers held in place with tape.

I am concerned that all the pressure holding the box against the fence is somehow influencing the process, since I am making large boxes and am paranoid about having something slip.

On the next one, I will position the body of the box against the fence and have the lid as the offcut.

Opinions?

Thank you @WoodenDreams 🙂

IMO you are chasing your tail. Seems my suggestion of using sandpaper was lost in the discussion about getting it perfect off the saw? It’s such an easy solution.

Dialing in a lid with a hand plane is not that easy. Sanding the mating edges is.

 

When I have to use the tablesaw, that is exactly how I do that.....smaller boxes, it is easier to just use a backsaw...do the 4 corners first, then the 2 ends THEN one face,,add a sawblade into the kerf, and saw the other face...saw blade is the same thickness as the saw I am using...and, I use the sawn corners to help start and guide the face cuts...

 

Put in a few shims to make sure the two parts don't move after the final side is cut.