Tolerancing slots for snap together joinery

I am playing around with one of the free cnc designs from Woodsmith’s CNC basecamp.
It uses tab and slot joinery to make a small tool tote. They demoed with Baltic birch. I am wanting to build one with the two sided white board/Chalk board sheet good from Home Depot (which is 0.1915 inch thick).
I had gotten the files imported last summer (then school started…and I was teaching an overload). Getting back to it, what tolerance should there be on the slot width versus tab thickness. The files from woodsmith have the slots at 1/4 inch. Since my board is a little over 3/16 that would leave me about a 1/16, will that be too much? What is a reasonable tolerance?


Link to CNC basecamp ep 10 with totes

The first time that I make a joint like this, I cut the first part at the actual/ideal size and cut it free completely, then I cut the joinery for the second part, but leave it fastened in place on the machine, and do a test fit — based on that, I then adjust the file for added clearance, or finish the cut.

I will note that if the parts are not all in the same orientation as will be used when fitting them together that you should calibrate for belt stretch:

Maybe this will help. I originally found this style from a Youtuber A Glimpse Inside I liked these and designed these to fit my tools.

I did the tab and slot joinery (box joint) and used 1/2 Birch ply for my holders. I sized everything based on the number of tabs and slots that fit the width of the holder and the thickness of the material I used. The brace was a tab and slot and was cut the same as the main holders and the slot was slightly over length to allow for the square corners. I did try a relief cut but found it was not needed when the slot was oversized.

The tabs were the same width as the opening. Example one of the holders had tabs that were .8 in width and fit into the opening that was also .8 wide. The fit was very good and even with glue it slid together easily. Also notice the relief radius cut to eliminate trimming the square corners.

I am working on a tool box much the same as the one you are doing for my grandson and will be doing the joinery the same way.

Hope this helps


The tab lengths are not where I am struggling (I’m doing a similar solution having tabs end where the curve starts.) My issue is slot width vs stock thickness. So if I had perfectly 1/4 inch thick stock. The slot, however long it may need to be in the design) needs to be 1/4 wide plus some tolerance so it will fit.

For an example, if i want a tab that is 2 inches long and the stock thickness in the other dimension. The slot would be two inches long plus the curve of the cutter. The question is how wide of slot (groove) do I need to cut. Stock thickness plus the tolerance.
I’m probably way overthinking this, but going from the dxf adjusting slot widths can be a bit on the tedious side.


Hi John

Measure the thickness of your divider and the length of the divider that will go in the slot.
Change the original slot measurements to the new measurements. I would not worry about the fit as long as the slot is at the same size as the divider. you will be sanding the divider to remove burrs and that will leave plenty of space for a good fit with glue.

If you measure the slots in my file you will see that all the slots are cut to the thickness of the material.This made for a tight fit after sanding everything and I didn’t need any clamps.

You can always make a test cut and if needed adjust if it is tight or loose.


I made these snap-together lanterns for a friend with a boutique shop.


The stock is 1/8" hardboard with one white side and measured 0.128 - 0.132 thick.
I went through a couple trials, but ended up with the slots set at 0.140, and the 1/8" tool undersized at 0.123.
So my slots ended up with a nominal dimension of 0.144" (or +0.014). This allowed for easy assembly without any slop after assembled.

This is what we used to refer to as, “Slop Fit” or “Clearance Fit”.

For the record, and “precision of language”, this is not “Tolerance”. This is generally “Fit”, or in this specific case “Clearance”. Slots would be dimensioned as 0.144" (±0.002"), with the 0.002 being the tolerance. Or more aptly 0.140 (-0.0 +0.004) in this situation, based on the design intent.
Your tolerance will come from mechanical properties of the setup. Tool deflection, machine calibration, etc…

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Tod would you please post the c2d file.

It was a commission for a customer. But here’s a similar 3x3x3 box

3x3_snap_box.c2d (248 KB)

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