Today’s focus is all about kerf. For those unfamiliar with the term, kerf is the sliver of material that is vaporised when cut with a laser or sawn away when cut with a saw. It’s usually the thickness of whatever is doing the cutting plus a smidge. By leveraging the kerf, it is possible to cut shapes that will then push-fit into others and remain there through friction. It’s particularly helpful when working with clear acrylic, which is tricky to glue without the glue showing up. That’s how I made these acrylic tealight holders using my xTool P2S laser and upcycled COVID screens for the main cube:

And some additional prototype designs for Christmas:

Measuring kerf

There are many ways of doing this: cut out a square and use digital callipers to measure the piece and the hole and divide by two; cut ten or so shapes in a row, squish them altogether, measure the gap and divide by 10. For the finger joint corners, there’s an easier way. By going to cuttle.xyx and using their free ‘finger joints kerf tester’, it is possible to work out the perfect kerf compensation for the material you are planning to cut with the machine settings (laser) you are cutting with. Keeping in mind that the bigger the kerf value, the tighter the fit. It is possible to choose the exact measurement to enable the pieces to wedge together without needing glue.

By entering in material thickness, tab sizes and a guess of your kerf, the widget creates a downloadable SVG file that you cut with the laser using your normal settings for the material in question. Here are the cut and scored shapes, showing the tightest fit pairing and the kerf compensation. By adjusting the size of the finger and slot using the kerf, you will get a perfect fit. I used the iPad app Box-O-matic to create my box, and it adjusts for the kerf value automatically.

Getting the perfect inset fit

It’s much the same process for combining materials and working out a friction fit for an insert. Again, cuttle.xyz has a widget for this, the free ‘press fit inlay kerf tester’. This time, you test the contrasting materials and adjust the inlay, offsetting by the kerf value. The laser carves a skewed cut, so there is a difference between the top and bottom of the cutouts. This can be taken into consideration for a tighter fit.

This is a test block for acrylic in walnut-veneered MDF to give you the gist. I used contrasting acrylics to test for this project, using 3mm inlays. On this test, 0.175 compensation gave the best fit, staying in place without needing excessive force to push in. In reality, with some of the shapes I used, it was necessary to reduce the kerf a little, mainly for the sharp-angled pieces.

Design considerations

The size of the cubes is 5cm internal measurement on each side. There’s therefore not a lot of real estate to play with once the finger joints are taken into consideration. You also don’t want to be handling tiny slivers of acrylic when inlaying. In the example above, Santa will not be going into production: the ears were a nightmare to place, with the moustache pieces not far behind in difficulty. Bold shapes are best, and I have again used the Noun Project icon library for ideas and inspiration.

santa clause by Joy from Noun Project (CC BY 3.0)
Cat by Kim Sun Young from Noun Project (CC BY 3.0)
Heart by sureya from Noun Project (CC BY 3.0)

I used translucent and frosted Perspex for my inlays to allow the candlelight to come through. There’s no reason that opaque varieties won’t work for this technique.

What do you think?

So, what are your thoughts? Do you like a particular design more than the others? Which of the Christmas ones should I make for sale?

These items, and a lot more, will be for sale at the Active Arts Craft Market (Countesthorpe Academy, UK) at the end of October. You may also request them as large-letter mail orders for self-assembly, just let me know.

If you’re a laser user and want an in-person demo, I’m happy to teach you – just let me know and we’ll make arrangements.