I combed through the scrap bin at a lumber yard and got some beautiful cracked pieces of walnut bound for the fire pit. This big slab needed some stabilizing keys to keep the crack from growing. Usually, butterfly or bow ties are used because of the opposing wedge shapes keep the wood in place. I decided to get tricky and use the CNC to carve some star inlays instead.
I plan on adding some simple hairpin legs to this slab and use it as a bench.
Step 1: Bug With Guys at the Lumber Yard
Lumber mills cut their own slabs and mill those pieces into dimensional lumber. When there is a piece that obviously won't make a clean piece of squares stock, it is cutoff and often discarded.
Go bug the cool guys at the mill and they may have some misfit pieces that you can get on the cheap. This gorgeous slab with amazing grain and crotch figure cost me only $10.
Step 2: Map Out Your Crack
You have to start with a definitive reference point. Since I am using Aspire as my CAM software, I can chose to reference my piece from a center point. If you use Easel from Inventables, you must take the measurements from the a clean reference point to the lower left of the crack.
I found the lowest point in the crack and drew a straight line across the slab as a latitude line to which all other points will be measured.
I marked the center point on that line as my start point and divided up the segments of the crack so that I could fit enough stars to properly bridge the gap evenly.
Step 3: Digitize Your Keys
In your CAM software, make some reference lines that correspond to the center of the crack on the slab. Again, measure from the reference point you made earlier.
After setting up my material in Aspire, I started with the center point (or the lower left for Easel users) and mapped out where the keys would go, making sure to match the crack measurements exactly.
I drew some stars on the center points making sure they were big enough to bridge the crack and hold firmly in the wood on either sides whilst not touching each other, I had to turn some.
Step 4: Cut Your Male Inlays
I did this out of order, but I'll explain the correct method to make life a little easier.
CUT THE MALE PIECES FIRST!
I used hard maple for my keys, they need to be a hard wood to ensure that they won't break against the movement of the slab.
I exported the male inlay G-code from Aspire into Easel using the new G-code app and carved it out using a 0.058 inch end mill bit. I cut the pieces 0.3 inches deeper than the pocket I intended to cut so that I could sand them flat later.
I didn't cut all the way through the material so I had to use my table saw to cut up to the bottom layer of the stars. After some light sanding, the stars separated from the maple.
Step 5: Cut the Inlay Pocket
Before you save the pocket inlay toolpath, you must add some space so that the males can fit. It is recommended to add a .02-.04 allowance in the pocket inlay toolpath area in Aspire.
The males were cut first so that you can test fit the pieces before you remove the slab from the CNC. If the fit is too tight, increase the allowance, re-zero, and cut the pockets again.
I used a .04 inch allowance and I could fit the pieces in by hand with light persuasion. I cut my pockets .25 inches deep, but if your bit allows go as deep as you want. (That's what she said)
Step 6: Glue It Up and Sand It Down
Add a generous amount of glue to the pockets, but be careful, it is a crack and you don't want a bunch of glue dripping down and looking sloppy. Tap in your snug-fitting keys and let the glue dry.
After you're satisfied with the glue-up, use a card scraper, a hand plane, or a sander to knock the keys flat and start finish sanding.
I started with 120 grit sandpaper to level the stars, card scraped the whole top, used a damp rag to raise the grain of wood, and sanded the roughened top with 220 grit sandpaper. I hand sanded the slab edges to soften up the sharp corners.
Step 7: Apply Finish
As a finish, I used an oil and beeswax polish I got from Woodcraft. It works really well at protecting and softening the wood, and it smells good too.
This is the time to marvel at your gorgeous slab. Getting a piece with amazing grain lines or figure really shows its true beauty after finishing. Adding keys of contrasting woods really sets off the whole project, walnut and maple always look great together.
The keys can be made of any shape, just remember they are a structural piece first and a decorative element second. Play around with different shapes and break the butterfly/bow tie stigma!