Hi There,
I wanted to have a go at making an inlay, using the V-Engrave plugin to cut a beveled edge to the inlay and the substrate. This allows the pattern to have sharp corners, which is not possible with a simple milling cutter, doing a vertical cut. Although this must be old hat for most of you, it turns out to be sufficiently complex that I needed to make notes of the procedure. I thought I would put these out on the forum in case they were of use to anyone.
The aim is to make a sandwich of the inlay material and the substrate, where the inlay penetrates into the substrate, and then, after the glue has set, to mill away the entirety of the inlay material and perhaps a little of the substrate, to leave a smooth surface. Quite different from the approach used in marquetry, where you simply cut a hole and fill it with a flat piece of the appropriate size.
The point is that the inlay piece should fit really snugly round the edges. Looking at a section through the sandwich,
- on top there is the block of inlay;
- below that is an air-gap, to allow for slop (A);
- below that is the thin surface of the substrate that is going to get sanded or milled off (S);
- then there is the inlay material that is going to be left behind (I);
- below which is a glue-filled gap (G)
- and finally the underlying substrate material. See picture!
The air gap and glue gap are there to allow for errors in the cutting. It is important not to ground the inlay into the bottom of the substrate pocket, so that the tight fit is obtained at the sloping edges.
The starting pattern consists of a number of closed polylines or, possibly, regions. The V-Engrave cut goes around the inside of the pattern for the substrate and around the outside of the pattern for the inlay. This is then followed by a conventional pocket with a milling cutter to hog out the centers of each region. The inlay is the mirror image pattern of the substrate.
My procedure was to make four complete copies of the pattern, each in a separate layer. The third and fourth copies are mirror images of the original pattern.
0 - The original pattern. Good to keep around!
1 - For the substrate V-Engrave, the ‘Offset’ tool in the Edit menu, specifying a negative offset, creates a new set of polylines inside the existing pattern. The offset specified should be -tan (theta / 2) * (I + G) * 2, where theta is the angle of the cutter (e.g. 60 degrees) and I and G are the chosen depths for the inlay and the glue gap. Then, select everything on the layer and ‘Convert to Regions’. Parts that are so small that the ‘Offset’ tool did nothing, remain polylines. These can be V-Engraved as they stand.
[Note: If the pattern includes Regions, and their offsets overlap, it may be necessary to do some manual tweaking using the Subtract tool.]
2 - The second layer is just a copy of the original pattern and will be used to rough out the larger areas of the substrate.
3 - For the inlay V-Engrave, the ‘Offset’ tool is used with a positive offset, creating a new set of polylines outside the existing pattern. The offset specified should be +tan (theta / 2) * (S + A) * 2, where S and A are the chosen depths for the sanding layer and the air gap. With the newly added polylines still selected (only), this must be followed by ‘Convert to Region’ and then the ‘Union’ tool, which joins them together just as needed. Something of a “suddenly a miracle happens” step! Then, select everything on the layer and ‘Convert to Region’ again.
[Note: If the pattern includes parts that surround other parts (e.g. a border) with a gap that is less than the offset, the border must be handled separately, since the ‘Union’ operation fails - this is due to the procedure generating a Region with two enclosed, overlapping holes, where ‘Union’ does the wrong thing for our purpose.]
4 - In the fourth layer, add an enclosing rectangle around everything, to be used to constrain the outer inlay pocket.
Note that tan (60/2) = 1/sqrt (3) = 0.577. tan (90/2) = 1.
OK, that takes care of the layers. On to machining!
We need four MOPS: Substrate V-Engrave and Pocket and Inlay V-Engrave and Pocket, each based on the geometry above.
1 - The Substrate V-Engrave follows the regions in the first layer. It is convenient to specify a Stock Surface as -S, the Sanding depth. This allows the cutter to be zeroed at the actual original surface. The tool diameter should be 0 for V-Engrave.
2 - The Substrate Pocket follows the polylines in the second layer and should specify: a Stock Surface of 0; a Target Depth of -(S + I + G) and a Roughing Clearance of tan (theta / 2) * I, or perhaps a bit less if there are tight corners to get into.
3 - The Inlay V-Engrave follows the regions in the third layer. The Stock Surface should be -I.
4 - The Inlay Pocket follows the polylines in the fourth layer, including the outer rectangle. It should specify a Stock Surface of 0; a Target Depth of -(I + S + A) and a Roughing Clearance of tan (theta / 2) * (A + S), or less if there are tight corners to get into.
Once everything has been cut, cleaned, glued and dried, the inlay layer needs to be milled off, down to the level of the Sanding depth on the substrate, if any. This requires a straight polyline with length equal to the height of the substrate and with Cut Width equal to its width. Standard stuff!
I hope that these notes are of practical use to someone!
Bob