CNC router bits and carbide cutting tools with tutorials and technical guides

Cutting Parts for Inlaying - techniques submitted by Dan Janes (Joss Cues) and Keith Josey (Josey Custom Cues)

Machining strategy and tool selection

The first step is to select a cutter diameter that is large enough to give you the desired cutting speed, but small enough to produce the level of detail necessary to accurately shape the part. Often multiple tools, used sequentially, are required to meet the demands of the job. This is shown in the aspen leaf inlay pictured at right.  The jagged, irregular contour requires the use of a finishing tool (clean-up bit) that is tool small to be effective in cutting out the entire part.

Once the tool (or tools) has been selected, a cutting strategy must be formulated. To achieve the highest level of precision, it is usually recommended that a part be initially "roughed out" with a large diameter tool to it's approximate size and shape.  This step should leave a minimum amount of "stock" that will need to be removed in a clean-up pass using a smaller diameter cutter to achieved the final size and shape. Although changing tools and making two passes may seem to be a waste of time, careful tool selection will allow high speed roughing and cleanup steps. Very often, this two step process is actually shorter than a single, lower speed pass, and the part fidelity is almost always higher. This is especially true when all tools are fitted with depth setting rings which remove the necessity to reset the cutting depth after a tool change.

Part Cut
Magnified Part Cut
A rule to remember when cutting most hardwoods is that the roughing pass should always be "climb-milled".  This means that the tool moves along it's path almost like a wheel rolling around the outside of the part (i.e. in a clockwise direction around the part). The reason for this is that the cutting resistance of the wood, combined with the rolling action of the cutter will deflect the bit outward, resulting in a part that is slightly larger than planned.  The deflection is small, so the roughing path will need to be enlarged to leave behind a small amount of stock that will be removed by the clean-up pass.  

The clean-up pass, often with a smaller diameter bit, will be "conventionally-milled" (anti-clockwise around the part).  The amount of stock left during rough-out needs to be calculated so that the cutting resistance of the wood is approximately balanced by the tendency of the bit to drift into part in conventional cutting, resulting in minimal bit deflection. Almost impossible to calculate, experience has shown that:

stock thickness = (clean-up bit diameter) / 8

A question that always comes up is "How do you hold the part in place after the rough cut? Doesn't it pop up and go spinning across the room?".  There are a number of ways to hold the part in place during both cutting operations.  Some woodworkers prefer glue or pressure sensitive adhesives.  If the part is big enough and you will be making more than one of each design, vacuum fixturing may offer a good solution.  In general, I do not like using any kind of glue or adhesive that I will have to clean off of the part before I can inlay it. There is also the very real danger that the adhesive or glue will melt and foul the bit.  Unfortunately, I rarely work on parts big enough to benefit from the effort needed to make a vacuum fixture. A very simple, and elegant solution to this apparent dilemma is to set your depth of cut a bit shallow so that you leave a thin web (0.003" - 0.005") of uncut material that connects your part the sheet you are cutting it from.  It is remarkable how much such a thin layer will contribute to the stability and fidelity of the final part during the clean-up pass.  This is especially true when cutting 2° - 3° inlaid points for pool cues with a high aspect ration clean-up or finishing bit. 

Unfortunately, shallow cutting does not work as well on brittle materials like mother-of-pearl or natural turquoise, where gluing to a sacrificial substrate with superglue or Durabond© is the method of choice.

Now it's time to cut a pocket for your part.

return to "Zero Glue-line Inlay"