April, 2021 

Chapter 12_   Inlay

​Play Safe, stay healthy...

​    I just got my second Moderna COVID shot Monday, March 15. No issues at all. The process makes one feel like they are a part of something much bigger... that each of us can add a couple of lumens to the light at the end of the tunnel.  



     Another challenge of this design is that every pointed leaf tip needs to be addressed in some way. There are 31 leaf tips in the design.  Most have been left sharp with the pocket very slightly overcut to allow the point to drop in. The drawings to the right show how these were handled.

     Two bits were used.  In the first cut, a larger bit (0.062") was used to hog out the center of the pocket as shown by the toolpath pictured on the left. A smaller bit was then used to widen the pocket at the tips so that the points on the pearl piece would fit into the pocket.

     The result is that 4 tool paths needed to be created instead of just one. Using only one toolpath would have meant that the pearl would have to be rounded off at the tips.  Woody likes the sharp points.​


​​​​​     One should understand that the sharp points on the key pocket vector cannot be cut from the inside with 100% precision as a router bit will always leave a rounded inside corner profile equal to the bit's diameter. The same is true of any inlay object that has sharp corners (like many mother of pearl designs). A simple square or rectangle will present the same issue at the corners. The best one can do is to use the smallest bit that you have and cut that part of the pocket just slightly oversize.

     By adding 1/10th of a mm to the toolpath profile when cutting the pocket, the key will fit more easily into the pocket but a tenth of a mm glue line might be visible. Can you see a 1/10th of a mm glue line from 3 feet away?  We are dealing here with a limitation of the mechanical system.

     An alternative is to cut the profile without oversizing and go back with an X-acto knife to cut out the corners by hand. This produces a tighter result IF one is good with an X-acto and has a steady hand. Some designs are just too small and intricate to cut out this way.

     A more fundamental alternative is to limit yourself to designs without points or corners so that all of the inside curves can be cut smoothly with your router bit. Think of designs with smooth curves and no radius smaller than the radius of your smallest bit.  The smaller the bit, the tighter the fit.      Cutting outside curves is a different story because the bit travels on the outside the sharp points (or corners) where there is free space (as when cutting out pearl inlays from MOP blanks). The square hole in the key is an example of this in our demo project.  The wood inside the square will be left in place and will fit snugly into the square hole in the key. The software will recognize that this square block inside the of the key pocket needs to be left in place. The user is prompted to select a tool for the inside of the key profile and a larger area clearance tool to clear the rest of the pocket. Two toolpaths are created.

​​​​     As noted above, the first step is to design the project.  The design shown was relatively easy for Woody because this design is in its third or fourth iteration.

  As mentioned above, originally, the cutting of the pearl and inlay pockets was done completely by hand with a Dremel tool. Although the peghead design on the left received many compliments over the years, I was never completely satisfied with the results. In part, this is the reason Woody invested in the CNC and changed to the peghead design on the right (and below).

      We need to convert the profile of this key into a vector. (A vector is a digital line that the software can read.) As the black/white silhouette of the key is relatively crisp, the software will attempt to create key's profile (outline) as one closed vector. Without the light box, or with less-than-opaque objects (like shell), a good bit of editing could be needed to get a crisp profile (explained below).

     The software will need to trace the outline of the key, that is, where the silhouette changes from dark (the key) to light (the background). Thus, the need for a sharp edge in the photo.

[Map Description automatically generated with medium confidence] ​     Once the photo has been imported into a new Vectric file, click on the photo and use the "Trace Bitmap" tool in the drop down on the left side of the screen.  The icon looks like a chicken (IMO).


​​​     Below is a photo of a peghead inlay that Woody has developed. The photos that follow are from a test-and-learn project that required more testing and learning than I expected.

     The design is somewhat traditional in that it is a climbing vine sort of thing. I like it well enough. I have modified it in the past couple of years.  It's balanced. It entertains the eye for a few seconds. But, IMO, it's no great work of artistic expression. However, when the inlay is tight and crisp, the whole presentation is attractive enough.

      One thing that all should realize about Woody is that the challenge of precise craftsmanship is the reason for just about everything that goes on at the Woody Strings shop. Once the inlay is finished or the guitar is strung up and hanging in the shop, that's it. I've had my fun and I'm moving on to the next project. An often-repeated analogy applies here: "It's not the destination, it's the journey." For Woody, guitar building, and inlay are that kind of journey.

     At Christmas, when my unfamiliar in-laws invariably ask me the price of my guitars, or how many guitars I've sold, or built, etc. I just reply... 'I don't know.'  This is the truth. I do not track that sort of thing because that's not really why most luthiers ARE luthiers. It is CERTAINLY not to become rich or famous.

     OK, enough philosophy. Bottom line_ inlay is a great outlet for those who seek a challenging method of creative expression. 

​​​     What I learned from that experience is that CNC inlay is doable even for someone who has no experience creating and executing toolpaths and has to teach themselves everything from scratch. I also learned that there are some small, legitimate-quality CNC machines on the market. So, I bought a new Nextwave, Shark II HD-4 for less than $3,500 in 2017.

  Learning to program a CNC machine requires one to dedicate themselves to a steep learning curve for several months... but the HD-4 comes with some very good software, online instructional videos, and live support. That machine is featured in the remainder of this presentation.


     The design of the headstock inlay pictured above is not for beginners. Noted here are some of the challenges that one will face when setting up the CNC to cut the pearl from blanks and cut the pocket into a veneer material like rosewood or ebony.

     First of all, in Woody's design, there are 12 pieces of pearl to cut accurately. In part this is because the shell stock that is readily available comes in 1" x 1-1/2" pieces, or smaller.  On the flip side, cutting pearl designs that have multiple twists, turns and switchbacks is better done in small increments. Unexpected things might happen when cutting pearl. It's better to have a re-do on a small piece, say, if a bit snaps off; or a pearl section flies off the spoil board and gets into a fight with the router bit; or the operator inadvertently dials up wrong tool path; etc.

     Woody has had all of these wonderful experiences and more.  One that must be noted immediately is the method of holding the pearl in place while it's being cut.  The design must be cut completely through the pearl stock. The finished piece must stay in place after it has been cut free from the rest of the pearl blank. Once, I thought that I would be clever and use spray contact adhesive on the back side of the MOP blank to hold it in place.

     Well, the spray adhesive works in the 'hold down' department, but the glue collects on the flutes of the router bit and changes the chip load significantly. As a result, the profile of the pearl piece was cut too large.  This is truly a PIA as each piece came out larger than was planned but not by a definite amount. Each pocket needed to be enlarged slightly.

     Bottom line: Now Woody uses CA glue to adhere the MOP to the spoil board. Be sparing with the super glue but be sure that there's glue on the entire area that will be the finished shape.

   Once the pearl cutting is finished, warm it, and the substrate, briefly in a microwave oven and be careful when slipping a sharp spatula under the work to pry it loose from the substrate. It doesn't take much heat to loosen the CA glue, but it cools rapidly. Work efficiently.

     Let the micro bit soak in a tray of acetone when finished. This will soften any CA glue in the flutes. Then, use a stiff tooth brush on the bit.


     Traditionally, inlays of mother of pearl in a guitar are common... but not really a skill that the luthier has in his bag. Truthfully, MOP inlay, if done by hand, is more of a jeweler's skill. Woody will confess that he has had only mild success attempting to inlay an artistic pattern with only a Dremel tool. Consequently, Woody's hand-tooled inlays have been limited to circles and dots with a Forstner bit. Or, when adventurous, Woody attempted to cut a limited number of easy shapes with rounded corners using a hand-held Dremel and a down-cut bit. None of these attempts yielded the results that satisfied Woody.

      There are many videos online which explain different techniques for hand-tool inlay of MOP. Woody has found, however, that his ability to inlay Mother of Pearl is not representative of the effort and quality of work required to build an acoustic guitar.

     Bottom Line: Woody has never felt like his hand-tooled MOP inlays were representative of his skills as a luthier... until he started using a CNC machine. The discussion of guitar inlays will begin there.

          Bottom Line: If you're breaking bits, the chip load is probably too high. Check your spindle speed and feed rate first.

     Most bit manufacturers publish charts of maximum recommended chip load for standard bits. You might have to ask your supplier for it.

     The CNC software should have well-developed tutorials on how to do all of this, in step-by step instructions and rules of thumb for safe operation. When learning to use the micro bits that are required for inlay, one is almost bound to break off a few bits. At $25 apiece, micro bit breakage will quickly teach one the limits of feed, speed, and chip load. Woody cannot teach you that in this format. Sometimes, experience is the best teacher (but always play safe).    

     Sleep on it and come back tomorrow.     


     Woody spent some money and had his last name cut out of MOP by a small shop down in Texas somewhere. They did a fine job. Then, it was up to Woody to carve the pockets that the MOP letters would drop into. For that I used a CNC machine that my son, Mark, built while at college. I still have that machine. It took a bit of coaxing through Autocad and two more unique programs to interface with Autocad and write the toolpaths that made it work. In the end, the system worked OK for carving out the pockets required to inlay the five letters of our last name... much better than I could have done with a Dremel.


     The photo above is of 2 of the 12 pieces of MOP that were cut for the design we're discussing (MOP-1, MOP-2). One can see that the bottom piece was cut from a different blank of MOP as it reflects light a bit differently. This is not really a problem. In fact, it adds the perception of texture to the design as a whole. 

     Although luthiery and inlay have been part of my life for quite a few years, CNC inlay is a New Beginning. I like both the challenge of creating the design in an electronic format and the precise, mechanical execution of the CNC.

     The above discussion is bound to raise some questions. For now, don't sweat the details, just understand that we are setting up the CNC to rout along a vector line. To do this, we have to show the software a picture or a drawing of the shape, and the software will program three stepper motors to drive the gantry along the appropriate X, Y and Z axes to follow the vectors. Because the router is attached to the gantry, it just goes where the gantry goes. Experience will tell the operator what feed and speed to program into the gantry movements.

     The feed, speed, depth of cut and the geometry of the bit determine the chip load which is an indication of the stress on the bit. Too much chip load on a micro bit and the bit will snap off. Calculating the chip load is simple arithmetic.

   Chip Load = Feed Rate (inch per minute travel)/ (RPM x number of flutes)

     There is more to know than I can describe here regarding setting up the proper feed and speed of the CNC machine; selecting the correct bit size and number of flutes; setting the best router speed; executing the proper sequence of toolpaths; and the resultant chip load. Fortunately, the expertise required for inlaying MOP in a guitar fretboard or headstock is minimal.     The effort here is to inform the reader that these factors do exist, and it will take a little time to see how they work together.  Below is a sample of feed and speed factors and Woody's calculated chip load for various micro bits used in the Woody Strings shop

      An ALTERNATE METHOD of cutting the MOP blanks into the finished pieces is to set up the blanks on a common centerline and then develop a different toolpath for each blank. To do this, one has to re-re-establish the zero point on each MOP blank and build the toolpaths around each one separately.


      Once the software converts the black/white lines in the bitmap to vectors, all of the various vectors (some of which you might not see in the on-screen graphic) need to be un-grouped.  This will allow you to delete the background noise and surrounding points until you're left with only two vectors...the profile of the key and the profile of the square hole inside the key.

​     In the picture below, you can see that the key leaked some light around its edge near the nose of the key. The vector line is, therefore, jagged and needs to be cleaned up manually.​ This is done in the node editing mode by selecting and eliminating extraneous points.  ​  


​     Anyone interested in CNC inlay should know that the process requires detail plan and execution in three steps. Each step must consider the next and, in the end, the process must be executed on a mechanical device which will convert electronically generated toolpaths into mechanically executed profiles and pockets. The materials used here will be selected for the demonstration purposes of this article.


       First step is the design drawing. This can be done at your kitchen table but steps two and three dictate some considerations that impact the design (like bit size and stock materials). Beginners really should keep the design as simple as possible. The word 'simple' here has specific geometric and programming recommendations... explained below.


       The second step is to convert the design (a line drawing) into vectors. This is done through a computer program designed for CNC milling. The software that Woody uses is called V-Carve by Vectric. This software may be purchased separately but the CNC that I bought (NextWave, Shark II HD-4) came with this software.  It is quite adequate for my purposes in instrument making as I think that you will see below.


     The final step is the mechanical considerations necessary to set up the CNC machine itself (i.e., bit type and size; feed and speed of cut; stock materials and layout on the spoilboard; etc.)

     It takes a while to make sure that the mechanical requirements of the machine (step 3) are accounted for in steps 1 and 2. As one is learning how the processes integrate, there is usually a good bit of going back and forth.  The discussion presented here will, hopefully, allow one to see the three steps as one integrated process instead of three separate steps (which it is not).

     The next few paragraphs and photos will attempt to make clear what I have outlined above.  The discussion here is for the novice but challenges of more complex design will be pointed out and easily recognized.  There are many tricks to learn in the inlay process presented here. So, for our discussion, we will keep the design SIMPLE.

Note: The CNC set-up process has many steps that the novice will not be familiar with. Don't let that bother you, the process is strange to everyone the first time through. The idea, here, is to just get an overview of a computer-controlled routing process.

      Since the computer cannot see, we start by giving it a picture of the shape that we want to inlay.

     Let's take a look at this key. It is an ordinary key with a square hole in it so that it can be put on a key ring. For demonstration purposes, we will inlay this key into a piece of rosewood. At first, you might be thinking, "What about keeping it simple?" Well, this key is simple in a couple of aspects.

     #1) it's only one piece...AND...
     #2) it’s a fixed shape ready for inlay, i.e., we don’t have to draw it first.
     All we have to do is create one inlay pocket that will accept the key. That is, we don't have to cut the design out of MOP and create a pocket in a piece of wood to accept it. We just need the pocket.
     #3) This ordinary key is opaque.

     That is, it's metal so it blocks all light. The photo that you see above was taken while the key was laying on a paper towel which was back-lighted on a Woody-made light box. (16"x16"x4" wood box equipped with two fluorescent lamps and a string of LED lights.) The box has a plexiglass top. We used this light box in checking the soundboard joint surfaces before gluing.
     Due to its opaque-ness, the key creates a sharp silhouette against the lighted background. Being opaque simplifies things. With the V-Carve software I can import this picture into the V-Carve program as a bitmap. As a screen shot of the Vectric software the imported photo/bitmap will look like this. The software will next convert the shape outline into vectors.  But there is some clean up to do first.

​​​​​     When you have clean, closed vectors, you must check that you have a router bit small enough to fit into the tight inside curves of the key profile.  ACTUALLY, this should have been done before choosing the key as a project.  If you don't have very small bits (less than 0.100"), they can be purchased at one of a few miniature bit manufacturers like Harvey Tools and Precise Bits online. The bit used here is a carbide End Mill that has a 0.040" diameter.