Intro to CNC Inlay
Machine Set UP
That's all for this month. Hope to see you here again next month.
The design is somewhat traditional in that it is a climbing vine sort of thing. I like it. 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 Woody Strings. 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.' Truly, 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.
Two clean, closed vectors, the key profile and the square hole.
Once the software converts the black/white lines in the bitmap to vectors, the various vectors (which you might not see in the on-screen graphic) need to be un-grouped. This will allow you to delete the background and surrounding trash (noise) until you're left with only one vector which is the outline (profile) of the key. That's what you want.
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 mainly done in the node editing mode by eliminating extraneous points.
0.040" router bit
Although luthiery and inlay have been part of my life for many 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.
Below is a photo of a peghead inlay that Woody is working on. The photos that follow are from a test-and-learn project that required more testing and learning than I expected.
As noted above, the first step is to design the project. The design drawing was relatively easy because I have used this design several times previously. Before now, however, the cutting of the pearl and inlay pockets was done completely by hand with a Dremel tool. Although this peghead inlay received many compliments over the years, I was never completlely satisfied with the results. In part, this is the reason Woody invested in the CNC.
Now we need to convert the profile of this key into a vector. As the black/white silhouette of the key is relatively crisp, the software will create key's profile 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. For now, we will take the simple route and let the software to do the work.
First, we have to trace the bitmap which will convert the lines on in the bitmap to vectors. "Trace" here does not mean by hand. The software traces the bitmap. Here's how. Once the photo has been imported into your working 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.
Leaked light... jagged vector edges need to be cleaned up.
The design of the pictured headstock inlay 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, 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. Things happen when cutting pearl and it's better to have a re-do on a small piece if a bit snaps off; or a pearl section flies out gets in a fight with the router bit; or the operator inadvertantly 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 so the finished piece must stay in place after it has been cut free from the rest of the pearl blank. I thought that I would be clever and use spray contact adhesive on the back side of the MOP blank.
Well, the 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 is cut too large. This is truly a PIA as each piece came out larger than was planned but not by a definite amount.
Bottom line: Use CA glue to adhere the MOP to the spoilboard. Be liberal with the super glue and be careful when slipping a spatula under the work once the pearl cutting is finished.
Another challenge of this design is that every pointed leaf tip needs to be addressed in some way as noted above. There are 31 leaf tips in this design. Most have been left sharp with the pocket very slightly overcut to allow the point to drop in. The photos below shows how most of these were handled.
Two bits were used. In the first cut, a larger bit (0.045") 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 which would have meant that the pearl would have to be rounded off at the tips. I like the sharp points.
Photo of 2 of the 12 pieces of MOP that were cut for the design we're discussing. 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.
Intro to Inlay with a CNC machine
Woody has been asked to demonstrate and discuss the luthiery inlay process as executed on a CNC machine. The presentation will take place at Woodcraft Supply in Richmond, VA in January. We're still working out the details. So........ the entire month of December was spent creating inlay materials that can be easily discussed and demonstrated to a small audience.
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.
The Design Drawing
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). One must keep the design as simple as possible to begin. The word 'simple' here has specific geometric and programming definitions... explained below.
The second step is to convert the design (a line drawing) into vectors. This is done in 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 HD-4) came with this software. It is quite adequate for my purposes in instrument making as I think that you will see below.
Machine Set Up
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.
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.
Once 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.
One should understand that the 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 or cut 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 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 with your router bit. Think of designs with smooth curves and no radius smaller than the radius of your 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). 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.
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...it's only one piece...AND...it is already a fixed shape ready for inlay. All we have to do is create one inlay pocket that will accept the key.
A third aspect of this ordinary key is that it 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.
Due to its opaque-ness, the key creates a sharp silhouette against the lighted background. This is important. With the V-Carve software I can import this picture into the program as a bitmap which the software will convert to vectors. This is VERY important. The imported photo/bitmap looks like this.