Official Luthiers Forum!

There are several techniques for cutting shell, from choice of bits to holdfast and zeroing. For those of us
using CNC I would like to hear your thought on your various techniques.
My biggest issue is how and where to set the zero when using irregular shape shell blanks.
Thanks,
Tom Armstrong

I'm sure there are better ways, but I glue my shell blanks to a scrap of MDF with Titebond, that I then hold it to the machine with vacuum. I take a sharpie and mark a dot in the approximate middle of each blank, and move the blank around until it is situated at my X0 Y0 location on the machine. Finally, I set my Z0 with a touch plate sitting on top of the shell. When I'm done, the MDF goes into a bucket of water and the shell comes loose.

I match up the reflective angle on all of the pieces of shell I'm going to use so that they all reflect the same direction, and I don't end up getting one position marker, for instance, that goes dark when all the others go bright. I mark the top of each piece of shell before cutting. Before I soak the MDF, I mark the top of each individual inlay.

I've attached a picture of how I normally do fingerboard markers. I nested them in VCarve Pro, and then try to situate the pearl to get as many inlays as I can out of the piece. As you can see, there are plenty of partial inlays , but I get enough complete ones that it's not a big deal. Odd shaped blanks present some issues.

Dave

Dave, thanks for sharing.


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_________________
"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

Thanks Dave. Similar to what I do when multiple pieces of the same shape and size is requires. My biggest issue is “one off” shapes and sizes that are “tight” fits for the shell available.

I still like cutting pearl by hand, so I don't use the CNC for one offs typically. But one thing that I have done in the past for bigger inlays is to cut the shape of the inlay into the spoil board I'll be using without having the pearl in place yet. Then I can glue the piece of pearl to the spoil board (without moving the spoil board on the CNC table) to a position where the outline is no longer visible. That has worked fine for me on more complex shapes, but shapes that I will use more than once.

Dave

That’s a good tip Dave. I’ll bookmark it for future reference.
Tom

I measure and mark one edge, hold the shell blank up to my monitor and simply trace around the piece. I typically use the upper left corner as my zero. I prefer medium CA to glue the shell to a piece of 1x. It needs to be long enough to clamp to my t-slot table, typically 18” or so. Once I’ve cut the part, I will use a sharpie to mark which way is up. I then take the piece of 1x to the bandsaw and cut the shell free, leaving a 1/16” or so of wood. I head upstairs, drop the piece in a pan and boil for a couple minutes. The CA will remain on the wood and leaves zero residue on the shell.

I take the time to do it this way because I want my fret-markers to come from the same piece and to have the same chatoyance.

Cheers, M

Took me a while to wrap my head around this but I like it. Didn’t know CA would release in boiling water.
Thanks,
Tom

Me either until I tried it.

I use Satellite City CA. Not sure if other products will behave similarly.

Best, M

Clearly worth the time to get the chatoyance the same on all of the inlays. A banjo building friend of mine, Kevin Enoch, did a fantastic Alice in Wonderland themed banjo a few years back. Different characters at each fret marker and in the peghead. Kevin is a great builder, inlay artist and engraver. He did the Cheshire Cat at the fifth fret out of black MOP with a white MOP face. By taking advantage of the chatoyance angles, he was able to make the cat disappear except for his grin into the ebony fingerboard when turning the fingerboard just so to the light. Told me that he had to go through a lot of pieces of black MOP to get just the right one. One of the best inlays I ever saw. Kevin first taught me how to engrave pearl, and I later followed up learning from Grit Laskin. I hope to some day get up to their level, but I'm getting old...

Kevin used/uses a lot of sea snail for inlays, as have several other great banjo makers. The chatoyance of that stuff is simply incredible.

Dave

The inlay sounds amazing! Would love to see it -

I have a picture of it somewhere--I just can't find it. The only picture of that banjo I can find on the Internet is the peghead, with the White Rabbit inlayed in it. Another very nice inlay...

If I can ever lay my hand on the picture, I'll post it.


Dave

Went out on Pinterest and found the headstock, incredible work!

M

For holding down....two sided tape.

For cutting?

Start by creating three successively deeper chamfer cuts WITH A SMALL BALL nosed mill. These aren't actually making chamfers....you'll be setting it ONLY to define the edge/corner. This will remove material that would otherwise chip out of you started by attacking the edges with a square mill.

Get this....normally a machinist will try to cut a chamfer only after the profile edges are cut. If you do the chamfer (again not a real chamfer - but rather - a chamfer type cut with zero chamfer occurring - but rather - only edge defining) kiss cut that defines the edge FIRST....there's no material to grab and chip in the during profile/contour cut.... it's already gone.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Thanks Stuart. I had not thought about chamfering first. I’ll order an appropriately sized ball end mill and give it a whirl.
Tom

...and do the same thing when you're developing the recess for it to glue into. I used this technique when cutting splintery woods (Wenge) and dropping in these logo pieces. Actually I use it for EVERY cut on wood that COULD tear out and on ANY intricate cut. It improves your odds against tear out by many times.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I like that shadow effect on the SK. Very nice.

Dave

I'm a bit late, since I check the forum once a year, but for posterity:
1- Glued shell down to hardboard backers with hide glue, drop it in water after and everything falls off clean. I'd used tape (can contaminate your mills) and CA (might also contaminate mills by melting) before I settled on hide. You can nest the pieces 1.5 cutter widths apart, which is really economical on material, and the hide will hold onto a piece of shell mostly reliably if it's 0.125" in diameter.

2- Used very high RPM air spindles (65-90K), but those have since gotten insanely pricey (Air Turbine Tools used to sell an all-aluminum version for 1/10 the price of the current 602JS). You can now get very high RPM electric spindles cheap! High RPM eliminates issues with chipout and allows smaller diameter cutters.

3- Did multiple depth cuts, blew out the dust with air in between passes or did a spring pass to clear it. That shell dust packs in there good. Cutters lasted waaaaaaaay longer. Same is doubly true for milling fret slots.

4- Tools from PreciseBits. Many places make cutters from better carbide now, but they've been making good ones for 20 years and you get a lot of inlays out of one cutter - at least a couple sheets of Abalam cut into mostly small pieces at production speeds with a 1/64" cutter.

5- If you're trying to align cuts precisely to a piece of material, draw a couple scale lines (like a 1" square) on the shell or backer then take a picture of it and use it as a background image in your CAD.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services