Custom Reproduction Guitar Tuners

[ESchenk]

Something kind of fun that I finally completed. I build a few guitars a year, and or my latest project I’m doing a reproduction of an 18th Century French classical guitar by the maker Rene Lacote. One of the interesting features of this guitar is that it was one of the earlier guitars with mechanical tuners, but the tuner mechanism was entirely concealed in the headstock. This requires a completely different design for the gearbox and tuner plates.

You can buy reproductions of this style of tuner for well over a $1000 a set. Or you can disassemble some $60 tuners, salvage the worm shaft, gear and tuning post, and use your CNC to make gear boxes and buttons, and your lathe to turn new fittings and modify the tuning posts.

Here is a picture of the final tuners

And installed on a test headstock

All the parts for one (minus the right gear box):

These were taken part way through the construction process. Some of the worm gears have been cut shorter, some have not. The tuning posts have not yet all been modified. Each one received a new fake ivory sleeve, a bearing, and got cut shorter by about 5mm.

The brass gear block is held to within about 1 thou in critical bore dimensions, and about 1-2 thou in the location and dimensions of the gear boxes themselves. This was a fair bit of work getting the backlash compensation dialed in, and dealing with multiple spring passes on the final cuts to deal with the flex in the gantry. Surprisingly, over the span of my vice (4" long, 5" with oversized softjaws) I can hold the accuracy, as long as I carefully probe and locate the setup at each step.

I did find that homing the machine generally requires me to re-probe the setup, so I stopped relying on the fixtures being at known fixed locations.

Second side machining for the boring was aligned by probing on hole centers. Overall the gear box requires machining from 4 sides.

Getting the stock squared up in the left vise [I have two side by side vises mounted on a carrier board on my rig]

After machining the gear blocks main shapes from the back side:

Hand polishing out the machining marks from the back side operation before moving on:

Mounted in the soft jaws, first side machining operation done. The critical bores are established now that will locate the worm and the gear. These are two two tolerance critical steps and holes. The location of the gear box was carefully probed to ensure alignment. The result is within 1 thou for critical diameter and location of centers. It took a lot of experimentation to learn how to get this right on the Avid machine.

Second side of the gearbox machined. This bores out the a housing for the gear. This dimension is less critical, but must be a bit bigger than the gear itself to avoid rubbing.

After this, the top side (visible face plate) is brought to thickness and engraved:

The same fixture is used for each of the last 3 steps.

I also made the buttons on the CNC, using a different soft jaw fixture. Maybe I’ll post a bit more about how those were made another day.

All in all, this was a fun project, but took a lot more time than I expected as I made my way through figuring out how to do metalwork on the avid. If I had more shop space, I’d probably be better off with a metal specific mill to do this kind of work, but as it stands, I don’t have a lot of space and I managed to make things work.

Cheers,

Eric

[ESchenk]

A few more pics of the finished product.

The sleeves for the worm posts and the end pips for the buttons were hand turned on my little Taig lathe. The original tuning posts were turned down to 6mm, fitted with the sleeve cut on the lathe, press fit with and end bearing (10x6x3mm roller skate sealed bearing). The posts were cross drilled for strings and a brass sleeve was installed to pin the core post and the outer imitation ivory together.

[da-kengineer-meister]

Great work, these look fantastic. From start to finish how much time do you have into these?

[ESchenk]

Thanks. In terms of time in on these, this was spread around a bunch over the year. I’m not sure I have a solid accounting. The main gearbox was about 10 attempts, each a full days work. The planning, buttons and all the other parts was probably another 20 half days (mostly evenings) total.
A good bit of this was overall cycles of “well that didn’t work, what went wrong” as I figured out how to work to the limits of the machine.

Definitely not worth it if I was getting paid for this, unless I went into production. I figure I could turn out a new set in about 2 days at this point.

I did do some foundational tuning on the machine a while back, shimming the linear rails and adjusting to get everything flat and square to < 1 thou as measured against a reference straight edge and checked with a machinists level. That probably took me about 2 weeks of work to get right. I recheck things every few months, but out of the box the z axis was bowed by something like 20 thou in the center, and getting the rail level as the axis transits required a fair bit of tuning with the level.

I needed to this tuning to get things set up even for the guitar working use cases I am doing. I do a bunch of 4 sided machining on necks, and I was needing to do a lot more finishing work than I wanted after the fact. I moved to fixed location fixtures and got rid of the twists and bows in the machine and that really cleaned things up.

Cheers,

Eric

[jjneeb]

Very impressive. This serves as a good example of what actually can be done with an AVID if you are detail oriented.

The engraving is a really nice touch. What kind of bit is that, and how did you darken the engraved area?

thanks for sharing!

[ESchenk]

Thanks! I feel like I pushed the machine to the limits here, but I’m very happy with the results and know that I know I can do this, I’ll probably use it more for fine detail projects as well. I was very please, for example, to find that the machined bores were round enough that I could get the vacuum pop pulling the right sized gauge pin back out. I was expecting I might need to ream those holes, but for my purposes I found I don’t need to go there.

The engraving was done with a micro engraving bit from precisebits.com: https://www.precisebits.com/products/carbidebits/scoreengrave.asp. I used a 45 degree 0.005" tip here.

A couple of critical things that need to be paid attention to:

1. I never touch off these bits. The tip is very fragile, and if anything goes wrong touch off will break the tip. Instead I lower the bit down to the surface in 0.1mm increments until I am about .2mm away (check by viewing the reflection of the top) and then lower by 0.01mm steps until a 0.03mm plastic shim catches under the bit. I basically wiggle the shim, drop 0.01mm, wiggle and see when that stops working. I set zero at that level. I’ve also found this works with other bits as generally precise depth cuts about 0.03mm deeper than the touch off depth.

2. Keep the RPMs up at maximum, and the feed really slow. 100mm/min, 24000 RPM. Depth of cut on those lines was between 0.025mm and 0.015mm (thick vs. thin lines).

I’ve also tried a diamond engraving bit, but the lines are too faint for my taste.

The lines here are highlighted with Laskin’s engraving filler. I get it from Stewmac: Laskin's Engraving Filler - StewMac. Rub it on like a crayon, then polish with a soft cloth. Works great.

Cheers,

Eric

[jjneeb]

@ESchenk

gh that I could get the vacuum pop pulling the

I know what you mean. I use a program called JointCAM to do different joints, and if I enter the exact measured end mill diameter for the tool, I can make mortise and tennon joints that are almost air tight, which is actually too tight because you can’t get any glue to escape, but they are fun to play with.

Thanks for the info on the engraving. I’ve done some stuff on brass with the diamond drag tip, but I wanted to try some mill engraving and I like the look of those after they are filled. I see that bit is for cutting circuit boards, thats also something I want to try, so I guess I’ll buy a few extras

[ESchenk]

One thing I forgot to mention, those flowers are cut with a different method. Those are cut with a 90 degree included angle 1/8" chamfer bit I got from mcmaster carr using a classic v-carve style engraving to drive a variable depth cut to meet the outline. The fine lines are with the engraving bit and are just traced. If you tried to cut those flowers with the engraving bit I think you would snap the tip off quite easily.