Cutting String Slots In Bridge Saddles...With A Little Manual Metal Shaper

[Bruce Johnson]

As you've seen on my other threads, I'm a hopeless machinery junkie. I collect and restore old antique woodworking and metalworking machines, and use them to build my basses. I also enjoy building my own specialty machines, for those special operations that I need to do.

In this case, I found a cute little home-made machine at a flea market 32 years ago. It's a miniature benchtop metal shaper. In the metalworking world, a shaper is a very old type of machine that goes way back into the 1800's, well before milling machines. A single tool bit is mounted on the end of a long horizontal ram. It cuts in long straight strokes, nothing spinning.

The guy I bought this machine from was a senior machinist at Borg Warner in York PA. He built this little tabletop shaper from scratch during his lunch hours, over a period of a year. He did some hobbyist gunsmithing, and he built this machine to use for cutting the slots for fitting sights to gun barrels. It worked, but he found that it was easier to do the job in one of his mills. So he hauled it to the big flea market at the York Fairgrounds in 1984. And I bought it from him. I couldn't resist.

I've had it all these years and used it for a few specialty jobs. But I hadn't found a real purpose for it, until this summer. I was making up the prototype bridge for Keith's 10-string bass, and I realized what a pain it was going to be to hand file all those slots in the saddles, to accurate spacing. And we hope to build a lot of those 10-strings.

So I came up with the idea of adapting this little shaper to do the specific job of cutting the slots in bridge saddles. I machined up an aluminum block that fits on the front carriage, to mount the whole assembled bridges. I ground up a small tool bit with a V tip, and fitted it on the ram.

It worked beautifully on the 10-string bridge, saved me a lot of time.

So, now I'm using it to cut the slots on all of my bridges. Here it is last night, cutting an AMB-2 bridge.

A closer view. The bridge is mounted tilted forward 7 degrees. The ram cuts a straight groove, so it ends up angled back in the saddle. The depth of cut is adjusted by the knob on the top of the ram. The machine is manual powered, no motor. I pull that big handle forward to make the cut, about 0.005" each stroke. It's about 10 strokes per slot.

The sideways location of the cut is adjusted with the crosswise leadscrew. The graduations on the knobs are thousandths of an inch, and it's pretty accurate as it is. I used the graduations when setting the string spacing on the 10-string. On this 4-string AMB-2 bridge, I just centered the cut on each saddle by eye.

This works well enough that I'm going to upgrade the machine. I bought a small digital readout, which I'll be mounting crosswise under the aluminum block. That will give me a direct readout, with zeroing, on the slot spacing. That will help a lot on the octave bridges.

And I'm thinking I'll make up another adapter block to hold aluminum and brass nuts. And a set of ground cutters to do round-bottom nut slots of specific widths. Being able to accurately locate the slots would be a big time saver over the usual hand filing.

Anyway, a cool old machine has found its calling in my shop!

 

[Matt Liebenau]

Fantastic. I love the specialty machinery and jigs you come up with. It gives me something to think about when I’m doing a similar job by hand.

 

[Rôckhewer]

That ....just too cool for words.
Color me sufficiently jealous.

BTW... That is pretty much the same footprint size of bridge I was planning to start machining for my builds.
I see you aren't using the typical dual setscrew height adjustment...
But you have a single setscrew in the front of the saddle...
Care to elaborate?

 

[Arie X]

that's cool! your set-up's kinda like horizontal broaching. last time i used a shaper was like when i was 14. i like that little oil cup. shapers are generally huge masses of pig iron and nobody wants them nor remembers how to use them. their claim to fame was planar milling and internal keyway or spline cutting plus whatever else you could think of.

way to go!

 

[T_Bone_TL]

Good for custom gears, as well, AFAIK. They come in small and expensive and too big to move easily but cheap for the size (sometimes cheaper than the small ones) though unless you can find one that was in use or put away carefully getting all the bits and pieces is chancy.

I don't have one (yet) but I've shopped them a few times, and considered calling my riggers at least once, but the right one has not come along.

(craigslist, ignore the wood ones and the "shaper bars for snowmobiles", whatever those are, here I come, but I don't expect much on any given day.)

 

[Bruce Johnson]

That ....just too cool for words.
Color me sufficiently jealous.

BTW... That is pretty much the same footprint size of bridge I was planning to start machining for my builds.
I see you aren't using the typical dual setscrew height adjustment...
But you have a single setscrew in the front of the saddle...
Care to elaborate?

You are looking at the back side of the bridge as it's being cut in the shaper. Here's a better view of the finished bridge assembly:

It's a simple, clean design, and it works very well. I've been using these bridges on all of my own model basses since 1996. It's patterned after the bridges in the old Ampeg Scroll Basses, but I've improved the saddles and the mounting. The saddles have full square surface contact on the bottoms of the slots, and I cut them for a tight sliding fit side-to-side. Stainless 6-32 cap screws for the intonation adjustment, with a little slice of silicone rubber tubing as the spring/retainer. Too much trouble with little buzzes from metal springs.

There's no individual saddle height adjustment. The heights of the saddles and the slots are carefully cut to the 7 1/4" radius that I use on this model. Action is adjusted with the two jacking screws that raise or tilt the whole bridge. The tips of the jacking screws fit into sockets in aluminum bars embedded deep in the body. The silicone rubber tubing sleeves on the tips of the jacking screws retain the bridge on the bass when the strings are off, and prevent any buzzing down in the sockets.

An important feature of this bridge design is that the bridge is able to rock slightly forward and backward on the points of the jacking screws as the string oscillates. Combined with the extra length of the strings back to the tailpiece, this rocking motion adds a small amount of "warble" to the note, right after the initial percussive pulse. Another small reason why my Scroll Basses don't sound like Fenders. These little engineering details matter.