Official Luthiers Forum!

This is kind of a longshot but I thought I'd cast the net and see what I can find.

I'm designing a re-tuning mechanism for my instruments and I'm going to require partially threaded rod to complete the design. The rod is of a very small diameter which is making it quite hard to find. I'm still in the early stages of design so I have some flexibility of dimensions. Here's what I'm looking for:

Diameter - at least .075" but no more than .1" (I'm thinking that .08 would be ideal)
Length of thread - minimum .250" a little more would be better
Length of smooth part - minimum 1"
Pitch - looking for as high a TPI I can find. This thread will be used for final intonation so more is better.

The threaded portion major diameter can be either the same as the smooth portion or a slightly greater diameter (like on a stud). I'm also willing to machine the parts e.g. if I could find a threaded stud that could be made to fit my parameters after machining.

I've found small diameter fully threaded rod that could fit the bill if I could machine the threads off and have a smooth shaft when I'm done. I don't know how I'd hold the part while machining though and how I could hold a tight tolerance on the shaft. If someone has ideas, let me know.

The final option would be to thread the rod by myself but, I imagine that it's not easy to thread 54 to 80tpi without some sort of special fixture.

Any and all advice is greatly appreciated.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Have you thought of using bicycle wheel spokes? The threaded portion is about .25" and I imagine you could find larger diameters if you look at motorcycle wheels.

Pedal steel pull rods come in different diameters, but are most commonly 3/32" (.09375"). Usually threaded with a 4-40 die. Seems like their mission in life on a pedal steel would be very similar to the re-tuning mechanism you're working on.

Dave

Is there a forehead slap emoticon?

I've been riding bikes for about the last 10 years, 6 of which I was racing and doing all of my wrenching. It looks like a 13 gauge SS spoke might be just what the doctor ordered. 54 TPI which might just cut it.

In researching spokes, I also learned of a tool called a "thread rolling head" which will indeed roll threads. If for some reason I find that cutting up spokes is too wasteful or the pitch is to coarse, I could always pick up something like that!

Keep the ideas coming though in case there's something even closer to what I'd like.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

JI Morris Miniature Screws may well have something close in stock, and if not, they can make it. The website says custom runs as small as a few hundred. Good company, with the widest array of small fasteners. I bought 0000 screws from them before, the smallest production screws out there. About the same diameter as a .5mm mechanical pencil lead.

I would just call them with your ideal specs, and let them find the closest match. Don't rule out a stock fastener with a head, you can always cut if off. If there are no good matches, ask for a quote on a minimum run of exactly your specs.

http://www.jimorrisco.com/

can you thread the rod yourself? I think you could make those threads without too much problem.

_________________
http://goatrockukulele.com

I have to make 4-40 threads from time to time, which are not all that small, and it's a royal pain, with much less quality than can be provided by a true screw company. I am going to go out on a limb and say making high quality high TPI screws would be extremely painstaking, with inadequate results compared to what can be furnished by the pros.

Andy : The bike spokes or the pedal steel rods seem like super ideas. Not sure what your up to but you seem concerned about the TPI. At 54 TPI a full turn will only advance you .0185". At 40 tpi its .025 and of course at 80tpi its .0125. Any of these seem like quite fine adjustment.But then again I don't know what you need.Good luck in your search.
Tom

_________________
A person who has never made a mistake has never made anything!!!

Just to explain myself a little better - basically what I'm thinking of is a movable "fret" where the rod comes from below the string and shortens the string by a halfstep in pitch. Most of the tuning will be done by the distance from nut to "fret" but the fine tuning will be done by adjusting the rod by a half turn at a time or .00925 with a 54 tpi or as little as .00625 at 80tpi. Half turn because I'll need some sort of saddle type thing for the string to rest on.

Attached is a pic of a very sloppy prop I built to describe the mechanism. I want to pick up some rods so I can build a working prototype. I too am thinking that spokes are the way to go if 54 tpi will work. It might turn out that 54 is ok for the longer strings but I might need 80 for the short ones. We'll see.

If you look at the first pic, you'll see the mechanism "disengaged".

In the next pic, you'll see that the lever at the bottom has been rotated around 45 degrees and the posts have come up a bit to the engaged position. Those posts will be at the "first fret" location.

The prop is just a prop and isn't to scale or anything. The posts are .125" and are just pressed into HDPE. In the actual version, the posts will be 6mm on center.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com