Tuning machine gear material

[3jaw]

I have volunteered to make a replacement set of gears (both worm and wheel) for a Gibson bass mandolin for a luthier that I know. I was wondering what kind of steel I should use for this. Would mild steel be okay or should I use something stronger like 4140? I am in the tooling design stage of the project since I will have to make tools to make tools, etc. I'm not even sure if my idea for making the worm will work although, theoretically it should.

Attached is a picture of the gears in question.

Any help would be greatly appreciated.

[garyphansen]

I would think that any steel would be fine. You might want to consider a leaded steel to make your work easier. It will make a nice smooth finish much easier to obtain than if you use crs or hrs. Gary P. Hansen

[dp]

Don't know if he mentions materials but Frank Ford has an excellent suite of pages discussing tuning machines:

http://www.frets.com/FretsPages/Musician/GenMaint/Gears/GearTune/geartune1.html

[Mike Burdick]

3jaw,

I guess I'll be a little different...

When I make small parts I like to use a material that is harder - not softer. Naturally I take smaller cuts but the final finish and accuracy seem to be much easier to achieve.

If you want some readily available material that those "worms and gears" can be made from, try some grade 5 bolts!

[SGW]

Personally, I'd probably use 1144 (Stressproof). It's about 2X the yield stregth of generic "mild steel" and it machines easily to a nice finish.

I wouldn't use leaded steel -- it has too much tendency to rust.

[Just Bob Again]

I'd be inclined to repair rather than replace. You probably already know that. "Pretty" for it's own sake isn't what you usually do on vintage instruments. I'd first look for a set of new generally similar tuners that I could remanufacture. Cutting those gears isn't easy. If you have to make replacements, the material most compatible with the original is what you should use. I'd use plain hot-rolled steel.

The worm is cut on special machinery. You can see how the root angle changes. The toolpost rotates and is geared to the spindle. The carriage is fixed lengthwise. Cross slide moves inward a few thousandths every revolution of the toolpost until full depth. I'd cut a straight worm from drill rod and harden it. Cut teeth in it. Use that to make a matching gear. Grind the gear to half thickness and harden it. Use that as a free-rotating hob on a bearing on the cross slide. Move the slide inwards until the right depth. The inverse of how people cut telescope gears using an acme tap.

[3jaw]

Thanks for the replies, folks.

I'd be inclined to repair rather than replace. You probably already know that. "Pretty" for it's own sake isn't what you usually do on vintage instruments.

I agree, but one of the tuners is beyond salvaging.

The worm is cut on special machinery. You can see how the root angle changes. The toolpost rotates and is geared to the spindle. The carriage is fixed lengthwise. Cross slide moves inward a few thousandths every revolution of the toolpost until full depth. I'd cut a straight worm from drill rod and harden it. Cut teeth in it. Use that to make a matching gear. Grind the gear to half thickness and harden it. Use that as a free-rotating hob on a bearing on the cross slide. Move the slide inwards until the right depth. The inverse of how people cut telescope gears using an acme tap.

I plan on making it as original as possible. I had thought of the rotating toolpost geared to the spindle idea as a last resort, but I have another idea that is practically identical to your second suggestion. Instead of cutting a straight worm and using that to make the cutter, I am thinking of making the cutter on the mill using a dividing head and a single point fly cutter ground to the proper geometry, adding some clearance angles, then hardening and sharpening. I do like your idea of cutting a straight worm first and using that as a "cutter to make a cutter", though.

Greg

[Just Bob Again]

I think making a cutter-cutter on the lathe is easier. Compound angles to deal with if you do it on the mill. And you still have to make a flycutter of sorts. If you have a thread pitch or feed on the lathe that's close enough, the straight worm cutter is simple. And you can use it to make the gears. This isn't precision stuff. Mostly visual as far as the curved worm goes. 3 teeth that don't fit real well isn't a lot better than one. I have mandolin tuners with a brass gear and straight steel worm that are just fine and a hundred years old.

When you make the cutter, use two disks sandwiched together for the blank. Then you don't need to grind it to get the cutting edge on center and the teeth won't have burrs.

[3jaw]

Sorry to drag up this old thread, but the gears are finished and here is a picture of them. I used 1144 stressproof for the worms and the wheels. The rotating cutter for the worms worked great except that the root diameter of the worms is pretty small and I bent and broke a couple of them before I was able to cut 4 good ones (think 18 short parting blades rotating with about 4 teeth in contact at all times )

If anyone is interested, I can post a picture of the cutter mounted in its tooling block.

[lenord]

Sorry ?

Don't be, it is good to see how a project turned out !
Nice looking set of gears there, congrats !

Lenord

[garyphansen]

"If anyone is interested, I can post a picture of the cutter mounted in its tooling block."

Are you kidding? Of course someone is interested! I will never make any but would still like to see your photos. Gary P. Hansen

[Bob Farr]

*** If anyone is interested, I can post a picture of the cutter mounted in its tooling block. ***

Beautiful work and yes, please post a picture of the cutter and tooling. I'd love to see how you did this.

[Tinkerer]

Make that two for the set up photos. You did real nice work on them. Will the thumb pieces be silver soldered in?

[tony ennis]

Very nice. I hope it wasn't a paying job 'cause if i was I bet you went broke!

[3jaw]

Okay,

Here are some pictures of the cutter and tool block.

Tinkerer,
Yes, the luthier will make brass knobs and solder them in.

This was a self-imposed challenge. I have the unfortunate attitude of "If one S.O.B. can do it, so can I!"

Pictures of the block and cutter:







The cutter removed from the block: