Electric Singing Saw - Spring Steel Alloy (Material Science)

[RRouse]

TL;DR Summary

Fatigue resistant spring alloy that responds to magnetism

This is a fun one, sort of!
I am working on a singing saw that can be amplified with a regular magnetic pick up, from an electric guitar.
I made a blade from laser cut1095 blue tempered spring steel (0.042" thick). It sounded great, but after a few months of service, the blade cracked.
What is a spring stock alloy that is resistant to fatigue, but still has enough magnetic permeability to be 'heard' by a regular pick up?
I'm looking into 301 stainless full hard. After being cold-worked to full hard state, the crystal structure changes to be magnetic.
I have been looking at lots of data sheets, but don't have a great sense for what values for permeability would be useful.
Has anyone worked with spring tempers that need to be magnetic?

 

[jrmichler]

Spring tempered 1095 steel is a good spring alloy with good fatigue resistance. Did it crack at a stress concentration, or at a laser cut surface? If so, you will get more improvement from reducing the stress concentration than from looking for a better alloy. If you want to look into spring alloys, here is the place to start: https://smihq.org/store/ViewProduct.aspx?id=8088348. Search stress concentration to learn more.

Or just post a photo of the part that shows the crack, and we can help you from there.

 

[RRouse]

jrmichler, thanks for your reply and recommendation. That text looks like the exact right thing.

The break did occur at an area of stress concentration, with cracks propagating from a laser cut feature. A wooden handle ("cheat") is attached near the end of the blade, for the musician to bend the saw and control pitch. A sigmoidal shape produces sound from the saw.

In my design, the cheat is attached with a carriage bolt, so that the saw player doesn't have to carry a tool to tighten the cheat. The bolt fits into a square laser cut hole in the blade.

Would switching to a round hole, drilled, not laser cut, modify the stress concentration enough to prevent breakage?

This project is replicating a singing saw that has been in use for 20+ years. I'm unsure what the alloy of that saw is. It looks like a more common alloy, with a dull gray lustre. The older blade has less resistance to deformation - it's floppier - but always returns to flat.

Pic of break is attached. Thanks for your interest!

 

[jrmichler]

I need to see exactly how you are clamping the blade, and a second photo showing what the blade looks like when it is being used. Are you springing it to a curve? Have you measured the thickness of the older blade? If it's floppier, I would expect it to be thinner. If so, do you flex the new and old blade to the same radius?

Does this image from Wikipedia correctly show how the saw is played?

 

[RRouse]

The 1095 blade is slightly thicker than the original: 0.042" vs. 0.035", IIRC. I found a pic of the saw in action, and an example of the cheat handle.
Some musical saw makers mention 1075 on their websites. Would there be a big difference?

 

[jrmichler]

Did it break at the cheat handle? Is the cheat handle what the hand is holding in the second photo? If yes and yes, then the head of your carriage bolt and the saw teeth are both stress concentrations. You have more to gain by reducing stress concentrations than by trying to find a better alloy.

When something develops cracks after extended use, and those cracks spread until it breaks, then we call that a fatigue failure. Fatigue cracks start at stress concentrations, and then propagate. The cure is to reduce or eliminate the stress concentration.

Three things you can do:
1) Change the tooth profile near the cheat handle to a rounded gullet (the groove at the base of the teeth). The teeth in that area will get pointier (more pointy). Excuse the term. Sharp inside corners are very bad for stress concentration.

2) Add a pair of wood blocks about 1" wide, and long enough to span the width of the blade, to the cheat handle. Use a hard wood, such as oak or ash. Use the blocks like washers, they are clamped between the bolt head and the blade, and between the blade and the handle. That will distribute the stress from the handle across the entire width of the blade, and thus reduce the stress.

3) Change the square hole to a round hole. This is probably the least important because it is clamped, so there should not be a lot of stress at the hole.

 

[Dr.D]

All of the saw teeth constitute stress raisers. Your failed blade pictures both show the crack starting from a tooth root. C0nsider getting rid 0f the teeth entirely.