Mostly-3D-printed stethoscope; looking for ideas to finish

I’m looking for a little help. I have a neighbor who is a nurse who works with COVID-positive patients. I’ve provided a few hundred ear savers for her department, and now she has a new problem: The disposable stethoscopes they are issued for working with COVID patients function poorly and are small, forcing close contact.

It turns out that there has been formal medical research into 3D printed stethoscopes, and she found this project that she asked me to help her try out for her unit:

This is a mash-up of the business end of a mostly-3d-printed classical stethoscope with an adapter for a microphone, and Android and iOS apps to listen and look (thus a stethogram).

It wasn’t too hard to print. It has to be 100% infill for acoustic reasons, so I had to tweak flow to avoid over-extrusion. But now the difficulty: It needs a precise 40mm circle of report cover material (the same thing we’ve been using for face shields) as the drum of the stethoscope.

I’m trying to figure out how to cut a precise 40mm circle without paying $125 for a punch, and without using a compass and putting a pinhole in the middle of the diaphragm for the drum.

I’d love ideas on how to cut out this nice precise 40mm hole. 1.5" punches are cheap in the US, but that’s 38.1mm; too small. So 40mm punches apparently have to be imported from Japan.

There has to be a better way!


Laser cut. Find someone with a laser local to you.


Here I sit with 75% or so of a design for a laser cutter, wishing I were a bit further along on that project.

The only problem I see there is that the report covers I’m using are PVC, and the C isn’t great for laser cutting. @NedMan am I overthinking this? Is cutting a few holes in a thin PVC sheet actually not that much chlorine and it’s just like hanging out at the pool?

After I posted, I did get the idea that I might be able to make a punch. I formed a 40mm die on the lathe, and I found a piece of steel that I can bore 40mm holes in for the next step.

There are a few forum members who are almost local to me, one direction or another.

If you could use PETG then you would be fine laser cutting it, the PVC is a definite no in my books.

A diaphragm was cut from a Staples brand PVC report cover (Swing-lock report cover, clear with black spine; UPC 718103160223) by turning a sharp caliper and creating a circular diaphragm with a 40 mm diameter.

The acoustical model was validated against a high-quality commercial stethoscope with PVC, and changing to a different type of plastic might affect frequency response.

I don’t know what they mean by “sharp caliper” though.

OK, I designed a circle cutter in FreeCAD that I hope to be able to use an M3 screw to clamp down on an exacto blade sticking out just far enough to cut the sheet. This is printing overnight:

If that doesn’t work, I also cut a 40mm punch die out of cold-rolled steel on the lathe. I can use it on a piece of sacrificial MDF or masonite for a first test, but I could also make a proper punch by boring some 40mm holes in steel and lining things up carefully. I don’t have a chunk of the right steel to harden, though, so if I use that the edge is probably going to wear quickly.

I’ll start with the circle cutter in the morning, and if it works I’ll post the design in gitlab.

You could find someone with a cricut cutter or another type of hobby knife cutter.

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Oh, that’s a thought. I guess what I’m printing overnight kind of is like a single-purpose cricut! :wink:


I wonder about using a modified hole saw as a punch.
I made something similar to cut sandpaper circles. Work marginally well.

Cut the diamonds off this and then sharpen the edge to a cutting bevel.
Put material on a piece of wood and use hammer or press to punch the material with this tool???

After a cup of coffee I said to myself… self just search for a 40mm punch on amazon…then there was this…

Now I need one of these to punch sandpaper :slight_smile:

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On a related note, my brother-in-law who was a doctor started doing video appointments when the pandemic started. He noted that stethoscopes were not provided to the patients he saw. He pointed out that an exam cannot be done without a stethoscope. I wondered why remote patients were not given an adapter for their phone so they could send an MP3 stethogram file to the doctor.
What I found is that digital stethoscopes cost >$200… seriously.
I started to think about a design using a cheap head connected to a phone, I missed this already completed design…


Yeah, I found those too, but the edges looked double-beveled in the pictures. Maybe it was a trick of light though? The more I think about it, the more that surprises me, so probably it’s just a trick of the light…

The cutter I designed last night worked great except that the pieces are just a tiny bit too big, because in one place I forgot to account for variation in extrusion width. I have some design ideas to make it easier to use, too. Other than that it works great, so I just have some design tweaks to do.

Yeah, that is the same thing that led my neighbor down this path, especially thinking about things that might come into contact with a COVID-positive patient and need disinfection. She found that some are as much as $500!

With this adapter, I’d think you could label a head for an individual patient and keep it in their room, and just hook up the microphone you carry with you to the head for the patient. And the plastic parts could be removed from the electric parts and the plastic parts sterilized.

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Third iteration circle cutter printing now, with feedback from beta testers (my kids):

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What I can’t figure out is even with this head, how do you generically across all types of phone acoustically couple the head to the mic in the cell phone?

The Stethogram still needs $25 in electronics to get the data from the head to your phone.

I wonder if there’s a bluetooth microphone that could substitute for the wired microphone in the stethogram?

Not sure I understand the question?

In the design you sited above, the phone is connected to the scope via a phone compatible microphone connected to the phone’s headset port?
The microphone is inserted in an adapter in the stethescope head the other end connects to the phone.

I understand how this one works.

This was in response to your comment about “a cheap head connected to a phone” with respect to your brother-in-law doing phone consults, and whether there was a way to not have to buy $25 worth of electrical components (microphone, cable, standards transposer adapter) but instead use the microphone in the phone, reducing the cost by an order of magnitude.

Ah… I see.
I don’t think an electronic head that was $50 would be to expensive.

Perhaps a chamber that works the opposite way that the acoustic amplifiers for phones work.

You probably don’t need electronics for 25$. A lavalier mic for less than 1$ might be enough:


Yeah, the $25 for mic + cord + adapter stuck in my craw a bit.

Needs the right plug for a phone, though. The TRS plug on the one you found won’t work with at least most phones. Here’s one that at least sports a TRRS plug:

At most that would need a CTIA-OMTP converter.

I kept trying to find a bluetooth lavalier mic and so far haven’t found one; that would be better and not need a physical phone connection.

Also, I have discovered that the microphone adapter in the Stethogram project probably needs to be redesigned from scratch. It was modeled in fusion 360, and the fusion 360 thread modeling doesn’t have proper thread termination for 3D printing, so the thread trails off to a thin wedge. It also has no accounting for necessary clearance; it offered only H7 and I couldn’t change it. So the only way I can screw the two halves together is to cut off the wispy ends of the threads, and then chase both sides with an M16x2 tap and die. The threadlib I use in OpenSCAD doesn’t have this problem and I’ve designed and printed things that have working threads using OpenSCAD before, but after over and hour mucking about trying workarounds in fusion 360 last night I gave up.

And if I’m redesigning it, I could design it around a more convenient microphone.

The head is modeled in OpenSCAD, I think, so possibly I could integrate microphone support directly. I need to redesign anyway, so why not just make it simple? :slight_smile: I’ll ponder that.

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