u/Solarbg

TL;DR: I designed a feeler gauge style device to measure the gap between your thumb and fingers (it's an occupational therapy and physiotherapy device prototype). Some people have trouble bending their fingers

I just designed and 3D printed Petal Pinch, a stackable petal gauge for thumb opposition testing.

The concept started with a milk bag tag and evolved into a gauge with rotating petals that gives tactile feedback while tracking thumb mobility.

How it works:

Hold it in one hand or place it on the table and try to pinch the exposed petal with your other. If you can touch it, slide that petal under the hummingbird and try again. The assembly stacks to about 50mm with 10 petals at 1mm and 4 petals at 10mm. I printed them in alternating colours to make tracking easier.

Learn more about thumb opposition scoring: https://mobilephysiotherapyclinic.in/kapandji-thumb-opposition-scores/

Why petals:

It's inspired by the "loves me, loves me not" petal game. Also added a little humming bird design to bring the theme all together (it's also used to store the petals).

About tolerances:

3D printing 1mm petals accurately is tricky. You'll likely get 1.05 to 1.15mm depending on your printer. I'm including a STEP file so you can adjust dimensions to match your output. So you might have to print the petals a few times to get it somewhat right.

Important:

Just to be clear this is not a medical device. I just design and 3d print devices that could potentially help people. There were no clinical studies or tests that were done when developing this device just a whole lot of trial and error and online research.

Hopefully this can help someone out. And by all means of you have any suggestions on how I can make it better, let me know. I'm always down to improve my designs.

Thank you,

Free Files available on:

Thingiverse: https://www.thingiverse.com/thing:7355682

Printables: https://www.printables.com/model/1725865-petal-pinch-thumb-opposition-gauge

MakerWorld: https://makerworld.com/models/2820455?appSharePlatform=copy

Creality Cloud: https://m.crealitycloud.com/en/model-details/6a0be3dee0b3fd73ac34b421

Edit 1: think of this device like a filer gauge. It measures the gap between the fingers when you try to make your thumb touch other fingers. The maximum distance this device does is 50mm which most people can do if they are suffering from a hand condition that limits their gripping mobility. The goal of this device is to keep reducing the gap between the fingers until you get to hopefully 0mm.

Edit 2: here's a video of it being used : Petal Pinch - Thumb Opposition Gauge (3d printed) https://imgur.com/gallery/GVWNwW8

Edit 3: to be more clear some people find it difficult to close their hand or bend certain fingers. So through Occupational Therapy and physio therapy they can do the exercise to practice the finger mobility to try and close them. In most cases they use the thumb as a reference point for contact with other fingers.

I'm working on a stackable functional assembly where dimensional accuracy matters, and I'm trying to dial in my Prusa Core One to get closer to nominal dimensions.

I printed simple test features designed as:

- 1.00 mm

- 10.00 mm

Measured with digital calipers, I'm getting:

- 1 mm feature → ~1.05 - 1.12 mm

- 10 mm feature → 10.05–10.14 mm

The problem shows up when stacking parts:

- 10 × 1 mm parts + 4 × 10 mm parts should equal 50 mm total

- Actual stacked height goes over 51.2 mm

That 1.2 mm cumulative error is too much for the assembly to work.

**Current settings:**

- 0.4 mm nozzle

- Creality Hyper PLA

- 230°C nozzle / 65°C bed (bed temp helped a lot with warping, even after washing the plate)

- 15% gyroid infill

- 4–5 perimeters (accidentally left high, usually run 2–3)

- Default Generic PLA profile in PrusaSlicer

**Questions:**

1. What should I tackle first to reduce the oversizing? Flow rate? Horizontal expansion? Pressure advance? Fewer perimeters?

2. How accurate can I realistically expect 1 mm features to be with a 0.4 mm nozzle and Hyper PLA?

I know FDM has limits, but I'd like to get as close as possible to nominal dimensions for functional stacking parts. Any direction appreciated.