In this post, I am going to show the process I went through to build SMD test measurement tweezers. Will they work?
I started with loose goals in mind, iterated, and made something physical daily with what was at hand.
18 July 2021 I have this idea for making Kelvin 4-Wire Tweezers using the 3D printer for connection to my LCR meter or milliohm meter. A look around the internet to see how others made tweezers on 3D printers showed several approaches but the ones with a "C" for the spring looked promising. My concern is the stiffness and perceived brittleness of PLA. A new design in Fusion 360 was drawn up rather than copying someone else's work. Some fancy "nubs" for holding were added but the main goal was to see if the tweezers had the right stiffness when printed in PLA.
They look like tweezers and act like tweezers, but they aren't stiff enough.
19 July 2021 This is the same basic design as yesterday but thicker and a small change to the tips to make them mate better when closed.
They still aren't stiff enough. The flex is so much that it is difficult to pick even small light things up which surprised me.
20 July 2021 I've started to think about cable management as well as stiffness. A box shape would be stiff and allow wires to be run to the tip in a protected fashion. The "C" spring didn't seem to be contributing that much - the flex is in the extending arms of the tweezers - so out with the "C" spring. The new design without tips was printed standing up on the printer bed which results in the weakest layer orientation when the tweezers are bent.
I was afraid this would be too stiff but it appears to be OK and it didn't break when bending a fair amount. The hole running to the tip is just large enough for two 22 AWG wires to pass. It isn't aesthetically pleasing but I can work on that.
21 July 2021 What might make more sense is to split the body which would allow proper layer orientation. I am fighting my inclination to fine-tune iterations.
Stiffness isn't bad and the "C" shaped cross-section allows it to be adjusted while providing a means to route wire and attach the tips.
22 July 2021 I gave it a more aesthetic shape today and lengthened the "arms" of the tweezers. The design is now fully parametric which makes it easy to adjust.
The hole for the 3mm screw and heat set threaded insert are too small. There need to be lateral tabs to keep the body from twisting and the tips don't close parallel. I wouldn't have made these "errors" if I spent the time I normally do on the design before printing. But it was getting late and so I set the printer to work and went to bed.
23 July 2021 Made the changes that I thought were necessary from yesterday and installed the 3mm screw and threaded insert.
It is getting there although some dimensional changes are probably in order. That should be easy enough as the model is parametric. I'm about ready to wire something up.
24 July 2021 The threaded insert hole was modified again for a better fit and the body was widened to allow larger wires to pass through. Here is the render from Fusion 360.
I have some copper foil with adhesive on the back leftover from lining the electronics cavity of an electric bass guitar made some 25+ years ago (never toss things out :-). Some foil was cut to shape and attached to the tips. Inexpensive spare multimeter leads were then cut off at the tips to allow soldering to the foil.
Resistance across the pads is low and grip is OK on large parts. However, it is awkward to pick up small parts like 0805 resistors. The plan to correct that is to fabricate some kind of "pointy" tips. Ultimately PCB tips could be used. The grandkids are visiting so it might be a while until I get back to this.
31 July 2021 The kids are gone so I made the updates yesterday. Two more tabs were added to position the halves and the tips were angled with a smaller flat tip.
I made a short 1-minute video using an LCR meter with the tweezers to pick up and measure a 100 ohm 0805 resistor.
I decided it wasn't worthwhile pursuing 4-wire tweezers. These work about as well the cheap SMD measurement tweezers I bought off Aliexpress a while back but are still fiddly. I believe that replacing the foil on the tips with PCB tips would improve functionality. I could also try fabricating metal tips but this was mostly a 3D printing design exercise and I don't really have a need for the tweezers. A bonus from the project was that I learned more about the flexibility of PLA. Thanks for reading! Comments and suggestions are welcome but I have another design in mind that I think I'll try next.