Sunday, June 10, 2012

Fastners for 3D Printing: Tenacity and robust pin joints. Part 4 of n

Failure is often a sign that you have successfully determined what doesn't work. We have been trying to figure out how to construct a pin joint that is compatible with fused filament fabrication 3D printed plastics, it has taken a few tries to get something that works.

On a previous project, we tried using screws which posed some design challenges and seemed ill suited to production, especially since the nuts kept falling off and the screws seemed prone to fatigue failures due to the rotating radial loads.

Next, we tried using pop rivets to construct a joint.

Parts for testing.


This washer is supposed to act as a spacer and prevent the rivet from expanding and cracking the link.

Ready for testing.

Failure. The parts are crushed together and the rivet is malformed.

Next, we redesign to support the rivet from both sides on the same link so that hopefully the joint can rotate.

A washer is placed on the far side in hopes that it will ensure that the rivet doesn't crack the joint.

You can see the stress risers in white where the outside is crushed together while the inside has expanded and cracked the link.

Next, based on a suggestion from the internet we decided to try tubular rivets. We purchased a HT-174 Hand Rivet Clincher for 1/8 Dia Tubular Rivets for $25 from Hanson Rivet, along with a handful of C4-18-ST Aluminium Rivet. I can highly recommend Hanson Rivet for all your rivet needs.

After a little practice and a delicate touch with a hammer, a reasonable joint can be formed.

A rivet press would probably make this a reasonable choice if you were producing parts in quantities of 250 or more. The way the joint is formed makes it a challenge to use these rivets to produce a joint the rotates freely without being loose. The other disadvantage is that the  joint can not be disassembled easily.

Our next attempt is to use what are apparently called 'Grooved Clevis Pins with Retaining Ring'. We are also using some wave springs to make sure everything fits tightly.

Product placement photo for e-ring pliers.

Assembled joints.

This looks like it will solve most of our design issues that have been holding back a few projects.

Success goes to those who are tenacious.


Bob Mottram said...

Using this sort of joint it might be possible to make grippers, or parts for small robot arms.

GEOFF said...

Check these put too...

I Heart Robotics said...

There may be a reason we are working on these things. ;)

Also, sex bolts are pretty nice but I haven't seen them available in smaller diameters and I am not sure how prone they are to loosening.

Jim said...

Don't forget interlocking shank rivets. They're plastic but hella strong when mated in the right size hole. Example: McMaster p/n 90243A741

"Identical parts with internally serrated prongs ratchet together. Two heads provide a finished look on both sides."

stevecooley said...

I'm so out of the loop... who is that in the background... ? it reminds me of what I've heard from Front 242.

I Heart Robotics said...

Nitzer Ebb

servant74 said...

You really should use aluminum vs steel pop-rivets.

Björn said...

Hi, does anyone know where I can get a hold of these Grooved clevis pins? I've spent days (literally) searching for them on the web. I can't access McMaster Carr from outside US unfortunately.


- Björn