So here’s a project that came in the door with a 24 hour turn around time. It’s a Delkin Fat Gecko suction cup mount rigged for iPad holding. Problem, the turn screw closest to the iPad holder had become frozen and would not tighten securely.
Now based on the position that it is usually mounted as you can see above, most of the stress is torsional. Also this unit is subjected to near continuous vibration, so requires regular tightening.
Combine regular tightening with a fixed position and the result is thread wear, spalling, freeze up, and finally failure.
While not a complete failure of the turn screw, it was pretty close and required significant working to release.
Once freed from the rest of the unit, the threaded side was clamped into a machinist vice for complete removal. This required careful application of force, as there was a significant risk to the remaining internal threading.
Finally free for layout and component inspection.
As you will see below there was thread destruction about .18″ in from the screw tip.
Luckily the extraction did only minor damage to the internal threads on the head.
Since a knob of that type using 1/4″ x 28 threading is not a common hardware store item, the first choice was to refresh the threads. This was also driven by the short 24 hour turn around requirement.
Cutting damaged threads requires careful attention and patience. Slowly lubricating and cutting a 1/4 to 1/2 turn at a time, with chip clearing along the way.
Unfortunately, things don’t always work out as you plan. The result was the removal of to much material from the threads, creating a loose fit and low clamping force. Also the stress popped the shaft from the knob head, revealing it to be a 1/4″ x 28 hex drive bolt.
Now what is common to the local suppliers are 1/4″ x 28 range hex head bolts. When compared to the original, a 1/2″ length bolt would give sufficient clamping force without bottoming out.
Speaking of bottoming out, based on a cursory inspection it may have played a role in the original bolt failure as well.
Now with a proper bolt, we need a way for it to be tightened by hand. Can’t tell the client to carry around a wrench, so we need to make a knob to fit from scratch. This also opened an opportunity for a better fitting solution, since the client expressed a desire for a lower profile on the unit.
Surprisingly enough, there wasn’t a 1/4″ hex screw / knob model in our project collection. It’s a simple enough object, so the total modelling time was negligible. Started with a basic round for draft printing speed, test fitting of bolt and head spacing.
With the clearances checked, time to move on to a more user friendly form. For this we added indents to the circular construct, creating leverage for tightening.
Once printing was completed, staged assembly of the replacement parts for comparison. Added a nylon washer between bottom of knob and top of head casing, easing tightening and reducing surface wear.
Also added a healthy dosage of grease to the screws prior to assembly for life extension.
Client declined surface finishing of the knobs as the tactile nature of the rough print provided better grip surface.
Unit returned to client ahead of schedule and received positive initial satisfaction of results. Awaiting testing and feedback for any desired changes.
Side notes :
While this was the best solution given the time constraints, it wasn’t what could be considered the best solution for longevity. The shorter bolt length could exaggerate the internal thread issues, leading to freezing failure again. Optimally we would have preferred a machined insert that passed completely through the head, creating a greater clamping force and allowing replacement as needed. Processing would have exceeded the return to service deadline, and was put on hold for client consideration.